Prediction of the Kamchatka July 29, 2025, earthquake by the evolution of low-magnitude seismicity recovered using waveform cross-correlation at IMS seismic arrays (Part 2)

 There are dozens of REB events in Figure 21 that are not matched in the XSEL. The reason behind this underperformance could be related to the WCC processing and the quality of the REB events obtained during the interactive review. Figure 22 displays the frequency distribution of SNR values for the first P-phases associated with all REB events on July 30, 2025. All SNR estimates are sourced from the IDC database. The pattern is controversial. There are 5241 arrivals with SNR>10, which are high-quality signals not shown in Figure 22 because they are well matched by the XSEL. The curve between 0 and 10 includes 15802 values in 0.2-wide bins. There are 4872 detections with SNR<3.4, with 244 of them having a default value of -1.0 and fixed at the SNR=0 point.  They fall below the automatic detection thresholds at the IMS stations and were added by IDC analysts. The peak of the distribution below 3.4 is around 2.0, with 2049 arrivals having SNR <2.0, and 321 with SNR <1.0. 

 

Figure 22. Frequency distribution of SNR values reported by the IDC for all P-phases associated with the REB events on July 30, 2025.

           

The quality of REB detections added by analysts is formally defined only by their experience and the task not to miss events of concern. The SNR estimates for the added signals, assuming they are correct, pose a challenge for any detector, including those based on the WCC. These detections were missed by the IDC automatic detector primarily due to their low quality. Furthermore, detections lacking SNR estimates should not be included in the database, as their quality is crucial for GA processing. The XSEL, when based on routine WCC processing, may encounter internal limitations in recovering events during periods of extremely high activity, which consequently affects the quality of the REB events.

 

Statistical power

The XSELs include numerous events with statistical significance controlled by the LA parameters for a given detection list. The question of their statistical power is related to the match of REB events in the XSELs with various LA versions and origin-time tolerance. The strict LA version with low tolerance has to be adjusted to create event hypotheses matching the REB and a small number of new XSEL events which could match the REB-potentially missed in the IDC processing and recovered by the WCC. Table 4 lists the match statistics for two LA versions and the origin-time tolerance of 2.0 s, as adopted by the REB.  The first number in the “XSEL matched” column is for the weak version and the second for the strict version. The “REB only” and “SEL3” columns split the “REB total” into automatic and interactive production stages. “XSEL LA matched” refers to the match by XSELs for individual MEs before the CR is applied.

 

Table 4. Statistics of REB and SEL3 match in the XSEL for the period 2025201-2025210.

Day of 2025	Q	REB total	XSEL matched	REB only	XSEL matched	SEL3	XSEL matched	XSEL LA matched	New XSEL
201	Q2	109	104 / 81	27	22 / 8	82	82 /73	108/96	113 / 15
	Q3	75	70 /63	14	12 / 8	61	59 / 55	74 / 69	120 / 17
	Q4	51	50 / 44	8	8 / 3	43	42 /41	51 / 48	113 /27
202	Q1	31	30 / 29	4	3 / 2	27	27 / 27	30 /30	120 / 24
	Q2	45	38 / 37	10	6 / 5	35	32 / 32	41 / 41	129 / 24
	Q3	44	42 / 41	10	9 / 5	34	33 / 33	44 /42	141 / 25
	Q4	23	22 / 18	5	4 / 1	18	18 / 17	22 / 18	121 / 23
203	Q1	55	52 / 46	20	17 / 13	35	35 / 33	55 / 50	102 / 16
	Q2	31	30 / 30	4	3 / 4	27	27 / 26	30 / 30	116 / 23
	Q3	20	20 / 18	8	8 / 6	12	12 / 12	20 / 18	118 / 24
	Q4	11	10 / 10	2	1/ 1	9	9 / 9	11 / 11	112 /18
204	Q1	4	4 / 4	0	0	4	4 / 4	4 / 4	101 / 24
	Q2	5	5 / 5	1	1 / 1	4	4 / 4	5 / 5	123 / 33
	Q3	11	11 / 11	0	0	11	11 / 11	11 /11	134 / 47
	Q4	9	9 / 9	2	2 / 2	7	7 / 7	9 / 9	91 / 19
205	Q1	7	7 / 7	1	1 / 1	6	6 / 6	7 / 7	99 / 24
	Q2	10	10 / 10	3	3 / 3	7	7 / 7	10 / 10	105 / 17
	Q3	6	6 / 5	3	3 /2	3	3 / 3	6 /6	133 / 37
	Q4	22	22 / 22	5	5 / 5	17	17 / 17	22 /22	131 / 28
206	Q1	16	16 / 16	4	4 / 4	12	12 / 12	16 / 16	164 / 45
	Q2	4	4 / 4	1	1 / 1	3	3 / 3	4 / 4	137 / 36
	Q3	2	2 / 2	0	0	2	2 / 2	2 / 2	129 / 33
	Q4	6	6 / 6	0	0	6	6 / 6	6 / 6	122 / 23
207	Q1	3	3 / 3	0	0	3	3 / 3	3 / 3	87 / 15
	Q2	4	4 / 4	1	1 / 1	3	3 / 3	4 / 4	107 / 28
	Q3	9	9 / 9	1	1 / 1	8	8 / 8	9 / 9	90 / 19
	Q4	4	4 / 4	0	0	4	4 / 4	4 / 4	107 / 37
208	Q1	0	0	0	0	0	0	0	110 / 24
	Q2	4	4 / 4	1	1 / 1	3	3 / 3	4 /4	89 / 16
	Q3	0	0	0	0	0	0	0	93 / 27
	Q4	5	5 / 4	2	2 / 1	3	3 / 3	5 / 5	89 / 24
209	Q1	3	2 / 3	2	1 / 2	1	1 /1	3 / 3	131 / 29
	Q2	2	2 / 2	0	0	2	2 / 2	2 /2	84 /  16
	Q3	1	1 / 1	0	0	1	1 / 1	1 /1	80 /  13
	Q4	1	1 / 1	0	0	1	1 / 1	1 / 1	135 / 38
210	Q1	2	2 /2	0	0	2	2 / 2	2 / 2	127 / 33
Total		635	604 / 555	139	119 / 81	496	472 / 454	626 /593	4103 / 921

 

The period between the J20 and J29 earthquakes is likely the best time to assess the statistical power of the XSEL events. Two days following the J20 are characterized by rates ranging from 100 to 30 REB events per 6 hours. The second quarter-day (Q2) of 2025201 includes 109 REB events. The XSEL for the weak LA version matched 104 out of 109, and the strict version matched 81. This difference between the weak and strict versions is relatively large, but it is decreasing with time from the J20. Since the second half of 2023203, there is no difference in the matches between the weak and strict versions for SEL3 events or for the pure REB. The watershed is likely 30 events per quarter-day. The larger the number of REB events the lower the match rate, and the reason for this deviation is discussed in the previous section.

The final column in Table 4 lists the number of new XSEL events. The weak LA version produces 114±19 events per 6 hours, whereas the strict version produces 26±9 events per 6 hours for the studied period. This difference is expected, given to the design of the detection threshold, which results in a decreasing number of XSEL events as the pair order number or the corresponding magnitude threshold increases. The number of new events in the strict LA version is slightly larger than 50% to 100% of the number of REB events with the expectation of approximate equality, as related to the exercises with interactive review of XSEL bulletins. For the 0.25-second tolerance case, the number of XSEL events after the 2025203 ranges between 2 and 11. This pair of the LA version and tolerance case is designated as the transition to the REB. The statistical power of the XSELs used in this study is demonstrated by a perfect alignment with the REB, encompassing both actual and potential REB-ready events.

The importance of the results in Table 4 lies in the equal statistical significance of XSEL events that match the REB and those that do not. This implies that a complete match of the REB events during the period of relatively high activity between July 22 and July 29, 2025, can be extended to other XSEL events, thereby assigning them the same quality as the REB.  In turn, the XSELs for the period between 201 and 210 of 2025 were obtained using the same procedure as before the J20 earthquake. The WCC processing introduced in [Kitov, 2026] and further developed in this study can be applied to IMS data for earthquakes across various regions. This approach guarantees both statistical significance and statistical power for the XSEL event hypotheses.

 

Results

            General aspects of WCC processing and its specific characteristics across different levels of seismic activity have been discussed. The seismicity within the studied region before and after the J20 and J29 earthquakes was also examined. The selection procedure and structure of the ME set were customized to fulfill the processing objectives. All calculations in this study were performed using identical WCC settings, ensuring direct compatibility throughout the entire processing period and across specific areas. The results include XSELs for individual MEs as well as the final XSELs obtained via the CR process. The full processing period spans from July 12 to July 31. Additionally, the best 100 MEs were subjected to an extended processing period from July 6 to August 3. Finally, two 3-day periods without REB events were processed using this 100 best ME set, providing a baseline reference for XSEL interpretation.

 

XSEL seismicity during low seismicity and periods without REB events

            The two 3-day periods without REB events allow for the estimation of recurrence curves exclusively for XSEL events. The first period, spanning November 29 to December 1, 2023 (jdate = 2023333-2023335), can be extended into the second quarter (Q2) of day 2023332. Additional calculations were performed for December 2 (5 REB events) and December 3 (1 event) to analyze the transition from a quiet period to the activity captured in the REB events. Similarly, the quiet period from February 24 (2024055) to February 26 (2024057), 2024, was also extended by two days exhibiting REB activity. Because this study focuses on rapid fluctuations in seismic activity below the IDC detection level, the data are processed in quarter-days intervals (Q1-Q4). Given that the activity levels can be exceptionally low, with zero XSEL events across several consecutive quarters, a four-quarter moving sum, denoted as MS(4), is calculated with a one-quarter step to ensure robust event statistics.

            The results of the WCC processing using the best 100 MEs are displayed in Figure 23. The upper panel shows the recurrence curves, where the x-axis represents the order numbers. These XSEL recurrence curves demonstrate the feasibility of converting these order numbers into magnitudes; furthermore, deviations from the regression lines are potentially related to fluctuations in seismic activity near a specific magnitude level. The curves between 333.125 (Q1/333) and 335.875 (Q4/335) follow exponential trends, exhibiting deviations from these trends that increase in amplitude as the order number increases. The summary curve for the entire 15-quarter quiet period yields an R²=0.98. On day 2023336, 5 REB events occurred, and two curves 336.875 and 337.125 - show a dramatic 3- to 4-fold increase in seismic activity throughout all the curves. This predominant growth is concentrated within the mid-range tolerance values of the strict LA version, peaking between 1.0 s and 2.0 s. A similar behavior is observed in the curves for some, but not all, quiet days.  The most prominent deviations occur toward the end of the quiet period, while the summary curve shows a low-amplitude deviation for a 1-second tolerance.

 

Figure 23. The recurrence curves of the moving sum of four consecutive 6-hours intervals for 12 XSELs from v₁c₁ to v₂c₆ with order number from 1 to 12.  Upper panel: days between Q2/2023332 to Q4/2023337. Each curve is marked by its end quarter:  curve 333.175 is the sum of 4 quarter-days between Q2/2023332 and Q1/2023333. Lower panel: the days between 2024055 and 2024059.

 

            The recurrence curves illustrate the integral characteristics of the XSEL event distribution over a linear scale, such as a magnitude-like one. This representation is important as it proves the quality of the observational tool and the unambiguous conversion of the order number of the version/case into a magnitude scale. The scale is not standard in terms of the measurement procedure underlying the values. There are many magnitude scales, each with slightly different characteristics, taken from different parts of the signal wavetrain from the first P-wave to Love waves. An inverse representation of these scales is also possible similar to that described for Figure 18.

The time evolution of the seismic process for a set of magnitude thresholds is crucial for understanding the earthquake preparation process. The numbers of XSEL events for 12 thresholds are shown in Figure 24 for the quiet period 2023332-2023335 with an extension into the days 2023336 and 2023337 with REB events. The XSEL curves, which represent the MS(4) of the previous four quarter-days, look smooth and synchronized for each LA version over the entire period including the quiet and active seismicity periods.

 

Figure 24. The evolution of the number of XSEL events in 6 weak cases (upper panel) and 6 strict cases (lower panel). Vertical dashed line – the origin time of the m_b=3.95 event occurred at 17:22:35 on December 2, 2023. Hypocenter is ~100 km due south of the J29 earthquake: 51.60ºN, 159.86ºE, d=0 km.

 

The origin time of the largest event on 2023336 with m_b=3.95 is indicated by a vertical red line. There is a noticeable increase in the XSEL numbers starting two quarter-days before this event. The only deviation is the drop in the 0.25 s curve for the strict LA version, which began approximately 20 hours before the earthquake. The ratios of the weak and strict LA versions for all six cases are displayed in Figure 25. These curves highlight the variances in seismic activity above the respective thresholds. The 0.25 s ratio curve begins to  rise three quarter-days before the earthquake, peaks roughly 8 hours prior to the earthquake, and then declines as it approaches the red line. A similar pattern was observed in the 0.25 s curve for the May 24, 2013, Sea of Okhotsk earthquake, while the other five curves exhibit different behaviors. The resemblance in the peaks in the 0.25 s curve is likely coincidental, but similar patterns before other major earthquakes are the objects of this study. For the 2023336 earthquake, the increase in the 0.25 s curve may be observable due to the absence of larger-magnitude seismic activity during the previous 96 hours.

 

Figure 25. The ratios of the XSEL for the weak and strict LA versions and the same origin time tolerance case. The curve for the 0.25 s tolerance has a peak approximately 10 hours before and then falls just before the earthquake at 17:22:25. This peak corresponds to the fall in the XSEL for strict LA version in Figure 23.

 

            The 3-day quiet period in 2024 is shown in Figure 26. It displays a pattern similar to that observed in 2023, with a peak in the 0.25 s ratio curve. Figure 27 depicts this curve starting to rise 5 quarter-days before peaking a day before the event on February 28 at 11:11:31. The epicenter coordinates are 53.93°N, 160.34°E, with a depth of 75.4 km, and m_b=3.85. This peak is observed long before the event origin time, and the event itself is not large. Therefore, there may be no causal link between the peak and the earthquake. Nevertheless, the peak and the events are measured independently and are where they are.

There were no significant changes observed in the XSEL numbers throughout the entire period between February 24 and February 29. However, the strict LA version demonstrated larger variations. The day of February 29 was added to the initial interval due to a sudden rise in the 0.25 s curve right up to the end of February 28, as observed in Figure 27. This rise is related to the decreasing 0.25 s curve for the strict LA version in the lower panel. The only REB event on February 29 occurred at 19:37:35 at a depth of 499 km, far to the west of the J29 epicenter (51.45°N, 151.28°E). The magnitude was 3.47, large enough for the event to be detected by almost all IMS stations worldwide.

igure 26. Same as in Figure 24 for the quiet period between 2024055 and 2024057 extended into 2024058-061. 

Figure 27. Same as in Figure 25 for the quiet period between 2024055 and 2024057 extended into 2024058-059.

 

Seismic activity prior to the J20 earthquake

            During the quiet periods, the seismic activity detected by the WCC is nearly identical to that observed during the periods of low activity periods preceding the J20 earthquake. Figure 28 displays the trajectories of the running four-quarter moving sum, MS(4), of the quarter-day-long XSELs between July 6 and July 20, 2025. This period was fully processed using the best 100 MEs and presents a baseline reference for relatively low seismicity prior to the J20 M7.4 earthquake (see Figure 19), which featured only one large REB event on July 14 (origin time: 21:38:01, epicenter: 46.66°N, 151.30°E, depth: 87 km, Mw~5.7-5.8). Results for both the weak (upper panel) and strict (lower panel) LA versions, each including six origin-time tolerance cases from 5.0 s to 0.25 s, are shown.

            Several intervals with significant variations in the MS(4) are marked by ovals. Between July 10 and 12, there was deep through, with all curves for both the weak and strict version changing almost synchronously. The strict/0.25 s curve dropped down to 1 XSEL per day during three consecutive quarter-days. There were four quarter-days without XSEL events in a row and only one XSEL event during a day and a half. There were also no REB events on 2025189. Given that the final REB event on 2025188 occurred at approximately 16:00 and the first event on 2025191 appeared at 06:00, the total event-free period lasted roughly 38 hours. The strict LA version with a 0.25 s origin time tolerance exhibits approximately the same sensitivity as the REB.

            This serves as an interesting example of a rapid 2- to 3-fold decline in low-magnitude seismicity within a very large geographical region, especially when compared to the subsequent 38-hour REB-event-free period between the morning of 2025197 and the late evening of 2025198. During this latter period, all curves for the weak LA version rise and then fall back to their initial levels. In this case, the strict/0.25 s curve also demonstrates growth, albeit emerging from a trough at the end of 2025196. 

Figure 28. The evolution of the running MS(4) of the quarter-day number of XSEL events between  2025187  and 2025201. The weak (upper panel) and strict (lower panel) with six origin time tolerance cases (from 5.0 s to 0.25 s) each are shown. Several intervals of with significant variations are marked by ovals.

 

            Figure 29 compares two LA versions and two cases, highlighting the periods of synchronous and asynchronous behaviors across various version/case permutations. When seismicity fluctuates simultaneously at all magnitude levels, the corresponding ratios evolve in sync.  Conversely, the asynchronous onset of growth and decline may be attributed to waves of low-magnitude seismicity passing through the magnitude thresholds that correspond to the respective version/case order numbers. The weak LA curves are nearly synchronous, with the exception of the final quarter-day (201.125, representing the first six hours of July 20). Specifically, the weak/0.25 s curve, which remains systematically below the weak/0.5 s curve, exhibited a sharp increase to align with the 0.5 s curve level. The strict curves did not mimic this behavior; consequently, the weak-to-strict ratio for the 0.25 s tolerance increased, as illustrated in Figure 30.  This is an example of low-seismicity progress through the magnitude range confined between the weak and strict versions.

           

Figure 29. Comparison of the 0.25 s and 0.5 s cases illustrating the synchronous in asynchronous behavior of the two LA versions and two cases.

 

Figure 30. The ratio of weak-to-strict LA versions for six origin tolerance cases between 2025187 and 2025203. The event at 21:38:01 on July 14 is marked by a black vertical dashed line.

 

Periods of significant decline in the strict/0.25 s curve compared to the weak curve were observed several times during the period between 2025187 and 2025201. The weak-to-strict ratio rose to high values, albeit against the background of decaying seismic activity. A prominent spike reaching a value of 38 occurred in the 0.25 s ratio curve in the second quarter-day of 2025189, when the XSEL count for the strict version fell to 1. A broader peak was observed on 2025195, when the strict curve dropped to a level of 3. Similarly, a sharp peak in the fourth quarter-day of 2025196 was induced by an abrupt decline in the strict LA version. This specific point marked the onset of growth in the strict version at a faster rate than in the weak version. This corresponding peak likely indicated the transition to the event on July 20.

            The two weeks preceding the J20 earthquake demonstrate a variety of peaks and troughs in the MS(4) of quarter-day XSELs and in the ratio of the weak and strict MS(4) XSELs. These peaks are most prominent in the curves for the 0.25 s origin-time tolerance, which defines the magnitude range closest to the corner magnitude of the REB recurrence curve and generates the most statistically significant XSEL event hypotheses. The XSEL ratio for this specific case may reveal the preparation for an impending earthquake, provided it is induced by an increasing number of events in the weak LA version, as was previously observed for the Sea of Okhotsk earthquake.

An ex post analysis allows for the differentiation of two distinct classes of peaks in the ratio curves. The first class is associated with a rapid drop in the number of events in the strict XSELs, accompanied by a much slower decline or no decrease at all in the weak XSEL version. The resulting peaks are sharp and exhibit large amplitudes, which are likely associated with periods of extremely low seismic activity. Statistically, recording only one to three events per day within a highly seismic region of a size of approximately 2000 km × ~1400 km is inherently characterized by statistical uncertainty. In contrast, the peaks in the weak-to-strict ratio curves observed prior to the J20 earthquake are supported by more robust statistics within the strict LA version.

The 0.5 s ratio curve also demonstrates peaks in nearly the same time intervals; however, these peaks are systematically lower, whereas the underlying MS(4) XSEL curves remain higher than the 0.25 s curves. The 0.5 s ratio curve exhibits a peak along the growth trend two quarter-days prior to the J20 earthquake and then decreases slightly. This behavior differs from that observed before the Sea of Okhotsk earthquake, where the 0.5 s curve began to rise a few quarter-days before the 0.25 s curve, after which both curves peaked synchronously right before the mainshock. This sequence was interpreted as a wave of events with an increasing magnitude progressing through a set of magnitude thresholds defined by the increasing order number of the LA version/origin-time tolerance case.

The four other curves also exhibit minor shifts toward the peaks of the 0.25 s curve. There is an episode of countermotion near 2025189, where the other five curves form a trough while the 0.25 s curve approaches its peak value. The significance of XSEL bulletins with the largest origin time tolerances depends on the sensitivity of the observational networks and the magnitude of the earthquake under study. For the Sea of Okhotsk earthquake, all six XSEL ratio curves contributed specifically to estimating the level and tracking the progression of the seismic activity wave from the lowest magnitudes to the mainshock. In contrast, for the study of the Kamchatka earthquakes, they likely serve only as a baseline reference, assisting in the reconstruction of the recurrence curve, such as those presented in Figure 18. 

 

Seismic activity between the J20 and J29 earthquakes

The period between the J20 and J29 earthquakes is of special interest because it involves seismic activity that is significantly higher than that observed prior to the July 20 earthquake. Although the J20 aftershock sequence decays rapidly toward the J29, as illustrated in Figure 19, it could pose a potential challenge for the WCC processing to distinguish between the REB matching and new XSEL events before the J29 mainshock. While the REB matching statistics in Table 4 illustrate the efficacy of the WCC in detecting REB events, it remains a question whether the WCC can differentiate between the mixture of low-magnitude J20 aftershocks and J29 foreshocks, or if this mixture represents a single, continuous process of earthquake preparation. Up to this point, the XSELs included all events, encompassing both the REB-matching and newly detected ones. The latter subset of XSEL events does not differ significantly from the total version for quiet days, and could therefore be utilized instead of the total number for earthquake prediction purposes.

The period between the J20 and J29 events, however, is not characterized by seismic quietness. Figure 31 compares the total number of events with the number of new XSEL events for both the weak and strict LA versions under the 0.25 s and 5.0 s origin-time tolerance cases. The difference between these curves represents the number of REB-matching XSEL events. It rises across both versions and for both cases. Within the strict version, the gap between the 0.25 s and 5.0 s cases is small on the first day after the J20 event and practically disappears after the J29 mainshock. For the weak version, the 5.0 s total and new curves virtually coincide after 2025207, whereas the 0.25 s curves converge only two days prior to J29. In contrast, the 0.25 s strict total and new curves retain the gap until the final minute prior to the J29. Overall, because the total XSEL counts inherently include J20 aftershocks up to the onset of the J29, these aftershocks and the J29 foreshocks cannot be distinguished. Nevertheless, the J20 earthquake failed to follow the J29 evolution steps, suggesting that the overall preparation for the mega-earthquake may not be disrupted by a relatively small loss of the total elastic energy accumulated in the entire region in J20 mainshock and its aftershocks.

 

Figure 31. Evolution of weak (upper panel) and strict (lower panel) MS(4) numbers in the total XSEL vs. only the new events XSEL. Two origin time tolerances are used: 0.25 s and 5.0 s.

 

The evolution of the MS(4) for the  total XSEL curves between 2025200 and 2025211 is presented in Figure 32. Six curves representing all cases within the same versions move nearly synchronously, displaying only a few minor deviations. Furthermore, curve pairs sharing the same origin-time tolerance case appear highly similar across both LA versions. The principal divergence among the curves in Figure 32 is observed during the day preceding the J29 mainshock. The 0.25 s curve for the weak version demonstrates a clear upward trend, whereas the strict version exhibits a downward trend common to all six cases, punctuated by a minor  peak two quarter-days prior to the J29 rupture.

The weak-to-strict ratios for the six origin time tolerance cases in Figure 33 illustrate a synchronous evolution of the MS(4) metric, characterized by an overall positive trend from 2025200 to 2025211. The most pronounced variations in these ratios are confined to the 0.25 s and 0.5 s cases. This pattern is common to all events analyzed thus far: the Sea of Okhotsk, the J20, and the J29 events, as well as the two quiet periods. Following the J29 earthquake, all six ratio curves drop sharply to a baseline level of 2, a feature that is directly linked to the abrupt drop in the MS(4) levels illustrated in Figure 20. Because of the smoothing effect inherent to the running sum window, the first three quarter-days following the mainshock still incorporate MS(4) values from the pre-seismic interval, thereby smoothing the transition to the lower post-seismic baseline. A ratio value of 2 corresponds to a regime where the number of newly detected XSEL events is negligibly small compared to those matching the REB bulletin. In other words, the magnitude threshold for the WCC processing is shifted very close to the corner magnitude of the total recurrence curves in Figure 15 or 17, depending on the respective depth estimates in the REB and XSEL.

 

Figure 32. The MS(4) curves between 2025200 and 2025211.

 

The weak-to-strict ratios for the 0.25 s and 0.5 s tolerance cases in Figure 33 rise from a level of 4.0-4.5 up to 7.0 during the last two to three quarter-days prior to the J29. This peak is observed within the last 5 hours preceding the mainshock, which occurred at 23:24:48 (IDC origin time). The last hour of 2025210 was excluded from the pre-event WCC processing. This last hour was also omitted from the post-seismic processing because both the REB and XSEL are severely biased by extraordinary noise levels. There is a study devoted to the recovery of aftershocks in the initial minutes following the J29 [Kitov et al., 2026]. The exclusion of this hour should not introduce any measurable bias into the weak-to-strict version ratio, as it affects both versions proportionally and exhibits no discernible influence on the three cases ranging from 2.0 s to 5.0 s. The 1.0 s case demonstrates an increase, peaking near 5.5.

 

Figure 33. The ratios of the number of XSEL events in the weak and strict LA versions for the period of time between 2025200 and 2025215.

 

The peaks in the 0.25 s and 0.5 s curves immediately preceding the J29 mainshock, with the depth fixed at the free surface, represent a potential short-term earthquake prediction trigger. A similar precursor was previously observed prior to the Sea of Okhotsk M8.3 earthquake.  The consistent behavior of low-magnitude seismicity in both cases provides compelling evidence in favor of such a hypothesis.

             

Recurrence curves for the ME sets

            The best 100 MEs constituted the initial configuration covering the entire studied region. Thus far, this specific set has been utilized in the calculation of the XSEL bulletins, as well as the corresponding time series of the MS(4) metrics and the weak-to-strict ratios. There is another ME set consisting of 195 REB events, each featuring a mandatory associated P-wave arrival at station PDYAR. This latter set also covers the same geographical area and depth range but was selected from the REB after 2023, following the upgrade of this station to an operational array at the end of 2022. A cumulative total of 295 MEs were utilized to create the XSEL events; both constituent sets are illustrated in the left panel of Figure 13.

The expanded set allows for the investigation of several critical problems related to the precursory peaks observed in the 0.25 s and 0.5 s weak-to-strict ratio curves in Figures 30 and  33. First, it is necessary to determine which of the two configurations exhibits greater efficiency in generating XSEL events, given their subtle variations in spatial distribution and sample size. The top 100 MEs are likely characterized by superior sensitivity and resolution, a feature supported by their selection procedure, which relies on a comprehensive pairwise WCC analysis with neighboring REB events. The ME set anchored by station PDYAR is larger and directly benefits from the high sensitivity of this array to seismic events within Flinn–Engdahl region 19 (Kuril–Kamchatka).

The subsequent question involves resolving the geographical provenance of the XSEL events that drive the precursory trigger shown in Figure 33. The earthquake preparation processes may structurally encompasse the entire regional volume bounded by coordinates 45°N - 65°N, 145°N - 165°N and a depth range from 0 km to 700 km. Alternatively, the preparation zone can be restricted to a small spot, such as the J20 aftershock sequence in Figure 12, concentrated in the lithosphere, or a broader spatial envelope corresponding to the J29 aftershock distribution. To test these hypotheses, the total 295 ME set is partitioned into four distinct geographic domains in the right panel of Figure 13:

·       The entire region including the subducting Pacific plate.

·       The localized zone surrounding the potential asperity that arrested the slip propagation of the J20 event along the eventual J29 rupture line (hereafter referred to as the "Asperity" zone, which excludes a single deep-focus event within its boundary and incorporates 49 MEs).

·       The remaining spatial domain outside this core zone, designated as "Out of Asperity".

·       The southwestern periphery situated beyond the immediate J29 aftershock zone, designated as the "Rim" region, which utilizes 79 MEs from the “PDYAR” configuration.

 

This spatial subdivision is baseline-exploratory, aimed at identifying potential regional accents in the precursory signal.  If necessary, this tentative division can be optimized during a subsequent technical calibration procedure. One such optimization parameter involves the recurrence curve, which characterizes the performance of the WCC pipeline in terms of how accurately the order numbers of specific version-case pairs correspond to a uniform sequence of magnitude thresholds. The upper panel of Figure 34 illustrates this parameter computed for the period between 2025193 and 2025201. The curve for the best 100 MEs is identical to that presented in Figure 16, whereas the «JOINT» curve represents the complete set of 295 MEs. The «JOINT» bulletin is not a simple cumulative sum of the best 100 and 195 “PDYAR” XSELs; rather, it is the direct product of joint processing incorporating a Conflict Resolution (CR) algorithm those cross-rejects redundant XSEL events between the two constituent bulletins. For instance, the «JOINT» XSEL yields 3752 events in the v₁c₁ pair (weak version, 5.0 s tolerance), whereas the standalone best 100 MEs generate 2625 events and the  “PDYAR” configuration produces 2642 events. For the v₂c₆ pair (strict, 0.25 s), the event counts are 140, 74, and 92 events, respectively. Thus, the value added to the «JOINT» XSEL bulletin scales from 50% for the v₁c₁ pair to nearly 100% for the v₂c₆ pair. 

The respective curves in the upper panel of Figure 34 evolve synchronously, displaying similar low-amplitude deviations within the weak LA version. However, the strict LA version for the “PDYAR” subset generates larger numbers of XSEL events across all six tolerance cases. This behavior is likely attributed to the larger number of MEs physically encompassing a broader spatial domain during the high-resolution grid search, which operates with a maximum radius of 43.2 km in the strict LA version.  For the weak version, the grid radius is 81 km, and the neighboring MEs have overlapping search areas leading to the CR process. This phenomenon is important for the future ME setting – the number of MEs should be sufficient to cover the whole region without detection-creation blind zones. The “PDYAR” master subset was partially introduced to address this issue, and the «JOINT» set is likely an important step on the way to an optimal ME number and distribution. The increasing number of MEs can yield additional XSEL events, but it would require higher computing power or longer processing time.

 

            There are minor deviations from the «JOINT» XSEL exponential regression line within the strict version XSELs. The lower panel of Figure 34 presents the recurrence curves from the upper panel, normalized to their respective maximum values for the v₁c₁ pairs. These relative variations effectively highlight the outliers across the configurations. The highest coefficient of determination, R²=0.9805, is yielded by the top 100 MEs set. This value is statistically significant enough to validate the correlation between the order numbers and the uniform sequence of magnitude thresholds. This robust correspondence represents a key feature underlying the reliability of the weak-to-strict version ratio as a predictive parameter.  The largest fluctuations are demonstrated by the “Rim” curve with R²=0.94. This specific curve exhibits a progressive deficit in XSEL events when approaching the highest order numbers. Both regression lines have the same slope of -0.31.

 

Figure 34. Recurrence curve characteristics evaluated across distinct Master Event (ME) sets. Upper panel: The recurrence curves for the 12 LA version/tolerance case pairs computed for different ME configurations. Lower panel: The corresponding recurrence curves from the upper panel, normalized to their respective absolute levels in the v1c1 configuration to highlight relative morphological variations and outliers.

 

Evolution of the number of XSEL events

            Because the weak-to-strict version ratio represents a measurable parameter designed to serve as an operational trigger for a major earthquake alert, the fundamental properties of the underlying calculations become crucial. For each monitoring interval, six distinct ratio curves are evaluated, corresponding to the six origin-time tolerance figures ranging from 0.25 s to 5.0 s. These metrics are derived from the respective XSEL bulletins generated by the WCC detection and LA procedures for individual MEs. The aggregation of these individual XSELs into a consolidated final XSEL is governed by a CR procedure, which is finely calibrated against the baseline daily event rates observed during the periods between major earthquakes. The backbone of this entire WCC processing pipeline comprises a spatial network of MEs combined with waveform templates of the first P-wave recorded across all stations associated with each respective ME.

            A standalone ME creates event hypotheses strictly within the spatial grid search radius relative to its own hypocentral position reported in the REB. While the probability of detecting an event situated outside this grid boundary is non-zero, its reconstructed location will be artificially shifted inside the grid.  This artifact can introduce significant errors in the absolute location, a limitation that is highly common in automatic bulletins. The REB events are also inherently mislocated relative to their true positions; however, the confidence ellipses derived from travel time residuals provide an estimate of the 95% probability contour containing the actual source. In general, a larger number of associated stations reduce the dimensions of this confidence ellipse. These structural parameters, alongside potential phase misassociation, render the choice of MEs highly vulnerable to location errors and template quality.

            The set of 295 MEs includes REB events of varying quality regarding absolute location accuracy, the number of associated stations, and the availability of valid templates of the first P-phase. The operational efficacy of individual MEs can vary significantly across the studied region. This variability can introduce measurable fluctuations in both the resolution and sensitivity of the WCC processing, directly affecting the final XSEL bulletins. Smaller ME subsets assigned to specific geographical areas may exhibit localized underperformance due to poor ME quality, which could be misinterpreted as low seismic activity. Genuine spatial fluctuations in seismic activity across the selected could be erroneously interpreted as instrumental or algorithmic underperformance.

            The partitioning into these geographical subsets is accomplished using simple straight lines that delineate quasi-rectangular areas. The probability space within which an individual ME can generate a valid event hypothesis can readily extend into adjacent zones. A discrete XSEL bulletin can incorporate events that geographically belong to a different ME subset. Within the complete ME network, however, these overlapping MEs would generate competing hypotheses that are ultimately resolved by the CR procedure. For example, the "Out of Asperity" domain contains numerous MEs situated immediately along the boundary of the "Asperity" core zone. This spatial overlapping introduces a non-zero bias in the independent XSEL estimates computed for each discrete subset. Within the integrated «JOINT» master set, this specific spatial bias is successfully suppressed by the presence of the MEs that explicitly belong to the "Asperity" set.

            The operational performance of all individual ME subsets can be systematically compared to the «JOINT» set as an aggregated output of all MEs in all subsets. Panel a) in Figure 35 depicts the six weak-to-strict ratios between 2025193 and 2025201 for the «JOINT» set. There are notable differences observed in the behavior of the 0.25 s and 0.5 s curves when compared to the best 100 MEs subset in Figure 30. The precursory peak amplitude in the «JOINT» 0.25 s curve decreases to 13.1, compared to 15.4. The double-peak structure preceding the J20 event in Figure 30 is smoothed, and the highest peak moved to the first place in their sequence. The peak at 2015199 lost its relative amplitude to 8.8 from 14.7 for the best MEs 100 set. The third peak near 2025197 is the highest for the presented interval. The period before 2025194 is not shown as the calculations for the «JOINT» set were conducted from 2025193 to 20252012. The 0.5 s curves are nearly the same for both sets with slightly lower amplitude relative to the highest peak. The remaining four tolerance curves are almost identical across both sets.

Overall, the 0.25 s curve has not lost its predictive capability, despite the fact that the weak MS(4) curve in Figure 36a exhibits only a single upward shift prior to the J20 event, while the strict MS(4) curve declines at a faster rate than the weak curve, followed by a sharp increase along the final leg preceding the J20 rupture, which is marked by the vertical red line. For the «JOINT» curve, the broader morphology of the precursory peak is driven primarily by an accelerated decline in the strict curve, mimicking the behavior observed during previous peak intervals. This underlying mechanism differs significantly from the forcing factors that govern the standalone best 100 MEs set.

The “PDYAR” subset in Figure 35b is characterized by a broader precursory peak that begins to grow three days before the J20 mainshock. The prominent trough near 2025200, which is clearly observed in the best 100 and «JOINT» ME sets, loses its depth in the “PDYAR” subset, appearing instead as a smooth transition between two individual maxima. Both of these peaks are driven by a faster decline in the strict curve relative to the weak LA version. The final quarter-day preceding the J20 event is characterized by a rapid amplitude rise in the strict curve – signaling an increasing activity of the XSEL events closer to the corner magnitude of the REB recurrence curve – which occurs alongside a more gradual increase in the weak curve. The peak level of the 0.25 s curve is 16.8, strongly supporting its precursory character.  The 0.5 s curve begins to decline three quarter-days prior to the J20 earthquake, consistent with the behaviors documented in the other two ME sets. This synchronous signature further reinforces the hypothesis of an impending major earthquake.

The 0.25 s ratio curve for the “Asperity” ME subset in Figure 35c exhibits features similar to those of the “PDYAR” subset, albeit with a few distinct deviations. The “Asperity” curve also starts to rise on 2025198, but reaches its peak value of 14.5 five quarter-days prior to the J20. Unlike in the «JOINT» and “PDYAR” subsets, there is a peak near 2025195 induced by a deep trough in the strict curve. There is a similar peak in the best 100 MEs curve. This empirical observation suggests that the sharp declines in the strict curves are driven by local conditions and depend on individual ME quality and spatial distribution, rather than encompassing the entire region. Such local peaks are effectively smoothed by the cumulative activity in the «JOINT» set. Only the largest anomalies remain visible in the curves that cover the entire region. For instance, the prominent peak near 2025197 is clear for the best 100, “PDYAR”, and «JOINT» sets, yet it remains statistically significant within the “Asperity” curve. Furthermore, the final precursory peak preceding the J20 event is significantly smoothed and displays a lower amplitude relative to the other maxima analyzed above.

The “Out of Asperity” 0.25 s ratio curve in Figure 35d demonstrates two distinct high-amplitude peaks: one near 2025197 and another three quarter-days prior to the J20. The latter maximum reaches an absolute value of 21.4. It could serve as an optimal short-term precursor to the J20 earthquake, especially given that its culmination is immediately followed by a systematic two-quarter-day decline leading directly to the J20 origin time. This peak is driven by a fast decline in the strict curve in Figure 36d, followed by an equally accelerated recovery that initiates three quarter-days before the J20. The growing number of events in the strict version XSEL is a feature potentially related to earthquake preparation. The geographical distribution of MEs within the “Out of Asperity” set suggests that the seismic activity prior to the coseismic phase was concentrated outside the zone of the “Asperity” subset. 

            The “Rim” subset is presented in Figure 35e. It was designed to cover the area beyond the aftershock sequence of the J29 mega-earthquake. It should inherently remain insensitive to any seismic activity related to the preparation of the J20 and J29 earthquakes. It has an extremely high peak near 2025199. While similar peaks are observed across the sets covering the entire region, they are much lower in relative amplitude, thereby confirming the strictly localized character of this anomaly. The “Rim” set in the southwestern periphery of the region undergoes its own independent seismic activity on 2025197, which should not be factored into the core J20/J29 nucleation process. There is another sharp peak with an amplitude of 42.0 four quarter-days before the J20. Its amplitude is directly related to the drop in the strict curve to the level of 1 to 2 per day, as illustrated by the corresponding MS(4) curves in Figure 36e. Despite its high amplitude, this peak is statistically insignificant due to the extremely small sample size. Mechanically, this anomalous spike can be attributed to the over-the-border spatial sensitivity of MEs positioned near the primary preparation zone of the J29 earthquake.

            a)

b)

c)

d)

e)

Figure 35. Evolution of the weak-to-strict LA version ratios calculated across six origin time tolerance levels preceding the J20 earthquake. Five distinct master event (ME) configurations are presented to resolve the spatial and geographical dependence of the precursory ratios: a) «JOINT» configuration comprising the complete macro-set of 295 MEs; b) Standalone “PDYAR” master subset incorporating 195 MEs with a mandatory associated arrival at station PDYAR; c) "Asperity" subset consisting of 49 MEs (excluding one deep-focus event located within the horizontal boundaries of the asperity area) clustered within the immediate epicentral zone of the J20 and J29 events, capturing the stable tectonic asperity that arrested the J20 rupture propagation; d) "Out of Asperity" subset comprising 245 MEs situated outside the core zone; e) "Rim" subset incorporating 79 MEs from the “PDYAR” configuration located along the southwestern periphery of the analyzed domain.

 a)

b)

c)

d)

e)

Figure 36. Comparison of the curves for the weak and strict LA versions and the origin-time tolerances of 0.25 s and 0.5 s for the same five ME configurations as in Figure 35.

 

Initially, the J20 earthquake was not the primary target of this study, with the original objective to analyze the J29 mega-earthquake. However, the fundamental physical mechanisms underlying major earthquake preparation remain consistent within a given seismogenic zone; consequently, the nucleation phase of a catastrophic rupture can overlap with and mask the preparation process of an intermediate event. As a result, the parameters used in the analysis of the J29 earthquake preparation may introduce a systematic bias when applied to the J20 process. Nevertheless, the observations presented in Figures 35 and 36 suggest that the weak-to-strict ratio curve for the 0.25 s origin time tolerance preserves its validity as an operational trigger for short-term earthquake early warning systems. The density and geographical distribution of MEs required to ensure the optimal resolution of this precursor can be systematically calibrated using the REB as a robust source. Furthermore, the core parameters of the WCC pipeline – ranging from individual station detection thresholds to the conflict resolution algorithm – remain fully adjustable.  Such a comprehensive network optimization is highly feasible given sufficient computational resources, and thus lies beyond the scope of the current study.

            For the J29 mega-earthquake, the weak-to-strict versions ratio for the 0.25 s origin time tolerance exhibits a sharp peak preceding the rupture onset time. The 0.5 s curve in Figure 33 is nearly synchronous with this 0.25 s curve, forming a more robust potential trigger for an earthquake early warning alert. Taken together, these two individual peaks would be sufficient to achieve the objective of the J29 earthquake forecasting. The Sea of Okhotsk earthquake also exhibits two similar peaks right before the mainshock. Their statistical significance is validated by a progressive increase in the number of XSEL events detected within the strict LA version for both the 0.25 s and 0.5 s cases during three consecutive quarter-days preceding the mainshock. The stochastic creation of these strict hypotheses is practically excluded, as the LA defining parameters are calibrated to suppress not only random phase alignments but also extraneous detections related to the side-sensitivity of the highest-weight IMS array stations.

            The «JOINT» set of MEs generated XSELs for all 12 LA version-case pairs computed for the period between 2025193 and 2025212. The corresponding weak-to-strict ratios of the MS(4) curves for the six distinct tolerance cases are presented in Figure 37a. The 0.25 s ratio curve exhibits a sharp peak in the fourth quarter of July 29, immediately preceding the mainshock, which is marked by the vertical black line.  The curve undergoes a rapid acceleration over two consecutive quarter-days, reaching a maximum amplitude of 7.0. This value is marginally higher than the respective peak in the best 100 MEs subset illustrated in Figure 33. This peak is driven by the rise in the weak curve, as observed in Figure 36a. The strict curve retains a practically constant level during the two days prior to the J29 earthquake. This behavior is nearly identical to that demonstrated by the respective curves of the 100 best MEs subset, despite the difference in the XSELs total numbers of events, as shown in Figure 34.

The 0.5 s curve peaks at a level of ~5 in the «JOINT» configuration, likely smoothed out by the competing MEs from the “PDYAR” subset. The geographical position of MEs generating extra XSEL events for the 0.5 s weak-to-strict ratio is not clear yet. Otherwise, the «JOINT» and the best 100 MEs curves are characterized by highly synchronous evolution between 2025202 and 2025212, punctuated by two distinct low-amplitude peaks on 2025204 and 2025207. These peaks are likely driven by the increased seismic activity after two earthquakes on 2025203 and 2025206, both featuring a body-wave magnitude (m_b) of approximately 5.5, as illustrated in Figure 19.

            The «JOINT» set and the best 100 MEs subset can both serve as effective predictive tools, displaying only minor differences. In contrast, the “PDYAR” subset in Figure 37b demonstrates a markedly different behavior. The 0.25 s ratio curve is characterized by a broader peak, with a maximum amplitude of 7.3 that is shifted chronologically to Q3/2025209. This peak is driven by a steady decline in the strict curve, which reaches its lowest point in Q3/2025209, as illustrated in Figure 36b. The strict curve then rises faster than the weak curve, causing the weak-to-strict ratio to decline continuously up to Q3/2025210. In Q4/2025210, the strict curve drops by 2 events relative to the preceding quarter-day, while the weak version retains its level. The Q4 lasts only 5 hours, as the J29 origin time is approximately at 23:25. A secondary, lower peak with an amplitude of 5.5 is observed immediately preceding the J29 mainshock, likely corresponding to the primary precursory maxima identified in the «JOINT» set and the best 100 subset. The 0.5 s ratio curve exhibits an isolated peak reaching an amplitude of 6.2 in Q4/2025209, followed by a gradual decline to the level of the remaining four tolerance curves right before the mainshock.  As for the 0.25 s curve, this 0.5 s peak can be explained by the decrease in the strict curve between Q1 and Q4 of 2025209. 

The predictive capability of both the 0.25-s and 0.5-s curves appears partially suppressed by the high ambient seismicity within the boundaries of the "Asperity" zone. This core domain represents a relatively compact area characterized by the highest level of historical seismicity and the nucleation of several extremely large historical earthquakes (Figures 12 and 13). While the primary mechanical source of the precursory indicator marking an impending rupture is clearly situated within this focal zone, the low-magnitude seismic activity recovered by the WCC pipeline likely incorporates events generated by alternative, long-term tectonic processes that operate concurrently with the immediate mainshock preparation cycle.          

The "Out of Asperity" ME subset yields results that are highly similar to those of the «JOINT» master set. Specifically, the 0.25-s version ratio curve exhibits a sharp peak immediately preceding the J29 mainshock, reaching maximum amplitude of approximately 7.0. Prior to the J29 event, the "Out of Asperity" 0.5-s curve reaches a level marginally higher than that recorded by the integrated «JOINT» configuration. This close similarity is primarily attributed to the absolute sample size of this subset; the "Out of Asperity" network incorporates 245 out of the total 295 MEs deployed in the «JOINT» configuration. Crucially, the operational performance and predictive capability of this subset do not suffer any significant degradation when the "Asperity" core events are entirely omitted from the processing stream. This empirical outcome demonstrates that the critical short-term earthquake preparation processes operate effectively across the broader spatial domain encompassed by these 245 master events, rather than being strictly restricted to the focal asperity itself.

a)

b)

c)

d)

e)

Figure 37.  Same as in Figure 35 for the earthquake on July 29, 2025

 

The "Rim" subset serves as a critical counterpoint to establish the statistical significance of precursory anomalies. The 0.25-s version ratio curve presented in Figure 37e demonstrates high-frequency fluctuations in amplitude, with its maximum peak emerging during Q4 of day 2025209. During the entire day preceding the J29 mainshock, this 0.25 s curve drops to a flat baseline level between 5.0 and 6.0, displaying no anomalous peak that could be interpreted as a precursor. This behavior demonstrates that the crustal volume encompassed by the "Rim" configuration remains structurally independent of the seismogenic and mechanical processes operating across the core preparation zone of the studied region. Figure 36e illustrates this conclusion by the effect of the weak (and strict) LA version curves for all origin-time tolerance cases fail to reach the post-seismic level observed after the J29 mega-earthquake in Figure 20. The other four configurations in Figure 36 demonstrate this essential feature of the XSEL recurrence curves. 

The 79 MEs defining this southwestern “PDYAR” subset can be safely excluded from the direct investigation of the seismic process leading to the J29 rupture. This spatial selectivity is robustly supported by the corresponding findings related to the preceding J20 earthquake. Mechanically, the largest peak observed in the MS(4) ratio curve during the fourth quarter of 2025209 is artificially induced by a sharp drop in the strict curve down to a baseline of only 4 events per day, as illustrated by the running sum trajectory in Figure 36e.

 

 

Discussion

The ultimate objective of this study is the recovery of low-magnitude seismicity prior to a major earthquake. The IMS array stations represent the best global seismic network, providing lower detection thresholds for remote regions. In the standard beamforming detection technique, this is achieved by the destructive interference of ambient noise accompanied by constructive interference of the sought signals. The WCC-based processing creates a pipeline that provides additionally reduced detection thresholds, effective suppression of detections not relevant to the studied localized areas, and accurate local association of the preselected detections with the event hypotheses created by high-quality master events. The conflicts for the same physical signals associated with independent hypotheses created by two or more neighboring master events are resolved in a Conflict Resolution procedure, which optimizes the resulting bulletin to the actual event rate – from very low to that observed during the first hours after the mega-earthquakes such as Sumatra in 2004, Tohoku 2011, and Kamchatka 2025.

The WCC-based approach was advocated by Schaff and co-authors [2025] as a technique to obtain information on the seismic processes not detectable by the standard methods. The WCC processing has been widely used to detect low-magnitude events at regional and teleseismic distances. One of the problems associated with low-magnitude events is that one cannot accurately estimate their standard magnitudes from amplitude and period measurements, as the detected signals are hidden in the ambient noise. In this study, we further developed the technique based on a set of threshold defined by the statistical significance of valid events introduced by Kitov [2026ab]. For the Kamchatka 2025 major earthquakes and their aftershocks, these thresholds were directly linked to standard magnitude estimates. This finding allowed us to follow the evolution of low-magnitude seismicity across a set of increasing magnitude thresholds toward the onset times of the J20 and J29 earthquakes.  The measurable parameter potentially useful as a mega-earthquake precursor is based on the ratio of the numbers of XSEL events above two thresholds. This parameter signals that the rate of such events increases in the corridor between the thresholds. Alternatively, a decreasing rate above the upper threshold also leads to a rise in the ratio, but does not indicate an approaching large event.

The absolute duration of the investigated interval was limited to approximately two weeks, primarily due to available computing power. However, this period was sufficient to resolve highly specific precursory features preceding the J29 mainshock and its potential foreshock, the J20 mainshock. A few days prior to the J29 megathrust failure, the acceleration in seismic activity progressed into the most statistically significant v₁c₁-v₂c₁ bin configuration – the parameter space situated closest to the historical corner magnitude of the Kamchatka recurrence curve. An identical manifestation was previously documented before the Sea of Okhotsk earthquake.

There are several important findings in this study made possible by the IMS array stations and the sensitivity and flexibility of the WCC processing. Firstly, the 12 independent version-case configurations, based on different Local Association criteria and varying origin time tolerances, can be mathematically converted into a standard body-wave magnitude scale. This transformation yields robust recurrence curves that successfully resolve actual seismicity down to a baseline of approximately magnitude 2.0, extending the catalog completeness into the sub-noise register where no official REB event hypotheses are available. Secondly, synchronous, high-amplitude peaks in the 0.25 s and 0.5 s weak-to-strict version ratio curves are consistently observed immediately preceding major seismic ruptures. The stability and predictive power of this dual-window operational trigger have been robustly verified across distinct geodynamic settings, encompassing the 2013 deep-focus Sea of Okhotsk earthquake as well as the 2025 Kamchatka major earthquakes. Thirdly, the partitioning of the comprehensive «JOINT» master event catalogue into discrete geographic subareas exposes the underlying spatial sensitivity of the WCC pipeline. The interaction between overlapping grid search radii and the automated Conflict Resolution cross-rejection process successfully suppresses spatial cross-talk and boundaries blurring between adjacent zones. Fourthly, the multi-parametric spatial test effectively divides the investigated subduction region into zones that are highly significant and zones that are insignificant for precursory signal localization. The analysis demonstrates that while the core tectonic asperity acts as the primary mechanical driver of the impending rupture, the accelerated micro-seismic preparation process is heavily concentrated outside this immediate core, mapping out a wide precursory activation envelope on the periphery of the fault plane. Conversely, remote reference configurations, such as the "Rim" subset, display a flat background variance, confirming the absolute spatial selectivity and statistical significance of the developed method.

There were several precursor-like peaks found in the data before the J20 and J29 earthquakes. They were all statistically insignificant and caused by a severe drop in the XSEL number of the strict LA version. However, it is not excluded that some statistically significant precursory signals might not end in a catastrophic earthquake. Further studies are needed to understand the abundance and properties of false positive related to the precursor-like signals. This methodological projection involves several critical aspects. First, a high rate of false alarms does not constitute a viable alternative to the unmitigated impact of a catastrophic event. Comprehensive false-alarm statistics must be systematically gathered over a multi-year baseline, encompassing both high-activity clusters and prolonged seismically quiet intervals. While this verification requires significant computational resources and time, isolating signatures similar to those reported before the 2013 Sea of Okhotsk and 2025 Kamchatka events would not only precisely constrain the empirical false-alarm probability but also unlock invaluable insights into the aggregate mechanical behavior of the subducting Pacific plate.

Tectonic preparation processes that fail to reach the final co-seismic rupture stage, alongside their precise spatial positions and hypocentral depths, are directly governed by the internal architecture of the slab and the space-time evolution of stresses under regional driving forces. On a macro-scale horizon, these observations may hold greater significance for structural geophysics than the isolated estimation of false-alarm rates. For instance, the J29 mega-earthquake was preceded by an almost collocated J20 event that failed to overcome the stable local asperity, terminating as a localized m_b=5.39 (IDC) event instead of immediately triggering the final M8.8 rupture that materialized ten days later. A highly analogous precursory sequence was documented two days prior to the 2011 Tohoku earthquake, whereas no such distinct foreshock activation was resolved before the catastrophic December 26, 2004, Sumatra-Andaman megathrust failure.

For future investigations of the Kuril–Kamchatka subduction zone, it will be highly informative to systematically assess the large aftershock that occurred on September 18, 2025. Furthermore, as a historic alternative to the modern, well-instrumented J29 case, the major earthquake of November 15, 2006, will be analyzed. That historical event nucleated within the southwestern boundary of the active domain, explicitly belonging to the "Rim" master event configuration. To achieve maximum operational efficiency, this localized "Rim" template database will be strategically extended using core anchors selected from the top 100 MEs configuration. In 2006, a significantly reduced IMS network topology was operational, completely lacking the critical near-regional array PETK (Petropavlovsk-Kamchatskiy, Russia; 53.108°N, 157.699°E) and the highly sensitive regional array PDYAR (Peleduy, Russia; 59.655°N, 112.441°E). Additionally, the regional events of May 19, 2013, and December 20, 2018, will be investigated to complete a comprehensive re-evaluation of all major earthquakes within this domain since 2001.

The Kamchatka Peninsula represents one of the most seismically active and well-instrumented subduction zones globally; however, some other regions capable of generating catastrophic earthquakes suffer from deficient instrumentation. For instance, the IMS network across South America comprises exclusively three-component (3-C) stations, a network that catches significantly fewer phases, elevates the baseline WCC detection threshold, and prevents the high-resolution recovery of low-magnitude sub-noise seismicity. Because Chilean megathrust earthquakes exhibit structural mechanisms highly analogous to those of the Kamchatka Peninsula, they warrant thorough investigation once the regional observational assets are upgraded to match the strict array configuration requirements of the WCC pipeline.

The March 11, 2011, Tohoku earthquake commands both a catastrophic magnitude and an exceptional instrumentation baseline, captured by multiple IMS arrays operating at regional distances. Conversely, the December 26, 2004, Sumatra earthquake exhibited a catastrophic rupture volume but was recorded by relatively poor network instrumentation during the early operational years of the IMS network. Both of these historical mega-earthquakes are currently under active investigation using the optimized WCC pipeline. The compelling structural divergence between them resides in the drastically different levels of ambient micro-seismic activity operating immediately prior to their respective mainshocks.

 

References

            Adushkin V. V., Kitov I. O., and Sanina I. A. (2025a). Further Development of the Matched Filter Method for Solving Seismological Problems // Doklady Earth Sciences. V. 523:13. https://doi.org/10.1134/S1028334X25606182

Adushkin V.V., Kitov I.O., Sanina I.A. (2025b). Detection of weak aftershocks at regional and teleseismic distances // J. Volcanology and Seismilogy. № 5, P. 3–18. DOI:10.7868/S3034513825050014 (in Russian)

Arrowsmith S. J., Eisner L. A technique for identifying microseismic multiplets and application to the Valhall field, North Sea // Geophysics. 2006. V. 71. P. 31‒40.

Baisch S., Ceranna L., Harjes H.-P. Earthquake Cluster: What Can We Learn from Waveform Similarity? // Bull. Seismol. Soc. Am. 2008. V. 98. P. 2806‒2814.

Bobrov D., Kitov I., Zerbo L. (2014). Perspectives of cross-correlation in seismic monitoring at the International Data Centre // Pure Appl. Geophys. V. 171. № 3–5. P. 439–468.

Bobrov D.I., Kitov I.O., Rozhkov M.V., Friberg P. Towards Global Seismic Monitoring of Underground Nuclear Explosions Using Waveform Cross-correlation. Part I: Grand Master Events // Seismic Instruments. 2016a. V. 52. № 1. P. 43–59.

Bobrov D.I., Kitov I.O., Rozhkov M.V., Friberg P. Towards Global Seismic Monitoring of Underground Nuclear Explosions Using Waveform Cross-correlation. Part II: Synthetic Master Events // Seismic Instruments. 2016b. V. 52. № 3. P. 207–223.           

Bobrov D., Kitov I., Rozhkov M. Studying seismicity of the Atlantic Ocean using waveform cross-correlation // NNC RK Bulletin. 2017. 2(70). P. 5-19.

Comprehensive Nuclear-Test-Ban Treaty. 1996. Protocol to the Comprehensive Nuclear-Test-Ban Treaty. https:www.ctbto.org.sites.default.files.Documents.treatytext.tt.html

Coyne J., Bobrov D., Bormann P., Duran E., Grenard P., Haralabus G., Kitov I., Starovoit Yu.( 2012). Chapter 15: CTBTO: Goals, Networks, Data Analysis and Data Availability / Ed. P. Bormann // New Manual of Seismological Practice Observatory. P. 1–41. DOI: 10.2312/GFZ.NMSOP-2_ch15 

Gibbons S., Ringdal F. (2004). A waveform correlation procedure for detecting decoupled chemical explosions // NORSAR Scientific Report: Semiannual Technical Summary № 2. NORSAR. Kjeller. Norway. P. 41–50.

Gibbons S.J., Ringdal F. (2006). The detection of low magnitude seismic events using array based waveform correlation // Geophys. J. Int. V. 165. P. 149–166.

            Gibbons S.J., Sorensen M.B., Harris D.B. and Ringdal F. (2007). The detection and location of low magnitude earthquakes in northern Norway using multichannel waveform correlation at regional distances // Phys. Earth Planet. Inter. Vol. 160. N. 3–4. P. 285–309.           

Gibbons S.J., Pabian F., Näsholm S.P., Kværna T., Mykkeltveit S. Accurate relative location estimates for the North Korean nuclear tests using empirical slowness corrections // Geophys. J. Int. 2017. V. 208. № 1. P. 101–117. 

Israelsson H. Correlation of waveforms from closely spaced regional events // Bull. Seismol. Soc. Am. 1990. V. 80. № 6. P. 2177–2193.

            Joswig M. (1990). Pattern recognition for earthquake detection // Bull. Seismol. Soc. Am. V. 80. P. 170–186.

            Joswig M., Schulte-Theis H. (1993). Master-event correlations of weak local earthquakes by dynamic waveform matching // Geophys. J. Int. V. 113. Issue 3. P. 562‒574. DOI: https://doi.org/10.1111/j.1365-246X.1993.tb04652.x

Kitov, I.O. (2026a). Low-magnitude seismic activity between the Kamchatka July 20 and July 29, 2025, earthquakes. Spatio-temporal evolution recovered using waveform cross-correlation // arXiv:2601.15302. https://doi.org/10.48550/arXiv.2601.15302

            Kitov I.O. (2026b) Spatio-temporal evolution of low-magnitude seismicity before the May 24, 2013, Sea of Okhotsk earthquake recovered by waveform cross-correlation. Is it an earthquake prediction case? // arXiv:2603.26717. https://doi.org/10.48550/arXiv.2603.26717

            Kitov I.O. (2026c) Seismic noise suppression: array stations, waveform cross-correlation, and noise stochastization // arXiv:2604.21939. https://doi.org/10.48550/arXiv.2604.21939

            Kitov I.O., Sanina I.A. (2025a). Recovery of the Aftershock Sequence of the North Atlantic Earthquake Using Waveform Cross-Correlation // Geodynamics & Tectonophysics.  16 (4), 0838. doi:10.5800/GT-2025-16-4-0838

Kitov I.O., Sanina I.A. (2025b). Quality assurance of seismological bulletins using waveform cross-correlation // Russian Seismological Journal. V. 7, N1. P. 26–41.  (in Russian) DOI: 10.35540/2686-7907.2025.1.02.

            Kitov I. O., Sanina I. A., Volosov S. G., Konstantinovskaya N. L.  (2025c). The 20th Anniversary of the Installation of the Small-Aperture  Mikhnevo Array for Monitoring Induced Seismicity // Izvestiya, Physics of the Solid Earth. V. 61. N. 2. P. 288–304. DOI: 10.1134/S1069351325700181

Kitov I.O, Sanina I.A., Sokolova I.N., Vinogradov Yu.A. (2026). Study of Seismic Activity between the Mainshock and the First Aftershock of the Kamchatka Earthquake on July 29 2025 //  Russian Journal of Earth Sciences, (in press)

Ringdal, F. (1994). GSETT 3: A Test of an Experimental International Seismic Monitoring System. // Ann. Geophys. 37 (3). https://doi.org/10.4401/ag-4203.

            Saragiotis C., Kitov I. (2020).  Tuning IMS station processing parameters and detection thresholds to increase detection precision and decrease detection miss rate // EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8949. DOI: 10.5194/egusphere-egu2020-8949 

            Schaff D.P. and Richards P.G. (2004). Repeating seismic events in China // Science. Vol. 303. P. 1176–1178.

            Schaff D. P. and Richards P.G. (2014). Improvements in magnitude precision, using the statistics of relative amplitudes measured by cross-correlation // Geophys. J. Int. Seismology. 197(1). P. 335–350. DOI: 10.1093/gji/ggt433

            Schaff, D.P., Kim, W.-Y. and Richards P.G. (2025). Background Seismicity for parts of the northern Korean peninsula //  CTBT Science & Technology Conference. P1.2-714. Vienna, Austria.

            Schweitzer J., Fyen J., Mykkeltveit S., Gibbons S.J., Pirli M., Kühn D. and Kværna T. // Seismic arrays, in New Manual of Seismological Practice Observatory. Bormann. P., Ed. 2012. Ch. 9. doi: 10.2312/GFZ.NMSOP2_ch9.

Selby, N. (2010). Relative location of the October 2006 and May 2009 DPRK announced nuclear tests using International Monitoring System Seismometer arrays // Bull. Seismol. Soc. Am. V. 100. P. 1779-1784.

Sultanov J.J. (1998). The role of G.A.Gamburtsev in creating a seismic monitoring method for nuclear testing. / M. OIPE RAS. V3. P. 188-193. (in Russian)

            Turin, G. L. (1960). An introduction to matched filters // IRE Transactions on Information Theory. V. 6. PP. 311–329. doi:10.1109/TIT.1960.1057571. S2CID 5128742

Waldhauser, F., Schaff, D. (2008). Large-scale cross-correlation based relocation of two decades of northern California seismicity // J. Geophys. Res. V. 113. B08311. doi:10.1029/2007JB005479.