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 mb=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 mb=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 mb=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
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.
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 (mb) 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 mb=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.
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