Virtual Real-Time (VRT) forecasting of the Kamchatka 29 July 2025 mega earthquake (Mw8.8) based on foreshock activity

Foreshocks, aftershocks, and swarms are common types of seismicity clusters. Foreshock patterns are recognized as of special value for earthquake forecasting. Beforehand discrimination of foreshocks from other clusters and from background seismicity is of great importance for short-term hazard assessment, but it remains a challenge. A promising prospect is that different seismicity clusters are characterized by distinct patterns in space, time, and magnitude, thus reflecting different underlying geophysical processes. In foreshocks, the count number increases at the inverse of time, but usually an activity lull is observed a few days before the mainshock; the b-value drops, while epicenters usually move towards the mainshock epicenter. In aftershocks, the epicenters expand away from the mainshock epicenter, the event count decreases exponentially with time, and the b-value increases. Swarms are not associated with specific patterns of epicentral and temporal distributions, while the b-value usually increases. On-time identification of statistically significant seismicity changes could be supportive towards real-time discrimination between different types of clusters. This approach was tested with the seismic sequence of the M_w8.8 megathrust mainshock that ruptured the subduction interface off eastern Kamchatka on 29 July 2025, based on classic earthquake statistics and advanced complex network tools. On 20 July 2025 an earthquake of M_w7.4 occurred; many smaller shocks followed. However, the foreshock sequence was only recognized a posteriori. We investigated if the foreshock sequence could be detectable beforehand. To examine this crucial issue, we introduced the concept of Virtual Real-Time (VRT) analysis, which is different from usual retrospective analysis because VRT utilizes incomplete knowledge of the earthquake sequence, i.e., the catalogue and other data available only up to each point of time T of the ongoing seismic sequence. This means the analysis is performed as if we were in the actual conditions of the sequence. VRT analysis was combined with a decision matrix based on the different patterns of different clusters and on testing appropriate null hypotheses. Considering 20 July 2025 as Τf=1 day, the VRT analysis detected the transition from the state of background seismicity to that of foreshocks on Τf=3 (23 July), if not earlier, and persistently on every subsequent day prior to the mainshock up to Τf=9 days (29 July). The imminence of an even larger earthquake became evident from the foreshock lull in about Τf=7 days, while its magnitude was approximated by an empirical relationship between magnitude and the area covered by the foreshocks. Setting the mega earthquake at time Τa=1 day, the transition from the state of foreshocks to that of aftershocks was detectable at Ta=2 days and at every subsequent day, thus signifying that the mega earthquake was the mainshock. All seismicity changes from one state to the other were found to be highly significant. The results obtained underline the important capabilities for earthquake forecasting from the recognition of foreshocks beforehand. The data used in this work were obtained from the large-scale research facilities «Seismic infrasound array for monitoring Arctic cryolithozone and continuous seismic monitoring of the Russian Federation, neighboring territories, and the world» (https://ckp-rf.ru/usu/507436/).