Study of Seismic Activity Between the Mainshock and the First Aftershock of the Kamchatka Earthquake on July 29, 2025

The high-amplitude seismic waves generated by the earthquakes with magnitudes similar to that of the July 29, 2025 Kamchatka event (MW 8.8) create the background noise that can potentially mask any post-seismic activity during the first few minutes following the mainshock. Detection of signals from potential aftershocks buried in the coherent noise can be enhanced using a matched filter detector based on waveform cross-correlation with high-quality waveform templates obtained from historical events in the same region. The array stations of the International Monitoring System (IMS) located at regional and teleseismic distances provide one of the most effective networks for detecting seismic events globally. However, the coherence between noise signals and the sought signals, as generated by very similar sources in the same area, makes less efficient the detection methods based on noise suppression by velocity filtering such as beamforming. For the event on July 29, 2025 near Kamchatka Peninsula, the noise reduction method based on adding stochastic noise to the actual data was tested. During the initial 10-minute period following the mainshock, several reliable event hypotheses were created. These hypotheses are similar to those reported by the International Data Centre (IDC) and are based on the data from the same IMS stations. The IDC and the International Seismological Centre had not reported any events during this time frame, despite extended automatic and interactive analyses.