Coseismic Effect of the 2025 Kamchatka Earthquakes

We investigate the conditions under which coseismic electromagnetic disturbances arise, caused by the propagation of elastic waves from distant earthquakes with magnitudes ranging from 6.9 to 8.7. The origins of these seismic waves were the 2025 Great Kamchatka Earthquake and several strong events in the vicinity of its source. The analyzed data are provided by magnetic observatories of the INTERMAGNET network, which support 1-second recording of the geomagnetic field and are located at distances from 23 to 87 arc degrees from the epicenter of the Great Kamchatka Earthquake. The primary study examines the dependence of the fact of coseismic signal generation on the earthquake magnitude and its epicentral distance. The mechanism underlying coseismic disturbances is not discussed. Effects produced by different seismic phases are considered separately, which determines the lower bound for the epicentral distances under consideration. We show that the electromagnetic signal accompanying the arrival of teleseismic waves is generated only by strong earthquakes with magnitudes M > 7. The coseismic effect of the strongest events with magnitudes M > 7.4, primarily the Great Kamchatka Earthquake (M = 8.7), manifests itself at extremely large epicentral distances, up to 90 arc degrees. We demonstrate that the main spectral component of the coseismic signal occurs in the high-frequency range from 0.05 Hz to 1.0 Hz.