The Occurrence of Precursors before Large (MW ≥ 6.6) Kamchatka Earthquakes

A review is presented summarizing the work done in the search for earthquake precursors in Kamchatka compared with the leading elements of seismicity in the territory during the period of detailed seismic observation, 1962–2022, namely, the cumulative plot of seismic energy release and the large earthquakes that have occurred in the region. One peculiarity of the observing network consists in the location of most “nonseismological” types of precursor observation in the restricted area of the Petropavlovsk-Kamchatsky Test Site (PKTS). An analysis of 14 types of seismic, geophysical, and geochemical precursors preceding seven shallow and medium-depth earthquakes of 2005–2022 with М_W = 6.6–7.7 showed the growth of the number of precursors N with increasing parameter M_W/log d_h (d_h is hypocentral distance to the PKTS center in km), which characterizes the relative intensity of earthquake precursory processes in the PKTS area. This relationship between N and M_W/log d_h can be seen for interplate (subduction) earthquakes in the Kamchatka segment of the Kuril–Kamchatka island arc, is consistent with the occurrence of precursors in 1987–2004, and reflects the property of their simultaneous occurrence before earthquakes that are the largest and near to the PKTS. The effect of simultaneous occurrence of precursors (ESOP) before such earthquakes was observed in at least 80% of precursors of all those considered in this study. For such earthquakes, the ratio between hypocentral distance d_h and rupture length L (km) is d_h/L = 3.8–1.6; that is, the occurrence of ESOP is characteristic for the near and intermediate zones of the future earthquake rupture zone. Four separate kinds of precursor were examined to show that their threshold values d_h/L for the М_W ≥ 6.6 events are 5.0–8.5. If ESOP has been detected during seismic prediction work in real time, then one can use the threshold value found for it d_h/L ≤ 3.8 to considerably diminish the estimate of the distance between the future large earthquake and the PKTS and the Petropavlovsk–Elizovo urban agglomeration compared with the approach in which data on separate kinds of precursor are used.