Body wave velocity distribution in the Benioff zone of central Kamchatka during aftershocks of the Kronotskii earthquake of 1997 (M = 7.9)

The distribution of the P and S velocities in the Benioff zone of central Kamchatka during the period of aftershocks (1997–2004) of the disastrous Kronotskii earthquake of 1997 (M = 7.9, MW = 7.7) has been determined. Based on the data for the foreshock period immediately preceding the earthquake (1991–1997), a sharp increase in the body wave velocities in the Benioff zone below the Kronotskii Peninsula (up to 9.5–9.7 km/s for V P and 5.1–5.3 km/s for V S) has been determined at depths of 55–140 km in the subvertical region. Based on observations during the period of aftershocks comparable with the last period of foreshocks (about 7 years), it has been established that the body wave velocities calculated for the Benioff zone below the Kronotskii Peninsula returned to the initial values typical of the beginning of that period. This indicates that stresses relaxed around the head part of the Kronotskii earthquake rupture zone after its origination. This conclusion is confirmed by a sharp decrease in the number of earthquakes with M = 2.3–4.9 in the Benioff zone below the Kronotskii Peninsula. Moreover, taking the velocity distribution during the period of aftershocks into account, it has been determined that a second stress relaxation zone is located at the southwestern flank of the Kronotskii earthquake rupture zone where the largest (M = 6.7) aftershock occurred. According to these data, it is concluded that two stress concentration centers could have existed during the preparation of the Kronotskii earthquake.