The Boundary Areas Structure of the Malko-Petropavlovsk Fracture Zone from Local Seismic Tomography and Earthquake Foci Mechanisms Data

Abstract

Abstract —This study continues analysis of the new seismic tomographic structure of the suprasubduction complex of the central zone of Kamchatka, obtained from the dense local networks data of 2018–2020, and is devoted to the analysis of the velocity structure in the Malko-Petropavlovsk fracture zone margins and around them. The seismic tomographic model involves about 98,000 P- and S-wave travel times from 2963 local earthquakes from August 2018 to July 2020. The resolution of this model makes it possible to trace the feeding systems of volcanoes of the South Kamchatka and East Volcanic Belt to the slab surface, as well as to identify subvertical structural faults. To construct the orientations of the compression and extension axes we used the foci mechanisms of 41 earthquakes with М ≥ 4.5 from the catalog of the International Seismological Center for the period 1979–2019. Along the Malko-Petropavlovsk fracture zone, the Avacha transform fault is clearly traced in the geometry and mutual arrangement of velocity anomalies almost throughout the entire depth of the model. Comparison of seismic anomalies with a map of the directions of the compression and extension axes distribution from the earthquake foci mechanisms showed the correlation between the change in the value of the velocity anomalies along the Avacha transform fault with the axes direction change by almost 180°. A near-surface low-velocity anomaly to the depths of 25–35 km was found along the western border of the Malko-Petropavlovsk zone under the southern tip of the Sredinny Ridge. This anomaly probably marks the axes junction zone boundary of the ancient volcanic front along the Sredinny Ridge and the modern active Eastern Volcanic Belt, which formed as a result of the Kronotsky paleoarc accretion. To the west from the Sredinny Ridge southern tip, another low-velocity anomaly was revealed. This anomaly was traced to a depth of ~150 km, has a contrasting southern boundary confirmed by the distribution of the compression and extension axes directions by the earthquake foci mechanisms and apparently marks the southern boundary of the West Kamchatka block.