Geodynamic Manifestations of Seismic Process in the Region of the November 15, 2006 and January 13, 2007 Simushir Earthquakes

Abstract—The paper analyzes the evolution of the aftershock process, GNSS (GPS) site displacements, and gravity variations in the region of the November 15, 2006 and January 13, 2007 Simushir earthquakes. Postseismic creep and viscoelastic relaxation modeling with asthenospheric viscosity of 10¹⁹ Pa s is conducted. It is shown that the pattern of the GNSS site displacements, the growth of the gravity anomaly, which began five months after the earthquake of November 15, 2006, simultaneously with the propagation of the aftershock activity to a depth of 100 km, are mainly associated with the postseismic creep in a vast area including the segments of the subduction zone adjacent to the source region and the zone’s continuation to a depth of 100 km. Approximately 20% of the displacements are attributed to the action of viscoelastic relaxation. It is concluded that the features of many processes in the southern part of the Kuril Arc may be shaped by the direction of motion of the Pacific Plate which is not perpendicular to the oceanic trench in this region, and the so-called oblique subduction takes place. The emerging right-lateral shear component is realized not only in the back-arc basin, as it occurs, for example, in the Andaman Sea, but also at the plate contact itself. This leads to the formation of a zone of extension and destruction revealed by geophysical studies on the segment of the subduction zone from the Bussol Strait to the Diana Strait. On the edges of this segment, sharp changes are revealed in the depth to the acoustic basement top and the Moho which could not be detected in the destruction zone itself. This area has been a zone of seismic quiescence for a long time and hosted two largest events that occurred on November 15, 2006 and January 13, 2007. The postseismic creep model constructed in the work shows that in the region of extension and destruction, the displacements, along with the thrust component, also had a right-lateral shear component. To the north of this zone, in the region of Ketoy and Matua Islands, pure thrust displacements occurred; further north, the magnitude of the displacements decreases.