Enigma of the Olyutorskii Earthquake Resolved by SAR Interferometry

Abstract

The M_w 7.6 Olyutorskii earthquake of April 20, 2006, struck the southern edge of the Koryak Highland, in a region of great complexity at the junction of the North American, Eurasian, and Pacific plates. This seismic event was notable for several remarkable features. Firstly, it had an unexpectedly large magnitude, leading to a reassessment of the seismic hazard in the Northern Kamchatka region. Secondly, the GCMT focal mechanism solution showed two nodal planes corresponding to nearly thrusting displacements on a 40–51° dipping rupture plane. However, geological field studies conducted in the epicentral area allow to map at the surface three primary segments of a 120 km long rupture zone. The central segment showed predominant right-lateral strike-slip displacements, while the NE and SW segments exhibited mostly thrusts steeply dipping SE with smaller strike-slip components. Thirdly, aftershocks were recorded in an area extending over 200 km southwestward from the surface ruptures mapped in the field, with an intense activity observed in a vast area 75 km to the NW from the surface ruptures. Fourthly, although the ruptures found during fieldwork were dipping to the SE, most aftershocks occurred NW of them, with their depth decreasing in the NW direction. SAR interferometry resolved this apparent discrepancy between seismological and field-geology data, indicating that the primary displacements occurred northwest from the central and southwest segments of the ruptures at the earth's surface. We present in our study a new rupture model based on SAR, GPS, and field geology data. The model consists of a major blind thrust extending NW and three rear subvertical ruptures that reached the earth's surface and were mapped during the field survey. Additionally, models of the main M_w 6.6 aftershocks that occurred on April 29, 2006, and May 22, 2006, are presented. Our new model reconciles all collected data, with a direction of movements on the major thrust agreeing with the rotation of the previously suggested Beringia microplate around a pole situated in the North of the Chukotka peninsula.