Deep structure and tomographic imaging of strong earthquake source zones

Abstract

This work generalizes the results of tomographic imaging performed by the authors for epicentral zones. Seismic events in North Africa (the M w = 5.8 earthquake of 1985 near the town of Constantine), eastern Anatolia (the Erzincan M w = 6.7 earthquake of 1992), the Lesser and Greater Caucasus (the 1988 Spitak M w = 6.8 and the 1991 Racha M w = 7.0 earthquakes), and northern Sakhalin (the 1995 Neftegorsk M w = 7.1 earthquake) are examined. It is shown how various morphokinematic types of active faults differ in the resulting tomographic images at various depths. A classification of tomographic images of strong earthquake source zones is proposed in accordance with the rank of their generating faults. The sources of the Spitak, Racha, and Erzincan earthquakes are confined to large boundary faults separating tectonic zones. Lower velocity bands are revealed in the tomographic images, and low velocity “pockets” 1–2 km or somewhat more in width penetrating to a depth of up to 15 km are observed near the fault zones. The Constantine and Neftegorsk earthquakes were generated by faults of a lower rank. The source zones of these events are imaged tomographically as narrow gradient zones.