Estimation of Stress/Strain State of Tectonic Structures Using VP/VS Ratios: A Case Study of Seismically Active Zones of the Greater Caucasus, Kura Depression, Transcaucasia, and the Western Caspian Region

Abstract

Earthquakes, which result from tectonic movements within the bowels of the Earth, reflect spatiotemporal changes in the stress field of the geologic medium. Weak regional earthquakes indicate the activation of some elements of seismotectonic zones and preparation of strong earthquakes. The travel times of Pand S-waves and V_P/V_S ratios are also a source of information on the regional tectonic setting; sometimes this information helps to reveal blind faults. The paper analyzes local earthquakes in a seismically active region, which comprises the Greater Caucasus, the Kura Depression, Transcaucasia, and the western Caspian Region. Based on a large database of hypocenter parameters, P- and S-wave travel times, and areal V_P/V_S ratio distributions, we identified seismotectonic zones with characteristic anomalous features and outlined gradient zones where V_P/V_S anomalies reverse sign. It is found that the V_P/V_S ratio field complies with the regional stress field and deep seismotectonic setting. Zones where anomalies reverse sign (gradient zones) are most likely confined to blind faults. Areas of the high gradient of the field can be put down to blind tectonic structures (faults) or the incipient sources of future strong earthquakes. V_P/V_S ratio monitoring and mapping will make it possible to identify the source zones of forthcoming earthquakes.