| Summary: | 碩士 === 國立臺灣大學 === 地質科學研究所 === 101 === The continental collision between Arabia and Eurasia created large strike-slip faults in Turkey, uplift of East Anatolia and mountains in the Caucasus. In previous studies, focal mechanisms in the Caucasus are determined primarily for large earthquakes using global waveform data. Small earthquakes are less studied and poorly constrained due to limited stations in regional distances. In this study, we use regional waveforms of new array to constrain the focal mechanisms and depths of the earthquakes with M>3.5 in the major seismic zones in the Caucasus, Javakheti Highland (Lesser Caucasus) and eastern Turkey.
Earthquakes in Racha of the Greater Caucasus are mainly thrust events with strikes in E-W direction confined in the upper to middle crust. Focal mechanisms and distribution of the 2009/09/07 earthquake sequence clearly show that they are the reactivation of 1991 Racha rupture fault zone. As for the eastern Turkey and Armenia, the earthquakes are upper crustal strike-slip events. Three of them may be directly associated with Sengaya-Gole fault in the eastern Turkey. Several focal mechanisms are in the Javakheti volcanic highland where swarms of small earthquakes are continuously occurring. Except for two small thrust events north of the swarm area, other earthquakes in this region are dominated by strike-slip faulting, which disagrees with normal structure inferred from the surface expression.
With our and Global-CMT results, the formal stress inversion show that the maximum stress (σ1) is consistently in 7-14°N direction for the entire study region, which is apparently controlled by the northward movement of Arabia. In the Greater Caucasus, the W-E median stress (σ2) is considerable comparing to the N-S σ1, allowing both sub-EW and sub-NS striking thrust events to occur. As for the eastern Turkey, the vertical σ2 are more comparable to W-E minimum stress (σ3), such that the major strike-slip earthquakes coincide with a few thrusting events. The stress ratio in Javakheti region is similar to that of eastern Turkey but the σ2–σ3 girdle is much pronounced, suggesting that the permutation between the two stress axes is unstable.
|
|---|
