Three-dimensional VP and VP/VS structure of the upper crust in the Erzincan basin (eastern Turkey)

Abstract

Using aftershocks of the 13 March 1992 Erzincan earthquake (Ms = 6.8) three dimensional VP and VP/VS velocity structure of the Erzincan basin and its surroundings were modeled by local earthquake tomography. Travel times of body waves, belonging to 1025 selected high-quality events recorded by 58 temporary stations, were inverted iteratively and simultaneously by using the SIMUL2000 algorithm. Synthetic and resolution tests were performed to analyze the sensitivity of tomographic results and model parameterization. The resulting 3-D VP and VP/VS tomographic images show velocity anomalies related to geologic and tectonic structures beneath the Erzincan basin. While low VP anomalies down to 2-3 km depths are associated to thick Neogene sediments deposited in the Erzincan basin, high VP anomalies indicate bed rocks and intrusive magmatic rocks beneath the mountain ranges. VP/VS anomalies are an important indicator of local tectonic structures, and the physical, mechanical, and compositional variations in the rocks. The low VP/VS anomalies (≤1.7) observed at shallow depths (0-4 km) track the main fault systems and thus the weakness zones in the basin. The high VP/VS anomalies (≥ 1.9) located between 4 and 6 km depth in the southeast of the Erzincan basin may correspond to fluid saturation, high pore pressure, and carbonate content in the rock matrix. The high VP/VS ratios play an important role for increasing seismicity in this area because of the fluid saturation and the high pore pressure. Finally, on the basis of the 3-D tomographic images, basement of the Erzincan basin deepens down to about 10 km depth and possesses an irregular and asymmetric shape. The thick and unconsolidated sedimentary layer increases seismic risk to Erzincan and other cities located on the basin as it magnifies ground motion caused by large earthquakes. Copyright 2008 by the American Geophysical Union.