Paleoseismologic data and seismic tomographic images of the 1992 Erzincan Earthquake along the North Anatolian Fault Zone, Turkey

Abstract:

The North Anatolian Fault Zone (NAFZ), spanning over 1200 km from Karliova in eastern Turkey to the Aegean Sea, defines complex active fault zone. The zone consists of network of subfaults, each of varying geometry, and associated failure properties. The Erzincan basin, one of a series basins along the NAFZ, is active basin surrounding bedrock which is Mesozoic and Tertiary units. Available geophysical data constrain the total thickness of Pliocene and Quaternary sediments in the Erzincan basin to more than 2 km. The basin was affected by the 1992 March 13 Erzincan earthquake (Ms=6.8) showing weakly developed surface ruptures.

We excavated 6 paleoseismic trenches along the southeastern termination of the 1992 rupture in Üzümlü area. A total of trenches is 294 m long and trends N20°-30°E. Exposed lithology in trenches is made up of alluvial fan deposites and fluvial facies with rare flood-plain sediments, which are characterized by stratified and/or lens-shaped pebble gravel to coarse-grained sand, silt, and clay. Our preliminary interpretation of trenches reveals evidence of 1992 earthquake features. Trench logs showing structural elements including normal faults, which are parallel to the strike of the trace of the NAFZ and dipping 40°-75° to the northeast and the southwest, and network of almost vertical fractures with a few centimeters displacement. Some of trenches contain well-developed flame structures, suggesting liquefaction of water-saturated sediments during earthquake.

Seismic velocity (VP and VP/VS) images obtained from the 3-D local earthquake tomography using the arrival time data of aftershocks of the 13 March 1992 Erzincan earthquake show several anomalies related to geological features of the Erzincan basin. The tomographic results indicate that (1) the major fault zones control the regional tectonics and the geometry of the Erzincan basin, (2) sediments of the basin show low seismic velocity, and (3) The high velocity units characterized by ophiolitic and metamorphic rocks in the north and carbonate rocks in the south bordering the basin.

We conclude that surface rupture of the 1992 Erzincan earthquake is much longer than surface ruptures shown in maps and 3-D VP and 3-D VP/VS anomalies represent the lithology and structure of the Erzincan basin, respectively.