Relocation of the 1997 Mw 6.1 Golestan, Ardebil, Earthquake Sequence: A Hint for the Fault Mechanism

Abstract:

The 28^th Feb. 1997 Golestan-Ardebil earthquake (Mw 6.1) struck an area around the southeast of Sabalan Volcano in NW Iran, where the regional shortening between Arabia and Eurasia mostly is accommodated by a complex system of right-lateral strike slip and thrust faulting. The region is surrounded by rigid South Caspian Basin (SCB) in the east, North Tabriz right-lateral strike-slip fault (NTF) in the west, thrust faulting in Caucasus in the north, and thrust faulting along Bozghoush fault in the south. Therefore, finding the causative fault plane of the event has a key role in understanding of the accommodation of the regional strain in the region. Based on CMT moment tensor solution, the causative fault plane could be either an E-W right lateral or a N-S left lateral strike slip fault, whereas the recognition of the causative fault plane is impossible due to the lack of a surface rupture and also large error in location of aftershocks. In this work, we significantly improved the location of the main shock and its large aftershocks by using a multiple-event relocation method (HDC). We also added more recent events recorded both by local and regional networks and critical nearby S-P phases from the accelerometer network of Iran. The relocated locations of the main shock and its larger immediate aftershocks is in a good agreement with a N-S trending fault plane which implies a left-lateral strike slip faulting for the Golestan event. The presence of a North trending left-lateral fault might be connected to the way the strain is accommodated within the block wedged among SCB, Caucasus and NTF.

The relocated aftershocks mostly lie north of the main shock and that implies an almost northward directivity of the rupture. The depth of main event is about 18 km and the depth of seismicity varies from 6 to 22 km. Furthermore, some events are aggregated around inward dipping to subvertical ring faults and other fractures associated with the caldera of Sabalan Volcano. Those seem to be smaller earthquakes triggered by the main shock.