Analysis on the Causative Fault of the 2021 Mw 6.0 Tehoru Earthquake in the South Coast of Seram Island: A Preliminary Result

Abstract

Seram Island is the northern part of the Banda subduction which is characterized by the existence of a thrust fault structure in the Seram Trough as the main contributor to seismicity in this region. Complex tectonic activity also forms a deformation zone of the Kawa Fault with a sinistral strike slip mechanism and high-angle normal faults in the mainland. On June 16, 2021 a Mw 6.0 earthquake occurred on the south coast of Seram, in Tehoru to be precise, which followed by a small tsunami wave which is thought to have been generated by an underwater landslide. This earthquake has a normal fault mechanism which is quite rare on Seram Island, however, the actual fault plane that caused the earthquake is still unknown. In this study, we investigate potential faults that may have caused the 2021 Tehoru earthquake by performing a seismicity analysis using well relocated hypocenter of the mainshock and the aftershocks sequence. We use the double-difference relocation method with the SVD algorithm performed in the hypoDD program. The results show the distribution of aftershocks that form a northeast-southwest trending lineation. The depth of the hypocenter has also improved from being dominated by fixed depth to being varied with dominance at depths of 8-14 km. An important feature of the relocation results is that the aftershocks distributed downward along a ~35° northwest-dipping plane which indicates a southwest-northeast fault orientation. This result confirmed by using geodetic observations that shows a horizontal displacement of 14 mm toward northwest direction and a land subsidence of 14 mm. This southwest-northeast oriented fault structure is thought to be associated with high-angle normal fault resulting from a north-south extension activity in the central part of Seram Island. The results of this study indicate the existence of other local fault structure in the Seram Island which can be used as a basis for seismic hazard assessment, other than seismic activities from Seram Trough and Kawa Fault.