Impact of The 2004 Sumatra-Andaman Earthquake to The Stress Heterogeneity and Seismicity Pattern in Nothern Sumatra, Indonesia

Abstract

Sumatra Island is a highly seismic active region due to its close location to the convergent zone between the Indo-Australian and Eurasian plates, and the existence of the Great Sumatran Fault (GSF). These conditions caused the island of Sumatra suffered from hundreds of earthquakes with Mw > 4 per year. Particularly, a 9.0 Mw Sumatra-Andaman 2004 earthquake occured in northern Sumatra which changed the in-situ stress and seismicity pattern. In such case, quantifying the tectonic stress perturbation prior and after 2004 Sumatra-Andaman earthquake is becoming key issue in assessing the seismic hazard. To invert the in-situ stress information in Sumatra, first we identify the fault plane from its auxiliary plane in the focal mechanism solutions using the fault instability criterion. There are 354 focal mechanisms data of earthquake with Mw > 5 and focal depths < 250 km along northern Sumatra Island from 1976 to 2010. This catalogue was compiled by combining the data from Global Centroid Moment Tensor (GCMT) catalogue and the International Seismological Centre (ISC) bulletin. We then divided the focal mechanism catalogue into two parts, i.e. prior and after Sumatra-Andaman 2004 earthquake. An iterative joint inversion is then implemented to estimate the in-situ stress orientation and its magnitude ratio. The in-situ principal stress orientation and magnitude ratio obtained from this study will be used to build a Sumatran stress map and its correlation with the seismicity pattern variations in the region. Our results produce remarkable changes of maximum principal stress orientation after 2004 megathrust earthquakes along the region. As the seismic hazard potential is controlled by stress concentration, the obtained results from this study could also be used further for seismic hazard mitigation in northern Sumatra.