Aftershock study of the 2007 Sumatran earthquakes: aftershocks and postseismic stress diffusion

Abstract

We operated a temporary seismic network comprising 10 broadband (Trillium 120P, Univ Liverpool) and 19 short period stations (CMG-6TD, SeisUK) from December 2007 to October 2008 on the Mentawai islands and the adjacent mainland provinces of West Sumatra and Bengkulu (Sumatra, Indonesia) in order to record aftershocks of the MW 8.4 and 7.9 megathrust earthquakes of 12 September 2007. 26 of the stations returned at least some useful data; the average recovery rate was approximately 70%. In spite of relatively high noise levels caused by surf and close vicinity of cultural noise sources, a large number of events was detected, including the MW 7.2 megathrust event of 25 February 2008 and its fore- and aftershocks. In total 1037 events comprising 12,600 P and 6,282 S arrivals were picked. An optimum 1D-model and station correction terms were determined based on a subset of 386 high quality events within the network (Gap<180°), and all events subsequently relocated with this model. The model shows unusually high velocities of nearly 6.4 km/s in the upper crust; the average Vp/Vs ratio is 1.74. Earthquakes occur primarily along the plate interface updip of the coseismic rupture zones of the main events, and in two arc-parallel linear streaks of seismicity below the central part of the forearc basin and along its eastern margin. The data from this network are archived at SeisUK and the IRIS-DMC with network code ZB (valid for years 2007-2009) alongside data from the consortium experiment in North Sumatra.