The 2008 M­w 7.2 North Pagai Earthquake Sequence and its Relationship to Ruptured and Unruptured Parts of the Mentawai Patch

Abstract:

The Mentawai patch is a 700-km long section of the Sunda megathrust, where great earthquakes commonly rupture most or all of the patch in decade-long sequences, every two centuries or so. In 2007, the patch sustained partial rupture during a M_w 8.4 earthquake. Several aftershocks failed in the M_w 7.0 to 7.8 range on the parts of the patch peripheral to the 2007 mainshock. However, the events released far less energy than expected, thus we anticipate other great earthquakes in the next few decades. We examine the kinematic details of one large aftershock, a M_w 7.2 earthquake in 2008. This event has not previously been studied due to its relatively low magnitude compared to other recent studies. Using daily solutions for nine continuous GPS stations, we estimated a coseismic slip distribution for the event. We then used 2.75-years of cumulative displacements for four GPS stations to model afterslip, both kinematically and dynamically in the framework of rate-and-state friction laws. Our preliminary results show that peak coseismic slip of ~2 m is concentrated under North Pagai Island. The modeled afterslip concentrates up- and down-dip of the estimated coseismic slip patch. The fact that this moderate size event did not cascade into the much larger event that has been forecast, and instead generated significant afterslip in surrounding areas of the fault, illustrates the potential of the megathrust to generate a spectrum of earthquake sizes. Their sizes are likely controlled by lateral segmentation and structural barriers on the fault.