Slow slip along the Sumatra-Andaman megathrust during various stages of earthquake cycle

Abstract:

Pre-seismic, co-seismic and post-seismic slow slip behavior along the Andaman portion of the Sumatra-Andaman megathrust has been detected by modeling GPS data. The campaign-mode acquisitions at Port Blair showed that the site started to subside between 2003 and 2004. In addition, during this period, the horizontal displacement of Port Blair with respect to Indian plate, deduced from 1996 to 2000 GPS data, changed its orientation to that obtained during the 26^th Dec 2004 co-seismic. This short term subsidence can be modeled as slip in the up-dip portion of the fault, a slip that is equivalent to an earthquake with moment magnitude of 6.3. Previously, slow slip was thought to appear at intermediate depths roughly 35–55km but simple models of the deformation at this single site suggest slow slip at much shallower depth than this.

The northern part of the Sumatra–Andaman plate boundary revealed the slow slip it experienced in response to the rapid coseismic stress change on the 26th of December2004. This earthquake started near Sumatra and the slip travelled upwards towards the Andaman region. Geodetic observations indicate that additional slow slip occurred in this region over a time scale of 50 min or longer. Early post-2004 of geodetic measurements were modeled for post seismic slip beneath the Andaman Islands that released a moment equivalent to a magnitude Mw = 7.5 earthquake, and suggested deep slip in the stable frictional regime accelerated to catch up to the coseismic rupture.

Eight years of post-2004 GPS data yielded models that have combination of after slip and viscouselastic relaxation. Both deep slip and flow respond to stress changes, and each can significantly change stress in the realm of the other, so it is reasonable to expect that both transient deep slip and viscoelastic flow will influence surface deformation long after a great earthquake.

In the case of the twin strike‐slip earthquakes that struck the northeast Indian Ocean on 11 April 2012, modeling of GPS displacements suggests that the en echelon and orthogonal fault ruptures of the intraplate oceanic earthquakes could have possibly accelerated the ongoing slow slip, along the lower portion of the thrust fault beneath the islands with a month‐long slip of 4–10 cm.