Andaman Postseismic Deformation Observations: Still Slipping after All These Years?

Abstract

More than six years after the great (M w 9.2) Sumatra-Andaman earthquake, postevent processes responsible for relaxation of the coseismic stress change remain controversial. Modeling of Andaman Islands Global Positioning System (GPS) displacements indicated early near-field motions were dominated by slip down-dip of the rupture, but various researchers ascribe elements of relaxation to dominantly poroelastic, dominantly viscoelastic and dominantly fault slip processes, depending primarily on their measurement sampling and modeling tools used. After subtracting a pre-2004 interseismic velocity, significant transient motion during the 2008.5-2010.5 epoch confirms that postseismic relaxation processes continue in Andaman. Modeling three-component velocities as viscoelastic flow yields a weighted root-mean-square (WRMS) misfit that always exceeds the WRMS 26.3 mm/yr of the measured signal. The bestfitting models are those that yield negligible deformation, indicating the model parameters have no real physical meaning. GPS velocities are well-fit (WRMS 4.0 mm/yr) by combining a viscoelastic flow model that best-fits the horizontal velocities with ~50 cm/yr thrust slip downdip of the coseismic rupture. 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.

A unified GPS‐based earthquake catalog for the Sumatran plate boundary between 2002 and 2013

“…According to satellite imagery [ Meltzner et al , ; Tobita et al , ] and one cGPS station ACEH, which was installed a few months after the earthquake [ Gunawan et al , ], a similar reversal from coseismic subsidence to postseismic uplift was also recorded in Banda Aceh on the northernmost tip of Sumatra. In addition, similar postseismic trenchward motion and uplift were observed in the Andaman and Nicobar Islands [ Paul et al , ; Gahalaut et al , ]. In contrast, postseismic trenchward motion and subsidence up to 1 cm/yr were observed in Peninsular Malaysia and Thailand [ Panumastrakul et al , ; Satirapod et al , ].…”

“…Therefore, poroelastic effects can explain only a small fraction of early postseismic deformation and are usually disregarded in postseismic studies of the 2004 earthquake. Most recent postseismic studies of the 2004 earthquake supported the idea that a combination of afterslip and viscoelastic relaxation is necessary to account for all the observations [e.g., Panet et al , ; Hu and Wang , ; Paul et al , ; Satirapod et al , ; Gunawan et al , ].…”

Spectral-finite element approach to post-seismic relaxation in a spherical compressible Earth: application to gravity changes due to the 2004 Sumatra–Andaman earthquake

“…Studies have revealed that afterslip occurred in periods between 2005.5 and 2011.5. Paul et al (2012) stated that inclusion of afterslip at depths immediately below the downdip terminus of coseismic rupture can explain the GNSS data obtained from 4 to 6 yr after the event better than models where only viscoelastic flow is considered. Panet et al (2010) showed that an M w 8.2 afterslip which is distributed at depths from 30 to 85 km is necessary to explain the misfit between the viscoelastic relaxation model and the observed geoid data during the period from 2005 April to 2007 September.…”

“…Mechanisms of post-seismic deformation by the 2004 Sumatra event have been discussed, using GNSS and GRACE data (e.g. Hashimoto et al 2006;Pollitz et al 2006;Chlieh et al 2007;Ogawa & Heki 2007;Panet et al 2007;Han et al 2008;Hughes et al 2010;Panet et al 2010;Hoechner et al 2011;Paul et al 2012) where afterslip and viscoelastic relaxation are proposed as the dominant mechanism. Nevertheless, a consensus has not yet been reached regarding their respective contributions (see also Section 4) and while the accumulated effect up to present might be small for the 2010 Maule and the 2011 Tohoku events, most studies focused on the analysis of these events concentrate on afterslip.…”

The Geodetic Signature of the Earthquake Cycle at Subduction Zones: Model Constraints on the Deep Processes

“…Initial afterslip was particularly significant in the Andaman and Nicobar Islands (Banerjee et al, ; Bilham et al, ; Gahalaut et al, ; Subarya et al, ). Afterslip continued here 2008.5–2010.5 (Paul et al, ). Far‐field observations of geodetic motions (Broerse et al, ; Satirapod et al, ) and gravity changes (Broerse et al, ; Han et al, ; Hoechner et al, ; Panet et al, ) find that (bulk) relaxation in the upper mantle has commenced.…”