By capturing the postseismic deformation following the 2004 Sumatra-Andaman great earthquake, this project continues a six-year effort to understand the evolution of transient strain and stress. Several dozen studies have examined geodetic measurements of the transient deformation that followed the event, but the dominant mechanisms and processes involved in near-field deformation remain murky. Most of these studies conclude that a small amount of accelerated downdip slip occurred in the first few weeks or months following the earthquake, but that subsequent postseismic deformation and uplift are dominated by deeper viscoelastic flow. The most recent two years of near-field data from the Andaman region however exhibit uplift and right-lateral shear that is not replicated in existing models of viscoelastic flow. Expansion of the network would optimize the greater network to isolate nonlinear viscoelastic flow response by (1) extending the network coverage across the along-strike region of largest postseismic signal present in the time-variable geoid response, and (2) pairing up closely-spaced sites, enabling us to fingerprint fault slip signals superimposed on the desired flow signals. U Memphis collaborators will perform the maintenance and measurements at both continuous and campaign sites, in partnership with Center for Earth Sciences, Indian Institute of Science, Bangalore. All data will be archived at, and openly available from, the UNAVCO Facility which is also loaning several receivers for this project. Utah State University will combine these measurements with other local and regional GPS network data, and with GRACE time-variable gravity data, to model the full evolution of transient stress and strain in the region. We will refine existing estimates of the coseismic slip and examine (separately and together) models of fault slip, viscoelastic flow and poroelastic flow to determine which deformation processes are the most likely mechanisms for observed transients.

The earthquake cycle is driven by a slow build-up of shallow stress within the earthquake zone. These shallow stress changes result from quiet slip on faults and flow of rocks at greater depths where temperatures are higher. Our understanding of the deeper fault slip and flow processes is limited by the difficulty of measuring small changes at great depths, but importantly these processes are briefly accelerated to measurable rates immediately following large earthquakes. The 2004 Sumatra-Andaman earthquake was the third-largest ever recorded, and GPS measurements indicate that it moved the ground surface in the Andaman and Nicobar Islands by up to sixteen feet toward India, and as much as three feet upward or downward. Since then, the islands have continued moving rapidly, totaling an additional foot toward India and a foot upward. Conventional scientific wisdom holds that the motion may be largely fault-slip driven in the first few weeks to months after an earthquake, but should be dominated by deeper flow in the years that follow. However, the most recent two years of measurements show anomalous motion that is not consistent with existing rock flow models, but would be consistent with predictions for fault slip. This project will combine GPS and gravity data with new modeling tools to try to understand this surprising observation, with special attention to how stress changes resulting from deep fault slip may influence the stress that drives deep rock flow, and vice-versa. The project?s scientific objectives have potentially far-reaching implications for our fundamental understanding of earthquake physics, seismic hazard, and the evolution of stress throughout the earthquake cycle.

Project Report

After nearly a decade of Sumatra-Andaman postseismic deformation studies, a general consensus view of the results posits that early transient deformation was dominated by fault slip; that the slip dwindled to nothing after a period of order weeks to months; and that subsequent movement represents the viscoelastic flow response to coseismic stress changes. However, the most recent epoch of transient motion in the Andaman Islands exhibits relatively significant rates of uplift and right-lateral shear at all sites. Intriguingly, these observed features cannot be replicated by radially-symmetric viscoelastic flow models. We hypothesize here that either (1) the dynamics of the accretionary wedge are being ignored by the existing models or hence a need for newer models or (2) the near-field deformation still continues to be dominated by postseismic fault slip. The Mw 8.6 and 8.2 strike-slip earthquakes that struck the northeast Indian Ocean on 11 April 2012 resulted in coseismic deformation both at near and distant sites. The slip distribution, deduced using seismic-wave analysis for the orthogonal faults that ruptured during these earthquakes, is sufficient to predict the coseismic displacements at the Global Positioning System (GPS) sites, such as NTUS, PALK, and CUSV, but fall short at four continuous sites in the Andaman Islands region. Slip modeling, for times prior to the events, suggests that the lower portion of the thrust fault beneath the Andaman Islands has been slipping at least at the rate of 40 cm/yr, in response to the 2004 Sumatra–Andaman coseismic stress change. Modeling of GPS displacements suggests that the en echelon and orthogonal fault ruptures of the 2012 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. The misfit to the coseismic GPS displacements along the Andaman Islands could be improved with a better source model, assuming that no local process contributed to this anomaly.

National Science Foundation (NSF)
Division of Earth Sciences (EAR)
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Eva E. Zanzerkia
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University of Memphis
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