Characterizing the spectrum of slip behavior at the Sumatran subduction zone with long-term geodetic records

I present three studies that depict the spatio-temporal spectrum of slip behavior on the Sunda megathrust. These studies illuminate new characteristics of slip behavior on the Sunda megathrust, which are important for gaining a comprehensive understanding of the earthquake cycle at the Sumatran subduction zone. First, from an ~1100-year-long paleogeodetic record at Simeulue Island, I discover that time-variable vertical deformation rates can be explained by changes in interseismic fault coupling over time. Also, the pattern of fault coupling preceding earthquakes between two seismic cycles may not necessarily be the same, even though the earthquakes themselves have a similar rupture pattern. Our results highlight the need for researchers to treat interseismic fault coupling maps as a snapshot of conditions on the fault that can change with time. Second, I present paleogeodetic time series that reveal a 15-year reversal in vertical displacements from 1966 to 1981. I examine a range of oceanographic and tectonic processes that can potentially explain these observations, and find that the most likely explanation is a 15-year-long slow slip event on the Banyak Islands section of the Sunda megathrust. This event is the longest lasting slow slip event reported at any subduction zone around the world, and is the first to be discovered in Sumatra. Third, I examine long GPS postseismic time series to unravel the spatio-temporal distribution of afterslip following the 2007 Mw 8.4 Bengkulu earthquake, and investigate its spatial relationship with the 2010 Mw 7.8 “tsunami earthquake” that occurred updip of the 2007 event. A key finding is that afterslip borders and overlaps the 2010 rupture area, and that localized increases in strain rates caused by afterslip may have advanced rupture of this patch in 2010.​Doctor of Philosophy (ASE

A paleogeodetic record of variable interseismic rates and megathrust coupling at Simeulue Island, Sumatra

An ∼1100 year long paleogeodetic record of land-height change along the Simeulue section of the Sumatran subduction zone reveals significant variations in vertical motion rates. From an ∼267 year long record, we develop models to explain rate variations in the decades before the 1861, 2004, and 2005 great earthquakes. The record shows that rates accelerated by a factor of 4 to 10 in the decades before the 1861 earthquake; one plausible explanation is a significant increase in the depth of interseismic coupling on the Sunda megathrust under Simeulue. Despite similarity of the 1861 and 2005 coseismic rupture patterns, the pattern of coupling during the decades before the two earthquakes may have been different. Most GPS observations of interseismic deformation at subduction zones span only a decade or two; our results highlight the need to treat GPS-derived coupling maps as only a snapshot of fault conditions that are temporally variable.NRF (Natl Research Foundation, S’pore)MOE (Min. of Education, S’pore)Published versio

Sibling earthquakes generated within a persistent rupture barrier on the Sunda megathrust under Simeulue Island

A section of the Sunda megathrust underneath Simeulue is known to persistently halt rupture propagation of great earthquakes, including those in 2004 (Mw 9.2) and 2005 (Mw 8.6). Yet the same section generated large earthquakes in 2002 (Mw 7.3) and 2008 (Mw 7.4). To date, few studies have investigated the 2002 and 2008 events, and none have satisfactorily located or explained them. Using near-field InSAR, GPS, and coral geodetic data, we find that the slip distributions of the two events are not identical but do show a close resemblance and largely overlap. We thus consider these earthquakes “siblings” that were generated by an anomalous “parent” feature of the megathrust. We suggest that this parent feature is a locked asperity surrounded by the otherwise partially creeping Simeulue section, perhaps structurally controlled by a broad morphological high on the megathrust.NRF (Natl Research Foundation, S’pore)MOE (Min. of Education, S’pore)Published versio