An ancient shallow slip event on the Mentawai segment of the Sunda megathrust, Sumatra

Abstract

[1] The outer-arc islands of western Sumatra rise during great megathrust earthquakes, due to large slip on the underlying megathrust. In contrast, the islands subsided up to a few centimeters during the recent tsunamigenic earthquake of October 2010, due to slip far updip, near the trench. Coral microatolls on one of the islands recorded a much larger subsidence, at least 35 cm, during an event in approximately A.D. 1314. We calculate a suite of slip models, slightly deeper and/or larger than the 2010 event, that are consistent with this large amount of subsidence. Sea level records from older coral microatolls suggest that these events occur at least once every millennium, but likely far less frequently than their great downdip neighbors. The revelation that shallow slip events are important contributors to the seismic cycle of the Mentawai segment further complicates our understanding of this subduction megathrust and our assessment of the region's exposure to seismic and tsunami hazards.

Assessing tsunami hazard using heterogeneous slip models in the Mentawai Islands, Indonesia

“…Therefore, while our model constrains slip on the main seismogenic portion of the megathrust based on previous rupture cycles, weak constraints trenchwards of the islands allow for more complex patterns of interseismic and coseismic slip near the trench (e.g. as suggested for Tohoku, Japan by Perfettini & Avouac 2014) that are not yet well resolved by the palaeogeodetic record (Philibosian et al 2012).…”

“…8b): variance also increases further away from the islands towards the most downdip portion of the fault. Constraining slip near the trench is further complicated as near-trench slip will generate subsidence at most locations on the Mentawai Islands (Hill et al 2012;Philibosian et al 2012): yet, all the coral data points record coseismic uplift. Therefore, while our model constrains slip on the main seismogenic portion of the megathrust based on previous rupture cycles, weak constraints trenchwards of the islands allow for more complex patterns of interseismic and coseismic slip near the trench (e.g.…”

Earthquake supercycles on the Mentawai segment of the Sunda megathrust in the seventeenth century and earlier

“…[], vertical deformation measurements on the Mentawai Islands are largely insensitive to the behavior of the shallow megathrust (see coseismic model resolution maps in Figures S19 and S21). The shallow portion of the Mentawai segment is known to rupture seismically, individually during the 2010 tsunami earthquake [e.g., Hill et al ., ] and an ancient shallow event [ Philibosian et al ., ], and likely in conjunction with the intermediate‐depth seismogenic zone in 1797 and 1833, based on historical tsunami reports summarized by Natawidjaja et al . [].…”

Afterslip following the 2007 Mw 8.4 Bengkulu earthquake in Sumatra loaded the 2010 Mw 7.8 Mentawai tsunami earthquake rupture zone

“…However, it is clear that the seismic behavior of the shallow portion of the megathrust appears to be more complex than previously thought. Specifically, the shallow region of megathrusts can both participate in large earthquakes, as exemplified by the 2011 M w 9.0 Tohoku‐oki earthquake, as well as host tsunami earthquakes, as exemplified by the 1314 and 2010 M w 7.8 Mentawai, 1992 M w 7.6 Nicaragua, 1994 M w 7.8 Java, and 1996 M w 7.5 Peru earthquakes [ Hill et al , ; Philibosian et al , ; Polet and Kanamori , ]. The question that needs to be addressed is, therefore, what physical conditions and properties control the spatial and temporal pattern of shallow seismic ruptures?…”