Abstract
Abstract. The Sumatran subduction zone exhibits strong seismic and tsunamogenic potential with the prominent examples of the 2004, 2005 and 2007 earthquakes. Here, we invert travel-time data of local earthquakes for vp and vp∕vs velocity models of the central Sumatran forearc. Data were acquired by an amphibious seismometer network consisting of 52 land stations and 10 ocean-bottom seismometers located on a segment of the Sumatran subduction zone that had not ruptured in a great earthquake since 1797 but witnessed recent ruptures to the north in 2005 (Nias earthquake, Mw = 8.7) and to the south in 2007 (Bengkulu earthquake, Mw = 8.5). The 2-D and 3-D vp velocity anomalies reveal the downgoing slab and the sedimentary basins. Although the seismicity pattern in the study area appears to be strongly influenced by the obliquely subducting Investigator Fracture Zone to at least 200 km depth, the 3-D velocity model shows prevailing trench-parallel structures at depths of the plate interface. The tomographic model suggests a thinned crust below the basin east of the forearc islands (Nias, Pulau Batu, Siberut) at ∼ 180 km distance to the trench. vp velocities beneath the magmatic arc and the Sumatran fault zone (SFZ) are around 5 km s−1 at 10 km depth and the vp∕vs ratios in the uppermost 10 km are low, indicating the presence of felsic lithologies typical for continental crust. We find moderately elevated vp∕vs values of 1.85 at ∼ 150 km distance to the trench in the region of the Mentawai Fault. vp∕vs ratios suggest an absence of large-scale alteration of the mantle wedge and might explain why the seismogenic plate interface (observed as a locked zone from geodetic data) extends below the continental forearc Moho in Sumatra. Reduced vp velocities beneath the forearc basin covering the region between the Mentawai Islands and the Sumatra mainland possibly reflect a reduced thickness of the overriding crust.
Highlights
The largest earthquakes on Earth occur along subduction zones where the oceanic plate is subducted beneath an upper continental plate and large stress is accumulated during the interseismic phase of the seismic cycle
In the shallow part of the vp velocity model we observe regions of reduced vp velocities alternating with higher vp values at shallow depths (Fig. 8, ∼ 10 km depth and Fig. 10a, b and c)
We present 2-D and 3-D velocity models from a local earthquake tomography (LET) using data from a dense network of seismic stations covering the onshore and offshore domain of the northern Sumatra forearc close to the Equator
Summary
The largest earthquakes on Earth occur along subduction zones where the oceanic plate is subducted beneath an upper continental plate and large stress is accumulated during the interseismic phase of the seismic cycle. For the Sumatran margin the seismogenic zone is suggested to reach below the continental Moho based on gravity surveys and wide-angle refraction and local earthquake tomography (Siberut: Simoes et al, 2004; Kieckhefer et al, 1980, ∼ 30 km Moho depth; Aceh basin and Simeulue: Dessa et al, 2009; Klingelhoefer et al, 2010; Tilmann et al, 2010, 21–25 km Moho depth; southern Mentawai Islands: Collings et al, 2012, less than 30 km Moho depth). Beneath the Batu Islands, both the forearc crust and the plate interface are characterized by enhanced seismicity levels with a number of persistent clusters This region hosted several major events during the last century (e.g., 1935 Mw = 7.7 and 1984 Mw = 7.2; Rivera et al, 2002) but was not affected by great earthquakes in the last 220 years at least (Konca et al, 2008). The main target of the seismometer network was to obtain velocity models of the complete marine and continental forearc in order to decipher down-dip and along-strike structural variations in the Sumatran subduction zone
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