Understanding the seismic structure beneath Sumatra and its surrounding regions

Abstract

The main objective of this research is to understand the seismic structure beneath Sumatra and its surrounding regions by using seismic tomography. The structure is interpreted as P wavespeed velocity anomalies relative to the ak135 model. The main data source is P wave traveltimes up to 2006 obtained from the International Seismological Centre with relocation using the EHB scheme. Summary rays are constructed for these ISC data to decrease the number of data and to give a more even rays distribution so that we obtain 1,750,070 ISC residuals. We also include arrivals reported to Geoscience Australia, data from RSES-ANU and the Indonesian Meteorological and Geophysical Bureau (BMG). The data recorded by the RSES arrays are analysed by using an adaptive stacking method and the BMG arrivals are picked by hand with an STA/LTA procedure used to speed up the analysis. The tomographic model using a 1{u00B0}x1{u00B0} degree grid parameterization extended from 70{u00B0}E to 193{u00B0}E and 45{u00B0}N to 45{u00B0}S which covers Himalaya to Tonga and Japan to Australia, is embedded inside a 2{u00B0}x2{u00B0} global model. Pseudo bending technique for 1D forward calculation is firstly performed and the conjugate least square is then applied to invert the residuals and obtain the velocity perturbation of the region. Subsequently, the 3D ray tracing and inversion scheme are reiterated to improve the 3D seismic structure of the region. This 1{u00B0}x1{u00B0} regional model is validated by resolution tests: with checkerboard and synthetic slab models. Within the region with sufficient ray coverage the checkerboard is resolved well. The 3D ray tracing and inversion reproduce the synthetic slabs well; even in complicated cases. This new regional model is very consistent with those produced by Widiyantoro et al., 1999 and Replumaz et al., 2004. In order to emphasize the understanding of the seismic structure beneath Sumatra and its surrounding regions a 0.5{u00B0}x0.5{u00B0} grid parameterization is embedded inside the 1{u00B0}x1{u00B0} regional model within the 2{u00B0}x2{u00B0} global model. The finer resolution model is chosen as the region from 80{u00B0}-125{u00B0}Eand 15{u00B0}S-l5{u00B0}N which covers Andaman, Sumatra, and eastern Indonesia. The 3D forward calculation and inversion are reiterated to obtain the smaller regional model with higher resolution. The higher resolution model offers better definition of the nature of the slabs and low velocity zones. Beneath the Aceh-Andaman region the old incoming lithosphere subducts steeply and terminates abruptly at ~500 km depth. A slab tear is found beneath this section that connects the Sumatran to the Himalayan subduction zone indicating a unified subduction zone a few million years ago. The Northern and Central Sumatran slabs dip shallowly with little deflection at ~500 km depth associated with younger, warmer incoming lithosphere. The slab-pull forces beneath Aceh-Andaman section may be less significant because the slabs terminate much shallower than beneath Sumatra. This behaviour may influence the frequency of large earthquakes. In the Java region with older lithosphere the slabs dip more steeply deflect backward and forward in the mantle transition zone.