Crustal structure and shallow-focus seismic activities in the Sumatra subduction zone: Constrains from receiver function inversion

The Sumatra subduction zone is located in the southwest of the Suna plate, between the Euro-Asia Plate and Indo-Austrilian Plates. With the obliquely subducting of the Indo-Austrilian Plate toward the the Euro-Asia Plate, complex tectonics, strong earthquakes and volcanoes have been observed in this area which has become a well experimental field used to study the subduction zone. In this work, we employed receiver function method to evaluate the S-wave velocity structure beneath 5 broadband seismic stations along the Sumatra subduction zone. We selected 332 receiver function waveforms with intelligent software and manual picking methods, including 130, 34, 42, 29 and 97 receiver function waveforms corresponding to BKNI, GSI, LHMI, MNAI, and PMBI stations, respectively. These stacked receiver function waveforms were applied to inversion to estimate Swave structure beneath each station based on a Neighborhood Algorithm (NA). Our results indicate that the sediment layers for GSI, LHMI and MNAI stations are more than 3 km thick, two stations of which are thicker than 6 km (e.g. GSI and LHMI). The difference of receiver function waveforms for NE, SW and W orientation at station GSI where is accompanied with strong thrust earthquakes suggests that there is a complicated structure beneath this station. Station BKNI and PMBI are located on the eastern side of the Sumatra fault and the thickness of their sediment layers is only ∼ 1 km. The crustal thickness for back-arc basin is within 30–36 km. However, the crustal thickness of forearc area varies from ∼ 26 km of the forearc ridge to 26–30 km of the forearc basin toward continent and its, which suggests that the down dip limit (slab-Moho intersection) of seismogenic zone is within 29–36 km in forearc and explains why the shallow-focus earthquakes play a dominant role in this area. The stable state for the inner wedge of forearc within a seismogenic circle provides a favorable environment for storing stress. Meanwhile, these faults caused by the subducting of Indo-Austrilian Plate constructed a condition (e.g. cracking of intact rocks and frictional sliding) in which it would trigger shallow-focus seismic activities (releasing stress).