The characteristics of body-wave records between forearc and backarc region at the Sumatra subduction zone from deep regional earthquakes

Abstract

The Sumatra subduction zones have been known to produce large destructive earthquakes and tsunami, such as the 26 December 2004 M9 earthquake at the western offshore of Aceh. This large destructive earthquake usually occurred at the shallow portion of the subduction zone between 10 – 35 km depth, while the intermediate-to-deep earthquake usually occurred at the greater depth, deeper than 65 km along the subduction, which produces small to moderate earthquake with a magnitude between M3-5. Thus, these deep events along the subduction zone usually being neglected. However, the recent study has shown that an anomalous large ground movement associated with a deep earthquake at the subduction zone has been observed along the forearc region while in the backarc or at its epicenter, it remains low. Hence, the characteristics of the body-wave at these regions are different, which could be utilized for hazard mitigation. For this reason, this study aims to identify the body-wave characteristics between the forearc and backarc regions at the Sumatra subduction zone. Several selected deep regional earthquake records with a depth greater than 100 km provided by the Agency for Meteorology, Climatology, and Geophysics of Indonesia (BMKG) were analyzed. The finding indicates that the body-waves recorded in the forearc region are significantly different, with the main characteristics are; 1. Amplitude: preserve high amplitude, 2. Arrival time: shows a delayed signal of P- and S-waves, and 3. Spectral: conserve energy at low to high frequency and shows as a dispersion. These findings suggest that the signal recorded at the forearc region travel within the subducting slab, and the late arrival of the signal indicates that the seismic waves record at forearc travel at the top of the subducting slab, i.e., the oceanic crust, which is having lower velocity to the surrounding mantle.