Teleseismic traveltime tomography of the Andaman subduction zone

Abstract

The Andaman subduction zone is characterized by great earthquakes and strong tectonic activity, which are closely related to the subduction of the Indian plate. To gain insight into the subduction processes, the geometry of the downgoing Indian plate must be investigated. At present, the geometry of the subducting Indian plate remains unclear owing to the historical lack of a seismic network deployed in the Andaman Sea and the surrounding area. In this study, we employed a teleseismic traveltime tomography method to image the deep structures beneath the Andaman Sea. The principle of the tomography method is that regional earthquakes are regarded as seismic stations, and seismic stations at teleseismic distances are treated as teleseismic events based on the reciprocity theorem. With this type of teleseismic traveltime tomography, we selected a total of 179 regional earthquakes that were recorded by 4515 stations, and we manually selected 40,369 high-quality P-wave first arrivals. From the inversion of these seismic traveltime data, we obtain P-wave images at depths from 300 km to 700 km. Our tomographic images reveal three distinct features. First, we observe high-velocity anomalies beneath the Andaman Sea at depths of 300–700 km. The orientation of the high-velocity anomalies is nearly consistent with that of the Andaman trench and volcanic arc. The high-velocity anomalies in the upper mantle are laterally discontinuous; a gap at approximately 9°N divides the high-velocity anomalies into two parts. Second, high-velocity anomalies are found in the mantle transition zone (MTZ) below the Andaman trench. Third, strong low-velocity anomalies are detected below the volcanic group in the Mergui Basin. Based on our tomographic results, we infer the currently subducting Indian plate beneath the Andaman Sea, an ancient slab segment in the MTZ, and upwelling hot materials below the Mergui Basin.