Spatial variation of crustal anisotropy in Simeulue Island, Indonesia, from shear wave splitting analysis

Abstract

Simeulue Island sits near the northern subduction margin of the Sumatran Megathrust, which is characterized by high tectonic activities and earthquakes. The oblique subduction along this margin has developed a complicated crustal deformation on the island, including faulting, uplifting and crustal segmentation. In the subduction zone, crustal anisotropy is often caused by stress-induced anisotropy in which the anisotropy direction is parallel to the stress direction. However, the complex crustal structure around the study area may produce a complicated anisotropy pattern. Here, we measure crustal seismic anisotropy from shear wave splitting analysis using the seismic data recorded at eight temporary stations spread across Simeulue Island. We apply the 2-D tomographic inversion and spatial averaging technique to map the splitting anisotropy patterns around the region. This research allows us to gain new insight into the crustal deformation pattern and its relationship with the complicated crustal structure beneath the island. The splitting result shows variations of anisotropy pattern around the study area. The spatially averaged fast directions at the northern region are trench-parallel, consistent with the strike of the geological features resulting from the strain partitioning deformation of the oblique convergence. Higher strength anisotropy is also observed in this area, indicating that the local fault system may strongly contribute to the crustal anisotropy. In the southern part of the island, the spatial averaging of fast direction gives a consistent pattern with the maximum regional stress direction, suggesting that anisotropy is mainly associated with stress-aligned microcracks. The central part of the island exhibits different splitting directions, marking the boundary of the geological structures between the areas in the north and south of the island. This pattern is also accompanied by high-strength anisotropy, suggesting that the source may be associated with the subducting geological structures beneath the area playing a significant role in the rupture barrier of the great events, as suggested by several previous studies.