Anisotropy variations in the continental crust of Central – East Java region, Indonesia from local shear wave splitting

The subduction of the Indo-Australian plate underneath the Eurasian plate causes the crust of the Central and East Java region to experience long-term deformation. We have analysed shear wave splitting using waveforms from local events recorded at 30 broadband stations of the BMKG network from 2009 to 2020. We select the events that meet the criteria for the crustal anisotropy study, i.e., event depths<30 km with a radius of 150 km
from each seismic station. We compare our results with the regional strain rates in Java, then divide the splitting
analysis into three regions: (A) Central Java, (B) western East Java, and (C) easternmost Java and Bali region. The results show that region A has a predominant NE-SW of fast polarization and experiences a relatively small strain rate except for the region near Yogyakarta, where the average fast direction is parallel to the strike of the Opak Fault due to the structural-induced anisotropy. In region B, the fast directions are considerably parallel to the principal strain, suggesting stress-induced anisotropy. Meanwhile, in region C, the fast polarizations are parallel and subparallel to the predominant principal strain at Madura and Bali, suggesting stress-induced anisotropy caused by cracks and microcracks as a response to the local tectonic stress. Moreover, in the area between East Java and Bali, the orientation of fast directions is perpendicular relative to the principal axis, suggesting the presence of a fluid-filled fracture zone may contribute to the crustal anisotropy.