Lithospheric mantle dynamics in Central and East Java Region, Indonesia from local shear wave splitting measurements

We have investigated the local shear wave splitting of 30–300 km depth earthquakes from 38 BMKG stations between 2009 and 2020 to determine upper mantle dynamics beneath the Central and East Java (CEJ) region, Indonesia. A total of 2338 measurements is obtained and divided the analysis into two focal depths, i.e., shallow (≤ 100 km) and deep (100 – 300 km) events. (1) Both individual station measurements and spatially averaged data using shallow events (≤ 100 km) show the trench-perpendicular fast direction in the northern CEJ region. Thus, anisotropy in this domain may be associated with the downdip subduction-induced 2-D corner flow in the mantle wedge allowing A-type olivine fabric to develop. Meanwhile, the trench-parallel fast directions in the southern CEJ region may reflect some possible causes of anisotropy: the presence of a serpentinized mantle wedge that promotes the development of B-type olivine fabric and anisotropy through alignment of the melt pockets. We also suggest a change in the hydration state of the subducting slab can cause the predominant trench-perpendicular fast directions in the eastern CEJ region. (2) For deep events (100 – 300 km), fast directions are relatively trench-parallel in the eastern CEJ region and trench-perpendicular in the western CEJ region, suggesting the presence of fossilized anisotropy and 2-D mantle flow-induced anisotropy, respectively.