Anisotropic structure beneath central Java from local earthquake tomography

In this study we present the new tomographic code ANITA which provides 3‐D anisotropic P and isotropic S velocity distribution based on P and S traveltimes from local seismicity. For the P anisotropic model, we determine four parameters for each parameterization cell. This represents an orthorhombic anisotropy with one predefined direction oriented vertically. Three of the parameters describe slowness variations along three horizontal orientations with azimuths of 0°, 60°, and 120°, and one is a perturbation along the vertical axis. The nonlinear iterative inversion procedure is similar to that used in the LOTOS code. We have implemented this algorithm for the updated data set of central Java, part of which was previously used for the isotropic inversion. It was obtained that the crustal and uppermost mantle velocity structure beneath central Java is strongly anisotropic with 7–10% of maximal difference between slow and fast velocity in different directions. In the forearc (area between southern coast and volcanoes), the structure of both isotropic and anisotropic structure is strongly heterogeneous. Variety of anisotropy orientations and highly contrasted velocity patterns can be explained by a complex block structure of the crust. Beneath volcanoes we observe faster velocities in vertical direction, which is probably an indicator for vertically oriented structures (channels, dykes). In the crust beneath the middle part of central Java, north to Merapi and Lawu volcanoes, we observe a large and very intense anomaly with a velocity decrease of up to 30% and 35% for P and S models, respectively. Inside this anomaly E‐W orientation of fast velocity takes place, probably caused by regional extension stress regime. In a vertical section we observe faster horizontal velocities inside this anomaly that might be explained by layering of sediments and/or penetration of quasi‐horizontal lenses with molten magma. In the mantle, trench parallel anisotropy is observed throughout the study area. Such anisotropy in the slab entrained corner flow may be due to presence of B‐type olivine having predominant axis parallel to the shear direction, which appears in conditions of high water or/and melting content.