Tomographic Imaging of P- and S-wave Velocity Structure Beneath Central Java, Indonesia: Joint Inversion of the MERAMEX and MCGA Earthquake Data

Abstract

Realization of local tomography is usually conducted by removing distant events outside the study region, because these events may increase errors. In this study, tomographic inversions have been conducted using combined travel time data of local and regional events in order to improve the resolution, especially for deep structures. We used the local MERapi AMphibious EXperiments (MERAMEX) data catalog that consists of 292 events from May to October 2004. The new data of regional events in the Java region were taken from the Meteorological Climatological and Geophysical Agency (MCGA) data catalog that consists of 882 events from April 2009 to February 2011. Each event has at least 10 recording phases at each seismographic station. We have conducted joint inversions of the combined data sets using double-difference tomography to invert for velocity structures and to conduct hypocenter relocation simultaneously. The checkerboard test results of Vp and Vs structures demonstrate a significantly improved structural resolution from the shallow crust down to a depth of about 165 km. Our tomographic inversions reveal a low velocity anomaly at the Lawu - Merapi zone, which is consistent with the results from previous studies. A strong velocity anomaly zone with low Vp, low Vs and low Vp/Vs was identified between Cilacap and Banyumas. We interpret this anomaly as a fluid content material with large aspect ratio or sediment. This anomaly zone is in good agreement with the existence of a large dome containing sediment as proposed by previous geological studies. A low velocity anomaly zone is also detected in Kebumen, where it coincides with the extensional oceanic basin toward the land. Vertical cross sections of Vp and Vp/Vs tomograms confirm that the Merapi’s magma source is not vertically inclined but comes from the south of Merapi. Vertical cross sections of the tomograms also depict a high velocity anomaly pattern beneath the western part of the central Java, which may represent the subducted Indo-Australian plate.