ALBERT

Description: Seismic and volcanic activity in Central Java, Indonesia, is related to the subduction of the Indo-Australian plate beneath the Sunda plate. The MERAMEX project (MERapi AMphibious EXperiments) studied the relationship between subduction zone processes and arc volcanism in Central Java. In 2004, a temporary seismological network was installed and seismic refraction and reflection profiles, bathymetric, gravimetric, and magnetic data was acquired. The airgun signals of the three profiles were recorded both at OBH stations deployed along the seismic profiles and within the onshore receiver array. The reflection and refraction data of the three seismic profiles were forward modelled and the P-wave velocity information was used to generate background velocity models for the tomographic calculations. Tomographic inversions of the active onshore data were performed using the 3DTH (Hole 1992) and LOTOS-06 (Koulakov et al. 2007) codes. The onshore active data and local earthquakes were also jointly inverted using LOTOS-06. The tomographic models show a low-velocity structure aligned in a NNE-SSW direction that separates the rigid forearc into two blocks. The hypocenter of the May 26, 2006 Java magnitude Mw=6.4 earthquake and its aftershocks are concentrated along the eastern edge of this elongated low-velocity zone. An anomalously-strong low-velocity anomaly is located in the backarc crust just north of the volcanoes Sumbing, Merapi and Lawu. The velocity perturbations, the attenuation of P- and S- waves, the high Poisson’s ratio of 0.3, its correlation with a gravity low and the location in the active Sunda Arc, all suggest that the low-velocity anomaly is caused by multiple magma reservoirs and ascending feeder systems below the volcanoes of Central Java.