Abstract
Merapi volcano located in central Java, Indonesia, is one of the most active stratovolcanoes in the world. Many Earth scientists have conducted studies on this volcano using various methods. The geological features around Merapi are very attractive to be investigated because they have been formed by a complex tectonic process and volcanic activities since tens of millions of years ago. The southern mountain range, Kendeng basin and Opak active fault located around the study area resulted from these processes. DOMERAPI project was conducted to understand deep magma sources of the Merapi volcano comprehensively. The DOMERAPI network was running from October 2013 to mid-April 2015 by deploying 46 broad-band seismometers around the volcano. Several steps, i.e., earthquake event identification, arrival time picking of P and S waves, hypocenter determination and hypocenter relocation, were carried out in this study. We used Geiger’s method (Geiger 1912) for hypocenter determination and double-difference method for hypocenter relocation. The relocation result will be used to carry out seismic tomographic imaging of structures beneath the Merapi volcano and its surroundings. For the hypocenter determination, the DOMERAPI data were processed simultaneously with those from the Agency for Meteorology, Climatology and Geophysics (BMKG) seismic network in order to minimize the azimuthal gap. We found that the majority of earthquakes occurred outside the DOMERAPI network. There are 464 and 399 earthquakes obtained before and after hypocenter relocation, respectively. The hypocenter relocation result successfully detects some tectonic features, such as a nearly vertical cluster of events indicating a subduction-related backthrust to the south of central Java and a cluster of events to the east of Opak fault suggesting that the fault has an eastward dip.
Highlights
Merapi volcano is located in central Java and surrounded by densely populated areas so that its eruption effect will be hazardous for the people living around the volcano
We incorporated events recorded by the BMKG seismometers outside the DOMERAPI network as shown in Fig. 3 to maximize azimuthal coverage
There are 464 earthquakes determined from the data recorded by the DOMERAPI and BMKG networks
Summary
Merapi volcano is located in central Java and surrounded by densely populated areas so that its eruption effect will be hazardous for the people living around the volcano. Detailed studies of Merapi have been performed using volcano-tectonic (VT) earthquake data to analyze seismicity and focal mechanism of events beneath the volcano (Ratdomopurbo and Poupinet 2000; Hidayati et al 2008; BudiSantoso et al 2013). They show that the maximum depth of VT earthquakes is about 5 km depth from the summit, so structures below this depth cannot be delineated. The P phase has much better onset quality than S leading to a less number of S phase than P
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