Relocation of hypocenters from DOMERAPI and BMKG networks: a preliminary result from DOMERAPI project
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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.
KeywordsEarthquake Relocation DOMERAPI BMKG Tectonic feature
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. Merapi has a relatively very short return period of eruptions, i.e., from 4 to 6 years with volcanic explosivity index (VEI) of 1–3 (Andreastuti et al. 2000; Voight et al. 2000). The eruption type of Merapi is dominated by pyroclastic flow caused by dome collapse. High VEIs (more than 4) with an explosive type occur in a longer return period, for instance the 2010 eruption event. Such an explosive type of eruption occurs every ±100 years (Surono et al. 2012). This eruption type has been controlled by deep magma sources with different mechanism (Costa et al. 2013). Based on the result of a petrological study (Costa et al. 2013), there are two magma bodies beneath the Merapi volcano, i.e., at ~30 and ~13 km depths. The DOMERAPI project was conducted partly to understand the internal structure and characteristics of the Merapi volcano.
Many earth scientists have conducted research on Merapi using various methods to understand the process related to physical, chemical and geological parameters beneath the volcano and its summit. Seismological approach can also be used to understand physical parameters beneath a volcano. For example, studies using MERAMEX (Merapi Amphibious Experiment) data have resulted in some publications explaining the relation between subduction zone and volcanic arc in central Java (Wagner et al. 2007; Koulakov et al. 2007; Rohadi et al. 2013; Haberland et al. 2014). These studies depict clearly a low velocity ascending from the subducted slab to the volcanic arc, but they do not show a detailed velocity structure of Merapi due to the lack of data coverage beneath 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; Budi-Santoso 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.
In this study, the data from the BMKG network in the same period of the DOMERAPI project were incorporated in the hypocenter determination and relocation to maximize azimuthal coverage. Epicenters from the DOMERAPI network have an azimuthal gap average of more than 300°. The joint networks have successfully minimized the azimuthal gap to <250° (Ramdhan et al. 2015).
2 Data and method
The earthquake hypocenters were first determined using the Geiger’s method (Geiger 1912) implemented with the hypoellipse software (Lahr 1999). The method was also applied to determine earthquakes along Cimandiri and Baribis faults in West Java based on BMKG data (Supendi and Nugraha 2016). We relocated the hypocenters using a double-difference earthquake location algorithm (Waldhauser and Ellsworth 2000; Waldhauser 2001). This method was successfully applied to relocate hypocenters in the Indonesia region based on data from BMKG network (Ramdhan and Nugraha 2013; Cahyaningrum et al. 2015; Sabtaji and Nugraha 2015; Utama et al. 2015). The hypocenter relocation was conducted carefully to obtain precise hypocenters before conducting seismic travel time tomography for our future work.
3 Results and discussion
There are 464 earthquakes determined from the data recorded by the DOMERAPI and BMKG networks. After hypocenter relocation, we end up with 399 events. There are some earthquakes that could not be relocated which may be due to poorly clustered events. A comparison between epicenter distributions from the DOMERAPI and BMKG networks before and after relocation is shown in Fig. 3. Most of the resulting hypocenters are located outside the DOMERAPI network, so they can be useful for imaging the deep structure of Merapi in our future work on seismic tomography. Our new data set indicates that there are still many events to the east of the Opak fault events after nearly a decade of the Yogyakarta earthquake in 2006. In addition, a thrust fault to the south of central Java, which may be related to a backthrust, is clearly delineated by the relocated events.
There are two tectonic features detected clearly by the relocated hypocenters. They are: (1) a nearly vertical cluster of events that may be related to a backthrust to the south of the study region and (2) the active Opak fault to the south of Merapi, which is indicated by small events even after a decade of the Yogyakarta earthquake of May 26, 2006, at 22:54 UTC (Ekström et al. 2016).
4 Conclusions and further work
The hypocenter relocation result reveals some important tectonic features. One of them is the observed nearly vertical cluster of events to the south of central Java, which is related to a backthrust fault. In addition, the relocated events also depict a cluster of earthquakes parallel to the Opak fault characterized by a contrast in wavespeed (Zulfakriza et al. 2014), and a possible double seismic zone in the study region.
For further work, we will use our hypocenter relocation result to perform seismic travel time tomography in order to construct 3-D v P, v S and v P/v S models. These models will be used to understand the deep structure of the Merapi volcano, which has not been detected clearly by previous works due to lack of data coverage. As previous volcanic studies (e.g., Lei et al. 2013), a multiscale seismic tomographic imaging will be carried out utilizing the DOMERAPI and BMKG data and by incorporating the data from MERAMEX as used in previous works (Wagner et al. 2007; Koulakov et al. 2007; Rohadi et al. 2013; Haberland et al. 2014) to reconstruct magma bodies of Merapi and ascending low velocity features from the subducting slab to the volcanic arc and some other tectonic features. We will also combine data from the Center for Investigation and Technology Development of Geological Disaster, Indonesia (BPPTKG), to obtain images, especially below the Merapi summit with unprecedented detailed resolution.
We are grateful to Institut de Recherche pour le Développement (IRD), France, for funding the DOMERAPI project; Center for Volcanology and Geohazard Mitigation as the main counterpart of the DOMERAPI project in Indonesia; the Agency for Meteorology, Climatology and Geophysics (BMKG) for providing the waveform data used in this study; and the Indonesia Endowment Fund for Education (LPDP) for granting a doctoral scholarship to MR. This study has been supported in part by the Indonesian Directorate General of Higher Education (DIKTI) research funding 2015–2016 and the Institut Teknologi Bandung (ITB) through a WCU research Grant 2016 awarded to SW.
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