Analysis of the seismic activity associated with the 2010 eruption of Merapi Volcano, Java
Introduction
Merapi is located on Java Island, about 30 km north of the city of Yogyakarta. It is considered as one of the most dangerous volcanoes of Indonesia because of densely populated surroundings and high levels of eruptive activity. The recent history of Merapi (Voight et al., 2000) is characterized by two eruptive styles: 1) effusive growth of viscous lava domes, with typical recurrence of 4 to 6 years, which collapse gravitationally to produce pyroclastic flows known as «Merapi-type nuées ardentes», 2) exceptional explosive eruptions of relatively large size, associated with column collapse and pyroclastic flows that reach or exceed distances of 10–15 km from the summit. The October–November 2010 eruption is the first large explosive event (VEI ~ 4) at Merapi that has been recorded by a multi-parametric monitoring network. It is also the first modern eruption that was not preceded by emergence of a lava dome. For example, previous instrumentally observed VEI = 1–2 eruptions in 1984, 1986, 1992, 1994, 1997, 1998, 2001, and 2006 were all characterized by lava extrusion followed by dome collapses. Consequently there is a unique opportunity to compare the seismic activity associated with the two types of eruption and to look for precursory evidence of a transition between effusive and explosive styles.
As for most volcanoes in the world, the seismic activity of Merapi is characterized by a large variety of events that correspond to different locations and physical processes of the sources. Since 1984, the classification of events at Merapi includes the following types: deep (VTA) and shallow (VTB) volcano-tectonic, multiphase (MP), low frequency (LF), very long period (VLP) events, tremor and rock fall (Ratdomopurbo and Poupinet, 2000). Hypocenter distributions of VT events from past eruptions display an aseismic zone at 1.5–2.5 km depth, which has been interpreted as a ductile high-temperature zone (Ratdomopurbo and Poupinet, 2000, Wassermann and Ohrnberger, 2001, Hidayati et al., 2008).
Eruptions at Merapi are generally preceded by VT and MP seismicity on varying time scales from weeks to months (Ratdomopurbo and Poupinet, 2000, Voight et al., 2000, Suharna et al., 2007). However, at least two of the past eruptions were not preceded by significant increases in seismicity (1986 and 1994) and these are interpreted as gravitational collapses of summit domes. During the 1991 eruption, about 25% of the shallow VT events were seismic multiplets. These were families of events with similar waveforms and are interpreted to indicate either sources very close to each other and with identical focal mechanisms or to non-destructive and repetitive sources. Ratdomopurbo and Poupinet (1995) analyzed multiplets during 1992 using a cross-spectral method comparison of coda waves. They detected an increase of 1.2% in seismic velocity, possibly related to the pressurization of the magma feeding system. Using records of a repeatable controlled source, Wegler et al. (2006) also observed an increase of the shear wave velocity before the 1998 eruption.
The velocity structure of Merapi is still poorly known. Active experiments using air gun shots have been carried out to investigate this structure (Lühr et al., 1998, Wegler et al., 1999). But, because the direct P- and S-waves generated by surficial sources were rapidly attenuated by scattering within the heterogeneous volcanic medium, no velocity model could be obtained. However, spindle-like shapes of seismogram envelopes were explained by a diffusion model (Wegler and Lühr, 2001). Lacking a more detailed velocity model, hypocenter determinations at Merapi generally use a homogeneous model with P-wave velocity of 3 km s− 1 (Ratdomopurbo, 1995, Hidayati et al., 2008) or 2.8 km s− 1 (Wassermann and Ohrnberger, 2001). At a broader scale, tomographic analysis (Koulakov et al., 2007, Koulakov et al., 2009, Wagner et al., 2007) reveals an exceptionally strong low velocity anomaly in the crust between the Merapi and Lawu (east of Merapi) volcanic groups. This anomaly is interpreted as a fluid and melt bearing magmatic feeder zone for the volcanoes in the area.
Focal mechanisms of VT events recorded in 2000–2001 were estimated by Hidayati et al. (2008) using both polarity and amplitude of P-wave first motions. VTA and most deep VTB events are of normal-fault types; whereas, VTB events located close to the surface yield both reverse and normal fault solutions.
Hidayat et al., 2000, Hidayat et al., 2002 studied very-long period (VLP) events that occurred in 1998 and determined that these have periods of 6–7 s, display similar waveforms from event to event, and are coeval with MP or LF earthquakes. They carried out moment tensor inversion of the waveforms and proposed a source model consistent with a dipping crack located at about 100 m depth under the 1998 dome. They suggested a source process involving the sudden release of pressurized gas through the crack over a time span of about 6 s. No VLP events were observed during the active periods of 2001 and 2006; whereas, a significant number of VLP events were observed in 2010 prior to and during the eruption (Jousset et al., 2013).
In this paper, we focus on seismic activity that accompanied the 2010 eruption, as well as activity during the year preceding the eruption. We describe the types of seismic events observed and provide a detailed chronology of the seismicity during this period. In particular, the spatial and temporal distribution of the VT earthquake hypocenters yields important information bearing on pre-eruptive processes. We apply the Material Failure Forecast Method (Voight, 1988) to the RSAM values and test the potential of this approach to forecast the time of the eruption onset. Comparisons of seismicity between 2010 and earlier eruptions give some insights into precursory seismic features that may be used to distinguish explosive from effusive eruptions.
Section snippets
Seismic network
The monitoring system of Merapi is operated by BPPTK (Balai Penyelidikan dan Pengembangan Teknologi Kegunungapian), which belongs to CVGHM (Center of Volcanology and Geological Hazard Mitigation). Monitoring of Merapi is mainly based on seismic, deformation and geochemical measurements. A permanent seismic network consists of four short-period (SP) stations equipped with L4C and L22 seismometers. Signals are transmitted to Yogyakarta by radio with VHF modulation and are digitized by a Güralp
Main features of the seismic events
For consistency, the same classification of seismic signals has been used at Merapi since the initial installation of a telemetered network in 1982 (Ratdomopurbo, 1995, Ratdomopurbo and Poupinet, 2000). The main types of signal are classified as volcanotectonic (VT), multiphase (MP), low-frequency (LF), rockfall (RF), and tremor. VT events are characterized by clear onsets and high frequency content (up to 25 Hz). These types of events are similar to common tectonic earthquakes and are
Chronological description of the pre- and co-eruptive seismicity
This section summarizes the history of the seismic activity during the year preceding the eruption, with a focus on the last few weeks preceding the eruption as well as the eruption itself. It mainly relies on routine manual counting and classification of events based on waveform shape. Daily statistics are reported in local time (GMT + 7). Seismic energy reported below is calculated using the Gutenberg–Richter equation:where M is the magnitude (Gutenberg and Richter, 1956) and E is
Source locations
From the database of seismic events of Merapi, 679 events recorded on 4 to 9 seismic stations were located for the period that between October 2009 and October 2010. In the present work, the Hypoellipse program (Lahr, 1999) was used with a homogeneous half space velocity model assuming Vp = 3 km s− 1 and Vp/VS = 1.86 (Ratdomopurbo and Poupinet, 2000). In order to estimate realistic uncertainties on hypocenter positions, a Monte-Carlo method of error simulation was applied. The observed arrival times
RSAM and hindsight forecasting
Real Time Seismic Amplitude Measurement (RSAM) is a robust tool for monitoring volcanic activity because it provides a simple indicator of the level of seismic energy released (Endo and Murray, 1991). At Merapi, real-time monitoring by RSAM was carried out during the critical period of the eruptive crisis of 2010. A module made by BPTTK calculated the RSAM value every 5 min from the discriminator output of station KLA (Fig. 1). This provided valuable information on the magnitude of the increase
Discussion
The 2010 eruption seismic activity of Merapi during pre-eruptive and eruptive periods presents both features commonly observed during previous monitored eruptions and some characteristics that had never been recorded before, such as an unusually high level of energy release. The types of events identified in 2010 are similar to those observed since seismic stations were installed on the volcano in 1982. Although empirically determined from waveform observations, the event classification also
Conclusion
After an exceptional eruption, it is of paramount importance to carry out a thorough analysis of the data from the monitoring network that could not be processed in detail during the crisis. Aside from early seismic swarms observed 12 to 4 months before the 2010 crisis, the seismic activity of Merapi increased almost monotonically during the 6 weeks preceding the eruption. The numbers of LF events, VLP events and tremors recorded in this period were larger than for smaller past eruptions. The
Acknowledgment
We wish to acknowledge the efforts of the BPPTK staff especially Ilham Nurdin, Febri Sadana, and Purwoto for their struggle to ensure the seismic stations work properly. We are grateful to VDAP USGS for the assistance and donation of short period stations and to MIAVITA project for the broadband stations. The MIAVITA project was financed by the European Commission under the 7th Framework Program for Research and Technological Development, Area “Environment,” Activity 6.1 “Climate Change,
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