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Other geophysical methods
Deformation
EDM measurements were conducted for the period of 1988 – 1994 on a summit trilateration network (Young et al., 2000). Cross-crater strain rates accelerated from less than 3 x 106/day between 1988 and 1990 to more than 11 x 106/day just prior to the January 1992 activity, representing a general, asymmetric extension of the summit during high-level conduit pressurization. During the effusive lava extrusion, strain decreased below the background level of less than 2 x 106/day. EDM measurements between lower flank and crater benchmarks during 4 years before the 1992 eruption revealed a long term displacements as high as 1m/year.
Later, in the period between November 1996 and March 1997, other deformation experiments were established using tiltmeters and GPS equipments (Beauducel and Cornet, 1999). An interpretation using a three-dimensional elastic model based on the mixed boundary element method and a near-neighbor Monte Carlo inversion lead to a suggestion of a magma chamber at the depth of 8 km below the summit and 2 km to the east of it. The estimated volume attributed to this magma chamber is about 11 x 106 m3.
Within the Indonesia – German joint research project MERAPI, four tiltmeter stations were installed on the flank during 1995 – 1997 (Rebscher et al., 2000; Westerhaus et al., 2008). In spite of the absence of strong volcano-induced tilt anomalies, rapid, step-like drift changes were detected with amplitudes of 15 to 80 µrad which are generally related to the alternation of wet and dry seasons. Finite-Element-Modelling showed that sign and amplitude of these perturbations are compatible with a pressure source located 1.2 km below the summit with radius of 1.7 km which is consistent with aseismic zone revealed by hypocenter distribution. These perturbations are interpreted as the effect of annual input of meteoric water to the pressure within deeper parts of the hydrothermal system on the central vent of Merapi (Westerhaus et al., 2008).
Geoelectric measurements
Based on DC resistivity survey, Friedel et al. (2000) developed resistivity models for the north, west, and south flanks with depth of investigation between 600 and 1000 m. For the high conductivity zones appearing in the West and South, a hypothesis was brought forward that the anomalies are caused by meteoric water penetrating highly permeable layers of volcanic deposits to great depth where it influences the extent of hydrothermal zones. In August 2000 a permanent SP and temperature monitoring station was established at the fumaroles field Woro. Correlations between SP, ground temperature anomalies, MP events and the appearance of lava dome during 2001 activity were observed (Friedel et al., 2004). Many SP anomalies and gas temperature coincided with the occurrence of Ultra Long Period (ULP) seismic events which are interpreted as the effects of gas emissions (Byrdina et al., 2003; Richter et al., 2004).
Muller and Haak (2004) derived a 3-D model of the electrical conductivity structure of Merapi volcano from magnetotelluric (MT) sounding and geomagnetic induction vectors (Fig. 1.6). The final model consists of two 3-D structures in the volcanic edifice, i.e. a central conductor (D) and a second conductor lying 5 km to the southwest of summit (E). The high conductivity material is probably hot saline water as suggested by position and lateral extent of the high conductivity material. Another conductive layer at the depth of 3.5 – 5.3 km (C) is attributed to a very porous regional layer containing seawater or fluid of comparable conductivity (Rittel et al., 1998; Muller and Haak, 2004).
Merapi seismic network
Historical Review
Monitoring volcanoes in Indonesia began in 1920 with the establishment of the Dinas Penjagaan Gunungapi by the Dutch East India Company (Dutch: Vereenigde Oost-Indische Compagnie, VOC). This establishment is a response to the eruption of G. Kelut in previous year which caused more than 5000 deaths. Shortly after, observation posts were established including at G. Merapi. In 1924 the first seismic station was installed at G. Merapi, with a Wichert type seismometer. It is an entirely mechanical seismometer, made in Gottingen (Germany). It is essentially an inverted pendulum, which records both components of horizontal motion on rolls of smoked paper. It weights 1000 kg, and has a natural period of 8 seconds. Damping is provided by two air-pistons on the top of the instrument. The pendulum is centered by placing a series of small weights on top of the main mass. This seismometer is no longer in operation, but is visible in BPPTK (Balai Penyelidikan dan Pengembangan Teknologi Kegunungapian). This seismometer was installed at 14 km west of the summit (Neumann van Padang, 1933). He observed an increasing seismic activity before the eruption of 1930. In 1968, Shimozuru performed seismic observations by installing seismographs at about 10 km south of the summit. Seismic signals were recorded on magnetic tapes (Shimozuru et al., 1969). Using 6 months of observation, he proposed the first classification of events of Merapi and their associated physical process. He suggested that multiphase (MP) events are related with dome growth.
Along with the development of monitoring technology, monitoring system of G. Merapi was also improved. In 1982 telemetry system began to be implemented. In cooperation between USGS and VSI (Volcanological Survey of Indonesia, former of CVGHM, Centre of Volcanology and Geological Hazard Mitigation), 6 short period seismic stations were installed around G. Merapi whose data were transmitted directly to Yogyakarta using VHF telemetry system (Koyanagi and Kojima, 1984). In 2 January 1991 the seismic network was digitized by using a Data Translation board, a dedicated PC, a stabilized power supply and the PCEQ – IAVCEI software (J.-L. Got, pers. comm.), in the frame of the cooperation with the French foreign office. In August 1994, 6 more short period seismic stations were installed and digitized, among which the 3-component summit station, in the frame of the cooperation with the French CNRS (J.-L. Got, pers. comm.). Then in 1994 Broadband seismometers started to be used in Merapi with the cooperation between the VSI and the GeoForschungsZentrum Potsdam (Beisser et al., 1996).
Recent seismic network
The monitoring system of Merapi is operated by BPPTK, which belongs to CVGHM. Seismic network at Merapi is a combination of short period and broadband stations. At the beginning of 2010 (February to April) a major renovation was carried out. All instruments of short period stations have been replaced.
Instrumental problems
Replacements were carried out because of signal quality degradations. Noise came from electronic self noise of the modulator producing significant distortion of the signal. Their sensitivities had also declined. After re-installation, some problems appeared. There were periods when the signal polarities were inverted for some reasons. Noise coming from outside the system such as interference with some amateur radio communication distorted the signals quite frequently. Even though this kind of distortion wasn’t continue, some treatment must be done before performing calculations especially those based on continuous data. The other problem is the limitation of a short period station i.e. amplitude saturation.
During the installation of broadband stations, new digital telemetry system was implemented. System based on TCP-IP protocol was chosen instead of conventional serial protocol. Despite the superiority in terms of transmission capacity, this system consumed much more power. In fact a typical power system consisting of a battery of 100 AH and 2 solar panels 40 W wasn’t sufficient to allow the battery to be always in stable capacity. Some breakdowns in stations reduced the amount of available records during the 2010 pre-eruptive period (Fig. 1.8). Therefore, analysis based on continues data such as RSAM and noise cross correlation is difficult to perform on the broadband data.
VTs at Merapi are sub-divided into deep (VTA) and shallow (VTB) events (Fig. 1.10). VTA events are characterized by hypocenters at depths greater than 2 km below the summit, and they have clear P- and S-wave arrivals. VTB events have depths less than 2 km and they have more emergent onsets at distant stations. For some VTB events, S-waves cannot be distinguished. VTA and VTB events are recognized principally by differences in amplitude ratios for the first arrivals between summit (PUS) and flank (DEL) stations. Differences in waveform and amplitude are probably related to greater degrees of scattering and attenuation for paths in the shallow parts of the structure (VTB) compared to deeper paths (VTA) (Wegler and Lühr, 2001).
Multiphase earthquakes are characterized by emergent onsets, maximum frequency of 4 to 8 Hz, and shallow depth (Fig. 1.10). These MP signals are similar to hybrid events in other classification schemes (McNutt, 1996). They are related to magma flow in the upper conduit and to dome growth (Ratdomopurbo and Poupinet, 2000). Their rate of occurrence is sometimes correlated with summit deformations (Beauducel et al., 2000).
Low-frequency earthquakes (LF), also sometimes called long-period (LP) events, have generally emergent onsets, lack S-wave arrivals, and have dominant peak frequencies in the range 1-3 Hz (Fig. 1.10). They are typically attributed to resonance of fluid-filled cavities resulting from pressure perturbations (Chouet, 1996). However, due to strong attenuation of the high-frequency waves, some events identified as LF at distant stations may be actually MP events (Hidayat et al., 2000). Very-Long-Period (VLP) events occurred at Merapi in 1998 (Hidayat et al, 2002) and 2010 (Jousset et al., 2013) but were not observed associated with the 2001 and 2006 eruptions. VLP signals correspond generally to the low frequency component of MP or LF events and they are interpreted as mass transfer of fluid (Ohminato et al., 1998; Legrand et al. 2000; Chouet et al. 2005; Waite et al., 2008, Jolly et al., 2012).
Tremor consists of long-lasting vibrations and is associated with resonance effects in cavities (Chouet , 1988; Konstantinou and Schlindwein, 2002), fluid flow (Rust et al. 2008), or degassing (Lesage et al., 2006). At Merapi tremor episodes are relatively sparse, of low amplitude, and their spectra contain a few regularly spaced peaks, with fundamental frequencies of 2-5 Hz (Ch. 2, Fig. 2.3). Rockfalls (RF) are characterized by progressively increasing amplitude at the onset, long duration and high frequency content (5 to 20 Hz). Pyroclastic flows (PF; Ch. 2, Fig. 2.5), usually generated by dome collapse, produce RF-type signals with fairly long duration (up to tens of minutes) and large enough amplitudes to be recorded at the farthest stations in the network.
Table of contents :
Chapter 1 GENERAL INTRODUCTION
1.1 Background
1.2 Previous studies
1.2.1 Structural geology
1.2.2 Seismic studies
1.2.3 Other geophyisical methods
1.2.3.1 Deformation
1.2.3.2 Geoelectric Measurements
1.2.3.3 Gravimetry
1.3 Merapi seismic network
1.3.1 Historical review
1.3.2 Recent seismic network
1.3.3 Instrumental problems
1.4 Main features of Merapi seismic events
1.5 Thesis Structure
Chapter 2 SEISMIC CHRONOLOGY ASSOCIATED WITH THE 2010 ERUPTION
2.1 Introduction
2.1 Data and method
2.3 Seismic chronology during 2010 crisis
2.4 Discussion
2.5 Conclusion
Chapter 3 SOURCE LOCATIONS
3.1 Introduction
3.2 Data and method
3.2.1 Absolute and uncertainty estimation
3.2.2 Relative location using double difference method
3.3 Results
3.3.1 Absolute locations
3.3.2 Relative locations
3.3.3 Depths versus arrival time difference models
3.3.4 Temporal evolution of the hypocenter distribution
3.4 Discussion
3.4.1 Aseismic zone in Merapi eddifice
3.4.2 Magma migration
3.5 Conclusions and perspective
Chapter 4 RSAM AND ERUPTION FORECASTING
4.1 Introduction
4.2 Data and method
4.2.1 RSAM and modified RSAM (MRSAM)
4.2.2 Hindsight eruption forecasting
4.3 Results
4.3.1 RSAM and MRSAM
4.3.2 Eruption forecasting
4.4 Discussion
4.5 Conclusions
Chapter 5 FAMILIES ANALYSIS
5.1 Introduction
5.2 Data and method
5.2.1 Recursive event detections
5.2.2 Extraction of families
5.3 Results and discussions
5.3.1 Event detections
5.3.2 Families of events
5.4 Conclusions and perspectives
Chapter 6 VELOCITY VARIATIONS BASED ON CODA WAVE INTERFEROMETRY
6.1 Introduction
6.2 Data and method
6.2.1 Velocity variations in the coda of multiplets
6.2.1.1 Multiplets data
6.2.1.2 Doublet method
6.2.1.3 Stretching method
6.2.2 Velovity variations using noise correlation
6.2.2.1 Noise data
6.2.2.2 Data selection and time synchronization
6.2.2.3 Corrections of rain effects
6.2.2.1 2D location of velocity perturbations
6.3 Results
6.3.1 Velocity variations obtained from multiplets
6.3.2 Velovity variations obtained from noise correlation
6.4 Discussions
6.4.1 Comparison of the methods used
6.4.2 Velocity changes prior to the eruption
6.4.3 Comparison with earlier studies at Merapi
6.4.4 AVV and tectonic events
6.5 Conclusions and perspectives
Chapter 7 GENERAL CONCLUSSIONS
