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Abstract: The link between seismicity and degassing is investigated during the VEI 4 eruptions of Merapi volcano (Indonesia) in October and in early November 2010. Seismicity comprised a large number and variety of earthquakes including Volcano-Tectonic events, a sustained period of Long Period Seismicity (LPS), i.e., Long-Period events (LP), Very Long Period events (VLP) and tremor. LPS seismicity is ascribed to the excitation of fluid-filled cavity resonance and inertial displacement of fluids and magma. We investigate here LPS that occurred between 17 October and 4 November 2010 to obtain insights into the volcano eruption processes which preceded the paroxysmal phase of the eruption on 4–5 November. We proceed to the moment tensor inversion of a well-recorded large VLP event during the intrusion phase on 17 October 2010, i.e., before the first explosion on 26 October. By using two simplified models (crack and pipe), we find a shallow source for this VLP event at about 1km to the south of the summit and less than 1km below the surface. We analyze more than 90 LP events that occurred during the multi-phase eruption (29 October–4 November). We show that most of them have a dominant frequency in the range 0.2–4Hz. We could locate 48 of those LP events; at least 3 clusters of LP events occurred successively. We interpret these observations as generated by different fluid-filled containers in the summit area that were excited while magma rose. We also observe significant variations of the complex frequency during the course of the eruption. We discuss these changes in terms of a variable ratio of fluid to solid densities and/or by possible conduit geometry change and/or permeability of the conduit or the edifice and/or by resonance of different fluid-containers during the release of more than 0.4Tg of SO2 and large but unknown masses of other volcanic gases. Finally, we also discuss how the major explosions of the eruption were possibly triggered by passing waves resulting from regional tectonic earthquakes on 3 and 4 November. [Copyright &y& Elsevier]

Highlights: [•] We briefly present the normal eruption of Merapi volcano. [•] We summarize the 2010 eruption. [•] We list the contribution of the Special Issue of JVGR on the 2010 eruption of Merapi volcano. [ABSTRACT FROM AUTHOR]

Abstract: Merapi volcano (Indonesia) is one of the most active and hazardous volcanoes in the world. It is known for frequent small to moderate eruptions, pyroclastic flows produced by lava dome collapse, and the large population settled on and around the flanks of the volcano that is at risk. Its usual behavior for the last decades abruptly changed in late October and early November 2010, when the volcano produced its largest and most explosive eruptions in more than a century, displacing at least a third of a million people, and claiming nearly 400 lives. Despite the challenges involved in forecasting this ‘hundred year eruption’, we show that the magnitude of precursory signals (seismicity, ground deformation, gas emissions) was proportional to the large size and intensity of the eruption. In addition and for the first time, near-real-time satellite radar imagery played an equal role with seismic, geodetic, and gas observations in monitoring eruptive activity during a major volcanic crisis. The Indonesian Center of Volcanology and Geological Hazard Mitigation (CVGHM) issued timely forecasts of the magnitude of the eruption phases, saving 10,000–20,000 lives. In addition to reporting on aspects of the crisis management, we report the first synthesis of scientific observations of the eruption. Our monitoring and petrologic data show that the 2010 eruption was fed by rapid ascent of magma from depths ranging from 5 to 30km. Magma reached the surface with variable gas content resulting in alternating explosive and rapid effusive eruptions, and released a total of ~0.44Tg of SO2. The eruptive behavior seems also related to the seismicity along a tectonic fault more than 40km from the volcano, highlighting both the complex stress pattern of the Merapi region of Java and the role of magmatic pressurization in activating regional faults. We suggest a dynamic triggering of the main explosions on 3 and 4 November by the passing seismic waves generated by regional earthquakes on these days. [Copyright &y& Elsevier]

Abstract: We present results of our study aimed at understanding the dynamics of fluids inside a “wet” volcano through the analysis of swarms of long-period (LP) events accompanying the 2011 seismic unrest at Papandayan volcano, West Java, Indonesia. Prior to this unrest, we measured an extremely high percentage (100 %) of CO2 in the ground at the summit crater, however with a very low value of SO2 flux (~6tons/day). Increase in volcanic activity was also observed from the records of a tiltmeter. A long-term inflation was followed by an abrupt deflation that took place concurrently with the swarms of LP events. Thereafter, swarms of local-tectonic (LT) and volcano-tectonic (VT) earthquakes started. We focus here on analyzing the LP events in the following manner. First, we estimate the source location of LP events by applying a 3-D non-linear hypocenter localization algorithm which includes topography. We then study the waveforms and spectral characteristics of LP events recorded at different stations and investigate whether or not these characteristics are due to source effects. Subsequently, we compute the oscillation frequencies (f) and the decay characteristics (Q factor) in the complex frequency domain of the coda part of the LP events by using the Sompi method which is based on a homogeneous auto-regressive (AR) equation. The rectangular fluid-filled crack model is used to estimate the physical processes related to the observed temporal variations in the complex frequencies. We divide the swarms of LP events into two intervals. The first interval occurred between June and July 2011 (48 LP events), while the second interval extended from September to October 2011 (36 LP events). The frequencies of LP events observed during these intervals range between 1.1 and 6.2Hz while the Q factors are widely scattered between 20 and 400. We estimate the compositions of fluids inside the crack during both intervals as either water foam (mixtures of water and H2O gas/steam) or misty gas (mixtures of water droplets and H2O gas/steam). We finally suggest that if an eruption were to have taken place following the 2011 unrest, it would have been in phreatic style rather than magmatic style. The results of our study therefore contribute to the effort in the prediction of the behavior of future eruptions, and to volcanic hazards assessment, and therefore to volcanic risk mitigation. [Copyright &y& Elsevier]

Abstract: Understanding precursory signals is essential to forecast eruptions hazards and mitigate risks. Satellite observations have been shown to increasingly contribute to this goal. We use Differential-Inteferometric Synthetic Aperture Radar (D-InSAR) and SAR backscattering intensity data from ALOS/PALSAR to define a time-series of ground surface displacements for the 2006–2010 inter-eruptive period at Merapi volcano. We correlate trends in the displacement data to trends for summit temperature (determined using data from the ASTER sensor) and to gas emission data. We show that processing of the satellite data must be performed carefully before meaningful interpretations can be drawn. For example, after careful removal of the topographic effect on phase delays, we detect only subtle inter-eruptive episodes of cyclic deformation in the D-InSAR and SAR backscattering data. These small (mostly sub-centimeter) displacements contrast with meter-scale pre-eruptive displacements along an electronic distance measurement (EDM) line on the south flank of the volcano. We suggest that this difference is a consequence of localized movement within the summit area — a result that has important implications for understanding Merapi's structure, for monitoring network design and for eruption forecasting. Although small, Merapi's inter-eruptive deformation cycles are also seen in thermal data from ASTER and gas emissions. Accordingly, we interpret these cycles as the result of successive movement of individual magma batches migrating upward from deeper to shallower storage systems located along a NE dipping conduit. In view of magma migration prior to eruptions, these cycles may also serve as a precursory signal for large eruptions at Merapi, such as the one that took place in early November 2010. [Copyright &y& Elsevier]

The 2010 eruption of Merapi is the first large explosive eruption of the volcano that has been instrumentally observed. The main characteristics of the seismic activity during the pre-eruptive period and the crisis are presented and interpreted in this paper. The first seismic precursors were a series of four shallow swarms during the period between 12 and 4months before the eruption. These swarms are interpreted as the result of perturbations of the hydrothermal system by increasing heat flow. Shorter-term and more continuous precursory seismic activity started about 6weeks before the initial explosion on 26 October 2010. During this period, the rate of seismicity increased almost constantly yielding a cumulative seismic energy release for volcano-tectonic (VT) and multiphase events (MP) of 7.5×1010 J. This value is 3 times the maximum energy release preceding previous effusive eruptions of Merapi. The high level reached and the accelerated behavior of both the deformation of the summit and the seismic activity are distinct features of the 2010 eruption. The hypocenters of VT events in 2010 occur in two clusters at of 2.5 to 5km and less than 1.5km depths below the summit. An aseismic zone was detected at 1.5–2.5km depth, consistent with studies of previous eruptions, and indicating that this is a robust feature of Merapi's subsurface structure. Our analysis suggests that the aseismic zone is a poorly consolidated layer of altered material within the volcano. Deep VT events occurred mainly before 17 October 2010; subsequent to that time shallow activity strongly increased. The deep seismic activity is interpreted as associated with the enlargement of a narrow conduit by an unusually large volume of rapidly ascending magma. The shallow seismicity is interpreted as recording the final magma ascent and the rupture of a summit-dome plug, which triggered the eruption on 26 October 2010. Hindsight forecasting of the occurrence time of the eruption is performed by applying the Material Failure Forecast Method (FFM) using cumulative Real-time Seismic Amplitude (RSAM) calculated both from raw records and on signals classified according to their dominant frequency. Stable estimates of eruption time with errors as small as ±4h are obtained within a 6day lapse time before the eruption. This approach could therefore be useful to support decision making in the case of future large explosive episodes at Merapi. [ABSTRACT FROM AUTHOR]