Dannie Hidayat

Deformation and seismic precursors to dome-collapse and fountain-collapse nuees ardentes at Merapi Volcano, Java, Indonesia, 1994-1998

Abstract Following the eruption of January 1992, episodes of lava dome growth accompanied by generation of dome-collapse nuees ardentes occurred in 1994–1998. In addition, nuees ardentes were generated by fountain-collapse in January 1997, and the 1998 events also suggest an explosive component. Significant tilt and seismic precursors on varying time scales preceded these events. Deformation about the summit has been detected by electronic tiltmeters since November 1992, with inflation corresponding generally to lava dome growth, and deflation (or decreased inflation) corresponding to loss of dome mass. Strong short-term (days to weeks) accelerations in tilt rate and seismicity occurred prior to the major nuees ardentes episodes, apart from those of 22 November 1994 which were preceded by steadily increasing tilt for over 200 days but lacked short-term precursors. Because of the combination of populated hazardous areas and the lack of an issued warning, about 100 casualties occurred in 1994. In contrast, the strong precursors in 1997 and 1998 provided advance warning to observatory scientists, enabled the stepped raising of alert levels, and aided hazard management. As a result of these factors, but also the fortunate fact that the large nuees ardentes did not quite descend into populated areas, no casualties occurred. The nuee ardente episode of 1994 is interpreted as purely due to gravitational collapse, whereas those of 1997 and 1998 were influenced by gas-pressurization of the lava dome.

Prototype PBO instrumentation of CALIPSO project captures world-record lava dome collapse on Montserrat Volcano

This article is an update on the status of an innovative new project designed to enhance generally our understanding of andesitic volcano eruption dynamics and, specifically the monitoring and scientific infrastructure at the active Soufriere Hills Volcano (SHV), Montserrat. The project has been designated as the Caribbean Andesite Lava Island Precision Seismo-geodetic Observatory known as CALIPSO. Its purpose is to investigate the dynamics of the entire SHV magmatic system using an integrated array of specialized instruments in four strategically located ∼200-m-deep boreholes in concert with several shallower holes and surface sites. The project is unique, as it represents the first, and only such borehole volcano-monitoring array deployed at an andesitic stratovolcano.

Unique and remarkable dilatometer measurements of pyroclastic flow–generated tsunamis

Pyroclastic flows entering the sea may cause tsunamis at coastal volcanoes worldwide, but geophysically monitored field occurrences are rare. We document the process of tsunami generation during a prolonged gigantic collapse of the Soufriere Hills volcano lava dome on Montserrat on 12–13 July 2003. Tsunamis were initiated by large-volume pyroclastic flows entering the ocean. We reconstruct the collapse from seismic records and report unique and remarkable borehole dilatometer observations, which recorded clearly the passage of wave packets at periods of 250–500 s over several hours. Strain signals are consistent in period and amplitude with water loading from passing tsunamis; each wave packet can be correlated with individual pyroclastic flow packages recorded by seismic data, proving that multiple tsunamis were initiated by pyroclastic flows. Any volcano within a few kilometers of water and capable of generating hot pyroclastic flows or cold debris flows with volumes greater than 5 × 10 6 m 3 may generate significant and possibly damaging tsunamis during future eruptions.

Unique strainmeter observations of Vulcanian explosions, Soufrière Hills Volcano, Montserrat, July 2003

gravity waves propagated at ∼30 m s −1 . Eruption volumes estimated from plume height and strain data are 0.32– 0.42 × 10 6 ,0 .26–0.49 × 10 6 , and 0.81–0.84 × 10 6 m 3 ,f or Explosions 3–5 respectively, consistent with quasi‐cylindrical conduit drawdown <2 km. The duration of vigorous explosion is given by the strain signature, indicating mass fluxes of order 10 7 kg s −1 . Conduit pressures released reflect static weight of porous gas‐charged magma, and exsolution‐generated overpressures of order 10 MPa. Citation: Voight, B., et al. (2010), Unique strainmeter observations of Vulcanian explosions, Soufriere Hills Volcano, Montserrat, July 2003, Geophys. Res. Lett., 37, L00E18, doi:10.1029/ 2010GL042551.

Aftershock sequence of the 1994, Mw 6.8, Liwa Earthquake (Indonesia): Seismic Rupture process in a volcanic arc

We present the aftershock activity following the February 15, 1994, Mw 6.8 earthquake which was strongly felt in southern Sumatra, Indonesia, near the Great Sumatran Fault (GSF). At this place, the slip rate is supposed to be low; neverthless, three M>6 events occurred along this segment during this century. No significant instrumental microseismicity has ever been recorded there. We use data from both the regional Indonesian network and a local seismic array operating 11 days after the mainshock during one month. Aftershocks mostly locate in a broad zone of 55 × 20 km² near two active NW-trending strike-slip segments of the GSF separated by a recent caldera, Suwoh. During the experiment, the NW segment (from Suwoh up to 15 km SE of the Ranau lake caldera, an old right-stepover pull-apart) was very active. As first suggested by the aftershock distribution and the lack of coseismic rupture at the surface, the 20 focal mechanisms determined provide evidence for various post-seismic stress adjustments on secondary faults located in the Ranau-Suwoh paleo-pull-apart graben. Less than 20% of the aftershocks are directly linked to the main rupture, a nearly pure right-lateral strike-slip faulting reaching 25 km depth. A narrow seismic gap underlines the active volcanic area of Suwoh. We conclude that the rupture process along the GSF is controlled both by volcanism and structures, and that the volcanic activity affects the mechanical properties of the crust only in a narrow zone.

Active Source Seismic Experiment Peers Under Soufrière Hills Volcano

Characterizing internal structures of active volcanoes remains an enigmatic issue in geosciences. Yet studies of such structures can greatly improve hazard assessments, helping scientists to better monitor seismic signatures, geodetic deformation, and gas emissions, data that can be used to improve models and forecasts of future eruptions. Several passive seismic tomography experiments—which use travel times of seismic waves from natural earthquakes to image underground structures—have been conducted at active volcanoes (Hawaiis Kilauea, Washingtons Mount St. Helens, Italys Etna, and Japans Unzen), but an inhomogeneous distribution of earthquakes compromises resolution. Further, if volcanic earthquakes are dominantly shallow at a given location, passive methods are limited to studying only shallow features. Thus, active source experiments—where seismic waves from the explosion of deliberately set charges are used to image below the surface—hold great potential to illuminate structures not readily seen through passive measures.

The SEA-CALIPSO volcano imaging experiment at Montserrat: plans, campaigns at sea and on land, scientific results, and lessons learned

Abstract Since 1995 the eruption of the andesitic Soufrière Hills Volcano (SHV), Montserrat, has been studied in substantial detail. As an important contribution to this effort, the Seismic Experiment with Airgunsource-Caribbean Andesitic Lava Island Precision Seismo-geodetic Observatory (SEA-CALIPSO) experiment was devised to image the arc crust underlying Montserrat, and, if possible, the magma system at SHV using tomography and reflection seismology. Field operations were carried out in October–December 2007, with deployment of 238 seismometers on land supplementing seven volcano observatory stations, and with an array of 10 ocean-bottom seismometers deployed offshore. The RRS James Cook on NERC cruise JC19 towed a tuned airgun array plus a digital 48-channel streamer on encircling and radial tracks for 77 h about Montserrat during December 2007, firing 4414 airgun shots and yielding about 47 Gb of data. The main objecctives of the experiment were achieved. Preliminary analyses of these data published in 2010 generated images of heterogeneous high-velocity bodies representing the cores of volcanoes and subjacent intrusions, and shallow areas of low velocity on the flanks of the island that reflect volcaniclastic deposits and hydrothermal alteration. The resolution of this preliminary work did not extend beyond 5 km depth. An improved three-dimensional (3D) seismic velocity model was then obtained by inversion of 181 665 first-arrival travel times from a more-complete sampling of the dataset, yielding clear images to 7.5 km depth of a low-velocity volume that was interpreted as the magma chamber which feeds the current eruption, with an estimated volume 13 km3. Coupled thermal and seismic modelling revealed properties of the partly crystallized magma. Seismic reflection analyses aimed at imaging structures under southern Montserrat had limited success, and suggest subhorizontal layering interpreted as sills at a depth of between 6 and 19 km. Seismic reflection profiles collected offshore reveal deep fans of volcaniclastic debris and fault offsets, leading to new tectonic interpretations. This chapter presents the project goals and planning concepts, describes in detail the campaigns at sea and on land, summarizes the major results, and identifies the key lessons learned.