Eylon Shalev

Non-double-couple microearthquakes at Long Valley caldera, California, provide evidence for hydraulic fracturing

Abstract Most of 26 small (0.4≲ M ≲3.1) microearthquakes at Long Valley caldera in mid-1997, analyzed using data from a dense temporary network of 69 digital three-component seismometers, have significantly non-double-couple focal mechanisms, inconsistent with simple shear faulting. We determined their mechanisms by inverting P - and S -wave polarities and amplitude ratios using linear-programming methods, and tracing rays through a three-dimensional Earth model derived using tomography. More than 80% of the mechanisms have positive (volume increase) isotropic components and most have compensated linear-vector dipole components with outward-directed major dipoles. The simplest interpretation of these mechanisms is combined shear and extensional faulting with a volume-compensating process, such as rapid flow of water, steam, or CO 2 into opening tensile cracks. Source orientations of earthquakes in the south moat suggest extensional faulting on ESE-striking subvertical planes, an orientation consistent with planes defined by earthquake hypocenters. The focal mechanisms show that clearly defined hypocentral planes in different locations result from different source processes. One such plane in the eastern south moat is consistent with extensional faulting, while one near Casa Diablo Hot Springs reflects en echelon right-lateral shear faulting. Source orientations at Mammoth Mountain vary systematically with location, indicating that the volcano influences the local stress field. Events in a ‘spasmodic burst’ at Mammoth Mountain have practically identical mechanisms that indicate nearly pure compensated tensile failure and high fluid mobility. Five earthquakes had mechanisms involving small volume decreases, but these may not be significant. No mechanisms have volumetric moment fractions larger than that of a force dipole, but the reason for this fact is unknown.

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.

Seismic monitoring and activity increase in California Caldera

Intense seismicity in the Long Valley caldera area of California last summer may have been hydrothermally driven and triggered by escalating magmatic unrest in the resurgent dome area, results from the MAMMOTH 97 Seismic Experiment suggest. The experiment involved enhanced seismic monitoring in an area that is already closely watched for earthquake activity. Sophisticated instruments were placed in the calderas south moat section, Mammoth Mountain, and the Casa Diablo geothermal area (Figure 1). In early July, just as the enhanced network became fully operational, there was a major increase in seismic activity. Preliminary analysis has yielded new insights into the structure and tectonics of the south moat and suggests the activity occurred above a vertical, east-west trending, dike-like structure beneath the moat. A series of vigorous swarms was accompanied by accelerated inflation of the resurgent dome (Figure 2). The permanent network in the area, using single-component analog instruments, recorded more than 1,600 earthquakes of magnitude up to 3.5 during July and August, but the MAMMOTH network, with three-component digital instruments, located about six times as many (Figure 3). About 63 “long period” (LP) earthquakes west of Mammoth Mountain were also recorded (Figures 1 and 4).

Shear-wave splitting and fracture alignments at the Northwest Geysers, California

Shear-wave splitting is commonly observed on three-component microearthquake seismograms recorded at the Northwest Geysers geothermal field, California. The polarization of leading shear waves around each recording station is predominantly N10°–40°E, suggesting that the shear-wave splitting is caused mainly by pervasive, north-northeast orientated anisotropy. Time delays between the leading and second shear waves can be as large as 10ms/km to 30ms/km. We propose that the direction and magnitude of this shear wave splitting is due to the presence of pervasive, stress-aligned secondary fractures related to several northwest-trending, right-lateral faults in the Geysers area.

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.

A Comprehensive Study of Fracture Patterns and Densities in The Geysers Geothermal Reservoir Using Microearthquake Shear-Wave Splitting Tomography

In this project we developed a method for using seismic S-wave data to map the patterns and densities of sub-surface fractures in the NW Geysers Geothermal Field/ (1) This project adds to both the general methods needed to characterize the geothermal production fractures that supply steam for power generation and to the specific knowledge of these in the Geysers area. (2)By locating zones of high fracture density it will be possible to reduce the cost of geothermal power development with the targeting of high production geothermal wells. (3) The results of the project having been transferred to both US based and international geothermal research and exploration agencies and concerns by several published papers and meeting presentations, and through the distribution of the data handling and other software codes we developed.

A comprehensive study of fracture patterns and densities in the Geysers geothermal reservoir using microearthquake shear-wave splitting tomography. Quarterly report for Sep-Dec 1998

We start organizing the computer programs needed for crack density inversion into an easy to follow scripts. These programs were collection of bits and pieces from many sources and we want to organize those separate programs into coherent product. We also gave a presentation (enclosed) in the Twenty-Fourth Workshop on Geothermal Reservoir Engineering in Stanford University on our Geyser and Mammoth results.