Phillip Dawson

United States Geological Survey

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Bernard Chouet

United States Geological Survey

Marcello Martini

Istituto Nazionale di Geofisica e Vulcanologia

H. M. Iyer

Naval Physical & Oceanographic Laboratory

A. Arciniega‐Ceballos

Universidad Nacional Autónoma de México

Javier Almendros

Universidad de Granada

Gilberto Saccorotti

Istituto Nazionale di Geofisica e Vulcanologia

Robin S. Matoza

University of California, Santa Barbara

John R. Evans

ARC Centre of Excellence for Transformative Meta-Optical Systems

R. Scarpa

University of Salerno

Gaetano De Luca

Istituto Nazionale di Geofisica e Vulcanologia

Cooperative Institutions

United States Geological Survey

University of California, San Diego

Scripps Institution of Oceanography

Cascades Volcano Observatory

Stanford University

Menlo School

Istituto Nazionale di Geofisica e Vulcanologia

Volcano Science Center

Hawaiian Volcano Observatory

University of Alaska Fairbanks

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Multiple seismic antennas: a tool to map the sources of complex seismic wave fields at active volcanoes

EGS - AGU - EUG Joint Assembly (2003)

Javier Almendros Bernard Chouet Phillip Dawson Christian Huber

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Citations (0)

Nonlinear teleseismic tomography at Long Valley Caldera, using three‐dimensional minimum travel time ray tracing

Journal of Geophysical Research Atmospheres (1995)

C. Weiland Lee K. Steck Phillip Dawson Valeri Korneev

We explore the impact of three‐dimensional minimum travel time ray tracing on nonlinear teleseismic inversion. This problem has particular significance when trying to image strongly contrasting low‐velocity bodies, such as magma chambers, because strongly refracted and/or diffracted rays may precede the direct P wave arrival traditionally used in straight‐ray seismic tomography. We use a simplex‐based ray tracer to compute the three‐dimensional, minimum travel time ray paths and employ an iterative inversion technique to cope with nonlinearity. Results from synthetic data show that our algorithm results in better model reconstructions compared with traditional straight‐ray inversions. We reexamine the teleseismic data collected at Long Valley caldera by the U.S. Geological Survey. The most prominent feature of our result is a 25–30% low‐velocity zone centered at 11.5 km depth beneath the northwestern quadrant of the caldera. Beneath this at a depth of 24.5 km is a more diffuse 15% low‐velocity zone. In general, the low velocities tend to deepen to the south and east. We interpret the shallow feature to be the residual Long Valley caldera magma chamber, while the deeper feature may represent basaltic magmas ponded in the midcrust. The deeper position of the prominent low‐velocity region in comparison to earlier tomographic images is a result of using three‐dimensional rays rather than straight rays in our ray tracing. The magnitude of our low‐velocity anomaly is a factor of ∼3 times larger than earlier models from linear arrival time inversions and is consistent with models based on observations of ray bending at sites within the caldera. Our results imply the presence of anywhere from 7 to 100% partial melt beneath the caldera.

Caldera

Magma chamber

Seismic Tomography

10.1029/95jb01147

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Citations (98)

Source mechanism of small long‐period events at Mount St. Helens in July 2005 using template matching, phase‐weighted stacking, and full‐waveform inversion

Journal of Geophysical Research Solid Earth (2015)

Robin S. Matoza Bernard Chouet Phillip Dawson Peter M. Shearer M. M. Haney

Abstract Long‐period (LP, 0.5‐5 Hz) seismicity, observed at volcanoes worldwide, is a recognized signature of unrest and eruption. Cyclic LP “drumbeating” was the characteristic seismicity accompanying the sustained dome‐building phase of the 2004–2008 eruption of Mount St. Helens (MSH), WA. However, together with the LP drumbeating was a near‐continuous, randomly occurring series of tiny LP seismic events (LP “subevents”), which may hold important additional information on the mechanism of seismogenesis at restless volcanoes. We employ template matching, phase‐weighted stacking, and full‐waveform inversion to image the source mechanism of one multiplet of these LP subevents at MSH in July 2005. The signal‐to‐noise ratios of the individual events are too low to produce reliable waveform inversion results, but the events are repetitive and can be stacked. We apply network‐based template matching to 8 days of continuous velocity waveform data from 29 June to 7 July 2005 using a master event to detect 822 network triggers. We stack waveforms for 359 high‐quality triggers at each station and component, using a combination of linear and phase‐weighted stacking to produce clean stacks for use in waveform inversion. The derived source mechanism points to the volumetric oscillation (∼10 m 3 ) of a subhorizontal crack located at shallow depth (∼30 m) in an area to the south of Crater Glacier in the southern portion of the breached MSH crater. A possible excitation mechanism is the sudden condensation of metastable steam from a shallow pressurized hydrothermal system as it encounters cool meteoric water in the outer parts of the edifice, perhaps supplied from snow melt.

10.1002/2015jb012279

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Citations (37)

Source characterization for an explosion during the 2009 eruption of Redoubt Volcano from very-long-period seismic waves

Journal of Volcanology and Geothermal Research (2012)

M. M. Haney Bernard Chouet Phillip Dawson John A. Power

Seismic moment

Point source

Seismogram

10.1016/j.jvolgeores.2012.04.018

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Citations (35)

Identifying elements of the plumbing system beneath Kilauea Volcano, Hawaii, from the source locations of very-long-period signals

Geophysical Journal International (2002)

Javier Almendros Bernard Chouet Phillip Dawson Tamzin Bond

We analyzed 16 seismic events recorded by the Hawaiian broad-band seismic network at Kilauea Volcano during the period September 9–26, 1999. Two distinct types of event are identified based on their spectral content, very-long-period (VLP) waveform, amplitude decay pattern and particle motion. We locate the VLP signals with a method based on analyses of semblance and particle motion. Different source regions are identified for the two event types. One source region is located at depths of ∼1 km beneath the northeast edge of the Halemaumau pit crater. A second region is located at depths of ∼8 km below the northwest quadrant of Kilauea caldera. Our study represents the first time that such deep sources have been identified in VLP data at Kilauea. This discovery opens the possibility of obtaining a detailed image of the location and geometry of the magma plumbing system beneath this volcano based on source locations and moment tensor inversions of VLP signals recorded by a permanent, large-aperture broad-band network.

Caldera

10.1046/j.1365-246x.2002.01629.x

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Citations (50)

Source Mechanism of Tiny Long-Period Events at Mount St. Helens in July 2005

AGU Fall Meeting Abstracts (2015)

Robin S. Matoza Bernard Chouet Phillip Dawson Peter M. Shearer M. M. Haney

Mount

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Full-Wave Ambient Noise Tomography of the Long Valley Volcanic Region (California)

AGU Fall Meeting Abstracts (2017)

A. F. Flinders D. R. Shelly Phillip Dawson D. P. Hill Yi Shen

Ambient noise level

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Magma under Long Valley Caldera

Eos (1988)

Phillip Dawson H. M. Iyer John R. Evans D. W. Steepels

Concerning the Long Valley Meeting Report (Eos, January 19, 1988, p. 43), in the section describing the presentation of results from the teleseismic imaging experiment, it is stated that “…subsequent questions by meeting attendees brought out the idea that the [low‐velocity] anomaly [beneath the caldera] could also be heavily influenced by fractured and hydrothermally altered rocks to depths of 4 – 5 km.” For several reasons, we think that this comment is a significant misrepresentation of facts.

Caldera

Misrepresentation

Section (typography)

Anomaly (physics)

10.1029/88eo00086

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Citations (5)

Three‐dimensional P and S wave velocity structure of Redoubt Volcano, Alaska

Journal of Geophysical Research Atmospheres (1996)

H. Benz Bernard Chouet Phillip Dawson John C. Lahr Robert A. Page

The three‐dimensional P and S wave structure of Redoubt Volcano, Alaska, and the underlying crust to depths of 7–8 km is determined from 6219 P wave and 4008 S wave first‐arrival times recorded by a 30‐station seismograph network deployed on and around the volcano. First‐arrival times are calculated using a finite‐difference technique, which allows for flexible parameterization of the slowness model and easy inclusion of topography and source‐receiver geometry. The three‐dimensional P wave velocity structure and hypocenters are determined simultaneously, while the three‐dimensional S wave velocity model is determined using the relocated seismicity and an initial S wave velocity model derived from the P wave velocity model assuming an average Vp / Vs ratio of 1.78. Convergence is steady with approximately 73% and 52% reduction in P and S wave arrival time RMS, respectively, after 10 iterations. The most prominent feature observed in the three‐dimensional velocity models derived for both P and S waves is a relative low‐velocity, near‐vertical, pipelike structure approximately 1 km in diameter that extends from 1 to 6 km beneath sea level. This feature aligns axially with the bulk of seismicity and is interpreted as a highly fractured and altered zone encompassing a magma conduit. The velocity structure beneath the north flank of the volcano between depths of 1 and 6 km is characterized by large lateral velocity variations. High velocities within this region are interpreted as remnant dikes and sills and low velocities as regions along which magma migrates. No large low‐velocity body suggestive of a magma chamber is resolved in the upper 7–8 km of the crust.

Slowness

P wave

Sill

S-wave

Dike

Earthquake location

10.1029/95jb03046

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Citations (261)

A seismological perspective of the shallow magma and hydrothermal systems under Kilauea Caldera

AGUFM (2011)

Bernard Chouet Phillip Dawson

Caldera

Magma chamber

Source

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Citations (0)