Gregory P. Waite

Evidence for gas and magmatic sources beneath the Yellowstone volcanic field from seismic tomographic imaging

The 3-D P-wave velocity and P- to S-wave velocity ratio structure of the Yellowstone volcanic field, Wyoming, has been determined from local earthquake tomography using new data from the permanent Yellowstone seismic network. We selected 3374 local earthquakes between 1995 and 2001 to invert for the 3-D P-wave velocity (Vp) and P-wave to S-wave velocity ratio (Vp/Vs) structure. Vp anomalies of small size (15×15 km) are reliably imaged in the northwestern part of the model outside the Yellowstone caldera; inside the caldera only Vp anomalies of large size extending over several grid nodes are reliably imaged. The Vp/Vs solution is generally poorer due to the low number of S–P arrival times. Only the northwestern part of the model is resolved with confidence; the Vp/Vs solution also suffers from strong vertical and horizontal velocity smearing. The tomographic images confirm the existence of a low Vp-body beneath the Yellowstone caldera at depths greater than 8 km, possibly representing hot, crystallizing magma. The most striking result of our study is a volume of anomalously low Vp and Vp/Vs in the northwestern part of the Yellowstone volcanic field at shallow depths of <2.0 km. Theoretical calculations of changes in P- to S-wave velocity ratios indicate that these anomalies can be interpreted as porous, gas-filled rock. The close spatial correlation of the observed anomalies and the occurrence of the largest earthquake swarm in historic time in Yellowstone, 1985, suggest that the gas may have originated as part of magmatic fluids released by crystallization of magma beneath the Yellowstone caldera.

VP and VS structure of the Yellowstone hot spot from teleseismic tomography: Evidence for an upper mantle plume

[1] The movement of the lithosphere over a stationary mantle magmatic source, often thought to be a mantle plume, explains key features of the 16 Ma Yellowstone–Snake River Plain volcanic system. However, the seismic signature of a Yellowstone plume has remained elusive because of the lack of adequate data. We employ new teleseismic P and S wave traveltime data to develop tomographic images of the Yellowstone hot spot upper mantle. The teleseismic data were recorded with two temporary seismograph arrays deployed in a 500 km by 600 km area centered on Yellowstone. Additional data from nearby regional seismic networks were incorporated into the data set. The VP and VS models reveal a strong low-velocity anomaly from � 50 to 200 km directly beneath the Yellowstone caldera and eastern Snake River Plain, as has been imaged in previous studies. Peak anomalies are � 2.3% for VP and � 5.5% for VS. A weaker, anomaly with a velocity perturbation of up to � 1.0% VP and � 2.5% VS continues to at least 400 km depth. This anomaly dips 30� from vertical, west-northwest to a location beneath the northern Rocky Mountains. We interpret the low-velocity body as a plume of upwelling hot, and possibly wet rock, from the mantle transition zone that promotes small-scale convection in the upper � 200 km of the mantle and long-lived volcanism. A high-velocity anomaly, 1.2% VP and 1.9% VS, is located at � 100 to 250 km depth southeast of Yellowstone and may represent a downwelling of colder, denser mantle material.

Modeling shock waves generated by explosive volcanic eruptions

Atmospheric shock waves induced by explosive volcanic eruptions can provide valuable information about eruption characteristics. Shock waves are manifested as pressure-density gradients that can be remotely observed with relatively little noise. Field measurements of expanding shock waves can be directly recorded by pressure transducers or imaged under the proper illumination and atmospheric conditions. In this paper, an open-ended shock tube was used to generate weak shock waves in the laboratory that are representative of explosive volcanic eruptions. They indicate that strong shock wave theory can be used for modeling moderate volcanic eruptions. Based on that finding, we use strong shock theory to estimate the sudden explosive energy released from several explosive eruptions. Our energy calculations are well correlated with total energy estimates derived from plume height or erupted mass.

Attenuation and scattering tomography of the deep plumbing system of Mount St. Helens

We present a combined 3-D P wave attenuation, 2-D S coda attenuation, and 3-D S coda scattering tomography model of fluid pathways, feeding systems, and sediments below Mount St. Helens (MSH) volcano between depths of 0 and 18 km. High-scattering and high-attenuation shallow anomalies are indicative of magma and fluid-rich zones within and below the volcanic edifice down to 6 km depth, where a high-scattering body outlines the top of deeper aseismic velocity anomalies. Both the volcanic edifice and these structures induce a combination of strong scattering and attenuation on any seismic wavefield, particularly those recorded on the northern and eastern flanks of the volcanic cone. North of the cone between depths of 0 and 10 km, a low-velocity, high-scattering, and high-attenuation north-south trending trough is attributed to thick piles of Tertiary marine sediments within the St. Helens Seismic Zone. A laterally extended 3-D scattering contrast at depths of 10 to 14 km is related to the boundary between upper and lower crust and caused in our interpretation by the large-scale interaction of the Siletz terrane with the Cascade arc crust. This contrast presents a low-scattering, 4–6 km2 “hole” under the northeastern flank of the volcano. We infer that this section represents the main path of magma ascent from depths greater than 6 km at MSH, with a small north-east shift in the lower plumbing system of the volcano. We conclude that combinations of different nonstandard tomographic methods, leading toward full-waveform tomography, represent the future of seismic volcano imaging.

Varying seismic-acoustic properties of the fluctuating lava lake at Villarrica volcano, Chile

Villarrica volcano outgasses through an open lava lake, with bubbles ranging in size from submillimeter to several meters, the largest of which produce strombolian bursting events that are visible from the crater rim. Thousands of shallow strombolian events identified through seismic waveform cross correlation were found to produce discrete and repetitive long-period seismic and infrasonic signals. We identified variations of up to 0.7 s in seismic-acoustic arrival delay times between April and July 2010 at a station ~750 m from the vent, which we interpret as due to fluctuations in the level of lava lake. During time periods interpreted as having high lava lake levels, based on reduced time delays, interevent times were also reduced, and average seismic amplitude measurements, seismic and acoustic event energies, and volcano acoustic-seismic ratios were all high as compared to times when the lava lake was lower. The crater is also a source of nearly continuous, monotonic infrasonic tremor. We found that the peak frequency of this infrasonic tremor, typically around 0.5–1.0 Hz, was inversely correlated with seismic-acoustic delay times and therefore an indicator of lava lake level. We use this correlation to propose a new model for infrasonic tremor generation, namely, using crater geometry to approximate a Bessel horn. We interpret the two clearest cycles of elevated seismicity and lava lake level as due to an increase in exsolved gas, resulting from an injection of volatile-rich magma or an overturn in a deeper magma reservoir.

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 RESEARCH ARTICLE 10.1002/2015JB012279 Key Points: • Source mechanism of small long-period (0.5–5 Hz) subevents at Mount St. Helens • Volumetric source consistent with shallow subhorizontal crack • Similar tiny long-period subevents likely part of source process at other volcanoes Supporting Information: • Figure S1 • Figure S2 • Figure S3 • Figures S1–S3 captions and Table S1 Correspondence to: R. S. Matoza, matoza@geol.ucsb.edu Citation: Matoza, R. S., B. A. Chouet, P. B. Dawson, P. M. Shearer, M. M. Haney, G. P. Waite, S. C. Moran, and T. D. Mikesell (2015), Source mechanism of small long-period events at Mount St. Helens in July 2005 using template matching, phase-weighted stacking, and full-waveform inversion, J. Geophys. Res. Solid Earth, 120, 6351–6364, doi:10.1002/2015JB012279. Received 11 JUN 2015 Accepted 11 AUG 2015 Accepted article online 14 AUG 2015 Published online 18 SEP 2015 Source mechanism of small long-period events at Mount St. Helens in July 2005 using template matching, phase-weighted stacking, and full-waveform inversion Robin S. Matoza 1,2 , Bernard A. Chouet 3 , Phillip B. Dawson 3 , Peter M. Shearer 1 , Matthew M. Haney 4 , Gregory P. Waite 5 , Seth C. Moran 6 , and T. Dylan Mikesell 7,8 1 Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California, USA, 2 Department of Earth Science and Earth Research Institute, University of California, Santa Barbara, California, USA, 3 U.S. Geological Survey, Volcano Science Center, Menlo Park, California, USA, 4 Alaska Volcano Observatory, U.S. Geological Survey Volcano Science Center, Anchorage, Alaska, USA, 5 Department of Geological and Mining Engineering and Sciences, Michigan Technological University, Houghton, Michigan, USA, 6 Cascades Volcano Observatory, U.S. Geological Survey Volcano Science Center, Vancouver, Washington, USA, 7 Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA, 8 Department of Geosciences, Boise State University, Boise, Idaho, USA 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. 1. Introduction Long-period (LP, 0.5–5 Hz) seismicity, observed at volcanoes worldwide, plays a central role in our ability to assess and forecast unrest and eruption [e.g., Chouet, 1996a; McNutt, 1996; Kawakatsu and Yamamoto, 2007; Kumagai, 2009; Neuberg, 2011; Nishimura and Iguchi, 2011; Zobin, 2012; Chouet and Matoza, 2013]. The term LP seismicity includes individual transient LP events and more continuous volcanic tremor signals. Over the past several decades, numerous competing hypotheses and models have emerged to explain LP seismicity [e.g., Chouet and Matoza, 2013, and references therein]. Among these hypotheses, LP events at shallow depth ( < 2 km) in a volcanic edifice are commonly explained by the impulsive excitation and resonance of fluid-filled cracks resulting from magmatic-hydrothermal interactions [e.g., Chouet et al., 1994; Chouet, 1996a; Kumagai et al., 2002b; Nakano et al., 2003; Nakano and Kumagai, 2005a; Waite et al., 2008; Matoza and Chouet, 2010; Arciniega-Ceballos et al., 2012; Maeda et al., 2013]. ©2015. American Geophysical Union. All Rights Reserved. MATOZA ET AL. The dome-building phase of the 2004–2008 eruption of Mount St. Helens (MSH) produced millions of repetitive seismic events with long-period codas and slowly evolving waveforms [Moran et al., 2008; Thelen et al., 2008]. Many of these events occurred with such precise regularity that they were termed “drumbeats” [Moran et al., 2008], a phenomenon that has been observed at several other volcanoes [e.g., Neuberg, 2000; MOUNT ST. HELENS SMALL LP SOURCE

Three-dimensional displacements of a large volcano flank movement during the May 2010 eruptions at Pacaya Volcano, Guatemala

Although massive flank failure is fairly common in the evolution of volcanoes, measurements of flank movement indicative of instability are rare. Here 3-D displacements from airborne radar amplitude images derived using an amplitude image pixel offset tracking technique show that the west and southwest flanks of Pacaya Volcano in Guatemala experienced large (~4 m), discrete landsliding that was ultimately aborted. Pixel offset tracking improved measurement recovery by nearly 50% over classic interferometric synthetic aperture radar techniques, providing unique measurements at the event. The 3-D displacement field shows that the flank moved coherently downslope along a complex failure surface involving both rotational and along-slope movement. Notably, the lack of continuous movement of the slide in the years leading up to the event emphasizes that active movement should not always be expected at volcanoes for which triggering factors (e.g., magmatic intrusions and eruptions) could precipitate sudden major flank instability.

Triggering of volcanic degassing by large earthquakes

Statistical analysis of temporal relationships between large earthquakes (M w ≥ 7) and volcanic eruptions suggests that seismic waves may trigger eruptions over great (>1000 km) distances from the epicenter, but a robust relationship between volcanic and teleseismic activity remains elusive. Here we investigate the relationship between dynamic stresses propagated by surface waves and a volcanic response, manifested by changes in sulfur dioxide (SO 2 ) emissions measured by the spaceborne Ozone Monitoring Instrument (OMI). Surface wave amplitudes for a catalog of 69 earthquakes in A.D. 2004–2010 are modeled at 12 persistently degassing volcanoes detected by the OMI. The volcanic response is assessed by examining daily OMI SO 2 measurements in 28 day windows centered on earthquakes meeting a variable peak dynamic stress threshold. A positive volcanic response is identified if the average post-earthquake SO 2 mass was at least 20% larger than the pre-earthquake SO 2 mass. We find two distinct volcanic responses, correlating strongly with eruption style. Open-vent, basaltic volcanoes exhibit a positive response to earthquake-generated dynamic stress (i.e., the earthquake triggers increased SO 2 discharge), and andesitic volcanoes exhibit a negative response. We suggest that the former is consistent with disruption or mobilization of bubbles, or magma sloshing, in low-viscosity magmas, whereas the latter observation may reflect more dominant controls on degassing in viscous magmas or a post-earthquake reduction in permeability. Overall this analysis suggests that the potential effects of large earthquakes should be taken into account when interpreting trends in volcanic gas emissions.

Source and Propagation Effects on Near‐Field Co‐Eruptive Ground Motion at Santiaguito Volcano, Guatemala

Abstract Data from a December 2008–January 2009 seismometer deployment show substantial variation in seismic signal characteristics among different sites and eruptive events at Santiaguito volcano. A station on one of the inactive domes typically records higher amplitudes and a more peaked spectrum than does another station closer to the vent on lower ground. Amplitude ratios between the stations and spectral peakedness of the station on the dome vary substantially and depend on vertical signal polarity, which is controlled by seismic source mechanism. Finite‐difference models considering two likely source mechanisms and several hypothetical subsurfaces show that the topography of the free surface and subsurface units can increase or decrease amplitudes by focusing or obstructing body waves. A homogeneous velocity structure is shown not to be a close approximation to the actual shallow subsurface, while a subsurface including a dipping reflective layer produces results that fit observations better. These models also indicate that path effects act more strongly on waves originating from an implosion than from a downward force, possibly due to differences in body and surface wave proportions and differing susceptibilities of body and surface waves to these path effects. Thus, recorded data and models both indicate that the level of wave‐field distortion associated with propagation effects is controlled by the source mechanism.

Nonlinear Moment-Tensor Inversion of Repetitive Long-Periods Events Recorded at Pacaya Volcano, Guatemala

Detailed models of low-frequency seismicity at volcanoes provide insights into conduit structure and dynamics of magmatic systems. We examine explosion-related long-period (LP) events from Pacaya volcano, Guatemala, that were recorded during a temporary installation of four broadband seismic stations from October 2013 to November 2013. The repetitive LP events are identified with the aid of infrasound measurements using a matched filter due to the high level of background tremor and the small magnitude of the recorded events. We derive a representative seismic signal from the phase-weighted stack of 8,587 of these similar events, and invert for a source moment tensor. To address the limitations posed by the limited number of stations of the local network, we employ a non-linear waveform inversion that uses a grid search for source type to obtain a quantitative measure of the source mechanism reliability. With only four stations, Pacaya represents a case of limited observational data, where a quantitative description of moment tensor uncertainty is needed before any interpretation is to be attempted. Results point to a shallow source mechanism somewhat like a tension crack, dipping ~ 40° to the east, consistent with the dominant E-W motion in the seismic records from stations west, north, and east of the source. The uncertainties determined from the non-linear inversion are not insignificant, but clearly constrain the mechanism to be a source dominated by isotropic components