T. Dylan Mikesell

Continuous Profiles of Electromagnetic Wave Velocity and Water Content in Glaciers: An Example from Bench Glacier, Alaska, USA

Abstract We conducted two-dimensional continuous multi-offset georadar surveys on Bench Glacier, south-central Alaska, USA, to measure the distribution of englacial water. We acquired data with a multichannel 25 MHz radar system using transmitter–receiver offsets ranging from 5 to 150 m. We towed the radar system at 5–10 kmh–1 with a snow machine with transmitter/receiver positions established by geodetic-grade kinematic differentially corrected GPS (nominal 0.5 m trace spacing). For radar velocity analyses, we employed reflection tomography in the pre-stack depth-migrated domain to attain an estimated 2% velocity uncertainty when averaged over three to five wavelengths. We estimated water content from the velocity structure using the complex refractive index method equation and use a three-phase model (ice, water, air) that accounts for compression of air bubbles as a function of depth. Our analysis produced laterally continuous profiles of glacier water content over several kilometers. These profiles show a laterally variable, stratified velocity structure with a low-water-content (~0–0.5%) shallow layer (~20–30 m) underlain by high-water-content (1–2.5%) ice.

Monitoring southwest Greenland’s ice sheet melt with ambient seismic noise

Researchers monitor southwest Greenland’s ice sheet mass changes by measuring seismic velocity variations in Greenland’s crust. The Greenland ice sheet presently accounts for ~70% of global ice sheet mass loss. Because this mass loss is associated with sea-level rise at a rate of 0.7 mm/year, the development of improved monitoring techniques to observe ongoing changes in ice sheet mass balance is of paramount concern. Spaceborne mass balance techniques are commonly used; however, they are inadequate for many purposes because of their low spatial and/or temporal resolution. We demonstrate that small variations in seismic wave speed in Earth’s crust, as measured with the correlation of seismic noise, may be used to infer seasonal ice sheet mass balance. Seasonal loading and unloading of glacial mass induces strain in the crust, and these strains then result in seismic velocity changes due to poroelastic processes. Our method provides a new and independent way of monitoring (in near real time) ice sheet mass balance, yielding new constraints on ice sheet evolution and its contribution to global sea-level changes. An increased number of seismic stations in the vicinity of ice sheets will enhance our ability to create detailed space-time records of ice mass variations.

Analyzing the coda from correlating scattered surface waves

The accuracy of scattered Rayleigh waves estimated using an interferometric method is investigated. Summing the cross correlations of the wave fields measured all around the scatterers yields the Greens function between two excitation points. This accounts for the direct wave and the scattered field (coda). The correlations themselves provide insights into the location of the scatterers, as well as which scatterer is responsible for particular parts of the coda. Furthermore, these measurements confirm a constant-time arrival in the correlations, not part of the Greens function, but which has previously been derived as a result of the generalized optical theorem.

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

Retrieving surface waves from ambient seismic noise using seismic interferometry by multidimensional deconvolution

Retrieving virtual source surface waves from ambient seismic noise by cross correlation assumes, among others, that the noise field is equipartitioned and the medium is lossless. Violation of these assumptions reduces the accuracy of the retrieved waves. A point-spread function computed from the same ambient noise quantifies the associated virtual sources spatial and temporal smearing. Multidimensional deconvolution (MDD) of the retrieved surface waves by this function has been shown to improve the virtual sources focusing and the accuracy of the retrieved waves using synthetic data. We tested MDD on data recorded during the Batholiths experiment, a passive deployment of broadband seismic sensors in British Columbia, Canada. The array consisted of two approximately linear station lines. Using 4 months of recordings, we retrieved fundamental-mode Rayleigh waves (0.05–0.27 Hz). We only used noise time windows dominated by waves that traverse the northern line before reaching the southern (2.5% of all data). Compared to the conventional cross-correlation result based on this subset, the MDD waveforms are better localized and have significantly higher signal-to-noise ratio. Furthermore, MDD corrects the phase, and the spatial deconvolution fills in a spectral (f, k domain) gap between the single-frequency and double-frequency microseism bands. Frequency whitening of the noise also fills the gap in the cross-correlation result, but the signal-to-noise ratio of the MDD result remains higher. Comparison of the extracted phase velocities shows some differences between the methods, also when all data are included in the conventional cross correlation.

Scanning for velocity anomalies in the crust and mantle with diffractions from the core-mantle boundary

A novel method, based on differential arrival times of diffractions from the core-mantle boundary, swiftly scans for seismic velocity anomalies in the crust and mantle below an array of seismometers. The method is applied to data from the USArray and the large-scale structural features in the western United States are resolved. High lateral resolution is achieved, but structure is averaged over depth. As such, this method is complementary to surface-wave and tomographic body-wave methods, where averaging takes place in the lateral sense. Processing and data-volume requirements involved are minimal. Therefore, this method can be applied during the early stages of array deployment, before the necessary data is acquired to obtain accurate inversion images. The quick scanner can be used to identify features of interest, upon which the array could be refined.

Local, regional, and remote seismo-acoustic observations of the April 2015 VEI 4 eruption of Calbuco volcano, Chile

The two major explosive phases of the 22–23 April 2015 eruption of Calbuco volcano, Chile produced powerful seismicity and infrasound. The eruption was recorded on seismo-acoustic stations out to 1,540 km and on 5 stations (IS02, IS08, IS09, IS27, and IS49) of the International Monitoring System (IMS) infrasound network at distances from 1,525 to 5,122 km. The remote IMS infrasound stations provide an accurate explosion chronology consistent with the regional and local seismo-acoustic data, and with previous studies of lightning and plume observations. We use the IMS network to detect and locate the eruption signals using a brute-force, grid-search, cross-bearings approach. After incorporating azimuth deviation corrections from stratospheric cross-winds using 3D ray-tracing, the estimated source location is 172 km from true. This case study highlights the significant capability of the IMS infrasound network to provide automated detection, characterization, and timing estimates of global explosive volcanic activity. Augmenting the IMS with regional seismo-acoustic networks will dramatically enhance volcanic signal detection, reduce latency, and improve discrimination capability.

Acoustic and Seismic Fields of Hydraulic Jumps at Varying Froude Numbers

Mechanisms that produce seismic and acoustic wavefields near rivers are poorly understood because of a lack of observations relating temporally dependent river conditions to the near-river seismoacoustic fields. This controlled study at the Harry W. Morrison Dam (HWMD) on the Boise River, Idaho, explores how temporal variation in fluvial systems affects surrounding acoustic and seismic fields. Adjusting the configuration of the HWMD changed the river bathymetry and therefore the form of the standing wave below the dam. The HWMD was adjusted to generate four distinct wave regimes that were parameterized through their dimensionless Froude numbers (Fr) and observations of the ambient seismic and acoustic wavefields at the study site. To generate detectable and coherent signals, a standing wave must exceed a threshold Fr value of 1.7, where a nonbreaking undular jump turns into a breaking weak hydraulic jump. Hydrodynamic processes may partially control the spectral content of the seismic and acoustic energies. Furthermore, spectra related to reproducible wave conditions can be used to calibrate and verify fluvial seismic and acoustic models.

Seismo‐Ionospheric Observations, Modeling, and Backprojection of the 2016 Kaikōura EarthquakeSeismo‐Ionospheric Observations, Modeling, and Backprojection of the 2016 Kaikōura Earthquake

We processed Global Navigation Satellite System (GNSS) time‐series data to extract total electron content (TEC) perturbations of the ionosphere caused by the Kaikōura earthquake. We used ray‐based modeling to infer which part of the Earth’s surface coupled significant energy from the solid Earth into the atmosphere. We compared modeled TEC data with the observed time‐series data and determined that significant coupling occurred north‐northeast of the initial slip. This work corroborates existing analysis made with geodetic and Interferometric Synthetic Aperture Radar (InSAR) data. The TEC data suggested that the initial rupture coupled some energy into the atmosphere, but later surface displacements ( ∼60  s after the initiation) caused more significant ionospheric perturbations. Using an array of GNSS stations, we were able to track the moveout of the acoustic wave through the ionosphere. We used a method commonly used in seismological studies called backprojection to estimate the location of the source of the TEC perturbation on Earth’s surface. This is the first time that this method has been applied to TEC data, and the results are promising. The backprojection results are slightly shifted in space from the known area of maximum uplift, and we attribute this small discrepancy to the fact that we did not account for horizontal winds in the atmosphere, nor the 3D heterogeneity of the real atmosphere in the travel‐time modeling.

Passive Icemology: fracture and model characterization using microseismicity

Microseismicity has seen a tremendous increase in popularity in exploration geophysics in the past few years, building on classical passive seismology techniques to infer reservoir parameters. Here we present a microseismic analysis on an alpine glacier. This outdoor laboratory provides the opportunity to asses the possibilities and limitations of microseismicity as a tool to provide time-lapse information on local stress states and (scattering) attenuation, as well as fracture location, orientation, and growth.