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USGS2012

Geophysical constraints on Washington convergent margin structure

Gravity and magnetic maps of western Washington reveal the lateral structure and fabric of the Washington Coast Range, Puget Basin, and southern Washington Cascade Range. The magnetic and gravity maps show large amplitude positive anomalies associated with the shallow but largely buried section of Washington Coast Range mafic rocks which are separated by negative anomalies over deep sedimentary basins. The positive anomalies indicate that the Coast Range mafic basement extends farther east than previously thought, at least as far east as the longitude of Seattle. Linear and steep gravity and magnetic gradients indicate many unmapped, often buried faults in the Washington Coast Range Province. Magnetic highs are also associated with mapped batholiths in the Cascade arc. Several magnetic highs observed east of the Coast Range rocks and west of these batholiths may be associated with buried Tertiary plutons or ophiolites. Two-dimensional gravity and magnetic modeling constrained with geological and other geophysical data indicate that the Coast Range Province rocks are about 1 km thick at the coast, thickening to as much as 30 km near their postulated eastern edge. A maximum boundary on the average density of the upper 15–20 km of the rocks that compose the Coast Range Province of 2920 kg/m 3 was established by the modeling, suggesting a composition largely of basalt and gabbro with little interbedded sediments. Under these rocks may be mantle or a subduction complex composed of dense mafic, ultramafic, and sedimentary rocks like that proposed to underlie Vancouver Island. Previous gravity models of the Washington margin include lower densities for the proposed subduction complex than for Vancouver Island, suggesting a lower component of mafic and ultramafic rocks than the rocks underlying Vancouver Island. However, my Washington model requires that the proposed subduction complex be more dense than the trench sediments and, therefore, that material denser than sediments be incorporated within it. The absence of continental mantle and the modeled wedge shape of the Coast Range Province upper crust suggest that erosion of the bottom of the overriding plate by subduction processes may have occurred.

Washington↗

USGS2012

Stratigraphic, lithologic, and major element geochemical constraints on magmatic evolution at Lassen volcanic center, California

The Lassen volcanic center is the most recent of several long-lived volcanic centers in the southernmost Cascade Range. These centers have erupted products ranging from basaltic andesite to rhyolite and are superimposed on a background of regional basaltic to andesitic volcanism. The evolution of the Lassen volcanic center is described in three stages. Stages I and II comprise the Brokeoff volcano, and 80 km 3 andesitic stratocone, active from 600 to 400 ka. Brokeoff volcano is compositionally equivalent to the regional basaltic andesite to andesite volcanism in the Lassen region and is the result of structurally controlled focusing of the diffuse regional mafic magmatism. Stage III comprises a silicic dome field and adjacent area of hybrid andesites and has a total volume of about 100 km 3 . Volcanism during stage III was episodic and is subdivided into four sequences of lithologically and temporally distinct lavas. Stage III began at 400 ka with a rhyolitic, caldera-forming pyroclastic eruption and chemically related lavas. Additional sequences of dacite erupted between 250–200 ka and 100–0 ka. Hybrid andesites erupted adjacent to the silicic dome field between 300 and 0 ka. Porphyritic andesite and dacite with high Al 2 O 3 , low TiO 2 , medium K 2 O and FeO/MgO ratios of 1.5–2.0 are the most abundant rock types in the Lassen volcanic center. However, the single most voluminous unit is sparsely phyric rhyolite pumice. In general, the lavas of Lassen volcanic center form a single coherent trend on major element variation diagrams and in pseudo-quaternary phase space, consistant with an origin either by fractional crystallization or magma mixing. In detail, however, the lack of systematic temporal change in silica and subtly crossing trends indicate a complex origin. A variety of statistically successful fractional crystallization models can be constructed that derive Brokeoff andesites from regional magmas. An important conclusion of the modeling is that if fractional crystallization is the process responsible for generation of Brokeoff andesite, then the parent magma must be low to medium K in geochemical affinity in order to explain the variation in K 2 O. However, although major element variation can be modeled by fractional crystallization, petrographic and stratigraphic evidence indicates that magma mixing is an important but subtle process in Brokeoff lavas and suggests that lavas evolved in small independent batches. Lavas erupted during stage III, while predominantly silicic, range from 53 to 75% SiO 2 . Disequilibrium mineral assemblages in the stage III lavas indicate that they are not directly derived from Brokeoff andesite by fractional crystallization. Mixing of silicic magma with regional mafic magma and disaggregation of andesitic quenched magmatic inclusions play dominant roles in the compositional diversity of stage III lavas.

Journal of Geophysical Research Solid Earth↗

USGS2012

Comparison of ground motion from tremors and explosions in deep gold mines

Seismic body waves, from tamped chemical explosions, two with yields of 50 and one of 150 kg, were compared with corresponding data from three mining-induced tremors with a view to testing methods of discriminating between the two types of events. Detonated at depths of about 2 km, all three explosions generated P waves for which the low-frequency spectral asymptotes agree well with corresponding results scaled down from nuclear shots at the Nevada Test Site. For the two smaller explosions, recorded underground at hypocentral distances ranging from 234 to 871 m, the P wave corner frequencies, defined by the intersections of the low- and high-frequency asymptotes, are also in fine agreement with scaled-down results from the Nevada Test Site; for the 150-kg explosion, which was recorded only at the surface, it appears that 2.39 km of upward propagation caused the corner frequency to be at least a factor of 4 lower than anticipated due to attenuation. All three explosions generated S waves that appear to be a consequence of deviatoric stress release in the immediate environs of the explosions. The three tremors analyzed here generated P and S waves whose spectra and source parameters agree well with standard earthquake source models. With regard to discrimination based on P wave spectra, we conclude that for events of fixed low-frequency spectral asymptotes, the explosions typically have higher corner frequencies than tremors or earthquakes, although counterexamples certainly exist. Interestingly, the 150-kg explosion was identified as such on the basis of P and S wave polarities that are incompatible with the normally expected double-couple source model; instead, these initial motions are consistent with an explosion in conjunction with normal faulting. The body wave spectra of this explosion and those of a nearby tremor, however, were indistinguishable.

Journal of Geophysical Research Solid Earth↗

USGS2012

Infragravity waves over a natural barred profile

Measurements of cross-shore flow were made across the surf zone during a storm as a nearshore bar became better developed and migrated offshore. Measured infragravity band spectra were compared to synthetic spectra calculated numerically over the natural barred profile assuming a white run-up spectrum of leaky mode or high-mode edge waves. As in earlier studies, the spectra compared closely; however, for some frequencies the energy of the measured spectrum exceeded the energy of the synthetic spectrum, suggesting that the run-up spectrum was not white but had dominating frequencies. Utilizing cross-shore flow data and synthetic spectra from a number of cross-shore locations, an equivalent run-up spectrum was calculated for each day. On the first day of the storm, the equivalent run-up spectrum indicated a dominant wave that had a node in velocity reasonably close to the bar crest. Later during the storm, when the bar had migrated farther offshore, there was no evidence for a dominant motion having a velocity node at the bar crest. The structure of the equivalent run-up spectrum compared well with spectra of direct measurements of run-up obtained several hundred meters away. We have no clear evidence in support of the theory that infragravity waves might form or force the offshore migration of a bar. To confirm this finding, longer records obtained synoptically over a developing bar are required. The dominant wave observed early in the storm was consistent with Symond and Bowen's (1984) theoretical prediction of resonant amplification of discrete frequencies over a barred profile.

Journal of Geophysical Research Solid Earth↗

USGS2020

Timescales and processes of methane hydrate formation and breakdown, with application to geologic systems

Gas hydrate is an ice-like form of water and low molecular weight gas stable at temperatures of roughly -10ºC to 25ºC and pressures of ~3 to 30 MPa in geologic systems. Natural gas hydrates sequester an estimated one-sixth of Earth’s methane and are found primarily in deepwater marine sediments on continental margins, but also in permafrost areas and under continental ice sheets. When gas hydrate is removed from its stability field, its breakdown has implications for the global carbon cycle, ocean chemistry, marine geohazards, and interactions between the geosphere and the ocean-atmosphere system. Gas hydrate breakdown can also be artificially driven as a component of studies assessing the resource potential of these deposits. Furthermore, geologic processes and perturbations to the ocean-atmosphere system (e.g., warming temperatures) can cause not only dissociation, but also more widespread dissolution of hydrate or even formation of new hydrate in reservoirs. Linkages between gas hydrate and disparate aspects of Earth’s near-surface physical, chemical, and biological systems render an assessment of the rates and processes affecting the persistence of gas hydrate an appropriate Centennial Grand Challenge. This paper reviews the thermodynamic controls on methane hydrate stability and then describes the relative importance of kinetic, mass transfer, and heat transfer processes in the formation and breakdown (dissociation and dissolution) of gas hydrate. Results from numerical modeling, laboratory, and some fields studies are used to summarize the rates of hydrate formation and breakdown, followed by an extensive treatment of hydrate dynamics in marine and cryospheric gas hydrate systems.

Journal of Geophysical Research-Solid Earth↗

USGS2021

Temporal seismic velocity variations: Recovery following from the 2019 Mw 7.1 Ridgecrest, California earthquake

We investigated seismic velocity changes ( dv / v ) associated with the 2019 Ridgecrest earthquake sequence with high‐frequency autocorrelations of ambient seismic noise data. Daily autocorrelation functions were computed for the entirety of 2019 and the first quarter of 2020 for broadband stations within the region, including the temporary broadband stations installed during the aftershock deployment. Travel time shifts in the daily autocorrelation functions, relative to the mean autocorrelation waveform, were computed to produce dv / v time series, which are sensitive to the evolving material properties of the shallow crust surrounding the Ridgecrest fault zone (RFZ). A short‐term velocity drop follows the M w 7.1 earthquake at stations in the vicinity of the rupture surface, while those greater than 50 km away showed no such drop. The maximum, absolute changes in seismic velocity are proportional to the logarithm of distance from the fault rupture and to the peak dynamic strain experienced during the earthquake. Near the areas of the highest coseismic slip within the RFZ, seismic velocities recovered over 3 months. However, in the vicinity of the nearby Garlock fault, where triggered slip manifested, and north of the RFZ, seismic velocities recovered within a month. We interpret the seismic velocity changes and their recovery to be largely due to changes in the physical properties of the shallow crust, such as fault zone damage recovery caused by the earthquake rupture process and in response to the large dynamic stresses of passing seismic waves from the mainshock.

California↗

USGS2022

Synthetic evaluation of infrasonic multipole waveform inversion

Acoustic source inversions estimate the mass flow rate of volcanic explosions or yield of chemical explosions and provide insight into potential source directionality. However, the limitations of applying these methods to complex sources and their ability to resolve a stable solution have not been investigated in detail. We perform synthetic infrasound waveform inversions that use 3-D Green’s functions for a variety of idealized and realistic deployment scenarios using both a flat plane and Yasur volcano, Vanuatu as examples. We investigate the ability of various scenarios to retrieve the input source functions and relative amplitudes for monopole and multipole (monopole and dipole) inversions. Infrasound waveform inversions appear to be a robust method to quantify mass flow rates from simple sources (monopole) using deployments of infrasound sensors placed around a source, but care should be taken when analyzing and interpreting results from more complex acoustic sources (multipole) that have significant directional components. In the examples we consider the solution is stable for monopole inversions with a signal-to-noise ratio greater than five and the dipole component is small. For most scenarios investigated, the vertical dipole component of the multipole explosion source is poorly constrained and can impact the ability to recover the other source term components. Because multipole inversions are ill-posed for many deployments, a low residual does not necessarily mean the proper source vector has been recovered. Synthetic studies can help investigate the limitations and place bounds on information that may be missing using monopole and multipole inversions for potentially directional sources.

Journal of Geophysical Research-Solid Earth↗