Robert S. Crosson

Seismic anisotropy in the upper mantle

Abstract From the study of ultramafic rocks it is clear that there is a strong tendency toward olivine orientation in which the b crystallographic axes show a distinct concentration normal to a schistosity or banding. Olivine a and c axes tend to lie in a girdle normal to this b axis concentration. Laboratory measurements of the elasticity of dunite with this fabric show behavior similar to transversely isotropic media for compressional waves. Evidence is sufficiently strong from both laboratory studies of elasticity and petrofabric studies to suggest that many ultramafic rocks behave macroscopic ally as transversely isotropic elastic solids. On the strength of prevailing theories regarding the composition and mechanical behavior of the upper mantle, these results suggest an upper mantle which is transversely isotropic to seismic wave propagation. The existence of laminar convection in the upper mantle may provide a mechanism to produce sub-parallel orientation of olivine. Where mantle flow is predominantly sheet-like over large regions such as beneath the oceans, a normal vertical axis transverse isotropy is suggested. In these regions there is no directional dependence of compressional wave refraction arrivals. However, where this pattern is disrupted by rising convection currents or transcurrent faulting, appreciable deviations from this configuration should occur. If seismic observations prove to substantiate the transversely isotropic behavior of the upper mantle, observable seismic anisotropy will be a powerful tool in determining the mantle composition, stress state, and existence and pattern of convection currents, as well as the type of mechanical behavior associated with convection.

Amplification of Seismic Waves by the Seattle Basin, Washington State

Recordings of the 1999 M w 7.6 Chi-Chi (Taiwan) earthquake, two local earthquakes, and five blasts show seismic-wave amplification over a large sedimentary basin in the U.S. Pacific Northwest. For weak ground motions from the Chi-Chi earthquake, the Seattle basin amplified 0.2- to 0.8-Hz waves by factors of 8 to 16 relative to bedrock sites west of the basin. The amplification and peak frequency change during the Chi-Chi coda: the initial S -wave arrivals (0–30 sec) had maximum amplifications of 12 at 0.5–0.8 Hz, whereas later arrivals (35–65 sec) reached amplifications of 16 at 0.3–0.5 Hz. Analysis of local events in the 1.0- to 10.0-Hz frequency range show fourfold amplifications for 1.0-Hz weak ground motion over the Seattle basin. Amplifications decrease as frequencies increase above 1.0 Hz, with frequencies above 7 Hz showing lower amplitudes over the basin than at bedrock sites. Modeling shows that resonance in low-impedance deposits forming the upper 550 m of the basin beneath our profile could cause most of the observed amplification, and the larger amplification at later arrival times suggests surface waves also play a substantial role. These results emphasize the importance of shallow deposits in determining ground motions over large basins.

Seismic survey probes urban earthquake hazards in Pacific Northwest

A multidisciplinary seismic survey earlier this year in the Pacific Northwest is expected to reveal much new information about the earthquake threat to U.S. and Canadian urban areas there. A disastrous earthquake is a very real possibility in the region. The survey, known as the Seismic Hazards Investigation in Puget Sound (SHIPS), engendered close cooperation among geologists, biologists, environmental groups, and government agencies. It also succeeded in striking a fine balance between the need to prepare for a great earthquake and the requirement to protect a coveted marine environment while operating a large airgun array.

Subsurface Geometry and Evolution of the Seattle Fault Zone and the Seattle Basin, Washington

The Seattle fault, a large, seismically active, east-west-striking fault zone under Seattle, is the best-studied fault within the tectonically active Puget Lowland in western Washington, yet its subsurface geometry and evolution are not well constrained. We combine several analysis and modeling approaches to study the fault geometry and evolution, including depth-converted, deep-seismic-reflection images, P -wave-velocity field, gravity data, elastic modeling of shoreline uplift from a late Holocene earthquake, and kinematic fault restoration. We propose that the Seattle thrust or reverse fault is accompanied by a shallow, antithetic reverse fault that emerges south of the main fault. The wedge enclosed by the two faults is subject to an enhanced uplift, as indicated by the boxcar shape of the shoreline uplift from the last major earthquake on the fault zone. The Seattle Basin is interpreted as a flexural basin at the footwall of the Seattle fault zone. Basin stratigraphy and the regional tectonic history lead us to suggest that the Seattle fault zone initiated as a reverse fault during the middle Miocene, concurrently with changes in the regional stress field, to absorb some of the north-south shortening of the Cascadia forearc. Kingston Arch, 30 km north of the Seattle fault zone, is interpreted as a more recent disruption arising within the basin, probably due to the development of a blind reverse fault. Manuscript received 23 August 2001.

Seismic velocity structure of the Puget Sound Region from 3‐D Non‐linear tomography

Seismic P-wave velocity structure is estimated for the Puget Sound basin region using iterative, non-linear tomographic inversion based on finite-difference travel-time calculations. Local earthquakes and explosion arrival times are used to image the three-dimensional velocity structure to a depth of approximately 60 km. Our earthquake data set comprises approximately 3000 well-located digitally recorded earthquakes collected by the Pacific Northwest Seismograph Network (PNSN) over the 17 year period from 1980 to 1996. Additional constraint on velocity is provided by including travel-times, with known locations and origin times, from controlled-source seismic profiles and also by including model interpretations along one profile directly in the inversion. We find good correspondence between the prominent features imaged by this study and previous geological and geophysical interpretations. The method is effective in resolving high resolution structure by combining earthquake, explosion, and active source interpretations into a single velocity structure model.

Seismic velocity structure across the central Washington Cascade Range from refraction interpretation with earthquake sources

A two-dimensional seismic P wave velocity model, extending to a depth of ∼50 km, is interpreted for a 425 km long profile extending from Hood Canal in western Washington across the central Cascade Range to Walla Walla in eastern Washington. Existing Pacific Northwest Seismograph Network stations and earthquake sources are employed in a refraction/wide-angle reflection interpretation. In the preferred model, crustal velocity decreases slightly from west to east in the depth range 10–25 km beneath the Cascade Range. The continental Moho is estimated to dip 2.7° to the west in eastern Washington and 4.4° to the east beneath Puget Sound forming a distinct crustal root for the Cascade Range. Crustal thickness ranges from ∼35.5 km on the west end beneath Puget Sound and 34 km on the east end near Walla Walla to 47 km under the high Cascades. Seismic velocity in the interpreted crustal root suggests rock of mafic composition which is consistent with a process of underplating resulting from dehydration of the subducting Juan de Fuca plate. Simplified analysis also suggests that the present topography of the Cascades in the vicinity of our profile is supported by the isostatic response of this root zone.

P-waveform analysis for local subduction geometry south of Puget Sound, Washington

The local subduction geometry at a site south of Puget Sound in western Washington is investigated using teleseismicP-waveforms recorded on a three-component event triggered seismograph. The data are processed using source equalization deconvolution in order to isolate locally convertedP-to-S arrivals and stacked to improve the signal-to-noise ratio. Stable arrivals in the radial component indicate an oceanic Moho within the subducted slab at a depth of about 53 km beneath the station. Observed amplitude variations with azimuth in the radial data, as well as qualitative aspects of the tangential data, are used to establish a slab dip of 16° to the southeast. Our results are compatible with previous results from a site 60 km to the west, and further confirm a substantial warp in the regional geometry of the subducted Juan de Fuca plate.

Symmetry of upper mantle anisotropy

Abstract Symmetry of the form of upper mantle seismic anisotropy observed by refraction measurements at sea reflects basic symmetry of mantle elasticity which in turn places constraints on models proposed to explain the origin of anisotropy. Observed mantle velocity variation appears to exhibit bilateral symmetry which can be explained by a fairly restricted class of elastic models for the upper mantle. Velocity data obtained off California are tested for a bilaterally symmetric model and a small component of asymmetry is found. Such asymmetry may be due to systematic small errors in the data or minor perturbations in the elastic symmetry.