Geology topics

R. S. Crosson

Publications and source records attributed to R. S. Crosson.

7 recordsLinked to original sources

USGS2011

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.

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USGS2003

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.

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USGS2002

Crustal structure and relocated earthquakes in the Puget Lowland, Washington, from high-resolution seismic tomography

The availability of regional earthquake data from the Pacific Northwest Seismograph Network (PNSN), together with active source data from the Seismic Hazards Investigation in Puget Sound (SHIPS) seismic experiments, has allowed us to construct a new high-resolution 3-D, P wave velocity model of the crust to a depth of about 30 km in the central Puget Lowland. In our method, earthquake hypocenters and velocity model are jointly coupled in a fully nonlinear tomographic inversion. Active source data constrain the upper 10–15 km of the model, and earthquakes constrain the deepest portion of the model. A number of sedimentary basins are imaged, including the previously unrecognized Muckleshoot basin, and the previously incompletely defined Possession and Sequim basins. Various features of the shallow crust are imaged in detail and their structural transitions to the mid and lower crust are revealed. These include the Tacoma basin and fault zone, the Seattle basin and fault zone, the Seattle and Port Ludlow velocity highs, the Port Townsend basin, the Kingston Arch, and the Crescent basement, which is arched beneath the Lowland from its surface exposure in the eastern Olympics. Strong lateral velocity gradients, consistent with the existence of previously inferred faults, are observed, bounding the southern Port Townsend basin, the western edge of the Seattle basin beneath Dabob Bay, and portions of the Port Ludlow velocity high and the Tacoma basin. Significant velocity gradients are not observed across the southern Whidbey Island fault, the Lofall fault, or along most of the inferred location of the Hood Canal fault. Using improved earthquake locations resulting from our inversion, we determined focal mechanisms for a number of the best recorded earthquakes in the data set, revealing a complex pattern of deformation dominated by general arc-parallel regional tectonic compression. Most earthquakes occur in the basement rocks inferred to be the lower Tertiary Crescent formation. The sedimentary basins and the eastern part of the Olympic subduction complex are largely devoid of earthquakes. Clear association of hypocenters and focal mechanisms with previously mapped or proposed faults is difficult; however, seismicity, structure, and focal mechanisms associated with the Seattle fault zone suggest a possible high-angle mode of deformation with the north side up. We suggest that this deformation may be driven by isostatic readjustment of the Seattle basin.

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USGS2002

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.

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USGS2001

Upper crustal structure in Puget Lowland, Washington: Results from the 1998 Seismic Hazards Investigation in Puget Sound

A new three-dimensional (3-D) model shows seismic velocities beneath the Puget Lowland to a depth of 11 km. The model is based on a tomographic inversion of nearly one million first-arrival travel times recorded during the 1998 Seismic Hazards Investigation in Puget Sound (SHIPS), allowing higher-resolution mapping of subsurface structures than previously possible. The model allows us to refine the subsurface geometry of previously proposed faults (e.g., Seattle, Hood Canal, southern Whidbey Island, and Devils Mountain fault zones) as well as to identify structures (Tacoma, Lofall, and Sequim fault zones) that warrant additional study. The largest and most important of these newly identified structures lies along the northern boundary of the Tacoma basin; we informally refer to this structure here as the Tacoma fault zone. Although tomography cannot provide information on the recency of motion on any structure, Holocene earthquake activity on the Tacoma fault zone is suggested by seismicity along it and paleoseismic evidence for abrupt uplift of tidal marsh deposits to its north. The tomography reveals four large, west to northwest trending low-velocity basins (Tacoma, Seattle, Everett, and Port Townsend) separated by regions of higher velocity ridges that are coincident with fault-bounded uplifts of Eocene Crescent Formation basalt and pre-Tertiary basement. The shapes of the basins and uplifts are similar to those observed in gravity data; gravity anomalies calculated from the 3-D tomography model are in close agreement with the observed anomalies. In velocity cross sections the Tacoma and Seattle basins are asymmetric: the basin floor dips gently toward a steep boundary with the adjacent high-velocity uplift, locally with a velocity "overhang" that suggests a basin vergent thrust fault boundary. Crustal fault zones grow from minor folds into much larger structures along strike. Inferred structural relief across the Tacoma fault zone increases by several kilometers westward along the fault zone to Lynch Cove, where we interpret it as a zone of south vergent faulting overthrusting Tacoma basin. In contrast, structural relief along the Seattle fault zone decreases west of Seattle, which we interpret as evidence that the N-S directed compression is being accommodated by slip transfer between the Seattle and Tacoma fault zones. Together, the Tacoma and Seattle fault zones raise the Seattle uplift, one of a series of east-west trending, pop-up structures underlying Puget Lowland from the Black Hills to the San Juan Islands.

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USGS1990

The Pacific Northwest; linkage between earthquake and volcano hazards

AS the title of the magazine Earthquake and Volcanoes suggests, these two geological phenomena are often closely associated. Earthquakes frequently precede volcanic eruptions, and volcanoes are often sources of small to intermediate size earthquakes resulting from the movement of magma within the volcano's plumbing system. In the Pacific Northwest, the association between earthquakes and volcanoes is much more fundamental- they both arise from the same large-scale interaction between two plates of the Earth's crust. The oceanic Juan de Fuca plate is being shoved beneath the edge of the continental North American plate in the process known as subduction. The result is the Cascade chain of volcanoes, the potential for very large earthquakes along the coastal margin, and the generation of stresses that produce other regional earthquakes. The Pacific Northwest (Oregon, Washington, and northern California) is experiencing rapid industrial and population growth. The same conditions that make the region attractive- close proximity to both mountains and oceans, volcanoes and spectacular inland waters- also present significant geologic hazards that are easily overlooked in the normal timetable of human activities. The catastrophic eruption of Mount St. Helens 10 years ago serves as a dramatic reminder of the forces of nature that can be unleashed through volcanism. other volcanoes such as mount Rainier, a majestic symbol of Washington, or Mount hood in Oregon, lie closer to population centers and could present far greater hazards should they become active. Earthquakes may affect even larger regions, prodcuging more cumulative damage.

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USGS1985

Seismotectonic map of the Puget Sound region, Washington

The Puget Sound region is a seismically active area with hundreds of earthquakes occurring each year (Crosson, 1974, 1975, Crosson and Millard, 1975, Crosson and Noson, 1978a, 1978b, 1979). Most of the earthquakes are so small that they can be detected only by sensitive seismographs such as those operated by the University of Washington. Not all of the earthquakes have been small, however, for several damaging ones have occurred in historic time. Little is known about the geologic structures responsible for generating these earthquakes, and knowledge of the causative structures is fundamental to an adequate evaluation of the earthquake risks for the Puget Sound region. The purpose of this map is to summarize the current knowledge about tectonic deformation, seismicity, and the tectonic framework of the Puget Sound region in order to provide a basis for detailed geologic and geophysical studies that will lead to better understanding of the structures and forces responsible for generating earthquakes in the area. This map is a compilation of all known and inferred faults, including what is known about their age of movement. Also shown are some deformed deposits of Quaternary age and earthquake epicenters. Few of the faults have been studied in sufficient detail to establish whether or not they have been active or inactive during late Tertiary or Quaternary time, but most of the faults are the result of tectonic forces that were active during early and middle Tertiary time. These faults may or may not be related to the tectonic forces responsible for the current seismicity. Approximately 5 percent of the earthquakes recorded in the Puget Sound region originated at depths of 40 Km or more (Crosson, 1972, fig. 11), and these are probably not directly related to exposed or near-surface structures. Most of the earthquakes occur at shallower depths and may be associated with near-surface structures, but no earthquakes have been definitely identified with mapped faults.

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