Geology topics

W. Stephenson

Publications and source records attributed to W. Stephenson.

5 recordsLinked to original sources

USGS2003

Lithology and shear-wave velocity in Memphis, Tennessee

We have derived a new three-dimensional model of the lithologic structure beneath the city of Memphis , Tennessee , and examined its correlation with measured shear - wave velocity profiles. The correlation is sufficiently high that the better-constrained lithologic model may be used as a proxy for shear - wave velocities, which are required to calculate site-amplification for new seismic hazard maps for Memphis . The lithologic model and its uncertainties are derived from over 1200 newly compiled well and boring logs, some sampling to 500 m depth, and a moving-least-squares algorithm. Seventy-six new shear - wave velocity profiles have been measured and used for this study, most sampling to 30 m depth or less. All log and velocity observations are publicly available via new web sites.

Tennessee↗

USGS2002

Simulation of broadband ground motion including nonlinear soil effects for a magnitude 6.5 earthquake on the Seattle fault, Seattle, Washington

The Seattle fault poses a significant seismic hazard to the city of Seattle, Washington. A hybrid, low-frequency, high-frequency method is used to calculate broadband (0-20 Hz) ground-motion time histories for a M 6.5 earthquake on the Seattle fault. Low frequencies (1 Hz) are calculated by a stochastic method that uses a fractal subevent size distribution to give an ω -2 displacement spectrum. Time histories are calculated for a grid of stations and then corrected for the local site response using a classification scheme based on the surficial geology. Average shear-wave velocity profiles are developed for six surficial geologic units: artificial fill, modified land, Esperance sand, Lawton clay, till, and Tertiary sandstone. These profiles together with other soil parameters are used to compare linear, equivalent-linear, and nonlinear predictions of ground motion in the frequency band 0-15 Hz. Linear site-response corrections are found to yield unreasonably large ground motions. Equivalent-linear and nonlinear calculations give peak values similar to the 1994 Northridge, California, earthquake and those predicted by regression relationships. Ground-motion variance is estimated for (1) randomization of the velocity profiles, (2) variation in source parameters, and (3) choice of nonlinear model. Within the limits of the models tested, the results are found to be most sensitive to the nonlinear model and soil parameters, notably the over consolidation ratio.

Washington↗

USGS2000

Three-dimensional simulations of ground motions in the Seattle region for earthquakes in the Seattle fault zone

We used the 3D finite-difference method to model observed seismograms of two earthquakes (ML 4.9 and 3.5) in the Seattle region and to simulate ground motions for hypothetical M 6.5 and M 5.0 earthquakes on the Seattle fault, for periods greater than 2 sec. A 3D velocity model of the Seattle Basin was constructed from studies that analyzed seismic-reflection surveys, borehole logs, and gravity and aeromagnetic data. The observations and the simulations highlight the importance of the Seattle Basin on long-period ground motions. For earthquakes occurring just south of the basin, the edge of the basin and the variation of the thickness of the Quaternary deposits in the basin produce much larger surface waves than expected from flat-layered models. The data consist of seismograms recorded by instruments deployed in Seattle by the USGS and the University of Washington (UW). The 3D simulation reproduces the peak amplitude and duration of most of the seismograms of the June 1997 Bremerton event (ML 4.9) recorded in Seattle. We found the focal mechanism for this event that best fits the observed seismograms in Seattle by combining Green's functions determined from the 3D simulations for the six fundamental moment couples. The February 1997 event (ML 3.5) to the south of the Seattle Basin exhibits a large surface-wave arrival at UW whose amplitude is matched by the synthetics in our 3D velocity model, for a source depth of 9 km. The M 6.5 simulations incorporated a fractal slip distribution on the fault plane. These simulations produced the largest ground motions in an area that includes downtown Seattle. This is mainly caused by rupture directed up dip toward downtown, radiation pattern of the source, and the turning of S waves by the velocity gradient in the Seattle basin. Another area of high ground motion is located about 13 km north of the fault and is caused by an increase in the amplitude of higher-mode Rayleigh waves caused by the thinning of the Quaternary deposits.

Bulletin of the Seismological Society of America↗

USGS1998

Faulting along the southern margin of Reelfoot Lake, Tennessee

Approximately 320 km of deep seismic-reflection profiles in northwestern Tennessee reveal the structure of a major portion of the southeastern margin of the Reelfoot Rift. This rift margin consists of at least two major down-to-the-west late Precambrian to Cambrian normal faults. Maximum fault displacement at one location is 3 km. These two faults strike N50°E, in their northern portions; over their southern extent they trend N30°E however. Numerous faults in these reflection lines displace Paleozoic, Cretaceous, and Tertiary reflectors. The dominantly reverse faulting, folding, and positive flower structures in the shallower section indicate Eocene or younger transpression. We believe the late Tertiary faulting is due to reactivation of the basement faults, because on some of the reflection lines basement normal faults can be traced into Tertiary reverse faults, the Tertiary faults parallel the basement faults, and the Tertiary faults overlie or are adjacent to the basement faults. Numerous faults displace the highest (youngest) reflectors and therefore we do not know how recently faulting has occurred. Previous studies have identified Quaternary faulting within the southeastern Reelfoot Rift margin of western Tennessee, however. Thus, we believe all of the late Tertiary faults identified in this research should be evaluated for possible Quaternary movement.

Tennessee↗

USGS1997

Seismic reflection images beneath Puget Sound, western Washington State: The Puget Lowland thrust sheet hypothesis

Seismic reflection data show that the densely populated Puget Lowland of western Washington state is underlain by subhorizontal Paleogene and Neogene sedimentary rocks deformed by west and northwest trending faults and folds. From south to north beneath the Lowland, features seen on the seismic data include: the horizontally-stratified, 3.5 km thick Tacoma sedimentary basin; the Seattle uplift with south dipping (∼20°) strata on its south flank and steeply (50° to 90°) north dipping strata and the west-trending Seattle fault on its north flank; the 7.5 km thick, northward-thinning Seattle sedimentary basin; the antiformal Kingston arch; and the northwest trending, transpressional Southern Whidbey Island fault zone (SWIF). Interpreting the uplifts as fault-bend and fault-propagation folds leads to the hypothesis that the Puget Lowland lies on a north directed thrust sheet. The base of the thrust sheet may lie at 14 to 20 km depth within or at the base of a thick block of basaltic Crescent Formation; its edges may be right-lateral strike-slip faults along the base of the Cascade Range on the east and the Olympic Mountains on the west. Our model suggests that the Seattle fault has a long-term slip rate of about 0.25 mm/year and is large enough to generate a M 7.6 to 7.7 earthquake.

Washington↗