Geology topics

William Stephenson

Publications and source records attributed to William Stephenson.

3 recordsLinked to original sources

USGS2012

Grizzly Valley fault system, Sierra Valley, CA

The Grizzly Valley fault system (GVFS) strikes northwestward across Sierra Valley, California and is part of a network of active, dextral strike-slip faults in the northern Walker Lane (Figure 1). To investigate Quaternary motion across the GVFS, we analyzed high-resolution (0.25 m) airborne LiDAR data (Figure 2) in combination with six, high-resolution, P-wave, seismic-reflection profiles [Gold and others, 2012]. The 0.5- to 2.0-km-long seismic-reflection profiles were sited orthogonal to suspected tectonic lineaments identified from previous mapping and our analysis of airborne LiDAR data. To image the upper 400–700 m of subsurface stratigraphy of Sierra Valley (Figure 3), we used a 230-kg accelerated weight drop source. Geophone spacing ranged from 2 to 5 m and shots were co-located with the geophones. The profiles reveal a highly reflective, deformed basal marker that we interpret to be the top of Tertiary volcanic rocks, overlain by a 120- to 300-m-thick suite of subhorizontal reflectors we interpret as Plio-Pleistocene lacustrine deposits. Three profiles image the principle active trace of the GVFS, which is a steeply dipping fault zone that offsets the volcanic rocks and the basin fill (Figures 4 & 5).

California↗

USGS2009

Sedimentary basin effects in Seattle, Washington: Ground-motion observations and 3D simulations

Seismograms of local earthquakes recorded in Seattle exhibit surface waves in the Seattle basin and basin-edge focusing of S waves. Spectral ratios of Swaves and later arrivals at 1 Hz for stiff-soil sites in the Seattle basin show a dependence on the direction to the earthquake, with earthquakes to the south and southwest producing higher average amplification. Earthquakes to the southwest typically produce larger basin surface waves relative to S waves than earthquakes to the north and northwest, probably because of the velocity contrast across the Seattle fault along the southern margin of the Seattle basin. S to P conversions are observed for some events and are likely converted at the bottom of the Seattle basin. We model five earthquakes, including the M 6.8 Nisqually earthquake, using 3D finite-difference simulations accurate up to 1 Hz. The simulations reproduce the observed dependence of amplification on the direction to the earthquake. The simulations generally match the timing and character of basin surface waves observed for many events. The 3D simulation for the Nisqually earth-quake produces focusing of S waves along the southern margin of the Seattle basin near the area in west Seattle that experienced increased chimney damage from the earthquake, similar to the results of the higher-frequency 2D simulation reported by Stephenson et al. (2006). Waveforms from the 3D simulations show reasonable agreement with the data at low frequencies (0.2-0.4 Hz) for the Nisqually earthquake and an M 4.8 deep earthquake west of Seattle.

Bulletin of the Seismological Society of America↗

USGS2005

Wagon loads of sand blows in White County, Illinois

Several anecdotal accounts provide compelling evidence that liquefaction occurred at several sites in Illinois during the 1811-1812 New Madrid sequence, as much as 250 km north of the New Madrid seismic zone (NMSZ). At one Wabash Valley location, sand blows are still evident near Big Prairie, Illinois, a location described in a particularly detailed and precise historic account. This account includes descriptions of substantial liquefaction (sand blows) as well as a two-mile-long east-west-trending “crack” along which two feet of south-side-down displacement occurred. An offset can no longer be seen at this location, which has been extensively farmed and plowed for decades. Field reconnaissance verifies many of the details provided in the account, however. We conducted a seismic-reflection experiment at this location and observed a modest offset in the Paleozoic strata at this location. The offset is opposite to that described in the historic account, consistent with the hypothesis that large midcontinent earthquakes occur on faults reactivated in a Holocene stress regime different from the one in which they were formed. Only two explanations can account for these observations: Either large NMSZ events triggered substantial liquefaction at distances greater than hitherto realized, or at least one large “New Madrid” event occurred significantly north of the NMSZ. We explore these possibilities and conclude that, while neither one can be ruled out, several disparate lines of evidence suggest that the 23 January 1812 “New Madrid mainshock” occurred in White County, Illinois, near the location of the m b 5.5 1968 southern Illinois earthquake and recent microearthquake activity.

Illinois↗