Geology topics

Lewis E. Hunter

Publications and source records attributed to Lewis E. Hunter.

3 recordsLinked to original sources

USGS2012

Airborne LiDAR analysis and geochronology of faulted glacial moraines in the Tahoe-Sierra frontal fault zone reveal substantial seismic hazards in the Lake Tahoe region, California-Nevada USA

We integrated high-resolution bare-earth airborne light detection and ranging (LiDAR) imagery with field observations and modern geochronology to characterize the Tahoe-Sierra frontal fault zone, which forms the neotectonic boundary between the Sierra Nevada and the Basin and Range Province west of Lake Tahoe. The LiDAR imagery clearly delineates active normal faults that have displaced late Pleistocene glacial moraines and Holocene alluvium along 30 km of linear, right-stepping range front of the Tahoe-Sierra frontal fault zone. Herein, we illustrate and describe the tectonic geomorphology of faulted lateral moraines. We have developed new, three-dimensional modeling techniques that utilize the high-resolution LiDAR data to determine tectonic displacements of moraine crests and alluvium. The statistically robust displacement models combined with new ages of the displaced Tioga (20.8 ± 1.4 ka) and Tahoe (69.2 ± 4.8 ka; 73.2 ± 8.7 ka) moraines are used to estimate the minimum vertical separation rate at 17 sites along the Tahoe-Sierra frontal fault zone. Near the northern end of the study area, the minimum vertical separation rate is 1.5 ± 0.4 mm/yr, which represents a two- to threefold increase in estimates of seismic moment for the Lake Tahoe basin. From this study, we conclude that potential earthquake moment magnitudes (M w ) range from 6.3 ± 0.25 to 6.9 ± 0.25. A close spatial association of landslides and active faults suggests that landslides have been seismically triggered. Our study underscores that the Tahoe-Sierra frontal fault zone poses substantial seismic and landslide hazards.

California;Nevada↗

USGS2010

Identification of the Polaris Fault using lidar and shallow geophysical methods

As part of the U.S. Army Corps of Engineers' (USACE) Dam Safety Assurance Program, Martis Creek Dam near Truckee, CA, is under evaluation for earthquake and seepage hazards. The investigations to date have included LiDAR (Light Detection and Ranging) and a wide range of geophysical surveys. The LiDAR data led to the discovery of an important and previously unknown fault tracing very near and possibly under Martis Creek Dam. The geophysical surveys of the dam foundation area confirm evidence of the fault in the area.

Conference Paper↗

USGS2007

Seismic Shear Wave Reflection Imaging at the Former Fort Ord, Monterey, California

At the former Fort Ord in Monterey County, California, contamination threatens an aquifer that provides drinking water for local communities. Assessment and remediation require accurate hydrological modeling, which in turn require a thorough understanding of aquifer stratigraphy. In order to help guide remediation efforts at the site, the U.S. Geological Survey, in cooperation with the U.S. Army Corps of Engineers, has undertaken seismic reflection surveys, testing compressional (P) and horizontally polarized shear (SH) waves. Sledgehammer-source SH data show reflections from interfaces up to approximately 60 m deep, which correspond with the major boundaries between aquifers and aquitards. In contrast, P-wave data show only the reflection from the water table at approximately 30 m depth. We collected SH data along two transects and processed these data to produce reflection images. The interpreted SH-wave images agree with available well information, constrain the geology for ground-water models, and provide guidance for future geophysical studies. These favorable results demonstrate the effectiveness of SH reflection methods for imaging unconsolidated aquifer layers at the former Fort Ord and at other sites with similar geologic conditions.

Open-File Report↗