Robert E. Abbott

Geophysical confirmation of low-angle normal slip on the historically active Dixie Valley fault, Nevada

The December 16, 1954, Dixie Valley earthquake (Ms = 6.8) followed the nearby Fairview Peak earthquake (Ms = 7.2) by 4 min, 20 s. Waveforms from the Fairview Peak event contaminate those from the Dixie Valley event, making accurate fault plane solutions impossible. A recent geologic study of surface rupture characteristics in southern Dixie Valley suggests that the Dixie Valley fault is low angle (<30°) along a significant portion of the 1954 rupture. To extend these observations into the subsurface, we conducted a seismic reflection and gravity experiment. Our results show that a portion of the Dixie Valley ruptures occurred along a fault dipping 25° to 30°. As such, the Dixie Valley event may represent the first large, low-angle normal earthquake on land recorded historically. Our high-resolution seismic reflection profile images the rupture plane from 5 to 50 m depth. Medium-resolution reflections, as well as refraction velocities, show a smoothly dipping fault plane from 50 to 500 m depth. Stratigraphic truncations and rollovers in the hanging wall show a slightly listric fault to 2 km depth. Gravity profiles conservatively constrain maximum basin depth and define overall geometry. Extension along the low-angle section may have occurred in two phases during the Cenozoic. Current fault motion postdates a 13 to 15 Ma basalt, imaged in the hanging wall, and inherits from a fault formed during an earlier extensional pulse, concentrated at 24.2 to 24.4 Ma. The earlier extension suggests extraordinary slip rates as high as 18 mm/yr, resulting in the formation of the low-angle fault break. Sections of the Dixie Valley fault where there is no evidence for current low-angle slip correlate well with areas where no pre-15 Ma slip has been documented.

Depth to bedrock using gravimetry in the Reno and Carson City, Nevada, area basins

Sedimentary basins can trap earthquake surface waves and amplify the magnitude and lengthen the duration of seismic shaking at the surface. Poor existing gravity and well‐data coverage of the basins below the rapidly growing Reno and Carson City urban areas of western Nevada prompted us to collect 200 new gravity measurements. By classifying all new and existing gravity locations as on seismic bedrock or in a basin, we separate the basins’ gravity signature from variable background bedrock gravity fields. We find an unexpected 1.2-km maximum depth trough below the western side of Reno; basin enhancement of the seismic shaking hazard would be greatest in this area. Depths throughout most of the rest of the Truckee Meadows basin below Reno are less than 0.5 km. The Eagle Valley basin below Carson City has a 0.53-km maximum depth. Basin depth estimates in Reno are consistent with depths to bedrock in the few available records of geothermal wells and in one wildcat oil well. Depths in Carson City are consiste...

Surface-wave and refraction tomography at the FACT Site, Sandia National Laboratories, Albuquerque, New Mexico.

We present a technique that allows for the simultaneous acquisition and interpretation of both shear-wave and compressive-wave 3-D velocities. The technique requires no special seismic sources or array geometries, and is suited to studies with small source-receiver offsets. The method also effectively deals with unwanted seismic arrivals by using the statistical properties of the data itself to discriminate against spurious picks. We demonstrate the technique with a field experiment at the Facility for Analysis, Calibration, and Testing at Sandia National Laboratories, Albuquerque, New Mexico. The resulting 3-D shear-velocity and compressive-velocity distributions are consistent with surface geologic mapping. The averaged velocities and V{sub p}/V{sub s} ratio in the upper 30 meters are also consistent with examples found in the scientific literature.

Sensor integration study for a shallow tunnel detection system.

During the past several years, there has been a growing recognition of the threats posed by the use of shallow tunnels against both international border security and the integrity of critical facilities. This has led to the development and testing of a variety of geophysical and surveillance techniques for the detection of these clandestine tunnels. The challenges of detection of these tunnels arising from the complexity of the near surface environment, the subtlety of the tunnel signatures themselves, and the frequent siting of these tunnels in urban environments with a high level of cultural noise, have time and again shown that any single technique is not robust enough to solve the tunnel detection problem in all cases. The question then arises as to how to best combine the multiple techniques currently available to create an integrated system that results in the best chance of detecting these tunnels in a variety of clutter environments and geologies. This study utilizes Taguchi analysis with simulated sensor detection performance to address this question. The analysis results show that ambient noise has the most effect on detection performance over the effects of tunnel characteristics and geological factors.

Permafrost Active Layer Seismic Interferometry Experiment (PALSIE).

We present findings from a novel field experiment conducted at Poker Flat Research Range in Fairbanks, Alaska that was designed to monitor changes in active layer thickness in real time. Results are derived primarily from seismic data streaming from seven Nanometric Trillium Posthole seismometers directly buried in the upper section of the permafrost. The data were evaluated using two analysis methods: Horizontal to Vertical Spectral Ratio (HVSR) and ambient noise seismic interferometry. Results from the HVSR conclusively illustrated the methods effectiveness at determining the active layers thickness with a single station. Investigations with the multi-station method (ambient noise seismic interferometry) are continuing at the University of Florida and have not yet conclusively determined active layer thickness changes. Further work continues with the Bureau of Land Management (BLM) to determine if the ground based measurements can constrain satellite imagery, which provide measurements on a much larger spatial scale.

Investigating the point seismic array concept with seismic rotation measurements.

Spatially-distributed arrays of seismometers are often utilized to infer the speed and direction of incident seismic waves. Conventionally, individual seismometers of the array measure one or more orthogonal components of rectilinear particle motion (displacement, velocity, or acceleration). The present work demonstrates that measure of both the particle velocity vector and the particle rotation vector at a single point receiver yields sufficient information to discern the type (compressional or shear), speed, and direction of an incident plane seismic wave. Hence, the approach offers the intriguing possibility of dispensing with spatially-extended received arrays, with their many problematic deployment, maintenance, relocation, and post-acquisition data processing issues. This study outlines straightforward mathematical theory underlying the point seismic array concept, and implements a simple cross-correlation scanning algorithm for determining the azimuth of incident seismic waves from measured acceleration and rotation rate data. The algorithm is successfully applied to synthetic seismic data generated by an advanced finite-difference seismic wave propagation modeling algorithm. Application of the same azimuth scanning approach to data acquired at a site near Yucca Mountain, Nevada yields ambiguous, albeit encouraging, results. Practical issues associated with rotational seismometry are recognized as important, but are not addressed in this investigation.