Summary of Research Highlights 1995/96

Simulation of 3-D wave propagation in the Los Angeles basin (Kim Olsen and Ralph Archuleta)

Olsen and Archuleta numerically simulated ground motion from hypothetical M 6.75 earthquakes on the Palos Verdes, Santa Monica, and Elysian Park faults, and the January 17, 1994 Northridge earthquake. A key factor in the models is a new 3-D determination of the deep sedimentary basin underlying the Los Angeles region. Their calculations showed strong amplification of motion due to the 3-D basin structure. Peak ground velocities were up to an order of magnitude higher above the basin (up to 67 cm/sec) compared to surrounding rock sites. While the Northridge earthquake was incredibly damaging to the Los Angeles area, the 3-D simulations show that earthquakes with the same magnitude on the Palos Verdes or Elysian Park faults produce more severe ground shaking in the Los Angeles Basin.

Using MIT's nCUBE2 parallel computer, Olsen and colleagues calculated two minutes of 3-D wave propagation in the Los Angeles basin for a magnitude 7.75 hypothetical earthquake on the San Andreas fault east of Los Angeles. Strong shaking was computed over the Los Angeles basin; the simulation showed peak velocities up to 1,400 cm/sec, almost 10 times as large as that for rock sites outside the basin at similar distances from the earthquake. Their simulation study demonstrates that 3-D basin structure can significantly amplify the ground motion. The results were published in Science (Olsen et al., 1995).

Evaluation of California's Leaking Underground Fuel Tank Program (Stephen Cullen and Lorne Everett)

Cullen and Everett recently co-authored two reports that have impacted traditional approaches to groundwater remediation in California and have had substantial national impact on the cleanup of petroleum hydrocarbons which have leaked to the subsurface.

A University of California team from several campuses and Lawrence Livermore National Lab issued two reports. One report entitled "Recommendations to Improve the Cleanup Process for California's Leaking Underground Fuel Tanks (LUFTs)" recommended that passive bioremediation be considered as a cleanup alternative. A second report entitled "California Leaking Underground Fuel Tank (LUFT) Historical Case Analysis" showed that the dissolved phase hydrocarbon plume length changes slowly and tends to stabilize at a relatively short distance from an underground storage tank release site. Plume length analysis showed that site plume lengths rarely exceed about 250 feet. Only one-half of one percent of the underground fuel tank groundwater contamination sites caused a drinking water problem. The potential volume of groundwater impacted by LUFT plumes with greater than one part per billion benzene was estimated to be .0005% of California's total groundwater basin storage capacity. They concluded that current LUFT decision-making processes do not result in cost effective site closures and that fuel hydrocarbons have limited impacts on human health, the environment, and California's groundwater resources.

The University of California report has resulted in substantial policy changes which will allow scarce remediation dollars to be applied to more immediate and serious problem sites.

Glacial marine stratigraphy in the eastern Ross Sea and western Marie Byrd Land, and shallow structure of the West Antarctic Rift (Bruce Luyendyk, Chris Sorlien, and Louis Bartek-University of Alabama)

Luyendyk and Bartek conducted a study of the Cenozoic glacial and tectonic history of western Marie Byrd Land (MBL) and the eastern Ross Sea, Antarctica, in winter, 1996. Their approach was an offshore marine geology and geophysics study of the unexplored continental margin here using the U.S. icebreaker N.B. Palmer (cruise NBP-9601).

A unique aspect of this project was that Luyendyk brought along 7 undergraduate students; six from UCSB and one from the Univ. of British Columbia. These students were enrolled in a class given at sea. They also have individual responsibilities for independent projects using 9601 data.

Cruise data reveal that the inner shelf in the Marie Byrd Land region has been stripped of sediment by glacial erosion and that much of this sediment has been deposited near the shelf edge in the region. Evidently a very different mode of glacial marine deposition is occurring in the inner shelf basins of the Marie Byrd Land region as compared to other regions of Antarctica. Geophysical data map faulted half grabens in the continental shelf of the eastern Ross Sea and western Marie Byrd Land trending subparallel to mountain ranges onshore. These grabens, or valleys, apparently route the flow of outlet glaciers from the continent onto the shelf. These structures are younger than the age of breakup of the Antarctic continent from Gondwana about 100 to 85 million years ago. Rather, they are associated with rifting and faulting in the Ross Sea prior to glacial development.

Estimation of Ground Motion Exposure from Large Earthquakes at Four UC Campuses in Southern California (Ralph J. Archuleta, Craig Nicholson, Jamison Steidl, Alexei Tumarkin)

This three-year project is part of the Campus Laboratory Collaborative (CLC) initiative to foster more collaborative research between the UC campuses and the national laboratories. The purpose of this project is to estimate the anticipated seismic shaking from future earthquakes at four UC campuses--Santa Barbara, San Diego, Los Angeles and Riverside. The location of faults, the activity of faults, and the style of faulting, will be determined for each campus. At each campus the subsurface geology will be determined from logs of boreholes and seismic profiling. Once the subsurface geology is known, boreholes will be drilled so that downhole seismometers and accelerometers can be placed in competent rock below a building. A matching set of sensors will also be placed at the ground surface. Geotechnical logs of the elastic parameters of the material between the surface and the deepest part of the borehole will provide parameters to theoretically predict seismic amplification. By comparing the borehole recordings with the surface recordings of earthquakes, the amplification of the seismic waves will be determined empirically. Assuming that the theoretical and empirical measurements are in agreement the measured subsurface structure over the entire campus can be used to extrapolate the amplification to other parts of the campus. The ground motion expected from large earthquakes will be estimated by summing the recordings of small earthquakes as well as using theoretical 3D simulation methods.

Natural Hydrocarbon Seepage in the Santa Barbara Channel (Bruce Luyendyk, Scott Hornafius, Libe Washburn, Jordan Clark, and Alan Trial)

Prolific natural gas seepage occurs offshore from Coal Oil Point near Santa Barbara, above the South Ellwood Offshore Field. Luyendyk and colleagues have been studying this seepage field in order to determine the volume of seepage, the time variation in seepage, and its possible cause. Seepage rates can be quantified by measuring sonar reflections from gas bubbles rising through the water column. In 1995 they digitally recorded 3.5 kHz sonar data in the same location as a 1973 survey. Comparison of the data indicates a substantial reduction in seepage within one mile of oil Platform Holly since 1973. The physical mechanism that is responsible for the natural seepage is the flow of hydrocarbon fluids from the oil reservoir to the sea floor through natural fractures in the rock. The flow rate is controlled by the pressure difference between the oil reservoir and the sea floor; as oil has been produced the reservoir pressure has decreased. In order to develop an empirical data set on time variations, they are analyzing sonar data acquired in 1980, 1981, 1982, 1983, and 1984 in the vicinity of Platform Holly. This would enable the timing of seepage reduction relative to the decrease in reservoir pressure to be compared against model predictions.