Institute for Crustal Studies
1996/97 Annual Report

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Summary of Research Highlights

Natural Hydrocarbon Seepage in the Santa Barbara Channel

Prolific natural gas seepage occurs offshore from Coal Oil Point near Santa Barbara, above the South Ellwood Offshore Field. The amount emitted to the atmosphere above the Coal Oil Point seeps is about 100 metric tonnes per day. Of this 30 tonnes are non-methane hydrocarbons that are precursors to smog. This amount is equal to the emissions from vehicles each day in Santa Barbara County.

Bruce Luyendyk and colleagues have been studying this seepage field in order to determine the volume discharge of seepage, the time variation in seepage, and its possible cause. 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 and also the seepage. Seepage rates were 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. The project results indicate that as the gas bubbles from the sea floor about 1/3 of the gas dissolves and is carried away by currents. Comparison of these data indicates a 50% reduction in seepage within one mile of oil Platform Holly since 1973.

Earthquake Hazards of Santa Barbara and Ventura

Discoveries by Ed Keller, Larry Gurrola and colleagues in the Santa Barbara Fold Belt are providing new, fundamental information about how young developing fold belts are produced and the seismic hazard they present. Of central importance is the location of active faults in the urban areas and estimates of their recurrence rates-how frequently they move and generate earthquakes. They have identified previously unmapped strands of the Quaternary Mission Ridge fault system as well as abandoned channels of Mission Creek in downtown Santa Barbara resulting from movement on this fault. They are also developing an exciting new method of correlating uplifted marine terraces using stable isotopes. Quantifying crustal uplift rates will yield estimates of crustal strain along the south coast. This research is supported by the Southern California Earthquake Center (SCEC).

Chris Sorlien and Marc Kamerling, supported by the USGS, are studying slip and associated folding on faults in the eastern Santa Barbara Channel and Ventura basin over the last 1.8 million years. One fault being studied is the Oak Ridge fault (thought to be the causative fault for the 1994 Northridge earthquake) that runs along the southern side of the Ventura basin and continues west offshore. By restoring the deformation of 1.0 million and 1.8 million year old sedimentary layers they can estimate the amount a deformation since these times. Their results show that a fold in the Santa Barbara Channel has increased in relief by ~2 mm/yr over the last 110-120 thousand years, and that the Oak Ridge fault has slipped vertically an average of 1.0 mm/yr over the last 1.0 million years.

With financial support from the USGS Art Sylvester and his students are studying deformation onshore at the northern side of the Ventura basin. The Ventura Avenue anticline is a geologically young, major fold that grew 4-5 mm/yr during the last 10,000 years. Comparison of three high precision leveling surveys across the anticline indicates that its crest rose 20 mm between 1978 and 1991 and about another 10 mm between 1991 and 1997, for an annual rate of uplift of about 1.5 mm/yr. All of this has happened without the benefit of nearby earthquakes. This observation of aseismic fold growth is critical to understanding how regional strain may be absorbed without earthquakes. See additional information at:

Strong Ground Motion on the UCSB Campus from Earthquakes

Through the Campus Laboratory Collaborative (CLC) project accelerometers have been installed at 75 m and at 0 m depth in a borehole just north of Webb Hall on the UCSB campus. The objective of the CLC project is to estimate the ground shaking likely to be experienced at UCSB during a large magnitude earthquake from recordings of smaller earthquakes. Preliminary data from two distant (and small magnitude) earthquakes indicate a factor of 3 to 4 amplification of weak ground motion. In the area bounded by Webb Hall, Broida Hall and Engineering I, the subsurface geology and elastic properties have been determined by seismic refraction experiments and cone penetration tests so that the ground motion observations at Webb Hall might be extrapolated to other locations on campus. The CLC project is collaborative with Lawrence -Livermore National Laboratory, UCSD, UC Riverside, UCLA and Cal State University, San Diego. This ICS project is led by Ralph Archuleta, Craig Nicholson and Jamie Steidl with both graduate student and undergraduate students participating. It is funded by UC Office of the President and UCSB.

Computer models of earthquake fault ruptures

Kim Olsen, Raul Madariaga, a visiting professor from Ecole Normale Superieure in Paris, and Ralph Archuleta have developed a computer method to model an earthquake including a complex variation of the friction on the fault plane. They have used their method to model the 1992 M 7.3 Landers earthquake as the propagation of a spontaneous rupture. The simulation used an initial stress distribution on the fault calculated from fault movements derived in a prior study. The simulation shows the rupture propagating on the fault along a complex path with highly variable speed and pulse width. The results have implications for the state of stress on the fault following an earthquake as well as the seismic waves radiated during the earthquake, and the method may provide the framework to estimate earthquake rupture parameters from recorded seismograms in the future. Their research is supported by the Southern California Earthquake Center (SCEC), NSF, USGS and UCSB.

Local Site Effects on Ground Motion From Earthquakes

The highly variable damage pattern that follows an earthquake has many components, but one major effect is the local geology. Fabian Bonilla, Jamie Steidl, Grant Lindley, Alexei Tumarkin and Ralph Archuleta completed a comprehensive study of site effects in the San Fernando Valley using an extensive data set from aftershocks of the 1994 Northridge earthquake. They showed that local site amplification could be a factor of 10 in the San Fernando Valley compared to sites on hard rock in the hills surrounding the basin. By using a variety of different methods on the same data they were able to isolate the strengths and weaknesses of some commonly used techniques for estimating site response. They determined robust methods for quantifying site effects using small earthquakes. This research was supported by SCEC, the U.S. Nuclear Regulatory Commission and the French Commission l'Energie Atomique.

Tectonic Studies in Tibet

Jeff Lee and Phil Gans have a collaborative project with a group of Chinese geologists from the Institute of Geology, State Seismological Bureau, Beijing to study the history of crustal deformation in southern Tibet, China. Their first, and quite successful, two month field season was completed Spring, 1997. They completed 1:50,000 scale mapping within the Kangmar dome in southern Tibet. These studies identified two primary deformation episodes since the beginning of mountain building of the Himalayas: - (1) north-south compression resulting in tight folds, and superimposed on that, (2) north-south extension. Finding evidence of crustal extension in a region undergoing regional contraction as the Indian subcontinent collided with Asia is enigmatic. A major project goal is understanding the dynamic processes that cause this crustal behavior. Numerous rock samples were collected for petrology, microscopic structural and kinematic analyses, U/Pb geochronology, and Ar/Ar and fission track thermochronology. Work over the next year will concentrate on these laboratory studies and compiling field data in preparation for their second field season in 1998. Their project is supported by the NSF.

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