Institute for Crustal Studies
1997/98 Annual Report

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Summary of Research Highlights

In May 1998, Kajima Engineering and Construction, Inc. , a California subsidiary of Kajima Corporation of Japan formally donated its Hollister Earthquake Observatory (HEO) to Archuleta and his ICS research group and to the University of California. This donation exceeds $1 M. The observatory consists of a vertical array of borehole 3-component accelerometers–depths 0, 10, 20, 50, 110 and 192 m (the deepest is in bedrock)–at a soil site in Salinas Valley and three accelerometers about three kilometers away–two being on surficial rock and another at 53 m depth. All of the accelerometers are recorded digitally; the data from the remote site is telemetered by a spread spectrum radio link to the main recording site co-located with the vertical array. The data can be transmitted to ICS via a telephone modem. Only three months after ICS took control of HEO the array recorded the most significant earthquake in its seven year history–a M W 5.1 earthquake on the San Andreas only 13 km from HEO. This array complements the Garner Valley Downhole Seismic Array (GVDSA) operated by the ICS borehole seismology group since 1988. The combination of HEO, GVDSA, the SCEC borehole initiative and the Campus Laboratory Collaborative (CLC) project (which installed two borehole accelerometers at UCSB near Webb Hall) puts ICS at the forefront of the quantification of the effects of near-surface geology.

With support from UNESCO, Dr. Zektser, a Russian visitor mentioned earlier, and Everett, Director of the ICS Vadose Zone Monitoring Laboratory completed the first world groundwater map. The map shows groundwater amounts, discharge areas and vulnerability. This map, which has taken five years to develop, is supported by one hydrogeologist from each country in the world. The map’s creation was spearheaded by Zektser; the final elements of the map were completed in the eight months that Zektser spent at the ICS Vadose Lab. The map is being published by UNESCO and is expected to be completed this coming year.

Drs. Gans and Bohrson recently published an article in Science documenting the interplay between volcanism and extension in the crust. They examined the details of the evolution of the Northern Eldorado Mountains in southern Nevada. What they found was voluminous volcanism preceding times of major extension (>= 100%) but suppression of volcanism during and following the episodes of major extension. Gans and Bohrson conjecture that effects of normal faulting could account for both suppression of volcanism and extension.

Simulations of earthquakes as fully dynamic ruptures are uncommon. Yet ICS researchers published two papers in Science on completely different dynamically simulated earthquakes. Olsen, Madariaga, and Archuleta simulated the dynamic rupture of the 1992 Landers earthquake. They found that the rupture front would break the regions of high stress with regions of low stress nearby filling in afterwards. The rupture velocity, the static slip and the slip rate are consistent with models based on inverting the strong motion data for the kinematic parameters of faulting. Oglesby, Archuleta and Nielsen clearly demonstrated that the ground motion is asymmetric in two important aspects for dip-slip faults, thrust and normal faults. First, the hanging wall of a fault moves much more than the footwall–an effect primarily due to the difference in mass. Second, the ground movement of the hanging wall of a thrust fault is much greater than that for a normal fault–all factors being equal except for the sign of the shear stress on the fault. This second result is particularly surprising but consistent with observations that thrust faults generate larger amplitude ground motions than normal faults. The results of Oglesby and others were the first dynamic simulations of earthquakes on faults with arbitrary dip.

What is the seismic hazard in Santa Barbara and Ventura counties? A simple question with a wide range of answers. Because Santa Barbara and Ventura are within a transpressional (combination of strike-slip and thrust faults) region, it is not straightforward to assess the seismic hazard without understanding how the large scale plate tectonic strain is partitioned. Drs. Nicholson, Kamerling, Pinter, Sorlien and Sylvester are attacking this problem using a variety of techniques: reanalysis of detailed 3D structure in the Ventura Basin, measurements of shoreline angles on the Channel Islands, unfolding of structure maps, and short-baseline leveling. Sorlien and Kamerling find that new, unmapped faults must exist in Ventura Basin for continuity of the Basin structure. Sylvester cogitates that aseismic folding, observed in the Ventura Avenue anticline, is clear evidence that not all strain is available for release in earthquakes, thereby making impending earthquakes smaller and/or less frequent.

In 1996/97 academic year, ICS scientists documented that natural hydrocarbon seeps off the coast of Santa Barbara (Cover Figure) are capable of producing ozone equivalent to the output of all the automobiles and trucks in the County. This stunning discovery has prompted a host of studies described below. Drs. Luyendyk, Alain Trial, Libe Washburn and Jordan Clark have combined efforts to measure the flux of hydrocarbons being released and their distribution in the Channel and to model these plumes as they dissipate in the ocean. Accurate measurements of the seeps and their contribution to air pollution are fundamentally important to realistic air quality goals in Santa Barbara county.

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