The Garner Valley downhole seismographic array (GVDSA) project, installed under the U.S. Nuclear Regulatory Commission contract NRC-04-87-108 in cooperation with the French Commissariat à l'Energie Atomique (CEA), has two main scientific objectives, first; understanding the effects of the near-surface soil conditions on seismic ground motion to improve the ground motion prediction capabilities for design, seismic hazard assessment, and hazard mitigation: and second; understanding the effect of earthquake ground motion on the hydraulic conductivity of ground water systems for the deep storage of nuclear waste.

The near-surface geological site conditions have been shown to be the dominant factor in controlling the amplitude and variation of strong ground motion, and the damage patterns that result from large earthquakes. A unique set of data collected from the Garner Valley project makes it possible to advance two major areas of engineering seismology. The first problem is how weak motions scale to strong motions. The second one is how the recordings at different soil types scale to each other, especially with respect to a competent rock ("reference") site. The understanding of competing effects of amplification and attenuation (including non-linearity) is of a vital importance for seismic design studies. The site is located near the Anza segment of the seismically active San Jacinto fault in Southern California, which has a relatively high probability for a large earthquake of magnitude 6.5 or greater in the near future.

At Garner Valley we measure the ground motion during earthquakes in the bedrock 500 and 220 meters below the surface, at 50 meters below the surface in a zone of weathered granite, and at 22, 15, and 6 meters below the surface in a layer of soft alluvium. The ground motion is also measured at the surface above these borehole instruments by 5 stations in a linear array, one of which is directly above the borehole instruments.

In the deepest borehole (500 meter), downhole pressure transducers are located within sealed off fracture zones. The effects of earthquake ground motion and the rock-mass hydraulic response to ground motion are important factors in the short- and long-term performance of a high-level nuclear waste repository. This part of the project was designed and undertaken to provide fundamental data regarding the influence of earthquake ground motion on dynamic and static changes to the pore pressure in the rock mass. In addition to the downhole pressure transducers, static and dynamic changes in the pore pressure at different levels within the bedrock borehole are also measured via tubes (sampling lines) which are connected to pressure transducers at the surface and extend into the borehole to various depths.

A remote rock site station consisting of 30 m borehole and surface accelerometers located 3km from the GVDA main station was installed in the past year. Data are recorded locally at the site and transferred via radio modem to the main station and eventually back to UCSB. Near-surface P- and S-wave velocities were determined from suspension logging to 30m before installation of the downhole sensor.

As the number of observations being made at the Garner Valley site continues to grow each year, the need for a more user-friendly interface to the data led to the development of a new database. The Garner Valley data was recently made available to our French collaborators via a relational database that has been set up to run on the World Wide Web. This database is also used internally by ICS researchers to allow for quicker access to particular records within the ever-increasing array of observations at Garner valley.

A new focus into the field of engineering seismology, specifically, modeling of the nonlinear behavior of soils, was undertaken. Nonlinear soil behavior can change the fundamental resonance frequency of the sediments, and modify the amplitudes and increases the duration of ground motions as the shaking level increases. These facts are very important in seismic hazard and building codes studies. With this in mind and the recent observations of nonlinear soil behavior from the 1994 Northridge and 1995 Kobe earthquakes, we are developing new computational techniques to model nonlinear soil behavior.

Other recent GVDA related research resulted in developing new approaches to empirical site response estimation. The idea is to study three-dimensional seismic radiation field instead of treating each component of ground shaking individually. A standard approach to estimating relative site response is to calculate the 1D transfer function between a rock and a soil sites as an average spectral ratio of individual components. We introduced a 3D-transfer function between all three components of motions at two sites as a 3x3-matrix transformation. This matrix accounts for complex propagation effects such as conversions of different types of seismic waves, differences in incidence angles, etc. It can be determined from observations of at least three earthquakes at both sites.