1997 SCEC Progress Report

Project Title: SCEC Borehole Instrumentation Initiative

PI's: Jamison H. Steidl and Ralph J. Archuleta

Institute for Crustal Studies
University of California, Santa Barbara

Project Description:

One of the major goals of the Center is to compute theoretical seismograms for scenario earthquakes in the Los Angeles and Southern California region. The existing strong-motion data are used to calibrate and improve our computational techniques. Ground motions recorded at strong motion stations throughout Southern California are a combination of the complex earthquake source process, the propagation path from the source zone to the station, and the local near-surface site conditions at the station. The separation of source, path, and site effects is limited by the current availability of data, the detailed knowledge of the crustal structure, and our understanding of the earthquake source process. The widespread and varied ground motions and damage patterns over short distances produces a large degree of uncertainty in our ability to predict ground motion from future earthquakes. Much of the variability is thought to be caused by the local near-surface site conditions.

In order to reduce the uncertainty in our ability to compute theoretical seismograms predicting the ground motion from future earthquakes, we will remove the near-surface site effect at a few select stations by installing borehole instrumentation below the surface. The SCEC borehole instrumentation project will produce data that has not been distorted by the effect of the surface materials. This will allow for direct estimation of site effects, provide a test for the calibration and improvement of physical models of soil response, and give us a much clearer picture of the incident ground motion which can be used to study the earthquake source process and the regional crustal structure in more detail. In addition the borehole data can be used as empirical Green's functions (the input motion) for predicting ground motion at surface sites in the region surrounding the borehole station.

Summary of Borehole Instrumentation Initiative Workshop (March 25, 1997):

The SCEC borehole instrumentation initiative workshop took place on Tuesday March 25, 1997 in Los Angeles at USC. The workshop was very successful in demonstrating the wide range of uses for borehole data and there was a general agreement on the scientific justification for the project. The main scientific issues can be broken down into three major topics:

Details of the Earthquake Source

-Earthquake nucleation and stopping.

-Rupture dynamics.

Ground Motion Prediction

-Borehole data as empirical Green's functions for the surrounding region.

-Borehole bedrock coherence.

-Difference between borehole ground motion and surface rock sites.

-Basin borehole coherence.

Nonlinear Effects on Ground Motion

-What is the degree of nonlinearity

Presentations were made on all of these topics, and while each presenter had their own order for them, with their own favorite listed as number one, most were agreed that each of these topics were important issues that could be addressed through borehole installations.

In addition to the discussion of the scientific just)fication for borehole instrumentation, a cadre of speakers from different organizations presented an update on the existing and planned borehole projects in the Los Angeles and Southern California region. It was clear the there is a great deal of work already begun with regard to collecting borehole geophysical and seismological data. The SCEC borehole project is working in close communication with the other organizations so as not to duplicate efforts, and to share resources where possible.

Science Addressed

Direct Estimation of Site Effects, Physical Modeling, and Understanding of Non-linear Effects

The new 1997 Uniform Building Code (UBC) to be used in the design of structures by the engineering community has placed a great deal of emphasis on the near surface soil conditions in the upper 30 meters. In fact, the site class)fication that will be used in this version of the UBC is determined by the shear-wave velocity or standard penetration tests in the upper 30 meters. Borehole geophysical data and seismic instrumentation for direct estimation of site effects at selected "typical" Southern California geologic site classes will help in calibrating and improving our physical models of soil response to different levels of ground motion. The degree of non-linear behavior in Southern California soils at large input ground motions is a critical issue for determining the maximum plausible ground motions from large earthquakes.

Borehole Data as Green's Functions for Predicting Ground Motion at the Surface

Results from a borehole study along the San Jacinto fault zone suggest that the input wavefield below the near-surface sediments is more coherent than at the surface, at soil or rock sites, even over distances as great as 5-20 km (Steidl et al., 1996). The implications of this result are that a small array of borehole recordings can define the input motion for physical modeling of site effects within a large region surrounding the borehole stations. Is borehole ground motion consistent over these same scales across the Los Angeles basin, where the instrumentation will in some cases, due to the great depth of the basin, be installed in stiff soil instead of granitic rock? The coherency of borehole ground motion is most likely a function of the shallow regional crustal structure. This borehole initiative will address this issue by placing borehole stations in the rock at the edges of the Los Angeles basin, and at different stations spacing away from the rock locations.

Details of the earthquake source

Results from borehole data in the Parkfield area show evidence for repeating characteristic micro-earthquakes (Nadeau and McEvilly, 1997; Nadeau et al., 1995) that provide a detailed look at what is happening on the fault zone in both spatial and temporal sense, on scales of meters and months respectively. With the increased signal to noise that the borehole data provide and an ultra low-noise sensor, small earthquake clusters which have events with identical waveforms are recorded. The individual earthquakes within each cluster occur at periodic and quasi-periodic time intervals on the order of weeks to months. This unique data we will attempt to collect will let us examine the earthquake source zones in the Los Angeles region in detail, with on scale recordings in the borehole from micro-g to 0.5 g accelerations.

Project Progress and Plan

Collaboration is the key word in describing the SCEC borehole instrumentation initiative. The borehole instrumentation plan will start with rock sites around the Los Angeles basin, and then move to soil sites within the basin. Currently, one site has been permitted (PVR-Figure 1), and two are still in the permitting phase (LA00 and GPK-Figure 1). The permitting phase of the project has taken longer than expected and puts the project about 6 months behind the original plan. Collaboration with the ROSRINE project on the drilling of all three of these boreholes may make it possible to case boreholes at other strong motion sites that are scheduled for drilling in early 1998. Some or all of these additional cased boreholes (WND, DHE, SAT, and OBG-Figure 1) will be instrumented using the savings from collaboration with the TRINET project on surface instrumentation. A dense 3-D borehole array located at one of the above rock stations is planned for the next year. The idea here is to place two instruments so that they form a triangle relative to the year 1 site location, and at the same depth as the year 1 instrument (100 meters) with the horizontal distance between stations of approximately 100-300 meters. A third instrument may be placed at an intermediate depth of 50 meters (or at a deeper depth of 200 meters) co-located horizontally with one of the corner stations of the
triangle (depending on budgetary constraints). Other possible locations for borehole instrumentation in future years will be stiff soil and rock stations further to the east and north (PTH, STA and PCD Figure 1) and sites within the basin (HOL, BHA and MDR-Figure l). However, the plan is somewhat flexible so that we can take advantage of collaboration and cost sharing with other ongoing projects in the Los Angeles basin, such as the ROSRINE and Los Angeles Water Replenishment District projects.

Instrumentation and Data

The borehole packages we will be using are a three-component design which uses a Wilcoxon model 7314a low-noise accelerometer (this package will also have an inexpensive Oyo/Geospace HS 1 4.5 Hz geophone for redundancy) or a more standard Kinemetrics FBA23-DH depending on the site location relative to the earthquake source zones. The downhole accelerometers will be recorded by a high dynamic range digital recorder at the surface. These digital accelerographs, coupled with the lownoise Wilcoxon borehole sensor, have enough dynamic range to record micro-g level ground acceleration to 0.5g acceleration. The surface sensor will be a Kinemetrics FBA-23 accelerometer, capable of recording ground motions of 2.0 g acceleration. The lower magnitude limit at the sensors will be dependent on the local noise conditions. The data will be integrated into the Caltech-USGS Southern California Seismic Network (SCSN) with telemetry so that the data is available through the SCEC data center.

An example of the Wilcoxon data from a borehole/surface station installed in Summer 1997 at the UC Santa Barbara campus under funding from the University of California is shown in Figure 2. The magnitude 3.3 Northridge aftershock is located ~100 km from the Santa Barbara station. The level of ground motion is 25 micro-g at ~75 meters depth in the borehole (Figure 2A) and -70 micro-g at the surface. In addition to the Wilcoxon data, a downhole Kinemetrics FBA23 has been installed at 75 meters depth co-located with the Wilcoxon borehole for comparison and in case of ground motion at depth greater than 0.5 g (the limit of the Wilcoxon). Shake table tests show that the two instruments are perfectly coherent for shaking up to 0.5 g. The data shown in Figure 2 was retrieved via the internet from the SCEC data center at Caltech. The SCEC borehole instrumentation will be recorded by the SCSN in the same fashion and be available for all to download via the SCEC data center.

References

Nadeau, R. M., and T. V. McEvilly (1997). Seismological studies at Parkfield V: Characteristic microearthquake sequences as fault-zone drilling targets, Bull. Seism. Soc. Am., Submitted.

Nadeau, R. M., W. Foxall, and T. V. McEvilly (1995). Clustering and periodic recurrence of microearthquakes on the San Andreas fault at Parkfield, California, Science, 267, pp. 503507

Steidl, J. H., A. G. Tumarkin, and R. J. Archuleta (1996). What is a reference site? Bull. Seism. Soc. Am. 86, No. 6, pp. 1733-1748.