Near-field Geodetic Study of the San Andreas
Fault
Jeffrey T. Freymueller
Geophysical Institute
University of Alaska, Fairbanks
Fairbanks, AK 99775-7320
Telephone: 907-474-7286
FAX: 907-474-5618
jeff@giseis.alaska.edu
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Investigations Undertaken
In the course of this project we are making GPS measurements at sites in five small-aperture geodetic networks along the San Andreas fault in northern California (Figure 1). These networks were established by the US Geological Survey (USGS) in the 1970s and early 1980s, and most have not been surveyed since the mid-1980s. The networks typically span about 5 kilometers on either side of the surface trace of the fault. Line length rates of change from these networks can be used to measure the near-fault strain rate at the fault, and to distinguish between shallow or deep locking depths. The combined line length and GPS data will be sufficient to determine the rates of change of line lengths with a precision of better than 1 mm/yr. With data of such precision we can address two important questions: (1) Is there systematic variation along strike in the near-fault strain rate? Variations along strike could be explained by spatial changes in the depth to which the fault is locked in the interseismic period, or by variations in the elastic properties of rocks near the fault zone. (2) How well can a single elastic model fit both the near-field and far-filed strain observed geodetically? We will develop a three dimensional elastic model (or a series of two dimensional models) to explain both the near field data and the existing far-field data, and expect that this data will improve our knowledge of current slip rates on the major strike-slip faults in the Bay Area.
Work on this project began in May of 1996. During calendar year 1996,
most of the site reconnaissance was completed, but only a part of the fieldwork.
All GPS data have been analyzed, although further refinement is expected
with the application of ambiguity resolution. A comparison of the GPS results
to the historical EDM data is underway. Fieldwork and GPS data analysis
will be completed in the spring of 1997.
Figure 1. Locations of the small networks being studied in this project.
Small black triangles show the locations of regional geodetic sites. Blowups
of the two southern networks are shown in Figure 2, and the Bodega and Tomales
networks in Figure 3.
Field Reconnaissance
Because many of the sites needed for this study have not been visited for up to a decade, a significant effort in field reconnaissance was required. The main part of the field reconnaissance was carried out in May 1996, with some additional sites found in October 1996. Existing descriptions for many sites were poorly written or out of date, and markers were found buried under as much as a foot of soil. The final report for this project will include a complete set of new descriptions for all of the sites used in this study, including up-to-date contacts and permitting info for all sites.
The great majority of sites in all networks were found in good condition,
although many had been buried. Because the descriptions as written generally
omitted mention of nearby points of reference, considerable time was spent
searching for markers with a metal detector. Several markers have not yet
been found. In particular, about half of the sites in the Bodega Bay network
and one critical site in the Tomales Bay network have not been found. However,
the missing sites are almost certainly buried rather than destroyed. Temporary
markers were surveyed in the probable vicinity of each of the missing marks,
and offsets between the temporary marks and the missing survey markers can
be inferred from the GPS coordinate solutions with a precision that should
make it possible to find them by a detailed search with a metal detector.
Figure 2.The Black Mountain and Lake San Andreas networks. Open triangles
show EDM sites known or believed to exist but not yet surveyed with GPS.
Solid triangles show sites surveyed with GPS during the course of this project.
Fieldwork
Fieldwork and GPS data analysis has been completed for the Black Mountain/Radio Facility network, the southernmost network in this project (Figure 2). One site in the network was destroyed in the 1980s, and a second site could not be visited because the owners could not be contacted in time to obtain permission. No description could be found for a third site, although USGS personnel may remember enough about the site to lead us to it during our next set of field measurements. In addition to the nearly complete occupation of this network, the largest network in our project, we made simultaneous occupations at several nearby sites that are part of a larger-scale GPS networks. Using these measurements, plus data from two of the Black Mountain network sites that are part of the larger-scale Black Mountain profile, it should be possible to determine vector velocities for all sites in the Black Mountain network.
We also made measurements to tie together the separate networks we surveyed, and help in connecting these sites kinematically with the larger-scale networks in the area. While this will not have an immediate benefit for this project, it should be possible to reoccupy selected sites in these networks in a few years and determine vector velocities for all of the sites.
Partial occupations have been made of the two networks north of the San
Francisco Bay. The status of these networks is shown in Figure 3.
Figure 3.The Tomales and Bodega networks. Open triangles show EDM
sites known or believed to exist but not yet surveyed with GPS. Solid triangles
show sites surveyed with GPS during the course of this project.
Results
The GPS estimated line lengths for most lines have a precision of about 2 mm, and an improvement is expected once all data are analyzed with ambiguities resolved. The uncertainty in the GPS line length estimates is comparable to that of the EDM. GPS line lengths are increased by 0.44 ppm based on an empirical scale difference determined by the USGS. Final analysis will include a complete check of all of the EDM data, since different instruments were used at different times.
Most lines in the Black Mountain network were surveyed regularly before
1982, then again in 1989, shortly before the Loma Prieta earthquake. The
lines were surveyed one or two times immediately after the earthquake. Line
length changes in the five to seven years since the last EDM survey range
from near zero to about 30 millimeters. For three lines, the comparison
of the GPS and EDM results for the Black Mountain network is clouded by
large (>1 meter) discrepancies, which can be explained if a reference
mark was used in the EDM surveys (but not clearly documented). Excluding
the three lines, with such large discrepancies, the results for the remaining
lines show line length rates of change ranging of up to 5 mm/yr. More typical
extension or contraction rates for faults crossing the San Andreas fault
are 1-3 mm/yr, corresponding to (directional) strain rates of up 1 microstrain/yr.
Preliminary analysis suggests that the near-fault strain rate at Black Mountain
is comparable to that at Point Reyes. More complete analysis is required
to determine whether there is any evidence for changes in rate over time,
for example whether the post-Loma Prieta rate is different than the pre-Loma
Prieta rate.
Non-Technical Summary
Repeated measurements of small-aperture geodetic networks across the San Andreas fault near Black Mountain (San Francisco peninsula), Point Reyes peninsula and Point Arena (100 miles north of the San Francisco Bay) show different near-fault strain. At Point Reyes, and apparently at Black Mountain based on preliminary results, a 3-4 km wide zone of concentrated strain is observed around the San Andreas fault; at Point Arena this zone is absent. This difference could be explained by either along-strike changes in fault behavior or in material properties. Measurements are continuing at other locations along the San Andreas fault determine the extent along strike of this zone of high near-fault strain. Our working hypothesis is that the presence or absence of a zone of high near-fault strain depends on the material properties of rocks on either side of the fault, and that the areas with Salinian granite to the west of the fault have high near-fault strain rates. If this hypothesis is correct, then any attempt to estimate the slip rate of the San Andreas fault in the San Francisco Bay area must account for the changes in material properties across the fault. Work is continuing using data from this project along with broader-scale geodetic networks to improve our estimate of the slip rate for the San Andreas fault. The fault slip rate, together with the history of past earthquakes, is a necessary input to long-term probabilistic earthquake hazard estimation.