Introduction:
Our proposal posed the scientific question: What are the current station velocities in the Tejon Pass/Gorman network regions, and what are the implications of the velocity field? To answer this question, we proposed to reoccupy about 20 SCEC-GPS stations spanning the Garlock-White Wolf-San Andreas fault system to study the current deformation north of the Big Bend segment of the San Andreas fault. The ongoing goals of this project are to: 1) develop a detailed station velocity map for the Big Bend segment of the San Andreas fault, 2) continue to monitor the regional post-seismic deformation from both the 1952 Ms 7.8 Kern County earthquake and the 1994 Ms 6.7 Northridge earthquake, 3) link the deformation in the Big Bend region to other regional studies, and 4) model the interseismic deformation from the major faults in the Big Bend region of the San Andreas fault. We have made significant progress on all of the goals of the project.
Progress to date:
During Summer, 1997, we conducted a GPS survey of 28 monuments in the Gorman and Tehachapi areas using GPS (Filled symbols on Figure 1; site names are listed in Table 1). The sites were chosen along three profiles that are roughly orthogonal to the Garlock and White Wolf Faults. All sites have been previously occupied by GPS, but not for several years. A number of sites had not been occupied since before the Northridge earthquake. When we complete the analysis of the data (during the second half of the granting period), these data will provide a detailed velocity map in the compressional region north of the Big Bend segment of the San Andreas. We coordinated the work from U.C. Davis with occupations conducted by students from Harvey Mudd College under the supervision of Greg Lyzenga. All sites were occupied for at least 48 hours using Trimble 4000ssi (from U.C. Davis and U.C. Santa Cruz) and TurboRogue (from Harvey Mudd) receivers. All receivers were equipped with Dorne-Margolin (choke-ring) style antennae. As much as possible, sites were occupied continuously, with a few sites occupied during daylight hours only for security reasons.
During Fall, 1997 we archived the GPS data from our 1993 occupation of the Tehachapi network in the SCEC archive. We began archiving the data from the 1997 survey. (Note: the 1993 occupation was not funded by SCEC; the current grant represents our first SCEC grant.)
In the Fall, we also developed viscoelastic and afterslip models of 4 decades of deformation associated with the Kern County earthquake (Bawden et al., 1997). Preliminary results were presented at the SCEC annual meeting in October, and at the Fall meeting of the American Geophysical Union. We used a layered earth model consisting of a faulted, elastic layer above a viscoelastic layer, to simulate viscoelastic relaxation after the seismic event (Rundle, 1982; Yu et al., 1996; Pollitz, 1997). A viscosity of about 4x1018 Pa s can account for most, but not all, of the signal; at least some of the high strains seen near the White Wolf Fault after the Kern County earthquake are likely due to afterslip. To determine whether all the strain can be obtained by afterslip, we also modeled the deformation using an elastic slip model. To account for our observed signal by afterslip alone requires slip comparable to that of the main shock. The magnitude of slip required by a fully-elastic model suggests that there are most likely components of both afterslip and viscous relaxation in the response of the crust to the Kern County earthquake. We are currently writing a manuscript on these results for submission to Journal of Geophysical Research. This aspect of the work is funded in part by a grant from NSF.
Educational accomplishments:
The work formed part of the Ph.D. dissertation of Gerald Bawden, who expects to complete his degree in June, 1997. In addition, 3 graduate students, one postdoc, and several undergraduates from U.C. Davis, U.C. Santa Barbara, and Harvey Mudd College participated in the GPS survey. One undergraduate involved in the survey, Eric Cannon, began graduate school in active tectonics at U.C. Davis this fall.
Tasks to be completed during the remaining 6 months of the grant:
During the remaining 6 months of this grant, we will finish processing the data using GIPSY-OASIS II on computer facilities at U.C. Davis. The results will be used to produce and interpret velocity maps for the Big Bend segment. We will also complete the process of archiving our data at the SCEC data center, and we will complete the JGR manuscript.
Publications supported by this award:
Abstracts
G. W. Bawden, L. H. Kellogg, and A. Donnellan, Geodetic measurements of horizontal strain near the White Wolf, Garlock, and San Andreas faults, 1926-1997, SCEC Annual Meeting, 1997.
G. W. Bawden, L. H. Kellogg, A. Donnellan, J. B. Rundle, Constraints on crustal rheology from viscoelastic models of time-dependent strain after the 1952 Ms 7.8 Kern County earthquake, Southern California, EOS, Trans. Amer. Geophys. Union, 46, F157, 1997.
Paper in preparation
G. W. Bawden, L. H. Kellogg, F. Pollitz, and A. Donnellan, Post-seismic strain following the 1952 Kern County, Southern California, earthquake: Results of models of viscoelastic relaxation and elastic post-seismic slip, in preparation for J. Geophysical Research, 1997.
References:
Bawden, G. W., A. Donnellan, L. H. Kellogg, D. N. Dong, and J. B. Rundle, Geodetic measurements of horizontal strain near the White Wolf fault, Kern County, California, 1926-1993. J. Geophys. Res., 102, 4957-4967, 1997.
Rundle, J. B., Viscoelastic-gravitational deformation by a rectangular thrust fault in a layered Earth, J. Geophys. Res., 87, 7787-7796, 1982.
Pollitz, F. F., Gravitational viscoelastic postseismic relaxation on a layered spherical Earth, J. Geophys. Res., 102, 17921-17941, 1997.
Yu, T.-T., J. B. Rundle, and J. Fernandez, Surface deformation due to a strike-slip fault in an elastic gravitational layer overlying a viscoelastic gravitational half-space, J. Geophys. Res., 101, 3199-3214, 1996.
Table 1: Monuments surveyed Summer 1997.
We surveyed 28 sites in the Tehachapi and Gorman regions. Each monument has a 4-character ID and a full name.
| Number: | 4 Character ID | Monument Name |
| 1 0618 | HPGN | CA 06 18 |
| 2 | AIRR | AIR |
| 3 | BED1 | Bed |
| 4 | BRUS | BRUSH MTN |
| 5 | CHAP | CHAPI |
| 6 | CUYA | CUYAMA |
| 7 | DEER | DEER |
| 8 | DMRM | DOUBLE MOUNTAIN RM 1 |
| 9 | DOBL | DOUBLE MOUNTAIN |
| 10 | FAIR | FAIRMONT |
| 11 | FLAN | FLANNAGAN |
| 12 | J976 | J976 |
| 13 | JACK | Jacks |
| 14 | KILN | KILN |
| 15 | MPNS | Mount Pinos |
| 16 | MRGO | MARGO |
| 17 | PAJA | PAJUELA |
| 18 | PORT | PORT |
| 19 | RKSP | Rock Springs |
| 20 | SMIT | SUMMIT |
| 21 | SOLE | SOLEDAD |
| 22 | TWST | TWISTED ROCK |
| 23 | WEED | WEED |
| 24 | WEED | WEED_1 |
| 25 | WHEE | WHEELER_2 |
| 26 | WSR2 | WARM SPRINGS RM 2 |
| 27 | Yam2 | Yam2 |
| 28 | 0617 | 0617 -- 'Rodeo' |
