We performed two GPS field surveys in 1997, and collected valuable data sets in the Los Angeles basin and the Landers areas. The first survey was carried out in early 1997 from days 006 to 066 in the Los Angeles basin area. A total of 52 station days of data were collected at 17 sites using Trimble 4000SST receivers (Fig. 1, sites denoted by squares). This experiment is a continuation of the Los Angeles basin experiment the UCLA group has been doing since 1987. The data have been processed and integrated into the velocity map solution v2.0.
The second experiment was a remeasurement of the Landers geodetic sites. This survey was started on day 315, and is still being carried out at the time this report is submitted. The plan is to survey about 40 sites in the region, and we have finished 11 sites so far (Fig. 1). We had to postpone the survey because we could not get enough Ashtech Z12 receivers until now. The Landers sites can be grouped into two categories. The first one is a group of about 25 sites (triangles in Fig. 1) which had multiple GPS measurements after the Landers earthquake. Reoccupation of those sites is part of a continuous effort to monitor their postseismic deformation. The second group is composed of about 15 sites (diamonds in Fig. 1), which were part of the USGS Mojave EDM network. These sites are in the vicinity of the Landers coseismic rupture. The western part of the network was occupied once post-Landers, but the eastern part was not because the stations are located in the 29 Palms military base and permission to work in the base did not get through right after the Landers earthquake. Non-occupation of the sites leaves a hole in the post-Landers deformation monitoring network. Reoccupations of those sites will give second-epoch measurement for the western part and first-epoch for the eastern part of the network after the Landers earthquake. Reoccupations of those sites will yield present deformation rates across the network. Such a result can be compared with the pre-Landers deformation rates derived from the Triangulation/EDM data (Sauber et al., 1994), and used to detect the rate changes in the region since the Landers earthquake.
Reference:
Sauber, J., W. Thatcher, S. C. Solomon, and M. Lisowski, Geodetic slip rate for the Eastern California shear zone and the recurrence time of Mojave Desert earthquakes, Nature, 367, 264-266, 1994.
