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National Information Service for Earthquake Engineering
University of California, Berkeley |
1989 Loma Prieta Earthquake Displacements Measured with the Global Positioning System
Chesley Williams and Paul Segall, Geophysics Department, Stanford University, Stanford, California.
Coseismic displacements during the 1989 Loma Prieta earthquake were measured using the Global Positioning System (GPS) During February and March of 1989, the California Highway Department (Caltrans) conducted a GPS survey in the Santa Cruz area and south to Watsonville using single-frequency Trimble 4000SL GPS receivers. In March of 1990, we reoccupied eight of the Caltrans stations with dual-frequency Trimble 2000SDT GPS receivers. Relative displacements were determined by differencing the two sets of station coordinates obtained from the pre- and post-earthquake GPS surveys. During both surveys, US Geological Survey GPS receivers were collecting data on Loma Priet a at the station LP1. Incorporation of the LP1 data allowed us to tie the relative displacements in the local network to the coseismic displacement of LP1 determined by GPS measurements from the VLBI site at Fort Ord. The observed horizontal displacements are as large as 41.3 (1.9cm), and the largest vertical displacement is 34.1 (2.7cm). The uncertainty in the displacements is dominated by the Caltrans survey because the data is single-frequency and the observation sessions are of short duration (15-90 minutes). Examination of repeated baseline measurements indicates that the precision of the pre-earthquake data is 1 cm in the horizontal components and is 3.5 cm in the vertical. Since our data was collected with dual-frequency receivers for six hour sessions, errors in the post-earthquake survey are significantly smaller (.5 cm level in the horizontal and 2-3 cm in the vertical).
These data are available on the Loma Prieta Data Archive CDROM.
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