Introduction
SOPAC provides ongoing infrastructure support for SCEC-related geodetic studies of crustal deformation in southern California under the umbrella of the Southern California Integrated GPS Network (SCIGN). We have primary responsibility for archiving all continuous GPS data in southern California (Figure 1), and parallel responsibility (with JPL) for generating daily position time series. We are responsible for maintaining SCIGN sites in Imperial, Orange, Riverside, San Bernadino, and San Diego Counties. YB is currently chairman of the SCIGN Coordinating Board and Executive Committee. Overall SCIGN progress is provided in detail in 1997 reports to the William M. Keck Foundation and to NSF, available from SCEC.
Accomplishments
Geodetic time series. In 1997, we analyzed data daily from over 50 SCIGN and BARD sites (BARD is the northern California continuous GPS array), and completed a re-analysis of all SCIGN/PGGA and BARD data collected since the 1992 Landers earthquake (Figure 2). The time series are available through our upgraded WWW home page, http://lox.ucsd.edu.
Post-Landers deformation. Bock et al. (1997) and Zhang et al. (1997) reported on the re-analysis of the PGGA time series between the Landers and Northridge earthquakes. After accounting for short-term postseismic displacements (Wdowinski et al., 1997), Bock et al. (1997) reported on significant changes in PGGA site velocities for the period between the Landers and Northridge earthquakes, compared to the pre-Landers velocities. An enhanced analysis of these data are shown in Figure 3a. Analysis of data since the Northridge earthquake indicates that the PGGA site velocities have returned to their pre-Landers earthquake rates (Figure 3c). The short duration of this effect, the observation that the post-earthquake deformations appears to be a continuation of the coseismic deformations (same direction, ~10% magnitude Figure 3b), and that the coseismic deformations are well modeled by an elastic dislocation model, indicates that all the elastic strain was not released instantaneously (coseismically) by the Landers earthquake (Bock et al., in preparation).
Los Angeles Basin. Walls et al. (1997) used our estimated velocities for sites in the Los Angeles area to construct a new geological model for the Los Angeles basin and to re-assess seismic risk.
Field operations. We installed the SCIGN/PGGA sites at Lake Skinner, San Clemente Island, and Wide Canyon (Figure 1), and installed Paroscientific Meteorological Packages at 10 sites.
Data Archiving and Dissemination. We archived data for all continuous GPS sites in California and all global IGS sites. In 1997, we accomplished a major upgrade of our hardware and software which has significantly improved access to the SOPAC archive. The number of data files being accessed per month is approaching 100,000 (compared to 30,000/month in 1996). See also Products.
Outreach activities
Our WWW home page supplies SCIGN/PGGA data to the public, particularly the surveying community. YB represents the academic community on the California Geodetic Control Committee. YB and Ken Hudnut are members of the California CORS committee which provides surveying community oversight for CORS (Continuously Operating Reference Stations) in California.
Products
(1) RINEX data archived for all permanent tracking stations in southern California, precise satellite ephemerides (IGS SP3 format), and daily solutions for geodetic coordinates (SINEX format); global and regional solution (h-) files and precise ephemeris (g-) files for GAMIT/GLOBK users.
(2) World Wide Web (WWW) home page (http://lox.ucsd.edu): Users can view and retrieve coordinate time series, station logs, archived data, and general information on SOPAC and SCIGN.
(3) Weekly data bulletin and weekly analysis bulletins mailed electronically. Also available through WWW.
(4) With MIT, development and testing of the GAMIT software used by several SCEC investigators.
Journal articles, Books, Reports - published, in press (1996-1997)
Bevis, M.,, Y. Bock, P. Fang, R. Reilinger, T. Herring, J. Stowell, and R. Smalley Jr., Blending old and new approaches to regional GPS geodesy, Eos, Trans. AGU, 78, pp. 64, 66, 1997.
Bock, Y., "Reference Systems" and "Medium Distance GPS," GPS For Geodesy, 2nd edition (Chapters 1 and 9), A. Kleusberg and P. J. G. Teunissen, eds., Springer Lecture Notes in Earth Sciences, Springer Verlag, 1997.
Bock, Y. and S. Williams, Integrated satellite interferometry in southern California, Eos Trans. AGU, 78, p.293,299-300, 1997.
Bock Y., S. Wdowinski, P. Fang, J. Zhang, J. Behr, J. Genrich, S. Williams, D. Agnew, F. Wyatt, H. Johnson, K. Stark, B. Oral, K. Hudnut, S. Dinardo, W. Young, D. Jackson, and W. Gurtner, Southern California Permanent GPS Geodetic Array: Continuous measurements of crustal deformation between the 1992 Landers and 1994 Northridge earthquakes, J. Geophys. Res., 102, 18,013-18,033, 1997.
Genrich, J.F., Y. Bock, and R. Mason, Crustal deformation across the Imperial Fault: Results from kinematic GPS surveys and trilateration of a densely-spaced, small-aperture network, J. Geophys. Res., 102, 4985-5004, 1997.
King, R.W. and Y. Bock, Documentation of the GAMIT software, MIT/SIO, 1997.
Walls, C., T. Rockwell, K. Mueller, Y. Bock, S. Williams, J. Pfanner, J. Dolan, and P. Fang, Escape tectonics in the Los Angeles metropolitan region and the implications for seismic risk, Nature, in press, 1997.
Wdowinski, S., Y. Bock, J. Zhang, P. Fang, and J. Genrich, Southern California Permanent GPS Geodetic Array: Spatial Filtering of Daily Positions for Estimating Coseismic and Postseismic Displacements Induced by the 1992 Landers Earthquake, J. Geophys. Res., 102, 18,057-18,070, 1997.
Zhang, J., Y. Bock, H. Johnson, P. Fang, J. Genrich, S. Williams, S. Wdowinski and J. Behr, Southern California Permanent GPS Geodetic Array: Error analysis of daily position estimates and site velocities, J. Geophys. Res., 102, 18,035-18,055, 1997.
Acknowledgements.
We thank the members of the SCIGN Executive Committee and Coordinating Board, and John Galetzka, Nancy King, Hadley Johnson, and Frank Wyatt for a productive year. Related activities at SOPAC were also funded by NASA, NSF, and USGS.