Faculty and Associates: Duncan Agnew (UCSD), Yehuda Bock (UCSD), Danan Dong (JPL), Andrea Donnellan (JPL), Brad Hager (MIT), Tom Herring (MIT), Ken Hudnut (USGS), Ken Hurst (JPL), Dave Jackson (UCLA), Louise Kellogg (UCD), Nancy King (USGS), Bruce Luyendyk (UCSB), Greg Lyzenga (Harvey Mudd), Mark Murray (UCB), Susan Owen (USC), Gilles Peltzer (JPL), Will Prescott (USGS), Rob Reilinger (MIT), Ross Stein (USGS), Wayne Thatcher (USGS), Steve Ward (UCSC), Mike Watkins (JPL), Frank Webb (JPL), Bill Young (Riverside County)

Postdocs: Don Argus (JPL), Luciana Astiz (UCSD), Gerald Bawden (USGS), Rick Bennett (Harvard), Peng Fang (UCSD), Mike Heflin (JPL), Hadley Johnson (UCSD), Bob King (MIT), Simon McClusky (MIT), Zhen-Kang Shen (UCLA), B. Souter (MIT), Li-Yu Sung (UCLA), Matt von Domselaar (UCSD), Simon Williams (UCSD), Frank Wyatt (UCSD)

Students: Greg Anderson (UCSD), Shirley Baher (UCLA), David Coblentz (Oregon), Rosanne Nikolaides (UCSD), David Palmer (USC)

Our goal is to develop highly accurate estimates of crustal strain and earthquake-induced displacements. These maps are then used with fault maps, geological slip rates, and seismicity catalogs, to enhance our understanding of seismogenic processes and mechanics of crustal deformation and to improve understanding of earthquake hazard. We have refined the analysis of trilateration, very long baseline interferometry (VLBI), and Global Positioning System (GPS) observations performed in southern California over the past two decades and organized additional GPS observations that support these efforts. We continue efforts on several main items related to our goal; 1) support of the Southern California Integrated GPS Network (SCIGN) collaborative effort, 2) support of the archive for campaign data, 3) additional analysis and refinements of the SCEC velocity field map, and 4) additional field data collection in support of scientific and/or velocity map objectives. As well, we carry out scientific investigations that employ crustal deformation data to address the overall goals of SCEC. These investigations have led to improvements in the understanding of GPS errors, and in the analysis of pre-1994 geodetic data in order to better characterize changes in strain associated with the Landers and Northridge earthquakes.

Postseismic deformation has been the focus of much recent research conducted by this group and others, some of whom have used the first release of the SCEC velocity field in their studies (e.g., Shen et al., 1994; Savage and Svarc, 1997; Bock et al., 1997; Donnellan et al, 1998; Savage et al., 1998). Others used geodetic estimates of deformation in the LA basin to test models that are plausible from both a geological and a continuum mechanics perspective (e.g., Souter, 1997), and to help substantiate tectonic interpretations (Shen et al., 1996 and Walls et al., 1998; Figure 3). Our Working Group contributed, in late 1996, the first release of the SCEC velocity map based on a combination of trilateration data acquired between 1970 and 1992 and survey-mode GPS data from 1986 to 1996. Despite the significant contribution made by this first release of the SCEC velocity map, our Working Group recognizes that continuing work is needed to realize the full potential of the available data sets. Accordingly, we have continued to improve the velocity map. During the past year we added to our analysis the VLBI data from 1980 to 1994, several additional GPS surveys, and the continuous data acquired between late 1992 and early 1998. In July 1998, the second release of the velocity map was presented to the SCEC Board to ask their approval of this as an official SCEC product. We will present this new version of the velocity map at the SCEC 1998 Annual Meeting (the web address for the second version is available below). We will continue to improve the velocity map by approaching the analysis in a different way. This "third release" effort will involve exercising the archive by including all campaign-mode GPS data with all of the PGGA and SCIGN data in a single set of re-analysis runs, and again combine those solutions using GLOBK as well as QOCA. Problems identified in producing release 2, especially that of reference frame issues, will be further investigated. It was hoped that release 2 would allow us to better constrain the suggested change in regional velocity field following the Landers earthquake (Bock et al., 1997), but another attempt is clearly needed still. We began our effort to characterize Landers-induced changes in local and regional strain by reanalyzing data from GPS surveys conducted between 1988 and 1992. As a result of this effort, we have improved the uncertainties in the estimated velocities of about a dozen stations to a level at which they may be usefully compared with post-Landers estimates. During the coming year we plan to complete reanalysis of the data from the three continuous stations deployed for a year or more prior to Landers and to perform a more rigorous error analysis on all of the pre-Landers data - VLBI, survey-mode GPS, and continuous GPS - in order to place more reliable constraints on the likelihood of region-wide changes. The upcoming work will also strive to remove the recognized regional common-mode drift that is seen in continuous GPS data but not normally removed from campaign-mode GPS data due to insufficient continuous data coverage. This data set offers an opportunity to extend the geodetic capabilities to combine various data sets and to remove common-mode data corrections for the first time. We anticipate finishing this third release of the velocity map by mid-1999, and decreasing the level of effort on our group's velocity map project substantially after its completion. We have investigated the quality of both the data and the analysis procedures for the SCIGN data as well. The MIT group has been looking at a subset of the SCIGN data and making available via the world-wide-web the detailed analysis of these data in the form of daily phase residuals and time series plots that can be interactively modified by a web user. The SIO analysis group has also introduced interactive time series plots on their analysis web page. The JPL analysis group is also developing an interactive display method to allow detailed analysis of the results from the SCIGN array. These analyses of the SCIGN data have been helping us to isolate noise sources and equipment problems, and have contributed information early enough so that as the new SCIGN stations are deployed they can be made better.

WWW sites for additional information:

Group E Information www-socal.wr.usgs.gov/hudnut/scec/SCEC_Group_E.html
Velocity Map (release 2.0) www.data.scec.org/group_e/release.v2
Archive Information www-pfo.ucsd.edu/scecgps/gps_page.html
SCIGN Project www.scign.org SIO
SCIGN analysis lox.ucsd.edu/permanentGPSSites
MIT's SCIGN analysis www-gpsg.mit.edu/~tah/cont98.html
Pinon Flat Observatory www-pfo.ucsd.edu


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