SCEC Annual Report (for 1996)
Group E - Crustal Deformation
by Ken Hudnut, Group Leader
February 1997
The premise of our Working Group is that geodesy contributes towards the main Southern California Earthquake Center (SCEC) goals by accurately estimating the displacement rate (velocity) field for southern California. Precise geodesy measures all inputs and outputs on the earthquake budget ledger sheet, and complements the fault maps, geological slip rates, and seismicity catalogs that had, in the past, formed the input to earthquake hazard maps. SCEC has innovated the combination of geodetic information into probabilistic seismic hazard estimation, and our Working Group strives to improve the geodetic observational basis for future improvements to hazard maps of the region.
Our Working Group continues efforts on several main infrastructure items; 1) SCEC support of the Southern California Integrated GPS Network (SCIGN) collaborative effort, 2) support of the archive especially for campaign data, 3) additional analysis and refinements for the SCEC Velocity Field Map, and 4) additional field data collection in support of scientific and/or velocity map objectives. As well, we fund several scientific investigations that generally employ crustal deformation data, in an innovation fashion, to address the overall goals of SCEC.
Our Working Group contributed an official SCEC product this year that resulted from the SCEC-motivated approach, and from the focus of our group. Notably, this first release of the SCEC Velocity Map incorporated diverse high-precision geodetic data that have been collected during more than two decadesŐ efforts conducted and/or funded by several federal, state, and local government agencies, as well as from SCEC-funded field work. The SCEC analysis effort has maximized return on the public's long-term investments into loosely coordinated surveys, some of which were performed for the purposes of survey control network maintenance, while others were conducted for tectonic or earthquake research. Trilateration data, along with abundant GPS data from the mid-1980's through the present, were rigorously combined to form the best set of velocity vectors yet obtained for the region.
Production of the SCEC Velocity Map can well be compared with efforts to produce a consistent and reliable seismicity catalog that covers a multi-decadal time interval. In our case, this process requires continuing refinement as we simultaneously develop the tools to properly analyze and understand the data. Specifically, we have to deal with the problematical yet valuable early data, and we must now test new models for atmospheric gradients, ocean loading, and antenna phase center variations. Also, to represent the actual errors in velocity estimates, we must improve our understanding of long period noise such as monument instability. 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.
For example, the research reports submitted by our Group on their work in 1996 included work in progress on the possibility of a regional variation in deformation following the Landers earthquake sequence. Work led by Yehuda Bock indicated a slight difference between velocities at several continuously operating GPS stations, before and after the 1992 event. Analysis of a longer run of solutions from these same stations, and with a different approach led by Tom Herring, has been performed as one means of testing the result. The particular significance of this issue is that the possible variation in rates appears to have occurred over a much larger region, and over a longer time interval, than the more typical short to intermediate wavelength (0 to 30 km) and short to intermediate time scale (days to months) postseismic phenomena associated with either the shallow or deep portions of the fault zone itself. This result, if confirmed, could lead to a variety of breakthroughs in earthquake research, and in understanding the earth's crust and its coupling to materials below it, as well as properties of those deeper materials. Because of the potential far-reaching implications of such a result, and because the claimed observations lie close to the measurement accuracy, our Working Group is motivated to perform additional studies in an attempt to evaluate this further.
Our Working Group recognizes that the campaign data will provide better spatial coverage of this possible deformation event. Also, unfortunately, the continuous GPS stations' time series begin less than one year before the Landers event. We recognize that it will be crucial to combine the older VLBI and early GPS data, as well as all the more recent campaign data and continuous data, into a single solution with a consistent reference frame. This needs to be done in order to resolve the issue of a possible regional deformation rate change, and we feel these efforts are of utmost importance. As well, the refined analyses of these data that will occur are bound to greatly improve over the first release of the SCEC Velocity Map. Regardless of whether the result is confirmed or denied, the efforts will benefit hazard mapping and other SCEC modeling efforts by resulting in improved estimation of the regional velocity field. Resolution of this issue will certainly lead to developments that will generally help our understanding of actual noise in the GPS and other geodetic data as well.
WWW sites for additional information:
Recent work either partly or fully funded by our Working Group, includes the following publications and manuscripts (submitted or in press):
Bock, Y., S. Wdowinski, P. Fang, J. Zhang, J. Behr, J. Gengrich, 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, submitted to JGR, 1996.
Dong, D., T. Herring, and R. King, Estimating regional deformation from a combination of space and terrestrial geodetic data, submitted to J. Geodesy, 1996.
Hudnut, K. W., Z. Shen, M. Murray, S. McClusky, R. King, T. Herring, B. Hager, Y. Feng, P. Fang, A. Donnellan, and Y. Bock, Coseismic displacements of the 1994 Northridge, Calif., earthquake, BSSA, v. 86, No. 1B, pp. S19-S36, 1996.
Langbein, J., F. Wyatt, H. Johnson, D. Hamman, and P. Zimmer, Improved stability of a deeply anchored geodetic monument for deformation monitoring, Geophys. Res. Lett., v. 22, pp. 3533-3536, 1995.
Shen, Z., X. Ge, D. Jackson, D. Potter, M. Cline, and L. Sung, Northridge earthquake rupture models based on the GPS system measurements, BSSA, v. 86, No. 1B, pp. S37-S48, 1996.
Shen, Z., D. Jackson, X. Ge, Crustal deformation across and beyond the LA Basin from geodetic measurements, in press, JGR, 1996.
Souter, B. and B. Hager, Fault propagation fold growth during the 1994 Northridge, California, earthquake, in press, JGR, 1996.
Wdowinski, S., Y. Bock, J. Zhang, P. Fang, and J. Gengrich, Southern California Permanent GPS Geodetic Array: Spatial filtering of daily positions for estimating coseismic and postseismic displacements induced by the 1992 Landers earthquake, submitted to JGR, 1996.
Zhang, J., Y. Bock, H. Johnson, P. Fang, J. Gengrich, S. Williams, S. Wdowinski, and J. Behr, Southern California Permanent GPS Geodetic Array: Error analysis of daily position estimates and site velocities, submitted to JGR, 1996.
Projects Funded in 1996: Agnew Data Archiving Agnew, H. Johnson, Wyatt Understanding and Reducing Monument Noise Bock PGGA Dong Horizontal Velocity Field in So. Calif. Donnellan and Lyzenga Geodetic Signals from Fault Models Gurnis Dynamic Models of Geodetic Signals Hager Models of Blind Thrusts Herring and King GPS Data Error Spectrum Analysis Humphreys 3-D Viscoelastic Faulting Response King and Herring GPS Analysis King, Herring, Reilinger Geodetic Constraints Shen and Jackson Tectonic Deformation in LA Shen, Sung, Jackson Geodetic Velocity Map Wyatt and Agnew Monitoring Structure Stability Wyatt and Agnew Pinon Flat