SCEC Past Research: 1999 Thrust Area 4

About SCEC

Thrust Area 4: Crustal Deformation

This group is responsible for one SCEC product, what has been called the SCEC Velocity Map (now the Crustal Motion Map or CMM); the purpose of this is to provide the scientific community with a set of current geodetic motions in southern California, based on high-quality GPS measurements. This group is also associated with several Infrastructure projects, notably Pinon Flat Observatory (PFO), which provides a detailed time history of strain variations in southern California, and the Southern California Integrated GPS Network (SCIGN), which provides continuous GPS measurements at over 100 points in the region. This group supports research efforts which apply high precision geodetic measurements to such problems as stress evolution, fault interaction, and post-seismic behavior.

In 1999 the SCIGN project (which is funded largely independently) continued to progress, with many new stations installed. The SCEC contribution to the UCSD component of the SCIGN data centers (SOPAC) continued to ensure the wide availability of continuous GPS data to SCEC researchers and others.

Much of the effort this year went into improvements and additions to the SCEC crustal-motion map (CMM). These can be classified into three general areas:

Additional GPS fieldwork, focussed on making additional measurements at points which had good prior measurements but none recently; this both improved the precision of some points already in the CMM, and added new points to it. The top panel of the associated figure (Figure 7) shows the locations of all GPS sites observed at least once since the Landers earthquake. From these, sites were selected according to how ``ripe'' they were, and observed by whichever SCEC institution was closest (to minimize travel costs): in practice, either by UCLA, UCSD, or Harvey Mudd college. The sites observed in 1999 prior to the Hector earthquake are shown as large red dots, and cover several areas in which the Version 2 CMM (released in 1998) needed strengthening.
Archiving the data collected by SCEC and by any other groups. In 1999 the archiving effort (at UCSD) finished the backlog of data from previous years for the area covered by the CMM. A total of 11164 RINEX files have been archived, including (this year) all data from southern California collected by the U.S. Geological Survey. 10,058 of these files have been associated with a particular geodetic monument, and placed in the SCEC portion of the GPS Seamless Archive (GSAC) system. The archiving effort is not complete, since there remain files from the northern Mojave desert, collected by MIT, NIMA, and Central Washington University, as well as the data around the Hector earthquake. However, the existing set is adequate for a greatly improved CMM.
Processing the data. This year a considerable advance was made in the processing of the data. Using scripts developed at MIT, MIT and UCLA have reprocessed all survey-mode GPS data from 1992.5 through (at this writing) the end of 1998 in an almost entirely automated fashion. This means that all data have been treated consistently, and also that it will be easy to redo the analysis in the future should improvements in processing be made.

The next update to the CMM will thus include all the important geodetic measurements collected in southern California during the past 30 years and available to us, including electronic distance meter (EDM), very long interferometry baseline (VLBI), and global positioning system (GPS) data. Data from both the survey mode and permanent (SCIGN) GPS networks will be processed together uniformly, with station velocities and coseismic displacements derived in a global reference frame. Stations whose post-seismic velocities may have been perturbed by the Landers earthquake will be allowed to have different secular velocities before and after the earthquake. All together the CMM will include about 400 stations covering most of the major faults in southern California, with horizontal station velocity uncertainties < 4 mm/yr. About 2/3 of the sites will have uncertainties < 2 mm/yr.

Members of the group have also been investigating possible mechanisms for postseismic motion seen after the 1992 Landers earthquake. This has involved a joint analysis of data collected by SCEC and the USGS over the months and years after this event. Previous analyses had looked at only SCEC or only USGS data, and had proposed two quite different mechanisms for postseismic deformation: from the SCEC data, afterslip below seismogenic depth on the fault plane, amounting to about 15% of the co-seismic moment in the first 6 months; from the USGS data and InSAR measurements, viscoelastic flow in the lower crust, with a relaxation time of 3 to 4 years. A joint analysis (at UCLA) of the two data sets suggests that a combination of afterslip (soon after the earthquake), and visco-elastic flow may be required to explain the post-seismic deformation.
SCEC has also supported work at MIT on this problem. Modelling results there suggest that to distinguish between these two mechanisms, data is best collected over a wide range of distances and off the ends of a rupture. Far-field sites bracket the depth interval of afterslip or viscoelastic deformation. Modeled displacement azimuths at sites off the ends of a rupture are most sensitive to the differences between mechanisms: for afterslip, displacements are more fault-parallel; for viscoelastic relaxation, displacements look more like the response to a double couple.

Response to the Hector Mine Earthquake

The Hector Mine earthquake of 16 October 1999 called forth a considerable response from the crustal deformation community, since it represents another opportunity to measure the kind of postseismic changes just described. Both to get a baseline for later measurements, and to get coseismic offsets, several SCEC institutions (USC, UCLA, and UCSD) began field measurements within 48 hours of the earthquake, though measurements close to the fault were not possible during the first week because of restricted access to the Marine Corps Air-Ground Combat Center (MCAGCC), within which the rupture lies. Because this earthquake was located on the edge of the SCIGN network (since extended to cover it), geodetic coverage of this earthquake required re-occupation of survey-mode GPS points; fortunately many of these had been occupied in the last 2 years for post-Landers monitoring. The lower panel of Figure 7 shows the survey-mode stations occupied, by SCEC and other agencies, and also the SCIGN sites operating at the time of the earthquake. In order to improve later postseismic measurements, a number of new survey-mode sites have been established over a wide range of distances and azimuths from the fault.

Southern California Integrated GPS Network (SCIGN)
1999 Accomplishments and Current Status

SCIGN has had a productive and exciting year, with many new stations coming on-line and the Hector Mine earthquake's co-seismic and post-seismic deformations to keep things very interesting. We are now well past the midpoint on our way to the goal of 250 continuously operating GPS stations in southern California, and we are well lined up to complete the final major phase of network installation in the year 2000. Nearly all of the items mentioned below are documented in greater detail through the SCIGN web page at: http://www.scign.org/

Major accomplishments of SCIGN in 1999:

Completed 'Phase II' installation work (the 100th site installed by the contractor was San Gabriel dam site, an event considered newsworthy by ourselves and also the national news media). This brings the number of installed SCIGN sites to a total of 150.

Finalized selection of sites for the full SCIGN array (a list of 200 sites put forward by the Siting Committee to the Coordinating Board was approved by vote on May 21, 1999). Began work on 'Phase IIIa' of the contract to obtain Site Evaluation Reports for the remaining sites.

Co-seismic static displacements of up to 190 mm and post-seismic motions of over 10 mm have been recorded by SCIGN for the Hector Mine earthquake (Mw7.1). Since the earthquake we've re-deployed SCIGN resources to establish 6 new sites so far since the event. An additional 5 new sites are expected to also be running by Dec. 22, 1999, and more will be built early in 2000. These are being installed to help record the post-seismic deformation field of the earthquake.

Delivered first reports by the SCIGN Analysis Committee and jointly derived the first SCIGN product, a combined set of co-seismic displacement vectors for the Hector Mine event (for 45 SCIGN stations in common between the three processing groups).

Finished SCIGN radome development, and brought the SCIGN-developed ASHCAN/EGADS data acquisition software system on-line for all SCIGN stations. Brought the Station Information Manager (SIM) into operational status and continued development of the SCIGN archive's relational database system.

Participated in formation of the California Spatial Reference Center, a new organization that will provide an interface between SCIGN and the Land Surveying profession.

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