The SCEC Crustal Motion Map, Version 3.0

Z.-K. Shen(1), D. C. Agnew(2), R. W. King(3)

(who should be contacted if any problems are found; click here for the full list of contributors, which should be used in citations).

(1) University of California, Los Angeles
(2) IGPP/SIO, University of California, San Diego
(3) Massachusetts Institute of Technology

Introduction

      The Southern California Earthquake Center (SCEC) has supported the development of a number of community models, one of which is the Crustal Motion Model, or CMM (formerly known as the Horizontal Deformation Velocity Map). Version 1 of this was released in October 1996, and Version_2 in July 1998. We here present Version 3.0, which includes a great deal of GPS data not available in previous versions: both survey-mode data from new sites, and the permanent sites of the SCIGN network. As before, the analysis also includes precise trilateration (Geodolite) data from the Crustal_Strain project of the US Geological Survey (1970-1992) and very long baseline interferometry (VLBI) data collected by the NASA Crustal Dynamics Program (1980-1994), from fixed and mobile systems. The GPS data begin in 1986; because of the need to have a fixed cutoff for processing, we have in general not yet included data after October 2001, except for a few stations.

      Our aim, as in the earlier versions, is to produce a best estimate of the current interseismic horizontal velocities at a large number of points in southern California. We assume steady motion for most sites, except for a few close to large earthquakes.

      We have written a_sketch of our analysis procedure and assumptions, which should be read before using the data. A more detailed technical discussion is in preparation.

Reporting Problems

      We have created, through USC, a mail-list server for reporting problems and making comments. See this_page for instructions on how to subscribe (which you need to do to post comments). An archive of past emails can be viewed here.

Results

The CMM information that will be of interest to most users is given in four tables:

• The velocity_file gives the estimated velocities at the sites.
• A list_of_possible_problem_sites describes sites which we have retained in the solution, but which tests suggest may not reflect tectonic motion.
• A list_of_coseismic_offsets describes the motion of stations cause by the Joshua Tree, Landers, Northridge, and Hector Mine earthquakes. For the last event, we did not include some near-fault stations, which had little data prior to the earthquake. Offsets for these (and other sites) are included in a_list_of_Hector_Mine_offsets as published in BSSA in 2002, though these are less accurate than the CMM versions.
• The station_list gives information about the individual sites.

In addition, we provide the following information, for those who wish to use it:

• A table_of_times_of_observation shows when particular survey-mode GPS stations were observed, and cases in which the data were not used. (A complete list of all survey-mode data can be found via the GPS_data_archive at the SCEC_Data_Center (though this does not include some pre-1990 data used in the model); for information on the continuous GPS sites, see the SOPAC web site.)
• A SINEX_file (compressed with gzip) give the full covariance matrix of the solution. (NOTE: this is a large file: 23 Mb compressed).

The format_of_these_tables (except for the SINEX) is described on this_page.

The plots below show first the distribution of stations, classified as EDM sites, and survey-mode and continuous GPS: note that because of the time at which the data were processed, and because of the Hector Mine earthquake, about half of the stations of the SCIGN network have not yet been included. The next two maps show the site velocities. The first one shows those away from the plate boundary, with each velocity referenced to its own plate. The second one shows all sites, with half of the plate motion subtracted. Both presentations aim to reduce the problem of large common-mode signals obscuring patterns of interest, such as the asymmetry across the Mojave segment of the San Andreas, visible in the second plot.