Updating HTDP for Two Recent Earthquakes in California
This paper describes revisions to the Horizontal Time Dependent Positioning (HTDP) software carried out to address the motion associated with two recent California earthquakes. NOAA's National Geodetic Survey (NGS) developed HTDP to enable its users to compensate survey measurements for the differential movement associated with tectonic deformation, so that surveys conducted at different epochs may be compared. We focus on the process of selecting and validating models for two recent earthquakes in California, prior to their inclusion in version 2.9 of this software (HTDP v2.9)—the Parkfield earthquake with a magnitude of 6.0, which occurred on September 28, 2004, and the San Simeon, California earthquake with a magnitude of 6.5, occurring on December 22, 2003. The process of modeling and correcting for deformation can be well illustrated by utilizing data collected in the vicinity of these earthquakes, because they represent contrasting styles of deformation—Parkfield exhibited strike-slip motion and San Simeon was predominantly a blind-thrust earthquake. Also, while both earthquakes were relatively small, they produced significant displacements (though restricted to a relatively small area in central California). The Parkfield earthquake, in particular, produced an unusually high proportion of post-seismic deformation. This deformation can accumulate during a period of a few months after the earthquake, whereas co-seismic deformation occurs suddenly during an earthquake. Quantifying and characterizing deformation at these disparate time scales represents a major challenge to developing the accurate models of deformation that are required to support modern, high-accuracy surveying in tectonically active areas such as California. This study also confirms the importance of including separate dislocation models of the co-seismic and post-seismic deformation, because, as was the case for Parkfield, post-seismic deformation can be comparable in magnitude to co-seismic deformation.
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