The COMPLOC Earthquake Location Package
Guoqing Lin, & Peter M. ShearerPublished July 2006, SCEC Contribution #974
This article describes the programs included in the COMPLOC computer program package that are designed to apply the source-specific station term (SSST) method to solve for local earthquake locations using P- and S-wave phase data. These programs can greatly improve the relative location accuracy of nearby events by applying empirical corrections for the biasing effects of three-dimensional velocity structure. They have been tested on data from both the Southern California Seismic Network (SCSN) and Northern California Seismic Network (NCSN)
The SSST method (Richards-Dinger and Shearer 2000; Lin and Shearer 2005) works by assigning each station a travel-time correction that varies as a function of source position. This approach provides relative location accuracy comparable to master event or hypocentroidal decomposition (Jordan and Sverdrup 1981) methods within compact event clusters, but is applicable to distributed seismicity. It has some similarities to the double-difference algorithm (Waldhauser and Ellsworth 2000, 2002) and can be shown to give comparable results in tests on synthetic data (Lin and Shearer 2005). However, the SSST approach has computational advantages for big data sets because the location and station term parts of the computation are separate, so that large matrix inversions are not necessary. In addition, our implementation of SSST provides the option to use L1-norm misfit measures, which are more robust than least squares in the case of occasional timing errors or bad phase picks.
We implement the SSST approach by selecting nearby events located within a sphere of specified radius rmax around the target event. The station term for the target event is then computed as the median (or mean) residual of these events. Different results will be obtained depending upon the size of the cutoff distance rmax. If rmax is set to a large enough distance, then the SSST method will give the same result as the static station term method (in which there is only a single timing correction term for each station). However, if rmax is set to a very small distance, the number of events may not be sufficient to obtain a reliable estimate of the true station term. Thus selection of the cutoff distance is a key factor in the application of the SSST method.
When Richards-Dinger and Shearer (2000) applied an SSST algorithm to locate southern California earthquakes, they obtained their initial locations using the static station term method and then used these station terms as a starting point for the SSST calculation. In this way, they achieved some improvement in the absolute locations of the events before focusing on the relative locations among closely spaced events. A generalization of this approach, adopted in COMPLOC, is to continuously shrink the SSST cutoff distance rmax between the first and final iterations. In other words, we start the cutoff distance with a large value to include all the events from which we calculate station terms, then decrease it to some specified minimum distance to calculate station terms using only the closest events.
More details about the shrinking-box SSST algorithm are contained in Lin and Shearer (2005). The method also was used to relocate 340,000 southern California earthquakes by Shearer et al. (2005), a study that went on to relocate events within similar event clusters using waveform cross-correlation times. The COMPLOC package does not include an option to use differential times from waveform cross-correlation, but we hope to add this in a future release.
This paper will describe the programs in COMPLOC, which is available as a UNIX tar file at http://igpphome.ucsd.edu/~glin/COMPLOC. We also will present plots of some of the example data sets contained in the package.
Citation
Lin, G., & Shearer, P. M. (2006). The COMPLOC Earthquake Location Package. Seismological Research Letters, 77(4), 440-444. doi: 10.1785/gssrl.77.4.440.
