Single Station Event Location: Epicentral Distance, Bearing, and Focal Depth

The most sensitive and reliable methods for locating small-magnitude regional seismic events using records from a single, three-component station require knowledge of the near-station and along-path crustal structure. Unfortunately, obtaining information about crustal structure is time-consuming and may be costly. Furthermore, different location methods may be required depending on the lateral complexity of the crust. Here we address a simpler problem: How often can single-station methods accurately determine event-station bearing and distance information if no information about crustal structure is available? • Neither bearing nor distance determination was generally possible for signals if the SNR was less than about 2.0; bearing determination was most successful at most stations if signals were high-pass filtered with a cut-off frequency of about 0.5 Hz. • Bearing determination for low-magnitude signals was possible even if they had no impulsive P phases if covariance methods were applied to the entire P-to-S time window. • For signals with SNR > 2.0, there were clear differences between stations with respect to how well they could determine bearing direction. At many ‘good’ stations, 66% of the bearings determined by the 2-D covariance method were within about 6-10 of the expected bearing; at many other stations, the 66 percentile of the bearings was 10-30. • There were no obvious differences in the geographic or tectonic character of ‘good’ and ‘other’ stations. Our original method for determining event-station distance relied solely on the ratios of short-term and long-term averages to find S-P times and thus distance; this method was able to determine distances to within 20 km of the true distance for only 10% of the records we studied. Currently, we are revising the phase=identification method to incorporate information about signal polarization. Methods specifically designed for determining focal depths of events in sparsely-instrumented regions are rare. Here we apply a method for determining the focal depths of smallto moderate-sized earthquakes via waveform correlation between radial and vertical components of relatively high-frequency (1-5 Hz) seismic data recorded at a single three-component station and their synthetic counterparts. In addition to our waveform correlation method, which minimizes the complicating effects of the source time function and focal mechanism, we have begun to explore a strategy that exploits the expected simplicity of source time functions for small magnitude tectonic events and explosions. In this study we modify the deconvolution method of Christensen and Ruff (1984) and evaluate its applicability for focal depth determination at regional distances through a series of synthetic experiments. OBJECTIVE We are trying to develop, apply, and evaluate methods for locating seismic events using only a single, broadband, three-component seismographic station. Most of our work has been directed toward estimating the bearing and distance to and the focal depth of small-magnitude events located at regional distances from a station using full waveform data. A large percentage of seismic events could be identified as tectonic in origin if either or both the epicenter and/or focal depth were reasonably well-constrained. While approaches that use travel times and/or waveform data from several stations are well-developed, there are many regions of the world in which station coverage is sparse and small-magnitude events are often recorded by only one station, if at all. Report Documentation Page Form Approved OMB No. 0704-0188 Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. 1. REPORT DATE SEP 2000 2. REPORT TYPE 3. DATES COVERED 00-00-2000 to 00-00-2000 4. TITLE AND SUBTITLE Single Station Event Location: Epicentral Distance, Bearing, And Focal Depth 5a. CONTRACT NUMBER