Single-Station Event Location Using P- and S- Waves




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Seismic data are currently available for the Earth and its Moon. Such data are the most valuable type of information for the determination of internal structure. On November 26, 2018, the InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) mission deployed a geophysical lander on Mars using a single seismic station to gather information about the planet’s internal structure and seismicity. An instrument composed of multicomponent broadband and short period seismometers was deployed to record seismic noise, marsquakes, and meteor impacts. With just a single station, traditional source location methods are impossible, and different techniques need to be used. Some form of polarization analysis must be used for single-station event location methods. Single-station location is based on the determination of station to event azimuth and epicentral distance. For the initial linearly polarized P-wave, a single eigenvector of the signal covariance matrix can determine the azimuth. Epicentral distance can be computed using either the incidence angle to compute slowness or the S-P time differences. If incidence angle is used, then the P-wave eigenvectors are used to compute an incidence angle. S-P times or other phase differences can also be used to determine epicentral distance. These methods are more reliable than computation of the incidence angle. With the azimuth and epicentral distance, the location of seismic events can be determined relative to the station. This thesis develops a processing methodology and investigates the ability of a single, three-component seismic station to determine event locations. The proposed process uses a variant of Samson and Olson’s (1981) polarization method in the time domain with a complex covariance matrix derived from East-North-Vertical (E, N, Z) component seismograms. Azimuth is derived from the (E, N) components. The system is then rotated into the Radial-Transverse-Vertical (R, T, Z) coordinate system. The apparent incidence angle is then determined from the (R, Z) components. Application of Snell’s Law yields the horizontal slowness, from which horizontal distance and traveltimes are determined by ray tracing in a global velocity model. Distance can also be determined from traveltime differences between different phase arrivals. The S and P arrivals are most commonly used. With these parameters, the latitude, longitude, and origin time of the event are determined, which is the epicenter. If depth can be determined, the hypocenter is obtained. Depth can be estimated from extracting the pP surface reflection and using the traveltime difference with respect to P. Seismicity maps based on single-station event locations are compared to nominal, or more accurate, United States Geological Survey (USGS) Global Seismographic Network (GSN) locations to see if plate tectonic structures can be identified from single-station results.



Seismic waves, Seismic event location, Seismic networks


©2020 Lauren Phillips. All rights reserved., ©2020 Lauren Phillips. All rights reserved.