Depth determination of small shallow earthquakes in eastern Canada from maximum power Rg/Sg spectral ratio
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For small earthquakes, focal depths can be estimated jointly when epicenters are located using the arrival times of Pg and Sg waves recorded at seismic stations close to the event. However, if regional network coverage is sparse, this approach does not give accurate results. An alternative solution is the use of the regional depth-phase modeling (RDPM) method when such depth phases are available. Small, shallow earthquakes can generate Rg waves, the amplitudes of which approximately attenuate exponentially with focal depth; whereas, the amplitudes of Sg waves are, on average, less dependent on focal depth. Based on these features, a method using the maximum power spectral ratio (MPSR) between the Rg and Sg segments was developed to determine focal depth. Tests show the focal depth solutions obtained by the MPSR and RDPM methods for five events in an earthquake swarm and one event acquired by inspection are in good agreement. The error in the MPSR-determined focal depth caused by the error in the epicentral distance is in the order of 0.1 km. The error in the focal depth when using a default focal mechanism is in the order of 0.5 km. The quality factor, Q does not generate a significant error. Using the average of focal depths can provide a more reliable solution. Using an azimuth of approximately 45° from the strike direction to generate the synthetic ratio curve can reduce the error. As with any other earthquake locating technique, a reasonable regional crustal model is required when the MPSR method is used. Case studies show that the MPSR method can be used to successfully determine focal depths for events as small as m N 1.6.
KeywordsDepth determination Small shallow earthquakes Rg/Sg spectral ratio
This research was supported by the Natural Sciences and Engineering Research Council of Canada under the Strategic Research Networks program and Discovery Grant. John Adams of the Geological Survey of Canada (GSC) and Tom Lam of the Nuclear Waste Management Organization (NWMO) developed and organized the Rg/Sg ratio project, supported by NWMO. The seismograms and earthquake catalogs used in this article were retrieved from the Natural Resources Canada official web site. Figures were generated using Geotool software, MATLAB and the Generic Mapping Tool (Wessel and Smith 1991). The waveform records were processed using SAC2000 and redseed programs. Two anonymous reviewers provided constructive comments and suggestions in the article revising process. The authors gratefully acknowledge the assistance mentioned above.
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