Depth determination of small shallow earthquakes in eastern Canada from maximum power Rg/Sg spectral ratio |
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| Authors: | Shutian Ma Dariush Motazedian |
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| Institution: | (1) Department of Earth Sciences, Carleton University, Room 2125, Herzberg Bldg, 1125 Colonel by Drive, Ottawa, ON, K1S 5B6, Canada; |
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| Abstract: | 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. |
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