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ENSCO, INC., 5400 PORT ROYAL ROAD, SPRINGFIELD, VIRGINIA 22151
Abstract
In this study, we have investigated regional-phase spectra, spectral ratios, and amplitude ratios as discriminants for case-based, regional seismic-event identification using western Norway explosions and earthquakes recorded at the NORESS array. All events were in a comparable local magnitude range (2 to 3) and distance range (300 to 500 km) from the NORESS array. Thus, the propagation paths from the earthquakes and explosions were similar but not identical. Many of the events, previously not identified as blasts, had time-independent spectral modulations indicative of ripple-firing. These include a number of offshore events, which appear to be underwater explosions. The modulations for the offshore events may have been caused by bubble pulse interference effects and/or underwater reverberations. Amplitude ratios of Pn/Sn and Pn/Lg, measured off of incoherent beams, were examined on a region-by-region basis for a group of confirmed blasts, events strongly suspected to be earthquakes, and other events that had time-independent spectral modulations. The blast and earthquake groups were well separated by both ratios, with the blasts producing more P-wave energy relative to S-wave energy than the earthquakes. However, the Lg spectral ratio provided no separation. This result agrees with others in shield regions that amplitude ratios between compressional and shear waves discriminate better than low-frequency to high-frequency spectral ratios for individual phases. The converse holds for events in orogenic regions, such as the Basin and Range tectonic region of the Western United States, for reasons that are not well understood but may relate to differences in crustal attenuation. This fact argues in favor of using a case-based approach to event identification, where events are identified by comparing them to previous events or cases that occur in the same or similar tectonic environments. In the absence of a well-understood, unifying theory for regional-event identification, characterizing events on the basis of geophysical and signal similarities to previous cases may be the only way of providing meaningful event identifications.
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