Spectral characteristics of regional phases recorded at Noress

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Abstract
The spectral characteristics of Pn and Lg are studied for regional events recorded at the NORESS array in Norway. The emphasis is on the potential value of spectral ratios for identifying events as earthquake, chemical explosions, and nuclear explosions. The events studied include a suite of explosions from the Titania Mine in southwest Norway, a suite of events of unknown source type from a site offshore about 90 km from this mine, suites of explosions from several mines in the Soviet Union about 1000 km from NORESS, events from several locations along the 90° azimuth from NORESS, an apparent earthquake in the North Atlantic, and a nuclear explosion (PNE) at a range of 1560 km. The event identification issues addressed are as follows: (1) Can earthquakes and explosions be identified based on the ratio of high- and low-frequency energy in their signal spectra? (2) Do spectral ratios separate mine blasts from earthquakes for all source areas? (3) Will spectral ratio discriminants be effective for identifying decoupled underground nuclear explosions? We conclude that spectral ratios can sometimes separate events. An example is separation of the Titania mine blasts from the events at the nearby offshore locations, although we cannot be sure how much this is due to path differences. However, in general, spectral ratios vary as much within classes of events as they do among different classes. The PNE in our data set allows the simulation of spectra from a decoupled nuclear explosion by simply applying the frequency-dependent decoupling factor to the observed Pn spectrum for this event. After applying a distance correction, the spectrum for this simulated decoupled nuclear explosion is similar to those for the Soviet Union mine blasts in our data set.

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