Understanding Scatter in Local-Distance Event Amplitude Predictions and Implications for Event Discrimination

Abstract:

The ability to discriminate between earthquakes and explosions, such as underground nuclear tests, is a global concern. Differences in seismic signatures between the two types of events have empirically been shown to be effective discriminants. For example, at teleseismic distances, large events can be discriminated using the difference between the body wave and surface wave magnitudes (M_b:M_s), and P/S amplitude ratios have been employed at regional distances (>200 km). However, the discrimination of small events that require local data (<200 km) is comparatively poorly studied. The few existing studies of P/S ratios at local distances present mixed results, suggesting that this discriminant may not be reliable when extended to these short distances. One possible reason for the inconsistent behavior of P/S ratios at these distances is the large scatter observed in the ratios of the earthquake population. As event-to-station distance increases, this scatter decreases. Because the amplitude ratios are formed after making corrections for event distance and magnitude, the change in scatter with distance suggests that there may be a breakdown in the assumptions that go into the source and/or path terms at short distances. We examine each of these aspects in turn to investigate what might be causing the observed scatter and its implications for the discrimination performance of P/S amplitude ratios at different distance ranges.

Additionally, we examine the appropriateness of chemical explosions as an analog for nuclear tests with respect to P/S ratios. We utilize data from the Source Physics Experiment, a series of chemical explosions designed to improve our understanding of shear energy generated by explosions. Seismic amplitude ratios from both Phase I in granite, and Phase II in dry alluvium, are compared with data from historic nuclear tests to better understand the similarities and differences of this discriminant from the two types of explosions.