Investigation of the Partitioning of Source and Receiver-Site Factors on the Variance of Regional P/S Amplitude Ratio Discriminants

摘要

In this project, we are investigating problems associated with applying regional-phase amplitude ratios, such as Pn/Sn or Pn/Lg ratios, for discrimination of explosions and earthquakes for monitoring the CTBT. Using multiple array recordings of groups of events in the same source region, the factors that contribute bias or the scatter of P/S ratio measurements after correction for path effects are characterized. These factors include both receiver site effects and source mechanism effects on P/S ratios. The study of site effects will be focused on arrays where we have seen site variations in P/S ratios, including the Scandinavian regional arrays (NORES, FINES, ARCES, FINES), and other new arrays in the International Monitoring System (IMS). The variance in the P/S ratio around regional arrays and large aperture arrays reveals the extent to which site affects cause variations in P/S ratios around different arrays in different regions. The partitioning of the variance between source, path, and receiver effects is examined by analysis of variance (ANOVA). We have performed an initial study of a group of presumed underwater explosions in the Gulf of Bothnia recorded by regional arrays in Scandinavia. We find that P/S and P/S amplitude ratios vary by as much as a factor of 3 around the FINES and NORES arrays, with apertures of 3 km, as well as similar variations for the different sources. These variations appear to be driven by variations in Pn and Pg amplitudes, whereas Lg amplitudes appear to be more stable. For source mechanism effects, we have been concentrating on earthquakes in the Zagros thrust belt of Western Iran, where, in a previous study, azimuthal variations in P/S ratios have been observed. Regional P/S ratios have been measured on these events and ANOVA is being used to determine to what extent the azimuthal variance can be partitioned between source effects, including focal mechanism and depth of focus, and propagation path effects. The results of this st.