Phase Identification and Event Location at Regional Distance Using Small-Aperture Array Data

Abstract

As part of ongoing research aimed at a better understanding of seismic wave propagation at regional distances, one of the NORSAR subarrays has been modified to a small aperture array with station distances from 125 to 2051 meters. Regional phases including Pn, P, Sn, Lg and Rg from reported events within 10° have been analyzed with respect to phase velocity and azimuth. This is done by computing frequency-wave number spectra for time series of 100 samples each (20 Hz data), which for most phases include both onset and amplitude maximum. Derived values for phase velocity and azimuth are in general accordance with expected values, even at frequencies of 6–7 Hz. Discrimination between fast (P) and slow types of phases (Sn, Lg) is possible using data from this array. A location alogrithm for the small aray data is described that enables us to locate most regional events with a difference from the location by the whole Fennoscandian network that is within the uncertainty limits of the latter.