Slowness power spectrum analysis of the coda composition of two microearthquake clusters in northern Switzerland

Abstract

The S-wave coda composition of two microearthquake clusters in northern Switzerland has been analysed using the slowness power spectrum analysis proposed by Spudich and Bostwick. The purpose of this paper is to estimate the directions in which individual coda wavelets leave the source region for lapse times more than six times the S-wave travel time. The data set consists of 12 and 16 events in the magnitude range of 1.9–2.6 and 1.4–3.4, respectively. The depths range from 7 to 8 km and from 6 to 7 km. Digital data from five vertical-component stations (ENB, RBF, GEF, GIF and TSB) and two three-component stations (ACB and CHE) were available. The hypocentral distances to the recording sites vary between 29 and 63 km for the first cluster (Günsberg) and 10 and 24 km for the second cluster (Läufelfingen), respectively. All events have been relocated using a master event technique with inter-event arrival times estimated by a cross-correlation method. The slowness power spectra for the Läufelfingen cluster at all stations show essentially two different patterns. In agreement with the results of Spudich and Bostwick, we found that the early coda following the direct S wave immediately was composed of wavelets leaving the source region with the same slowness vector as the direct wave. We interpret this coda part as being very probably produced close to the recording site. However, with increasing lapse times we observe a different coda type which receives its energy from wavelets leaving the source region in a variety of directions. The transition between the two coda ‘types’ takes place often at 1.5–2 times and always before three times the S-wave arrival time.