Periodic ripple of coda envelope observed in northeastern Japan

Abstract

We have investigated the SH envelopes (bandpass filtered and mean squared seismograms) in the early coda of local and regional earthquakes observed at five stations in northeastern Japan. The most remarkable feature we have found is a periodic increase in coda amplitude with nearly a constant time interval of several seconds. We call this phenomena ‘ripple’ of coda envelope. We examined four possible origins of periodic ripple: an artifact due to inadequate data handling, repeated source radiation, site effect, and path-originated phase. The possibility of artifact due to inadequate filtering or smoothing is low because the ripple is visible in both unfiltered seismograms and filtered traces with various filtering parameters. Repeated source radiation is not responsible for the ripple, because the ripple has longer time duration than that of source time function of the investigated events, and because the appearance of ripple at five stations is not synchronous. Local site amplification factors determined by the spectral ratio of horizontal to vertical motions suggest the contribution of surficial low velocity layers (LVL) to the periodic ripple. The events with prominent ripple are predominantly distributed in shallow range off the Pacific coast and intermediate-depth range along the volcanic front of the Japanese Islands. This trend is commonly observed for all the investigated stations having different site amplification factors. This observation is suggestive of a path-dependent origin such as trapped waves in the LVL in the upper part of the subducting slab. Thus the guided waves trapped in the LVL are the probable origin of the periodic ripple. The LVL is either the surficial sedimentary layers under the recording site or the subducting oceanic crust.