Detection of anomalous seismic phases by the wavelet transform

Strong motion data at La Union, one of three near-fault stations of the Michoacan, Mexico, earthquake in 1985, are analysed by a wavelet transform in order to identify a clear later energy arrival whose frequency content and particle motion are quite different from the main part of the seismograms. We used an orthonormal set of analysing wavelets, a discrete wavelet transform, proposed by Meyer and Yamada, with which an efficient computational procedure can be achieved utilizing the fast Fourier transform. Results of an application of the above wavelet transform to three-component velocity data of La Union are summarized as follows. A vertical seismogram does not contain any distinguishable later energy arrivals .for the entire frequency range recorded. The major energy arrives in the time interval between 10 and 30 s. In contrast, an east-west component seismogram shows a peculiar later arrival more than 10 s later than the main energy arrival, and our wavelet analysis shows that this later phase is characterized by an arrival time of 38 s and a frequency range from 0.1 to 1Hz where the amplitude of the later phase is even larger than the main part. This example clearly demonstrates the potential of wavelet transforms to identify objectively any phase in seismograms,localized in both time and frequency. From the information on its particle motion, the above phase corresponds to a heterogeneous and delayed break of one stronger portion on a fault on which the rupture was very smooth or 'crack-like' elsewhere.