Crustal heterogeneity as inferred from seismic coda wave decomposition by small-aperture array observation

Seismic coda waves have complex waveforms and are considered to be caused by 3-dimensional structural heterogeneities. We carried out 3-component small-aperture array observations of seismic coda waves from natural earthquakes in Tsukuba, central Japan, to decompose complex coda waveforms and to clarify the nature of crustal heterogeneity. Epicentral distances in the present study are less than 200 km. The observational results are summarized as follows: 1) The remarkable feature of the coda from natural earthquakes is the difference between the P-coda and the S-coda especially in the vertical component. The P-coda consists mainly of coherent P-wave phases which have almost the same slowness vector as the initial P-wave motion, whereas the S-coda is decomposed into incoherent phases with random propagation directions. 2) Coherency of waveforms among the array sensors of the vertical component rapidly becomes low from the high value of coherency of the direct S-wave. 3) In the two horizontal components, radial and transverse ones, both the P-and S-coda are decomposed into incoherent random phases. 4) Coherency in the early S-coda of the horizontal components gradually becomes low compared to the vertical component. These features indicate that S to P converted waves at horizontal boundaries of the structure are dominant in the P-coda of the vertical component, while the P-coda of the horizontal components and the S-coda consist of scattered waves from the lateral heterogeneity. This interpretation leads to a model of crustal heterogeneity in which the degree of the lateral heterogeneity is smaller than the degree of vertical heterogeneity of layered structures. (C) 1997 Elsevier Science B.V.