The Earthquake Engineering Online Archive

Analysis of near-source waves: separation of wave types using strong motion array recordings

Darragh, Robert B.

UCB/EERC-88/08, Earthquake Engineering Research Center, University of California, Berkeley, 1988-06, 150 pages (410/D21/1988)

Algorithms are developed for the separation and identification of seismic wave types in the near-source region. Two large Taiwan earthquakes recorded by more than 25 digital, three-component accelerometers that make up Taiwan's SMART 1 array are used to demonstrate the methods: the Jan. 29, 1981 (M subscript L 6.3) earthquake and the Jan. 16, 1986 (M subscript L 6.2) earthquake. A sub-array of the SMART 1 array, consisting of the center station and the inner and middle rings, is used to analyze the ground acceleration wavefield. Two complementary analyses of the cross-spectral matrix are applied to the three rotated components of ground acceleration; the incident wavefield is first decomposed by frequency-wavenumber spectral analysis into either broadband plane waves or broadband spherical waves when the source of the waves is within the array. Each spectral peak may be interpreted as a body, surface, or scattered wave propagating across the sub-array with its own apparent horizontal velocity and direction. Secondly, polarization analysis estimates the broadband or narrowband polarization of the ground acceleration wavefield and provides a lower bound estimate of the coherence ("incoherency") of the wavefield. A frequency-wave number spectral analysis is performed which demonstrates that the mixture of wave types that propagated across a 2-km radius sub-array was different for the two earthquakes. Results are described in detail.

Available online: http://nisee.berkeley.edu/documents/EERC/EERC-88-08.pdf (10 MB)