Physics
Scientific paper
Oct 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011geoji.187..338k&link_type=abstract
Geophysical Journal International, Volume 187, Issue 1, pp. 338-354.
Physics
Time-Series Analysis, Guided Waves, Wave Propagation, Acoustic Properties, Pacific Ocean
Scientific paper
We have investigated the wave propagation characteristics of a seismic T wave on land in northeastern Japan using the seismograms recorded by a dense seismic network. The analysed T wave is a unique one that was generated by a large intermediate-depth earthquake beneath the network that backpropagated from the Emperor Seamount Chain in the northern Pacific. The T wave has a predominant frequency of 1.5 Hz, which is lower than the previously reported values. Spatiotemporal variation of an rms envelope bandpass filtered with a passband of 1-2 Hz demonstrates multiple arrivals of the T wave and inhomogeneous distribution of amplitude. Judging from the spatial pattern of first arrival times and the polarization characteristics, such as the relatively large dip angle and high planarity, we regard the seismic T wave as a Rayleigh wave. A possible conversion area, estimated by a method similar to the source-scanning algorithm, is located to the northeast of Honshu Island and may have a dimension of ˜100 km. The most prominent feature of the spatiotemporal variation of the rms amplitude is the localization of T-wave energy in the northeastern part of Honshu Island. This localization can be attributed either to the close proximity of the conversion area to land or to the large extent of the area. However, these factors are only supplementary because they cannot explain a sharp contrast in amplitude between the two areas with comparable distance but different azimuth from the conversion point. An additional and essential factor in explaining the observation is the non-isotropic radiation from the conversion points. Since the conversion occurred on the continental slope, the maximum amplitude of converted energy is expected to appear either in the propagation direction of incoming acoustic waves or in the steepest direction of the local topography. These factors, in particular a convex shape of bathymetry, probably caused the focusing of converted energy into the northeastern part of Honshu Island.
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