Broadening of S Wave Seismogram Envelopes

Abstract

Long duration or spindle-like shape of S wave seismogram is well known phenomena in the case of long travel distances. This is characterized by the broadening of seismogram envelope. The broadening has been studied based on the scattering in random media. The diffraction and multiple forward scattering is modeled using the parabolic approximation for the wave propagation through the randomly inhomogeneous structure whose correlation distance is much longer than the seismic wavelength. The hypocentral distance dependence and frequency dependence of the envelope broadening are affected by the type of the auto-correlation function, correlation distance, and velocity fluctuation of the random medium. Recently, the model of envelope broadening is formulated by using spherically outgoing scalar-wave instead of plane wave. During the last decade, many observations for envelope broadening are carried out in order to investigate the random inhomogeneity. The envelope broadening characterized by a frequency dependence have sometimes recognized as a dispersion. At some subduction zones, the dispersion has been observed and studied based on the deterministic approach. It must be important to combine the stochastic approach with the deterministic approach for the analysis of dispersion phenomena.