ANOMALOUS AMPLIFICATION OF THE EARTHS NORMAL-MODES NEAR THE EPICENTERDUE TO LATERAL HETEROGENEITY

The broadband seismograms recorded at stations near the epicenter of t he Landers, California earthquake of 28 June 1992 exhibit observed amp litudes of the spheroidal modes of the Earth that are nearly one order of magnitude larger than those calculated for a spherically symmetric Earth model. We show that these anomalous observations at stations cl ose to the epicenter can be explained by coupling between the normal m odes due to the earth's asphericity. We calculate and sum fully couple d normal modes for three-dimensional shear-wave velocity models of the Earth's mantle to calculate synthetic seismograms. Our coupled-mode s ynthetic seismograms explain the observed anomalously large amplitude of spheroidal modes and indicate that the coupling between the normal modes due to laterally heterogeneous elastic structure of the Earth ca uses this anomalous observation. Our results also show that the presen t three-dimensional Earth models give qualitative fits to this observa tion.