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Published December 14, 1993 | Published
Journal Article Open

An analysis of nearfield normal mode amplitude anomalies of the Landers Earthquake


The 1992 Landers earthquake (M_w=7.3) occurred in the middle of the TERRAscope network. Long-period Rayleigh waves recorded at the TERRAscope stations (Δ≤3°) after traveling around the Earth show large amplitude anomalies, one order of magnitude larger than spherical Earth predictions up to a period of about 600 s. The ground motions over the epicentral region at and after the arrival of R4–5 are in phase at all stations. These observations are inconsistent with the nearly vertical strike slip mechanism of the Landers earthquake. Synthetic seismograms for a rotating, elliptic and laterally heterogeneous Earth model calculated by the variational method agree well with the observed waveforms. Calculations for various 3D Earth models demonstrate that the amplitudes are very sensitive to the large scale aspherical structure in the crust and the mantle. The anomalies for modes shorter than 300 s period can be explained by lateral heterogeneity shallower than the upper mantle. Rotation of the Earth and lower mantle heterogeneity are required to explain mode amplitudes at longer periods. Current whole mantle seismic tomographic models can fully explain the observed amplitudes longer than 300 s. To assess the effect of the high order lateral heterogeneity in the mantle more precise estimate of the crustal correction is required.

Additional Information

© 1993 by the American Geophysical Union. Received July 7, 1993; revised October 12, 1993; accepted October 13, 1993. We thank T. Tanimoto and S. Tsuboi for useful discussions. We also thank W.-j. Su and R. L. Woodward for providing us with their whole mantle seismic models, A. Dziewonski for bringing the importance of the crustal correction to our attention, and an anonymous reviewer for valuable comments. A part of computations was supported by the JPL/Caltech supercomputing project. This research was partially supported by the U. S. Geological Survey Grant 1434-93-G-2287. This research was conducted under the TERRAscope project supported by the L. K. Whittier and Arco Foundations. Contribution No. S29S, Division of Geological and Planetary Sciences, California Institute of Technology.

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