Seismic, acoustic, and ionospheric wave kinematics associated with moderate to large earthquakes

Abstract

Unusually detailed Doppler radar records of ionospheric disturbances following two moderate‐to‐large dip‐slip earthquakes were reported in 1984. These indicate that earthquakes can perturb the ionosphere by means of acoustic waves launched by the moving ground surface. For one earthquake {magnitude 6.5, Coalinga, CA, 2 May 1983 [J. H. Wolcott et al., J. Geophys. Res. 89, 6835‐6839 (1984)]}, a seismic‐aeroacoustic‐ionospheric kinematic wave analysis was performed to explain radar signature timing. Surface Rayleigh waves were modeled and interpolated from main and after‐shock seismic records, acoustic wave trajectories by acoustic ray tracing in a standard atmosphere were established, and radar detection altitudes were determined from radio rays traced through an ionosphere calculated from an ionogram measured close by. Our calculated results agree well with Doppler signature times observed on several 5‐ and 10‐MHz radar beam paths 160–300 km from the epicenter. Similarities and differences were identified in the other earthquake {magnitude 7.1, Urakawa‐Oki, Japan, 21 March 1982 [T. Tanaka et al., J.Atm. Terr. Phys. 46, 233–245 (1984)]} and its signatures were studied to assess commonalities in perturbation mechanisms. It is tentatively proposed that nonlinear acoustic propagation effects could be significant in shaping observed Doppler signatures. [Work performed under the auspices of USDOE by LLNL under contract W‐7405‐Eng‐48.]