Thunder-induced ground motions:1. Observations

摘要

Acoustic pressure from thunder and its induced ground motions were investigatedusing a small array consisting of five three-component short-period surface seismometers,a three-component borehole seismometer, and five infrasound microphones. We used thearray to constrain wave parameters of the incident acoustic and seismic waves. Theincident slowness differences between acoustic pressure and ground motions suggest thatground reverberations were first initiated somewhat away from the array. Using slownessinferred from ground motions is preferable to obtain the seismic source parameters. Wepropose a source equalization procedure for acoustic/seismic deconvolution to generatethe time domain transfer function, a procedure similar to that of obtaining teleseismicearthquake receiver functions. The time domain transfer function removes the incidentpressure time history from the seismogram. An additional vertical-to-radial ground motiontransfer function was used to identify the Rayleigh wave propagation mode of inducedseismic waves complementing that found using the particle motions and amplitudevariations in the borehole. The initial motions obtained by the time domain transferfunctions suggest a low Poisson's ratio for the near-surface layer. The acoustic-to-seismictransfer functions show a consistent reverberation series at frequencies near 5 Hz. Thisgives an empirical measure of site resonance that depends on the ratio of the layer velocityto layer thickness for earthquake P and S waves. The time domain transfer functionapproach by transferring a spectral division into the time domain provides an alternativemethod for studying acoustic-to-seismic coupling.