Microseisms: Coastal and pelagic sources

Abstract

Wave‐number frequency spectra of seismic background recordings from the large aperture seismic array (LASA) in eastern Montana have been used to study the source locations of different wave types in the frequency band from 40 to 500 mHz. Microseisms in this band consist of surface waves of the Rayleigh and Love type and compressional body, waves. The peak power band near 140 mHz (7‐sec microseisms) and the lower frequency band near 70 mHz consist of fundamental Rayleigh waves, which often come from the same direction. This is especially true for directions from coasts in the vicinity of large storms. The average directional properties of the two bands are similar, indicating coastal sources for both. Love waves and higher mode Rayleigh waves in some instances come from the same coastal directions as the fundamental mode. Compressional body wave sources, pinpointed by using horizontal phase velocity to measure range, occur near storms both in coastal and pelagic regions. Pelagic storm sources were found only at frequencies that were high compared with double the frequency of ocean waves having a group velocity equal to the storm velocity. Located in the wake of a moving storm, such sources appear to be due to the oppositely traveling waves set up when a storm moves faster than its waves.