Microseisms from huge Indian Ocean storms in May 2007

Abstract

Ocean waves are acknowledged to be the cause of microseisms ubiquitous in seismograms. The excitation and source locations of microseisms, however, remain enigmatic. In this study, the characteristics of microseisms generated by extraordinary storms in the Indian Ocean in the middle of May, 2007 were investigated. Spectral analysis showed that two huge ocean-swell systems, one that began around May 9 and the other around May 14, generated strong microseisms: single-frequency (SF) microseisms ranging from 0.04–0.06 Hz and double-frequency (DF) microseisms ranging from 0.08–0.12 Hz. The dispersion of ocean waves generated a progressive frequency shift of microseismic peaks. A comparison among microseisms at deep-ocean islands and coastal seismic stations indicated that most of the dispersed DF microseisms followed SF microseisms generated in coastal areas. However, a dispersed DF microseism can also occur with no preceding excitation of an SF microseism; this has been observed at deep-ocean islands and coastal stations simultaneously, and implies that some of the DF microseisms recorded at inland stations may have been generated in the deep ocean. Frequency-wavenumber analysis of the DF band indicated the presence of non-dispersed body-wave microseisms when the swells were in the middle of the ocean. However, microseisms showing a progressive frequency shift of spectral intensity propagated dominantly as surface-waves and were observed when the swells were close to the coast and when a favorable condition for wave-wave interaction reached in the deep ocean.