Oceanic Path Effects of Microseismic Waves

Abstract:

Microseismic surface waves originating from sources in ocean would propagate along parts of oceanic path before being recorded by on-land seismic stations. Studying the path effects on waveform, travel-time, magnitude and other properties of these microseismic signals is important in accurately determining the location, strength and generating mechanism of the sources. Strong effects are observed in the microseismic signals generated by Hurricane Sandy in 2012, and verified by synthetic seismograms. We find that Sandy-related seismic signals are significantly affected by oceanic path: only seismic signals share a similar length of oceanic path are cross-correlated and a large portion of the correlated signals can be traced back to sources at the ocean-continent boundary within a narrow azimuthal range from the hurricane center. In this presentation, we report that these observations can be explained by strong path effect of wave propagation from a seismic source in the hurricane center. The strong directionality of waveform cross-correlation can be explained by the propagation effect that waveform characteristics of Rayleigh wave are mostly controlled by transitional propagating path from ocean to the continental region, resulting in seismic signals being correlated only among stations sharing similar length of oceanic path; the sources at the ocean-continent boundary can be attributed to strong seismic scattering in the ocean-continent boundary, generating apparent seismic “sources” there. We also compare the synthetic vertical/transverse magnitude ratio of Rayleigh waves in an anisotropic velocity model with observations. Our results indicate that these types of seismic observations would be particularly useful for studying seismic structure of crust and upper mantle in the ocean-continent area.