Locating the Source Regions of the Single and Double-Frequency Microseisms to Investigate the Source Effects on HVSR in Site Effect Analysis

Evaluating the seismic site effect by the ambient noise based horizontal-to-vertical spectral ratio (HVSR) method is strongly affected by the spatial and temporal variations of the ambient noise sources. Therefore, it is necessary to locate the source regions of ambient noise and investigate the relationships between the source energy and HVSR values at the predominant frequency (HVSR f 0 ) of the site. The generation mechanisms of the single- and double-frequency microseisms (SFMs, 0.05–0.085 Hz and DFMs, 0.1–0.5 Hz) in ambient noise are better understood than the noise in other frequency bands and they are dominantly composed of fundamental Rayleigh ( R g) waves. With this advantage, the recordings of SFMs and DFMs at 30 stations in the east coast region of the United States are used to demonstrate a study on locating their source regions with reasonable certainty and constructing the functional relationship between the HVSR f 0 and the source energy of SFMs and DFMs. The recordings are processed in four sub-frequency bands ( F s) of SF and DF bands and a polarization analysis is carried out to select the ellipsoids approximating the particle motions of R g waves. Then the probability density functions of the back azimuths of the ellipsoids’ semi-major axes are computed for each F and station, and are projected on the ocean to determine their possible source regions. These regions are further constrained by (1) the correlation coefficients between the SFMs and the WAVEWATCH III (WWIII) hindcasts of ocean wave spectra in the SF band, or between the DFMs and the modeled DF energy on ocean surface in the selected time windows in the DF band, (2) the energy contribution defined by (i) the average WWIII ocean wave energy and the ocean bottom topographical gradient in the SF band, or (ii) the average modeled DF energy on ocean surface and a frequency and water depth dependent coefficient measuring the conversion efficiency of DF energy from water to solid earth in the DF band, and (3) the percentile retained energy of R g waves in both the SF and DF bands. Results of source regions reveal that (1) the SFMs recorded in eastern US result from the interactions of low frequency (0.05–0.085 Hz) ocean waves with the continental slope and shelf of western North Atlantic Ocean; (2) the source regions for long- (0.1–0.2 Hz) period DFMs are located in the deep ocean close to the continental slope; and (3) the short- (0.2–0.5 Hz) period DFMs are generated in the continental shelf. Finally, the correlation analyses between the simulated source energy and the HVSR f 0 values at the stations whose f 0 s fall in DF band are carried out revealing significant source effect on thick sediments at low frequencies.