Microseisms are dominant seismic noise in the frequency range from
0.1 to 5 Hz and they are generated by pressure changes on the ocean
bottom. Since understanding of the coupling mechanism between the
ocean and sea floor is limited, the reliable simulation of the
microseism wavefield is quite difficult. Fortunately, Dolenc
and Dreger (2005) found that FDP is independent of the level of
microseism excitations. This observation indicates that the H/V
ratio is not dependent on the detailed source process of the
microseisms because taking the H/V ratio inherently cancels the
source effects and enhances the propagation effects. We have
developed a simple method to simulate FDP due to microseisms or any
continuous sources for given 3D velocity models. Our new method is
tested by comparing the synthetic FDPs to observations from SCVSE
(Figure 2.16). The method is very straightforward and consists of 5
steps. First, we compute Green's functions using a 3D
finite-difference code (E3D; Larsen and Schulz, 1995). Here we
assume a shallow (500m depth) vertical CLVD source as the source of
the microseisms in the continental margin, and a simple Gaussian source
time function is applied. Second, the Gaussian source time function
is deconvolved from the synthetics. Third, in order to reduce
grid-dispersion effects (e.g. Levander, 1988) a low pass filter with
a corner frequency of 0.8 Hz is applied to synthetics. Fourth,
continuous monochromatic sine waves for discrete frequencies over
the range from 0.025 to 0.8 Hz, with an interval of 0.025 Hz, are
convolved with the low-pass filtered synthetic waveforms for each
station. Finally, maximum amplitudes of convolved waveforms of three
components for each frequency are taken in the time window after
amplitudes become stable. The final horizontal maximum is determined
by taking the geometric mean of two maximum horizontal amplitudes.
In this study, the definition of the H/V spectral ratio is just the
ratio of maximum vertical and horizontal amplitudes as a function of
frequency. Since synthetic vertical and horizontal amplitudes are
not stably varying with frequency, and sometimes abnormally small
vertical amplitudes cause unrealistic peaks in the H/V ratio, the
moving average over 5 adjacent data points is applied to the
horizontal and vertical amplitudes before taking the H/V ratio.

Figure 2.16:
(a)The range of 3D
models used for synthetic Green's function computation. Small
triangles indicate the stations deployed during Santa Clara Valley
Seismic Experiment (SCVSE). Solid circle represents assumed location
of the microseism source. (b) The background shading indicates the
depth of the Santa Clara Valley obtained from the 3D USGS velocity
model