Subsections

Analysis of Long-Period Noise at the Farallon Islands Broadband Seismic Station FARB

David Dolenc (U of Minnesota), Robert Uhrhammer, and Barbara Romanowicz

Introduction

We present a preliminary analysis of the long-period noise observed at the broadband seismic station FARB (Streckeisen STS-2) located on the South Farallon Island, 43 km off the coast of San Francisco, CA. Results from our previous work (Dolenc et al., 2005) showed that long-period noise (20-500 s) observed at the ocean-bottom broadband station MOBB, located offshore Monterey Bay, CA, is mainly due to seafloor deformation under the pressure forcing by infragravity waves (IG; long-period ocean surface gravity waves). A similar type of long-period noise is observed on vertical and both horizontal components of the Farallon Islands station FARB, but not on the nearby mainland stations.

Results

The long-period noise at FARB is best observed on stormy days when it extends all the way to 1000 s and is even stronger than at MOBB (Figure 2.32). The long-period noise at FARB is stronger on the E-W than on the N-S component, suggesting that it results from the IG waves that propagate from the nearshore region into the deeper ocean.

The comparison of the power spectral density (PSD) at FARB and spectral wave density (SWD) at the nearby buoys for the 2004-2008 period showed that IG waves are generated in the nearshore region close to FARB from the shorter period ocean waves (Dolenc et al., 2008). The energy in the IG wave band at FARB is, as previously observed at MOBB, modulated in phase with tides. The phase of the modulation observed at FARB agrees with the phase of the local tides, suggesting that IG waves observed at FARB are generated locally (Dolenc et al., 2008).

An example of long-period signal observed at FARB and MOBB following the arrival of a dispersed swell on days 2006.037-039 is shown in Figure 2.33. The swell arrived from the NW direction and it can be observed in the 10-30 s period band at FARB and MOBB, as well as the two nearby ocean buoys. The swell signal at the buoys can be seen only when the swell periods fall below $\sim\!\!25$ s, suggesting that buoys can only record ocean waves with shorter periods. The PSD of the swell signal at MOBB ends once the periods fall below $\sim\!\!20$ s. Since MOBB is in 1000 m water depth, the 20 s is the short-period cutoff due to hydrodynamic filtering.

The observations at FARB in the 30-500 s period band show that IG waves are generated following the arrival of the swell. Since the incoming swell is dispersed and has a very narrow period band, it generates IG waves that also appear dispersed. A more detailed analysis reveals that IG waves are generated only once the period of the ocean swell falls below 23 s. The comparison of the incoming swell dispersion and the frequency of the resulting IG waves observed at FARB further shows that nonlinear interaction between a pair of swell components with frequencies f1 and f2 results in an infragravity wave with the difference (f2-f1) frequency.

Strong horizontal noise observed at FARB in the IG wave band suggests that passing IG waves tilt the island. The swaying of islands and underwater mounds driven by tilting due to infragravity waves could contribute to the recently observed horizontal hum of the Earth (Kurrle and Widmer-Schnidrig, 2008).

Conclusions

The IG waves are the main source of the long-period noise at FARB. They are generated locally and result from nonlinear interactions between the ocean waves with periods shorter than 23 s. Strong long-period horizontal noise at FARB is probably due to tilting of the island due to passing of IG waves as they propagate away from the shore. The swaying of the islands due to IG waves could contribute to the horizontal hum of the Earth.

Acknowledgements

The MOBB observatory instrumentation and deployment were supported by funds to the Monterey Bay Aquarium Research Institute from the Lucile and David Packard Foundation, the NSF (grant OCE-9911392), and UC Berkeley funds to the BSL.

References

Dolenc, D., B. Romanowicz, D. Stakes, P. McGill, and D. Neuhauser, Observations of infragravity waves at the Monterey ocean bottom broadband station (MOBB), Geochem. Geophys. Geosys., 6, Q09002, doi:10.1029/2005GC000988, 2005.

Dolenc, D., R. Uhrhammer, and B. Romanowicz, Analysis of long-period noise at the Farallon Islands broadband seismic station FARB, Seism. Res. Lett., 79, 293, 2008.

Kurrle, D. and R. Widmer-Schnidrig, The horizontal hum of the Earth: A global background of spheroidal and toroidal modes, Geophys. Res. Lett., 35, L06304, 2008.

Figure 2.32: Power spectral density (PSD) at FARB, MOBB (Monterey Bay ocean-bottom seismic station), and YBH (560 km north of MOBB, one of the quietest BDSN stations) for the vertical (left) and horizontal (right) component. Results for a stormy day (2005.027) are shown. The USGS high- and low-noise models for land stations are shown as dashed.
\begin{figure*}\begin{center}
\epsfig{file=dolenc08_2_1.eps, width=14cm}\end{center}\end{figure*}

Figure 2.33: PSD for the vertical FARB and MOBB channels as a function of period and time. Bottom panels show spectral wave density (SWD) of the ocean waves calculated at the nearby buoys. The arrival of a dispersed swell on days 037-039 is observed in all 4 panels in the 10-30 s period band. The signal due to infragravity waves generated in the nearshore region following the arrival of a dispersed swell can be seen in the top panel in the 30-500 s period band.
\begin{figure*}\begin{center}
\epsfig{file=dolenc08_2_2.eps, width=17cm}\end{center}\end{figure*}

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