Measurements of ambient seabed seismic levels below 1.0 Hz on the shallow eastern U.S. continental shelf

Abstract

Measurements of ambient seismic noise levels in the range 0.03–1.0 Hz were made using ocean-bottom seismometers (OBS) at four shallow-water (<100 m) locations on the New Jersey Shelf and George’s Bank. Surface gravity-wave-induced seabed motion (single-frequency microseism) was found to be dominant in the frequency range 0.03–0.3 Hz, with the high-frequency cutoff strongly dependent on water depth. The peak seismic level in the water wave band was measured at 2.0×10−8 (m/s2)2/Hz in 12 m of water. This level was observed to decrease rapidly with greater water depth. Seismic interface waves (microseisms) of power level approximately 5×10−10 (m/s2)2/Hz were observed in the range 0.25–1.0 Hz. This microseism power level was found to be roughly constant in water depths from 12 to 70 m. A quiet ‘‘notch’’ between the two wave bands, in the range 0.15–0.3 Hz, was observed. The background seismic level in this notch was determined to be less than 5×10−12 (m/s2)2/Hz. Extrapolations of the observed pressures and seabed motions into deeper water conditions are made.

On the microseisms associated with coastal sea waves

“…Furthermore, Fig. 3(d) gives results that are comparable with those from more recent analyses (Trevorrow et al 1989), thus suggesting that the phenomenon of wave reflection along coastlines contributes significantly to the spectral distortions observed in the intermediate frequency range of the spectrum. However, it should be noted that the data and subsequent calculations in Fig.…”

“…A relationship between the phenomena of wave reflection along the shoreline and microseisms is suggested by this study. In addition, the far‐field microseismic energy computed from this model is considerably larger than our two previous calculations (Darbyshire & Okeke 1969; Okeke 1972) and, interestingly, closer to that obtained from recent measurements (Trevorrow et al . 1989).…”

“…3 were based on far‐field microseismic events. Those of Trevorrow et al (1989) were from seafloor microseismic records. Although fairly similar, the two studies were based on two different Earth structures.…”

“…Fig. 1 depicts the form of the vertical structures of the elastic medium to a depth of about 100 m below the Earth’s surface in the locality (Trevorrow et al 1989).…”

Bottom scour observed under Hurricane Ivan

“…Surface wave‐induced seabed motion is common in the single‐frequency range of .05–.5 Hz (periods of 2–20 s), and is several orders of magnitude greater than the motion induced by seismic energy in the double‐frequency band [ Trevorrow et al , 1989]. A typical displacement caused by pressure variations under surface waves for the coastal seabed is on the order of 0.1 mm.…”