Directional spectra observations of seafloor microseisms from an ocean-bottom seismometer array

Abstract

Observations of the directional spectra of seabed motion in shallow water were conducted off the New Jersey coast during the summer of 1987. Using a six-point ocean-bottom seismometer array, each instrument supporting a pressure transducer, and two horizontal and vertical accelerometers, measurements of gravity and seismic waves across the ULF/VLF band were collected in 12.5 m of water. Array dimensions were tuned particularly for directional spectra observations of short-period seafloor microseisms. Directional spectra analysis indicates that in the short-period microseismic band, 1.5–2.5 s, motion of the seafloor is primarily a result of slow seismic waves traveling at apparent velocities near 200 m/s. These propagation velocities for the microseismic band in shallow water are an order of magnitude less than microseismic velocities from similar studies on land. Contemporaneous measurements of the directional spectra of long-period ocean gravity waves, 15–85 s, show an eastern direction of origin; short-period ocean gravity waves, 5–9 s, measured using particle motion analysis, are from the south. The direction of propagation of the microseisms, found from the maximum response of the array, is shown to be approximately N150E—a direction midway between long- and short-period ocean-wave propagation directions. Correlation of particle motion and directional spectra analysis indicates that microseisms have retrograde motion. These results suggest that the microseisms are most likely Scholte interface waves.