Waves, weather, and ocean bottom microseisms

Abstract

The latest model of the Lamont Geological Observatory Ocean Bottom Geophysical Station (OBS III), located approximately 200 km WNW of San Francisco at a depth of 3.9 km, has been in operation since May 1966. In addition to long- and short-period seismic data, this station provides data on water current direction and speed, water temperature, long- and short-period pressure variations, and horizontal and vertical accelerations at tidal periods. Twenty-three microseismic storms have been detected on the seismic and pressure sensors during the first eight months of operation (May-December 1966). The characteristics of two of these storms have been studied in detail, and the results have been compared with data from the Berkeley seismographs. Both peaks in microseismic activity appear to be related to specific weather systems, one local and one distant. Correlation between local water waves and microseismic amplitudes is observed in one case but not in the other. Particle motion amplitudes on the ocean bottom were 3 to 5 times larger than those at a near-land station (Berkeley) for the vertical component and 7 to 10 times larger for the horizontal component. Predominant periods (6 to 8 sec) were the same at both sites. Ratios of horizontal to vertical particle motion and pressure to vertical motion were measured for microseisms and compared with theoretical values for Rayleigh waves. The combination of these two ratios proves to be a very sensitive method of determining sediment properties. Coherence between pressure and vertical motion is consistently high. It is concluded that (1) the observed microseisms propagate primarily as Rayleigh waves of the fundamental mode, and (2) the thickness of the sediment layer is 0.65 km. The predominant direction of microseismic propagation during both storms was approximately perpendicular to the coastline. Study of the average phase relationship of pressure, vertical particle motion, and horizontal particle motion shows that the direction of propagation is from sea to land if fundamental mode Rayleigh waves are assumed. The microseismic energy flux was approximately 4 times larger in the oceanic structure than in the continental structure near Berkeley for both storms.