Ocean-bottom seismic observatories

Abstract

Observations of seismic motion and other geophysical parameters on the ocean bottom over extended periods of time and over a wide range of frequencies (periods) provide information on the suitability of the ocean bottom as a site for the detection of seismic events. Instrument systems consisting of long- and short-period seismographs, long- and short-period pressure sensors, a temperature sensor, and a water-current sensor were operated on the ocean bottom. Experience and calculation indicate that these systems can be well coupled to the sediment surface, mechanically stable, and relatively insensitive to water currents. Local and distant earthquakes, tides, microseisms (3-8 seconds period), long-period (2-6 minutes period) pressure disturbances, and temperature fluctuations have been well recorded from the ocean bottom at depths greater than 2000 fm. Instrument systems were implanted at two locations: south of Bermuda (2400 fm depth, seismic measurements only) and west of San Francisco (2200 fm depth). Data were telemetered acoustically to a ship (Bermuda) and via deep-sea cable to a shore station (California). Results to date indicate that the ocean bottom can be a favorable location for the detection of seismic events.