INVESTIGATION OF MICROSEISM SOURCES WITH OCEAN‐BOTTOM SEISMOMETERS

Abstract

Recordings to depths of 5 km have been made on the Pacific Ocean bottom with self‐rising internally recording seismometers. Simultaneous recordings have been made at land stations. The ocean‐bottom noise spectrum is between one and five orders of magnitude higher power than the land spectrum in the region from 0.1 to 9.0 cps. Coherence between two simultaneous instruments separated one‐quarter kilometer is above the 95 percent confidence level from 0.1 cps to 0.6 cps. Attempts to associate narrow‐beam Love and Rayleigh peaks with large storm‐generating areas or with heavy swell striking shore have not produced consistent results. Although some records show the bulk of the microseism peak energy in well‐defined modes, the energy is carried in different modes at different times and locations. Some of our data fit a model of microseism generation in a 100‐mile strip, by a statistical superposition of incident waves and waves reflected from shore; and the subsequent conversion of the energy to Rayleigh and Love modes propagating away from the generation zone. However, the shapes of the mid‐ocean spectra strongly imply additional sources far from shorelines or recognized storms, unless microseisms attenuate far less in the ocean than on land.