Excitation and Propagation of Short-Period Surface Waves in Young Seafloor
Abstract
In seafloor younger than about 20 Ma, the lithosphere is not well-developed and Sn does not propagate efficiently. In young seafloor, however, S waves or normal modes of 3-to-6-s period trapped in the crust are strongly excited by shallow earthquakes and propagate to large distances with little attenuation. Young seafloor typically has few sediments, so there is no low-velocity, low-Q layer at the surface to trap and attenuate these crustal phases. These waves are essentially the equivalent of Lg in continental settings, although because there is no granite in the oceanic crust, they might better be dubbed Lb, with b for basalt. Like T-phases, Sn, and 15-to-20-s period Rayleigh waves, these Lb waves are dispersed and/or scattered arrivals. Unlike these other phases, Lb is close to the microseism peak, yet, because they are so effectively excited by shallow earthquakes and so little attenuated, they stand out above the noise level as one of the most prominent signals on ocean-bottom seismometers (OBS). In the MELT Experiment, 51 ocean-bottom seismometers were deployed for a period of approximately six months, from November 1995 to May, 1996, in two linear arrays extending about 800 km across the East Pacific Rise at 15 to 18?S. Recordings of regional earthquakes on this array shows that short-period Love and Rayleigh waves (~3-6 s) are excited that travel with velocities characteristic of the upper crust (~ 3 km/s). The Love waves are normally dispersed with continuous frequency variation from 3 s to long periods, but Rayleigh waves appear to have a gap between 12-to-15 s fundamental mode waves with much of their energy concentrated in the water column and the 5-to-6 s higher mode waves. These latter waves, the short-period, crustal surface waves, Lb, show very little attenuation. Even at 3200 km away from an earthquake on the northern East Pacific Rise, they still are a prominent phase, despite lying in the peak of the microseismic noise.
Document Details
- Document Type
- Technical Report
- Publication Date
- Sep 01, 2000
- Accession Number
- ADA527179
Entities
People
- Donald W. Forsyth
Organizations
- Brown University