Recent results in T-wave seismology

Emile A. Okal
Northwestern University

Thursday, June 18, 1998
12:00 noon—Pacific Forum

T waves, trapped in the ocean's Sound Fixing and Ranging (SOFAR) channel, can be converted from and to seismic waves at a continental or insular shoreline, and hence, they can be both excited by earthquakes and recorded by seismic stations in the vicinity of the coastline. I will present two aspects of my recent research using records of T waves at seismic stations in the Pacific:

Monochromatic T waves from the Southern Ocean: Ringing witnesses to geyser phenomena at the Hollister Ridge

For about two years in the early 1990s, the Polynesian seismic network detected sustained episodes of T-wave activity in the Southern Pacific ocean characterized by an extremely monochromatic spectrum. This led to the discovery of a massive volcanic structure, located on the western flank of the Pacific-Antarctic Ridge, and culminating at only 135 m below sea level. This ridge is believed to be related to the Louisville hotspot, possibly in the framework of the tearing of the plate in the vicinity of the Eltanin Fracture Zone system. The monochromatic waves could have been generated by the resonance of a cloud of bubbles venting during eruptive episodes.

T waves from deep earthquakes and their inference on the state of slabs at subduction zones

Despite its very deep focus (638 km), the great 1994 Bolivian earthquake generated powerful T wavetrains into the Pacific oocean, which were recorded as far as the Bonin islands, 16,000 km from the epicenter. The timing of the T waves across the basin indicates that they were generated mostly at the Arica Bight on the Peru-Chilean coastline, upon arrival of the S waves from the earthquake focus. Because only high frequencies can be converted into T energy, and given the generally high attenuation of S waves in the Earth's upper mantle, this observation needs an exceptionally efficient propagation of the S waves to the ocean, which in turn requires the mechanical continuity of the downgoing slab in the subduction zone, even though a large gap of seismicity is observed between 300 and 570 km. Application of the same technique in other locations shows that most deep earthquakes previously described as "detached" are in fact connected to the surface by continuous slabs.

Next: Calibrate drifting sediment traps