151st ASA Meeting, Providence, RI

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When Katrina Hit California

Peter Gerstoft - gerstoft@ucsd.edu
Marine Physical Laboratory
University of California, San Diego

Mike Fehler
Los Alamos National Laboratory

Karim Sabra
University of California, San Diego

Popular version of paper 2aAO6
Presented Tuesday morning, June 6, 2006
151st ASA Meeting, Providence, RI
Scientific version of paper is available here

From half a continent away, we made an unusual seismic observation of a killer hurricane on Aug. 29, 2005 as Katrina bore down on New Orleans and the Gulf Coast. By using an array of 150 seismic stations in Southern California and a signal processing technique called beamforming to identify the seismic signal, we recorded a signal strength 1,000 times greater than that generated by volcanic tremor.

The energy of waves in the deep ocean is usually attenuated well before it reaches bottom. But in the shallow waters of the Mississippi River delta, Katrina’s waves interacted directly with the seafloor. Though the exact mechanisms behind the transfer of energy from ocean waves to seismic waves is not well understood, it appears that the same breaking and shoaling waves that dramatically changed the look of the Mississippi and Louisiana coastlines were strong enough to generate P, or primary waves, through the Earth, in a clear pattern. We observed the P waves that propagated at depths of 1,100 km (700 miles) even before the hurricane made landfall.

Because those body waves travel at between 3,000 and 12,000 meters per second, we could see them reach California about 10 minutes after they struck. We were able to see them using beamforming, a standard approach for comparing and averaging out signals across an array of seismic stations. Beamforming enabled us to locate the origin of the P waves and surface waves being generated by Katrina.

The findings demonstrate that ocean microseisms can be detected at great distances. They also open the possibility of further use of seismic noise for studying the Earth and Earth processes, even at very low signal level. Though hurricane researchers employ a number of tools — from satellites to aircraft to buoys — to model and track hurricanes, seismic instruments could stand to improve our physical understanding of them, especially of the dynamics of the often violent ocean waves they generate.

Figure 1: Schematic of travel paths of observed surface and body waves.

Figure 2: Path from hurricane Katrina to the seismic array in California (triangles). Track of Katrina is shown in red.

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