HR: 16:55h
AN: S12F-04    [PDF]
TI: Jet Stream, Roaring Ocean Waves, and Ringing Earth
AU: * Tanimoto, T
EM: toshiro@geol.ucsb.edu
AF: Institute for Crustal Studies, University of California, Santa Barbara, CA 93106 United States
AU: ReVelle, D
EM: revelle@lanl.gov
AF: Los Alamos National Laboratory, EES-8, LANL MS-J577, Los Alamos, NM 87545 United States
AB: During the last few decades, it has become clear that Earth's components, i.e. the atmosphere, the ocean and the solid Earth interact in complex ways on various time scales. Seismograms have been known to show such interactions but one of the surprising observations in the last 5-6 years is the discovery of continuous oscillations for frequencies 2-7 mHz; a sequence of fundamental spheroidal modes are excited continuously and display seasonal variations. The cause of these oscillations has been speculated to be an atmosphere-solid Earth interaction until recently (e.g., Kobayashi and Nishida, 1998; Tanimoto and Um, 1999; Fukao et al., 2002). In this talk, we present a case for an alternative mechanism that these oscillaitons are caused by oceanic (infragravity) waves. The original source of energy is in the atmosphere, because ocean waves are generated by atmosphere-ocean inteactions, but the essential point is that this energy must be filtered through an ocean process in order to explain the characteristics in seismic signals. We present two main points that support the oceanic infragravity-wave hypothesis. The first is a theoretical modelling of spectral amplitudes. We will show that, using a typical infragravity wave source, one can explain not only the amplitudes of continuous oscillations but also the broad noise peak that exists at 0.01 Hz. This broad noise peak between 0.003 Hz and 0.015 Hz has been known before continuous oscillations were discovered (e.g. Peterson, 1993). Oceanic infragarvity wave hypothesis can explain not only continuous oscillaiton peaks but also this broad noise peak simultaneously. This is the major difference from previous atmospheric excitation models, because previous atmospheric hypotheses treated the background broad noise peak as unknown (Gaussian noise) and did not explain its origin. Secondly, modal peaks of continuous oscillations display a predominant 6-months periodicity (Tanimoto and Um, 1999; Ekstrom, 2001). This can be easily explained by the oceanic hypothesis due to its hemispheric, mid-latitudinal ocean-wave activity that generate large-amplitude ocean waves with 6-months periodicity. Using a satellite ocean wave data (TOPEX/POSEISDON), we will show that seismic modal amplitudes display similar seasonal variations to ocean wave data both in amplitude and phase. The scenario that emerges from this analysis is the following; the atmospheric winds generate ocean waves that fills the oceans in the world. Among those waves, long period waves (oceanic infragravity waves) perturb pressure at sea bottom and exert pressure on the solid Earth. This pressure fluctuation all over the oceans results in generation of ambient seismic noise for frequencies between 3 and 15 mHz, which is observed in the vertical component seismograms at all quiet broadband stations. Spheroidal modes for this frequency range are excited but clear modal peaks are seen only for 2-7 mHz mainly because of attenuation (which can be easily examined by simple calculations). These activity reaches semi-annual peaks in December-January-February and in June-July-August because of strong atmospehric activities at mid-latitudes in each hemisphere (North and South) that generate high amplitude ocean waves, including infragravity waves.
DE: 7255 Surface waves and free oscillations
DE: 7260 Theory and modeling
SC: Seismology [S]
MN: 2003 Fall Meeting