HR: 09:00h
AN: S11D-05    [Abstracts]
TI: Seismological Evidence for Increasing Oceanic Storm Intensity
AU: * McNamara, D E
EM: mcnamara@usgs.gov
AF: USGS NEIC, 1711 Illinois St., Golden, CO 80421, United States
AU: Aster, R
EM: aster@ees.nmt.edu
AF: Dept of Earth & Environmental Science New Mexico Tech., New Mexico Tech, Socorro, NM 87801, United States
AU: Bromirski, P
EM: pbromirski@ucsd.edu
AF: Integrative Oceanography Division, Scripps Institution of Oceanography, University of California, San Diego, CA 92093, United States
AU: Hutt, C
EM: chutt@usgs.gov
AF: USGS ASL, 10002 Isleta Rd., Albuquerque, NM 87117, United States
AU: Gee, L
EM: lgee@usgs.gov
AF: USGS ASL, 10002 Isleta Rd., Albuquerque, NM 87117, United States
AB: Several major tropical cylones during the 2007 hurricane season have generated wave-induced seismic signals detectable by seismic instrumentation in the Global Seismographic Network (GSN) and Advanced National Seismic System (ANSS) (ex. Flossy, Dean, Felix, Henrietta). From these storms, seismic background energy "noise" between 6 and 25 s period is dominated by a persistent "microseism" arising from energy transferred from ocean gravity waves to elastic Rayleigh waves. Microseism power spectral density (PSD) is dominated by a primary peak (10-20s) that is generated by waves breaking on coastlines and by a (much stronger) secondary peak centered near (5-10s) that is generated by the half-period periodic variation of sea bottom pressure due to standing wave components generated from wave-wave interaction of the ocean gravity wave field. The microseism peaks can vary in amplitude by several orders of magnitude due to station proximity to coastlines and wave amplitudes, which have a strong seasonal dependence. Such observations demonstrate the utility of microseisms as an integrative proxy for assessing long-term and regional scale sea swell changes induced by changes in global storm activity. We examine changes in the microseism amplitude and use it as a proxy for decadal-scale changes in storm-wave amplitude, a topic of considerable interest in the debate about the impact of global climate change on oceanic storm frequency and intensity. High-quality continuous digital records from the GSN and its precursor networks now extend back over 30 years at the longest-operational sites. In this abstract, we demonstrate the development of an oceanic storm trigger algorithm by observing the current storm season using data from the ANSS and GSN and then apply the resulting methods to an investigation of oceanic wave climate changes over three decades. Limited spatial distribution and length of long-term seismic observational records causes some ambiguity for climate change detection and analysis. Inferior instrumentation, inconsistent techniques, and incomplete data can lead to artificial trends and, consequently, to incorrect interpretations. An advantage of our technique is that it is independent of seismic data age, quality and completeness. We measure the change in microseism intensities over time by computing a measure called the microseism index (Grevemeyer et al., 2000) at 14 long operating GSN and precursor stations. The oldest station in our study, HG.ALQ, began operation in 1972 and is a precursor to IU.ANMO in Albuquerque, NM. For the period 1972-2007 we detect clear microseismic signal and demonstrate an increasing trend through time. The positive trend in microseism levels suggests that oceanic wave maximum heights have increased, and correlates with other metrics showing increasing surface sea and air temperatures and oceanic storminess.
DE: 7255 Surface waves and free oscillations
DE: 7299 General or miscellaneous
SC: Seismology [S]
MN: 2007 Fall Meeting