[1] Seismic noise is an indirect source of information on ocean waves. Using a model of noise generation and propagation, seismic stations can be separated into those that are mostly sensitive to local sea states, and those that integrate sources from a large oceanic area. The model also provides a classification of noise-generating sea states into three classes. The analysis of Central California seismic noise data, well correlated with local waves, reveals that class I events dominate in summer, caused by a single wind-sea system, and for which ocean wave spectral levels are proportional to seismic spectral levels to an exponent b ≃ 0.9. In winter, noise is dominated by class II generation, for which coastal reflection is important, with a wave spectral density roughly proportional to the seismic spectral density to an exponent b ≃ 0.7. Sporadic events of class III probably produce some of the strongest noise events in Central California and need to be properly screened. These events are caused by opposed wave systems that are usually the wind-sea and a swell. This noise classification can be used to improve on the correlation between measured and estimated wave heights (up to r = 0.93 for daily averages). For other locations, where remote oceanic sources are recorded, a significant wave height estimated from the seismic noise compares well with area-averaged satellite data or wave model results (r > 0.85 for daily averages). These analyses pave the way for quantitative uses of seismic records, including the reconstruction of past wave climates, and the calibration of wave hindcasts.Citation: Ardhuin, F., A. Balanche, E. Stutzmann, and M. Obrebski (2012), From seismic noise to ocean wave parameters: General methods and validation,
Swells radiating across ocean basins are fingerprints of the large ocean storms that generated them, which are otherwise poorly observed. Here we analyze the signature of one swell event in the seismic noise recorded all around the Pacific and we show that it is a natural complement to the global coverage provided by the Synthetic Aperture Radar wave mode data from ENVISAT. In particular the seismic stations are much more sensitive to low frequency and amplitude signals than buoys and SAR, capturing swell forerunners a couple of days before they can be detected from space or in situ data. This information helps detect in the SAR measurements the presence of very long swell, with periods of 22 s in our case example, that were otherwise excluded.
S U M M A R YFor more than 15 yr, the recording of hydroacoustic signals with hydrophones moored in a minimum sound-velocity channel, called the SOFAR (SOund Fixing And Ranging) channel, has allowed for detection and localization of many small-magnitude earthquakes in oceanic areas. However, the interpretation of these hydroacoustic signals fails to provide direct information on the magnitudes, focal mechanisms, or focal depths of the causative earthquakes. These limitations result, in part, from an incomplete understanding of the physics of the conversion, across the seafloor interface, from seismic waves generated by subseafloor earthquakes to hydroacoustic T waves. To try and overcome some of these limitations, we have developed a 2-D finite-element mechanical model of the conversion process. By computing an exact solution of the velocity field of the waterborne T waves, our model shows that a double-couple source mechanism of a subseafloor earthquake generates T waves, whose take-off angles are adequate to allow penetration into the SOFAR channel and efficient trapping by this waveguide. Furthermore, our model confirms that a double-couple source with a high S-wave content produces higher-amplitude T waves than a simple explosive source, which only generates P waves.
RésuméEn zone de levée, l'énergie contenue dans la bande infragravitaire (fréquences typiquement inférieures à 0.05 Hz), est généralement négligeable par rapport à celle contenue dans les vagues (< 1%). Cette tendance s'inverse fortement en zone de surf où l'énergie infragravitaire peut devenir plus importante que celle contenue dans les vagues. Dans ce papier, nous montrerons, à partir de données in situ, que les processus physiques qui contrôlent l'énergie infragravitaire en zone de surf sont très différents de ceux des vagues et de ce fait que la structure spatio temporelle de cette bande se différencie de celle observée habituellement dans les vagues. AbstractInfragavity energy (typically frequencies less than 0.05Hz) is generally negligible in the shoaling zone compared to swell energy (<1%). Nevertheless, this trend drastically changes in the surf zone where infragavity energy can become more important than swell energy. In this paper, based on in situ data, we will focus on the differences, which can be observed between swell energy and infragravity energy. We will show that the physical processes, which rule these two energy bands, are deeply different. This induces deep differences between the spatial and temporal variability of energy contained in each frequency band.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.