On the Influence of Slopes, Source, Seabed and Water Column Properties on T Waves: Generation at Shore
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Pure and Applied Geophysics
On the Influence of Slopes, Source, Seabed and Water Column Properties on T Waves: Generation at Shore ALEXIS BOTTERO,1
PAUL CRISTINI,1 and DIMITRI KOMATITSCH1
Abstract—The term T waves is generally associated with acoustic waves generated by seismic events that subsequently travel horizontally in the ocean at the speed of sound. In this paper, we use a time-domain spectral-element method to perform a parametric study of the influence of seafloor slope, source position and media properties for a typical (downslope) T-wave generation scenario. We find that the energy and duration of these waves are particularly sensitive to the environment. In particular, the slopes and physical characteristics of the seabed play a crucial role for both the generation and the conversion of these waves. Likewise, the depth and position of the earthquake relative to the slope is of great importance, with the presence of privileged areas for the generation of T waves, which we map. Keywords: T waves, ocean acoustics, parametric study, fullwave numerical simulation, high-performance computing.
1. Introduction When a seismic event occurs near an ocean basin, part of the energy produced can be channeled into the water layer and then travel horizontally in this natural waveguide at the speed of sound in water. In ocean acoustics, T waves have been the subject of much attention since their discovery in the 1940s (Linehan 1940). They typically have a frequency range between 1 and 100 Hz and can be generated from seismic waves in two main ways: by successive reflections between the sea surface and a sloping seabed (downslope conversion), or by diffraction by roughness or by heterogeneities (see Okal 2007 for a review on the topic). In this article we will let aside T waves created by diffraction and focus on T waves generated by downslope conversion. Dimitri Komatitsch: Deceased January 21, 2019. 1
Aix Marseille Univ., CNRS, Centrale Marseille, LMA, Marseille, France. E-mail: [email protected]
Once channeled into the water layer, T waves can travel particularly far for several reasons. First, by propagating in the ocean, they spread in an almost cylindrical fashion, which causes less decay than in the case of spherical geometrical spreading in free space as for P and S waves (see Fox and Dziak 1998). Second, the attenuation of acoustic waves in water is particularly low at the frequencies considered. Third, due to the fact that temperature and pressure vary with depth, the speed of sound waves in the ocean typically presents a minimum around 1000 m in the Atlantic ocean. This feature, known as the Sound Fixing and Ranging (SOFAR) channel, makes it possible, under certain conditions, to facilitate energy transmission. For all these reasons, T waves can therefore propagate over very large distances, in practice only limited by the size of ocean basins (see Okal and Talandier 1997; Metz et al. 2016 for example). Even a moderate seismic event can be detected thousands of kilometers away if it has generated
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