Horizontal Motions of Water in the Vicinity of a Tsunami Source
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Pure and Applied Geophysics
Horizontal Motions of Water in the Vicinity of a Tsunami Source MIKHAIL A. NOSOV,1 ANASTASIA V. MOSHENCEVA,1 and SERGEY V. KOLESOV1 Abstract—Because it is based on an initial seismic analysis and preset criteria, tsunami forecast often fails in assessment of tsunami danger. The level of danger can be determined more or less reliably only when observed sea level data became available. Along with the sea level data, i.e., vertical motions of free water surface, we suggest considering horizontal motions of water that accompany the formation and the propagation of a tsunami. The amplitude of horizontal motions is normally much higher than the amplitude of the vertical motions. Detection of the horizontal motions may provide tsunami warning centers with additional in situ data that can be used for estimation of tsunami strength. In this study, taking the 2011 Tohoku-Oki event as an example, horizontal motions of water in the vicinity of the tsunami source are theoretically examined by means of dynamic and static numerical models developed within the framework of linear shallow-water approximation. It is shown that in the vicinity of the tsunami source within a wide area of about 0.5 million square kilometers, the amplitude of horizontal motions exceeded 10 m, whereas in some shallowwater areas the amplitude amounted to hundreds of meters. Possible methods of in situ detection of the horizontal motions are discussed. Key words: Tsunami source, coseismic bottom deformation, water displacement, initial elevation, shallow-water approximation, finite difference method, finite element method.
1. Introduction Devastating tsunami waves usually originate from strong bottom earthquakes (e.g., GUSIAKOV 2011). The initial response of a tsunami warning center must be issued very promptly, preferably within a few minutes after the earthquake. The first tsunami bulletin is always issued on the base of an initial seismic analysis and preset criteria (Operational Users Guide 2009). By this time, neither the tsunami strength nor even the earthquake parameters can be determined
1 Faculty of Physics, M.V. Lomonosov Moscow State University, Leninskie Gory, Moscow 119991, Russia. E-mail: [email protected]
with a reasonable accuracy. This is why the initial estimation of the level of tsunami danger often fails. Only when observed sea level data became available the level of danger can be determined more or less reliably. In this study, we suggest considering additional data, namely the horizontal motions of water that accompany tsunami evolution. Along with the vertical motions of the free water surface (sea level variations) the horizontal motions of the water column can be used, at least in principle, in tsunami warning systems. We recall that in propagating tsunami waves that can be considered as long oceanic waves, the amplitude of horizontal motions essentially exceeds the amplitude of vertical motions of the free water surface (e.g., LIGHTHILL 1978). This important feature of long waves should certainly pr
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