Evolution of Tsunami Science Evolution of Tsunami Science Pavel Tkalich Pavel Tkalich
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Asia Pacific Mathematics Newsletter Evolution of Tsunami Science Evolution of Tsunami Science Pavel Tkalich Pavel Tkalich 1. Introduction tsunamis. The JMA has been further updating the system and now can issue the forecast 2 to 3 minutes after Although tsunamis have been leaving tragic traces in occurrence of an earthquake. Still, the 2011 Tōhoku human history from ancient times, all earlier tsunami earthquake and tsunami, which claimed 20,000 lives in descriptions were based on anecdotal evidence of a few Japan, wiped out several nearshore cities, and critically survivors, embedded in myths, folklore, and art. Thus, the damaged Fukushima Nuclear Power Plant. The overall classical Plato description of death of legendary Atlantis cost could exceed US$300 billion, making it the most nowadays looks like a typical sequence of geological expensive natural disaster on record. events triggered by a destructive earthquake and followed Due to the half-century efforts by PTWC and JMA, by a giant tsunami. More recent artwork by Hokusai most of the tsunami modelling and forecasting (1760–1849) keeps inspiring tsunami scientists and wave capabilities were focused on the Pacific Ocean; in other professionals by the beauty and realism of depicted regions, tsunami science and awareness were not breaking waves. developed. Not surprisingly, the 2004 Indian Ocean In modern history, the Japan Meteorological Agency Tsunami caught off guard the coastal communities along (JMA) initiated tsunami warning services in 1952, and the Indian Ocean shores, killing almost 230,000 people. NOAA’s Pacific Tsunami Warning Centre (PTWC, The recent tragic events drew attention to the lack Honolulu) was established in 1948 following the deadly of tsunami-warning infrastructure, and triggered a 1946 Aleutian Island earthquake and tsunami. Until worldwide movement to develop tsunami modelling and about 1980, semi-empirical charts (connecting tsunami forecasting capabilities. The number of scientists and threats to seismic sources) were the only quick students migrating from different areas into the tsunami forecasting tools available. field has increased significantly, resulting in a re- During the 80’s and 90’s, due to pioneering work of examination of established approaches and perceptions, F. Imamura, N. Shuto, C. E. Synolakis and many othe rs, and in the development of novel ideas and methods. fast computers and efficient models have been employed In Singapore, a similar movement has led to the for tsunami modelling. In the early stages of the development of national earthquake and tsunami computing era, it was not possible to solve the two- predictive capabilities, and of a tsunami-warning system. dimensional Boussinesq equations with nonlinear and This publication highlights some of the most important dispersion terms; instead, simplified alternatives became historical milestones that have led to our modern popular. In 1999, JMA has introduced the computer understanding of tsunami behaviour. aided simulation system for quantitative tsunami forecasting, in which tsunami arrival times and heights are simulated and stored in the database for forecasting 2. Soliton Theory 2.1. The first scientific encounter of solitons One may start the description of tsunami behaviour using soliton theory, which is a simplified substitute for a full- scale tsunami model. In mathematics and physics, a soliton is a self-reinforcing solitary wave (a wave packet or pulse) that maintains its shape while it travels at constant speed. The soliton phenomenon was first described by John Scott Russell [12, 13] (Fig. 2), the British hydraulic engineer Scott Russell who observed a solitary wave in the Union Canal, Edinburgh (UK). He reproduced the phenomenon in a wave tank Fig. 1. “Behind the Great Wave at Kanagawa” by Katsushika (Fig. 3) and named it the “Wave of Translation” [13]. The Hokusai (1760 –1849). discovery is described here in his own words: July 2011, Volume 1 No 3 11 Asia Pacific Mathematics Newsletter Fig. 2. John Scott Russell (1808– 1882). “I was observing the motion of a boat which was rapidly Fig. 3. Russell’ s (1844) wave tank to study solitons. drawn along a narrow channel by a pair of horses, when the boat suddenly stopped—not so the mass of water in the not be explained by the then-existing theories of water channel which it had put in motion; it accumulated round waves. Subsequently, the observations were reinforced by the prow of the vessel in a state of violent agitation, then theoretical work of the French mathematician and suddenly leaving it behind, rolled forward with great physicist J. Boussinesq [3]. velocity, assuming the form of a large solitary elevation, a rounded, smooth and well-defined heap of water, which continued its course along the channel apparently without 2.2. Behaviour of solitons change of form or diminution of speed. I followed it on Soliton propagation could be understood by means of a horseback, and overtook it still rolling on at a rate of some simple convective wave equation eight or nine miles an hour [14 km/h], preserving its original figure some thirty feet [9 m] long and a foot to a ηηtx+=c 0 (1) foot and a half [300−450 mm] in height. Its height where the wave speed c= c(η , xt ,) is a function of gradually diminished, and after a chase of one or two miles surface elevation η , space x, and time t. [2–3 km] I lost it in the windings of the channel. If c = const, this equation has travelling wave solutions, Such, in the month of August 1834, was my first chance and all waves propagate at the same speed c. Particular interview with that singular and beautiful phenomenon interest for the subsequent examples attaches to the initial which I have called the Wave of Translation.” condition illustrated in [4] at t = 0 Following this discovery, Scott Russell built a 9 m η(,0)sech()xx= 2 , (2) wave tank in his back garden and made observations of for which the exact solution of Eq. (1) at time t for the properties of solitary waves, with the following c = const is conclusions [12]: η(x , t )= sech2 ( x − ct ) . (3) • Solitary waves have the shape a sech2 ( k ( x− ct )), where a is the wave height, k is the wave number, and Here sech(x )= 1/ cosh( x ) = 2 / ( eexx + − ). c is the wave speed; If the wave speed is dependent on the wave elevation, • A sufficiently large initial mass of water produces two cc= ()η , initial wave profiles are generally not self- or more independent solitary waves; preserving. The simplest example is given by c = η , • Solitary waves can pass through each other without which being substituted into the linear, non-dispersive change of any kind; wave Eq. (1) yields • A wave of height a and travelling in a channel of η+= ηη 0 . (4) depth h has a velocity given by the expression tx c= ga() + h, where g is the acceleration of gravity, This equation governs a nonlinear wave propagation. implying that a large amplitude solitary wave travels Using the initial wave profile Eq. (2), solutions for η(,)xt faster than one of low amplitude. describe waves such that the profile eventually becomes multi-valued and gradient blowup occurs (Fig. 4a). Throughout his life Russell remained convinced Dispersion behaviour of the waves is described with a that his “Wave of Translation” was of fundamental dispersive wave equation importance, but 19th and early 20th century scientists thought otherwise, partly because his observations could ηηt+= xxx 0. (5) 12 July 2011, Volume 1 No 3 Asia Pacific Mathematics Newsletter This equation has travelling wave solutions 3. Boussinesq-type Equations ∞ To draw a more complete and accurate picture of η(x , t )= a ( k )exp( ik3 t+ ikx ) dk ∫−∞ tsunami behaviour in the ocean, one can use the nonlinear water-wave model involving Laplace’s equation where a(k) is the component amplitude of the Fourier combined with boundary conditions, nonlinear at the transform of the initial profile. If the initial wave profile is free-surface and linear at the sea bottom [5], which can be again in the form of Eq. (2), one can observe that a single rewritten in dimensionless form as propagating wave splits (disperses) from the tail and resulting in oscillatory waves of different frequency that δϕ(xx+ ϕ yy )0 += ϕ zz in fluid (8) continue to propagate at different speed as in Fig. 4b. This ε 22ε 2 behaviour is explicitly embedded in the dispersive wave ϕt+() ϕϕ xy + + ϕηz +=0 at z = εη (9) solution depicting shorter harmonics (with larger k) 2 2δ propagating left relative the peak of the wave. Hence, δη[t+ εϕη ( xx + ϕη yy )] −= ϕ z 0 at z = εη (10) the solutions η(,)xt do not describe localised travelling waves of constant shape and speed. ϕz = 0 at z = −1. (11) Wave propagation exhibits both nonlinear and Here ϕ is the velocity potential, giving fluid velocity dispersive behaviour if described with the Korteweg–de ∂ϕ ∂ϕ ∂ϕ Vries (KdV) equation: components u = , v = , w = and η(x,y,z,t) ∂x ∂y ∂z ηt++= ηη x η xxx 0 . (6) is the free surface. The scale parameters ε = ah/ and 22 The equation is named after Korteweg and de Vries δ = hl/ are introduced to represent nonlinearity and [9], though the equation was in fact first derived by dispersion, respectively. The horizontal length-scale of Boussinesq [3]. This equation has localised travelling the sea bed non-uniformities L is assumed to be much wave solutions (solitary waves) in the form of larger than the wave length l (i.e., γγ≡lL/, << 1), resulting in the sea bed being “mild slope”, and the η(x , t )= 3 c sech2 ( ( x− ct ) / 2) .