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Chapter XXX the Generation of Tsunamis k Trim Size: 216mm x 279mm EMOE emoe523.tex V1 - 03/29/2016 4:33 P.M. Page 1 Chapter XXX The Generation of Tsunamis David R. Tappin British Geological Survey, Environmental Science Centre, Keyworth, UK in March 2011, which killed 19,000 people. These suggest 1 Introduction 1 that major tsunamis are now more common than before, but 2 Perspective 1 over the longer timescales this is not supported by statistical 3 What are Tsunamis? 1 evidence (Ben-Naim, Daub, and Johnson, 2013). Tsunami is a Japanese word (in kanji = , which translates into 4 Earthquake Tsunamis 3 harbor (‘tsu’) and wave (‘nami’)). It derives from the first 5 Landslide Tsunamis 5 indication of a tsunami wave approaching the coast some- 6 Volcanic Tsunamis 7 times being a “whirlpool” effect within a harbor. 7 Meteotsunamis 8 k 8 Extraterrestrial (Bolide) Impacts 9 k Acknowledgment 9 2 PERSPECTIVE Glossary 9 Here, we consider tsunamis from their different generating References 9 mechanisms, with “generation” including all processes from initial wave generation through propagation to final onland run-up. Since 1998, when a submarine landslide was proven to have generated a tsunami 15 m high that killed 2200 1 INTRODUCTION people (Tappin, Watts, and Grilli, 2008), there has been an increased realization that not only earthquakes generate Tsunamis are gravity-driven water waves, generated at the destructive tsunamis. In addition, there are also different water/air interface from a vertical perturbation of the water earthquake rupture mechanisms that generate different column, usually from seabed movement. They are gener- magnitude tsunamis. Recent events have resulted in major ated mainly along convergent margins and predominantly advances in understanding of how tsunamis are generated. from undersea earthquakes. Build-up of stress along these margins, generated by outward “push” from rock intruded into the crust along oceanic ridges and gravitational “pull” along convergent margins from oceanic crust descending 3 WHAT ARE TSUNAMIS? into the mantle, results in earthquake (seismic) rupture. The Tsunamis are quite different from wind waves, generated rupture uplifts the seabed and the overlying water column, by the frictional effect of wind acting on the sea surface, thereby generating a tsunami. A number of recent catas- because they are caused by vertical seabed movement. The trophic earthquake tsunamis include the Indian Ocean event movement results in a wave at the sea surface that, by in 2004, where over 230,000 people died, and Japan event gravitational attraction, collapses and travels (propagates) Encyclopedia of Maritime and Offshore Engineering, print © 2017 John Wiley & Sons, Ltd. outward (Figure 1). Apart from earthquakes other, “bottom Edited by John Carlton, Yoo Sang Choo, and Paul Jukes. up,” tsunami generation mechanisms include submarine This article is © 2017 John Wiley & Sons, Ltd. ISBN: 978-1-118-47635-2 Also published in the Encyclopedia of Maritime and Offshore Engineering (online edition) and subaerial landslides and volcanic eruptions, including DOI: 10.1002/9781118476406.emoe523 caldera collapse and pyroclastic flows entering the sea. k k Trim Size: 216mm x 279mm EMOE emoe523.tex V1 - 03/29/2016 4:33 P.M. Page 2 2 Offshore Shallow water Water column pushed up Seafloor Motion of fault block Figure 1. Tsunami generation, propagation, and run-up from a hypothetical earthquake generated event. Vertical elevation of the seabed travels up through the water column and creates a surface wave that travels out with low elevation that increases as the wave approaches the shore. 44°N +8 m The initial elevation wave of the tsunami is dependent on 1 the source mechanism. Large earthquakes generate tsunamis 2 with initial wave elevations of 10–20 m along hundreds of +4 kilometers of seabed—for example, the Indian Ocean rupture 3 was 1300 km long. Landslides have the potential to generate 40°N 4 initial wave elevations of hundreds and potentially thou- 5 0 6 sands of meters, but the area of wave elevation is much 7 21418 8 smaller. With landslides, the maximum initial wave eleva- k −4 tion is limited only by the ocean depth. A further difference k 9 between tsunamis from earthquakes and landslides is that 36°N at source the initial wave at the former is linear whereas at −8 the latter the wave is circular as the landslide acts as a point ° ° ° 142 E 146 E 150 E source (Figure 2). After initiation, the tsunami wave elevation from all Figure 2. Comparison of tsunami wave surface elevations from the earthquake and submarine landslide generated 15 min after the mechanisms decreases as it travels outward from the source. Japan, 2011 earthquake. The earthquake tsunami (to the south) is Tsunamis are shallow-water waves, defined as having almost linear compared to the circular wave train from the subma- extremely long wavelengths (of hundreds of kilometers) rine landslide (to the north). Note the highly dispersive nature of in comparison to oceanic water depths of kilometers.√ waves generated by the landslide source, as compared to the longer Shallow-water waves travel at velocities given by c = gh, wavelength, long-crested, non-dispersive, earthquake-generated 2 tsunami waves. Labeled black dots mark the locations of offshore where c is the celerity, g the gravity (=9.8 m/s ), and h tsunami buoys. (Reproduced from Tappin et al. (2014). © Elsevier, the water depth. Thus, tsunamis travel at very high veloc- 2014.) ities (160–250 m/s to 600–900 km/h) and have very long wavelengths (hundreds of kilometers) (Figure 3b, lower). Tsunamis are also generated from bolide impacts and rapid These velocities compare to wind waves of about 15 m/s pressure changes that accompany storms—termed meteot- (50 km/h). The deeper the water is, the faster the tsunami sunamis, but these are “top-down” mechanisms, with the travels (Figure 3a). In the deep ocean, a tsunami wave may tsunamis generated at the sea surface. be insignificant (less than a meter in elevation) and hardly There are three phases of a tsunami: (i) initial wave gener- noticeable, unlike storm waves that may be terrifically ation, (ii) propagation (travel) across the ocean and, (iii) destructive, for example, freak waves, tens of meters high. finally, onland incursion or run-up (wave elevation atthe The controls of water depth and seabed morphology coast) when the tsunami strikes and then flows across land on velocity thus determine tsunami direction of travel (Figure 1). When the source mechanism is large enough, the (Figure 4). A notable aspect of tsunami travel is that the incursion of the tsunami onto the land can be extensive (kilo- energy is within the whole water column, which results meter) and enormously destructive (e.g., the Indian Ocean (from the combination with seabed morphology/water tsunami of 2004 and Japan tsunami in 2011). depth) in the control on velocity from seabed friction. k k Trim Size: 216mm x 279mm EMOE emoe523.tex V1 - 03/29/2016 4:33 P.M. Page 3 The Generation of Tsunamis 3 1/4 (hs/hd) = {Hd/HS} where hs and hd are wave heights in 250 900 shallow and deep water and Hs and Hd are the depths of h = 6 km the shallow and deep water. A tsunami with a height of 1 m /s) 200 720 m h = 4 km in the open ocean where the water depth is 4000 m would Ocean depth Phase 150 velocity 540 have an increased wave height of 4–5 m in a water depth of h = 2 km km/h 10 m. Offshore seabed morphology has a significant control 100 360 h = 1 km in focusing and defocusing the tsunami, which may appear 50 h = 500 m 180 Wave velocity ( as a series of breaking waves. 25 90 h = 100 m Group 10 36 During onland incursion, tsunamis slow further due velocity to (i) wave energy being reflected offshore and (ii) 1000 shoreward-propagating wave energy being dissipated h = 6 km ) through bottom friction and turbulence (Figure 5). Tsunamis, m 100 T gh however, still retain tremendous amounts of energy and can 10 h = 100 m Beach waves be very destructive. They have great erosive power, stripping 1 beaches of sand, undermining trees and other coastal vege- T 2g tation, and crushing homes and other coastal infrastructure. Wavelength (k 0.1 Tsunami 2π window They can inundate and flood kilometers inland. Tsunamis 0.01 may reach major elevations (termed run-up height) above 10000 1000 100 10 1 sea level of tens of meters. Depending on whether the first Wave period (s) part of a tsunami to reach the shore is a crest or a trough, the tsunami may appear as a rising or falling tide. Figure 3. Relationship between tsunami wave period and wave velocity (a) and wavelength (b). Phase velocity c() (solid lines) and group velocity u() (dashed lines) of tsunami waves on a flat earth covered by oceans of 100 m to 6 km depth. Wavelength asso- 4 EARTHQUAKE TSUNAMIS k ciated with each wave period. The “tsunami window” is marked. k (Reproduced from Ward (2010). © Springer, 2010.) Tsunami magnitude generally scales to earthquake magni- tude, except for “tsunami earthquakes,” where a tsunami is much larger than earthquake magnitude would predict The rate at which a wave loses its energy is inversely (Kanamori, 1972). Satake and Tanioka (1999) identify three proportional to its wavelength, thus tsunamis not only prop- types of earthquakes: (i) at the plate interface (interplate), agate at high speeds but also travel great, transoceanic, (ii) earthquakes at the outer rise, within the subducting slab distances with limited energy loss. Tsunami travel distance or overlying crust (intraplate events), and (iii) tsunami earth- (as with initial wave height) is again dependent on the source quakes (Figure 6).
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