Tsunami

Physical Dynamics

A tsunami is a series of waves in water (not a single wave) with very long wavelengths (or distances between pairs of wave crests or between pairs of wave troughs). Wavelengths range from 10 to 500 km. Wave periods, or times between one wave and the next, are long, up to about an hour.

• Like all water waves, the water molecules move in an orbital motion and, the longer the wavelength, the larger the radius of this orbit.

• This radius decreases rapidly with depth, dwindling to just 1/23 of the surface value at a depth one half the wavelength. Think of a vertical column of orbiting water molecules: This column of disturbance thins exponentially to nearly nothing by half the wavelength downward.

• Because tsunami have such long wavelenths, they are "shallow-water" waves, because, even in the deepest oceans, the disturbance reaches all the way down to the ocean floor: The wave "feels" the ocean bottom. The language can be a little confusing, especially since wave height approaching shore is related to the contrast between "deep" water out at sea and "shallow" water close in to shore. In this context, "shallow-waves" mean that the bottom of the column of disturbed water reaches the sea floor.

• With regular wind-driven ("deep-water") waves, with their short wavelengths (~5-250 m), you get to a depth below which there is virtually no energy disturbance (so the ocean floor may not be disturbed even by something like a hurricane's waves). This is "wave base." Here is a very nice animation of the relationships between wavelength and wave base for typical wind-generated sea waves: http://earthguide.ucsd.edu/earthquide/diagrams/waves/swf/wave_wind.html.

• Tsunami travel very fast. As shallow-water waves, their speed (c in meters per second) is a function of water depth (H in meters) and gravity (g or 9.8 m/s²):

  c = (gH)^1/2 or c = square root of (g times H)

• So, if a tsunami were moving through an ocean floor of pretty typical depth (4,000 m), 4000 x 9.8 m/s = 39,200. The square root of 39,200 is 198 m/s. 198 m/s = 712,800 m/hr (multiply 198 by 60 seconds in a minute and again by 60 minutes in an hour, or 3,600). Divide that by 1,000 to get the speed in km/hr, or 713 km/hr (~493 mph).

• As a tsunami approaches a shore, however, the ocean depth decreases, so the wave must slow down. Do the math for 2,000 m depth; 1,000 m depth; and 100 m.

• As with earthquake waves, a slowing of the waves is compensated by an increase in wave amplitude (and a shortening in wavelength as wave crests (or troughs) "scrunch" up). in order to conserve the same energy flux. This slowing and compensation with greater wave heights (and decreasing wavelengths) is called shoaling.

  

• The buildup in wave height with shoaling is a function of the depth of the deep water (H_d) and the depth of the shallow water (H_s). That is, buildup is a function of the ratio of the shallow shore wave height (h_s to the deep water wave height (h_d):

  h_s/h_d = (H_d/H_s)^1/4 or h_s/h_d = the 4th root of (H_d/H_s)

• So, if we were to compare the heights of a tsunami wave out at sea, where the water is about 4,000 m deep, and close to shore, say, at 50 m depth, we'd divide 4,000 m by 50 m and then take the fourth root of the answer (that is, raise it to the 0.25 exponent). From this, we'd learn that a tsunami that was only about 1 m high out at sea, when it approaches shore in 50 m of water, would build up to a wave hight of about 3 m (10 feet), . By the time it gets into water only 2 m deep, that wave would be 6.7 m (or 22 feet high!).

• Only the very largest tsunami will crest on breaking like a classic wind-driven wave. Rather, they come in like a sudden rise in sea level, kind of like a high tide on steroids. This tide-like appearance of most tsunami may be why they were often called "tidal waves" in the past, even though they are not caused by daily (high and low) tides and monthly (spring and neap) tides. If you're curious about tides, which are actually really long period waves: http://www.srh.weather.gov/srh/jetstream/ocean/tides.html.

• Inundation of the shore occurs when the tsunami breaks onto it, and water molecule motion converts from the orbital motion-in-place out at sea to horizontal, advancing motion. The tsunami will reach a maximum runup height, which is the maximum elevation of local sea level above the normal base sea level. This maximum is reached at the maximum run in distance or the limit of the water's reach inland or the zone of normally dry land inundated.

• A tsunami's inundation zone can extend pretty far inland (dozens of meters up to a couple of kilometers), depending on the size of the tsunami, the dissipation of wave energy with distance of travel, and local offshore and onshore topography. The upwash can go farther on gently sloping shorelines and those with relatively little surface roughness. High tides would increase the reach of inundation (and such coïncidences as storm surge or high wind-driven seas could make tsunami landfall a "perfect storm" situation). Modelling inundation is not for the faint-hearted! Here is a four minute video clip showing tsunami hitting a variety of modelled coastal situations: https://www.youtube.com/watch?v=Fee_nqqOygM.

• Here are maps of predicted inundation distances for California:
  http://maps.conservation.ca.gov/cgs/informationwarehouse/index.html?map=tsunami.

• A further plot complication is the difference in hazard levels of near source and distant source tsunami.

  • The most common (but not only) cause of a tsunami is an earthquake on a submarine normal or reverse fault, which produces vertical displacement on the fault and sudden seismic uplift or subsidence on the sea floor.

  • This initiates the tsunami by pushing a column of water up or down. This geological "work" creates tremendous potential energy by repositioning a column of water above or below the normal sea level. Gravity then pulls down elevated water and fills in depressed water, converting the potential energy into the kinetic energy of vertical movement of the water and then horizontal movement of the energy through the water, creating the wave. It's akin to what happens when you drop a pebble into a quiet pond, the vertical displacement of water converted into concentric ripples of horizontal energy movement outward.

  • This wave splits into a tsunami that travels outward into the sea and another one that travels inward toward the nearest coast. Both will attenuate or diminish with distance from the epicenter as the energy spreads outward and disperses.

  • The inbound wave cannot attenuate significantly, however, because it encounters the shore within a relatively short distance.

  • So, the inbound tsunami is already inherently more dangerous due to its energy retention.

  • It gets worse: Pretty immediately it starts slowing down due to contact with the rising ocean floor, meaning it slows down, bunches up, and its amplitude whomps up.

  • Remember how fast these waves move as they start out, and the hazard to the nearest shore is tremendous: little attenuation, rapid shoaling, and almost no time to evacuate coastal communities.

• It is important to realize that a tsunami isn't a single big wave: It is a wave train, just as the pebble in the pond sets off multiple rings of outward moving waves. The leading crest is not necessarily the highest one in the series, either. Each wave crest will come in at longer intervals than wind-driven waves, depending on wavelength, with periods anywhere from five minutes apart to an hour apart. There will be interactions between waves, with the backwash from one flowing into the swash from the next and possibly making the next runup even higher. This is going to go on for hours.

• Like any wave, tsunami will be focussed and reflected by obstacles, such as islands and peninulas, in their way, creating complex patterns of wave interference. Islands struck by a tsunami will focus and compress the wave front on their leading side and then the wave fronts will wrap around the island. There can be surprisingly high runups on the lee side of the island as the deflected wave fronts converge again behind the obstacle. A famous case of this occurred on Babi Island in Indonesia during the 1992 Flores earthquake and tsunami. The 7.8 M_w earthquake struck north of the island and three minutes later a 7 m high tsunami completely obliterated two small villages on the south side of the island, killing more than half the people living on the island.

• Even as the pen on a seismograph can be jolted up or down with the first arrival of an earthquake's primary wave, depending on whether the first part of the wave cycle to arrive is a crest or a trough, the first wave of a tsunami series can arrive at a coast as either a crest or a trough.

  • If the first part of the wave is a crest, the water level will just suddenly rise.

  • If the first part of the wave is a trough, however, an odd premonitory phenomenon will occur: The ocean will draw back as water is pulled in by the incoming tsunami, suddenly exposing the sea floor and stranding fish and other marine organisms. People living on coasts with cultural memory of tsunami often recognize or can be educated to recognize drawback as a warning of immediate tsunami danger and to run for their lives inland or upward. The warning may be only seconds before the arrival of the wave or up to fifteen minutes,if you are very lucky (basically half the wave cycle).

    

    It is quite common for people with no exposure to the idea of drawback to be intrigued by the phenomenon or go out to capture stranded sea creatures and be caught by the tsunami. Public education campaigns are essential in any coastal community.

Tsunami can have a variety of causes and they can develop in a variety of water bodies: They are not all caused by earthquakes in oceans.

• Most tsunami are seismic sea waves. That is, the most common cause of tsunami is a submarine earthquake produced by a fault with a strong vertical motion: normal faults or reverse/thurst faults. An underwater earthquake on such a fault will cause a substantial section of seafloor to drop suddenly or rise suddenly, producing the requisite vertical displacement of the overlying water. A tsunamigenic earthquake is generally at least 7.5 on the moment-magnitude scale.

• A little-known tsunami type but one that is garnering increasing attention by earth scientists, planners, and emergency managers is an earthquake-induced tsunami and seiche in a lake!

  • If a lake bottom is underlain by a normal or reverse/thrust fault, an earthquake can produce sufficient vertical displacement of the overlying water, particularly in deep lakes, to set off a tsunami train.

  • Such an occurrence could be devastating, given the necessarily near-field source of the tsunami to the shores: There would be almost no warning, other than the earthquake itself, and almost no time to head for the hills.

  • Even an earthquake of M_w 4 could create a tsunami in these confined water bodies.

  • An odd plot complication is seiching. Where a tsunami is a progressive wave (that is, the wave energy progresses outward), a seiche is a standing wave. Lakes (and swimming pools) can develop this rocking back-and-forth motion. Its amplitude can be quite high, in fact, the seiche that follows the earthquake and tsunami can be larger than the tsunami itself! Here is a nice interactive diagram of a seiche:
    http://earthguide.ucsd.edu/earthguide/diagrams/waves/swf/wave_seiche.html

  • Optional tangent! I can't resist sharing two incidents in my backyard that illustrate seiching. Seiching is common in swimming pools after an earthquake.

    • In the 1971 Sylmar earthquake, my dad managed to run outside, fearful (correctly) of the collapse of a brick-and-board bookcase in the room he was in. He got onto our side yard by the driveway and was clinging for dear life to a clothesline post there. The swimming pool seiched and several feet of water in it disgorged and ran down the driveway, soaking his feet to the ankles. The water then ran back UP the driveway and some of it, dirty from its trip, went into the pool (and all over the yard). After the quake stopped, he went over to look at the pool, which was depressed about two feet. He immediately turned on the water and got the pool filled (we have our priorities!) before the water began coming out of the mains really filthy. Hours later, neighbors started trying to refill their pools but by then the water was really nasty, and they had major cleaning bills (an unexpected cascading effect of a disaster?). We, meanwhile, had water to use to flush toilets and sponge bathe.

    • In 2003, there was a M_w 6.6 earthquake near Paso Robles in San Luis Obispo County, which was quite noticeable in the San Fernando Valley, where I live. I was outside facing the pool, watching a feral pigeon deciding whether to fly across the pool to check out food opportunities there. The quake struck and my then-tenant yelled "EARTHQUAKE!!!" The pigeon had JUST started a low, lazy flight skimming above the pool to get to the deck on the other side. The water in the pool suddenly started bucking up and down (probably more of a micro-tsunami than a rocking back-and-forth seiche). There were, apparently, reflection effects and the result was a rogue wave (their actual name) that popped up like a mini-mountain on the shallow end of the pool, where the bird was. It swatted the pigeon upside the backside, and the bird tumbled over but managed not to fall in the drink. It got to the other side and just stood there, dripping, with a posture of sheer astonishment looking back at the now rocking/seiching pool. It was one of the funniest moments in my personal earthquake history! Just had to share!

  • Adding to the amplitude can be any embayment in the lake border. Reflection of waves around these irregularities will set up interference patterns with the seiche: Some crests will be minimized by reflected troughs, but some crests will be amplified by reflected crests, creating a rogue wave situation (as in my swimming pool).

  • The textbook example is Lake Tahoe.

    • Here is an article laying out three scenarios for earthquakes in and near Lake Tahoe that model tsunami and seiches as high as 3 to 10 m!
      Ichinose, Gene A.; Anderson, John G.; Stake, Kenji; Schweickert, Rich A.; and Lahren, Mary M. 2000. The potential hazard from tsunami and seiche waves generated by large earthquakes within Lake Tahoe, California-Nevada. Geophysical Research Letters 27, 8 (15 April): 1203-1206. doi: 10.1029/1999GL011119.

    • Here is a news story: https://www.livescience.com/25287-lake-tahoe-tsunami-earthquake-risk.html

• Another cause is a massive landslide into an ocean, lake, or reservoir or a submarine landslide. This can raise the largest tsunami wave heights known, as in 250-500 m tall! These are on such a different order of magnitude that they are sometimes called "megatsunami."

  

  • What happens here is that a large mass of rock and soil drops either into water or within water and produces a concentrated vertical displacement of water, which sets off the tsunami wave series. This event is highly concentrated in space, unlike the displacements all along a long normal or reverse/thrust fault. So, the tsunami are much more localized, their source being more of a point than a line. They attenuate quickly within a smaller radius from the event. Unfortunately, anyone in that radius is not going to get much warning or opportunity to evacuate this much higher amplitude tsunami.

  • Here are a couple of famous examples:

    • The 1958 Lituya Bay incident in Alaska. Lituya Bay is a small fjord, or narrow glacier-carved bay with steep walls (glacial erosion produces valleys with steep walls and a U-shaped cross-section, like Yosemite Valley). It is located at the northern end of the Alaska Panhandle (southeast Alaska), near Glacier Bay National Park. A M_w 7.8 earthquake struck on July 9th. Two and a half minutes later, a huge rockfall occurred near the snout of the Lituya Glacier. Witnesses reported it as looking like an explosion. This raised up a wave 524 m tall (1,720 ft.), which then raced down the fjord toward its narrow inlet. Three trolling boats were in the bay fishing at the time, each with two persons aboard. Stunningly, two of the three crews survived, and their accounts are riveting.

      • One pair of survivors, a man and his 8 year old son (the Ulrichs), saw the vertical wall of water bearing down on them and had enough time to put on life-vests and play out their 40 fathom (240 ft.) anchor chain to its maximum. The wave pushed the boat sky high, snapping the anchor chain, and they witnessed the forest far below them as they flew on the wave toward the entrance of the bay. The wave passed and their boat was drawn back on the ensuing trough back to the middle of the bay. They were somehow able to control the boat and headed out the inlet on something like an ebb tide.

      • The other pair were a married couple (the Swansons). They were positioned near the La Chaussée Spit that blocked about two-thirds of the entrance to the bay. At first, they thought that the earthquake had actually picked up the glacier itself, because they saw a white, solid mass jumping and shaking at the head of the bay. They then saw the tsunami climb up the opposite wall of the fjord and then come bearing down on them four minutes later. Their boat was picked up and thrown across La Chaussée Spit, riding backward just below the wave crest, as though they were surfing. They could also see the forest on the spit way below them. The wave broke and their boat hit bottom and basically was wrecked. They were able to launch a small fishing skiff and escape their stricken vessel and were rescued by another fishing boat a couple hours later.

      • The third boat was near the entrance and was swamped by the wave, causing the boat to sink and killing the two people on board.

      Here are some sources about this event:

      • Miller, Don J. 1960. Giant waves in Lituya Bay Alaska. Shorter Contributions to General Geology, Geological Survey Professional Paper 354-C. Available at: http://dggs.alaska.gov/webpubs/usgs/p/text/p0354c.pdf.

      • BBC. 2008. Alaskan super wave - mega tsunami - BBC. YouTube clip, 4 minutes long (sorry about the ad preceding it). Available at: https://www.youtube.com/watch?v=2uCZjqoRLjc.

      • BBC. 2008. Alaskan super wave - mega tsunami - BBC. YouTube clip, 4 minutes long . Available at: https://www.dailymotion.com/video/xhqagp. The rest of the show: an interview with the Ulrichs in Lituya Bay.

    • Another famous example is the Vajont (or Vaiont) Dam incident of October 9th, 1963.

      • The Vajont Dam was built between 1957 and 1960 on the Piave River in northeastern Italy, one of the tallest dams on Earth at 262 m (860 ft.) high.

      • It was built in order to supply hydroelectric energy to support industrialization.

      • The construction of the dam was delayed and embroiled in the politics of post-fascist Italy, resulting in a "disaster by management" situation. Mussolini's minister of finances owned the monopoly that was building the dam, and construction couldn't begin until the government legitimated the monopoly. Local communities opposed the dam, but the government and police sided with the now-legitimated monopoly and suppressed the opposition. One of the issues leading to community resistance was concern about the geology of the gorge supporting the dam and the history of landslides in the surrounding mountains.

      • These geological problems became evident during construction, triggering three new geological studies, which all expressed concern about landslide potential. All three were ignored, and the dam began filling.

      • All sorts of things started going wrong: a landslide into another dam that raised a 20 m wave that killed a person, a landslide into the Vajont Dam reservoir that stopped the filling process while the company started building a support structure for the Monte Toc face that was generating the slides, schemes to raise the water in order to stabilize Monte Toc, substantial creeping motions on Monte Toc.

      • Some journalists began reporting on these controversies and local concerns, and the government sued them for their efforts, accusing them of "undermining the social order."

      • In the late evening of 9 October, the side of the mountain detached in only 45 seconds, and 260 million cubic meters of rock, soil, and vegetation fell into the reservoir behind the dam, completely plugging that end of the reservoir.

      • 50 million cubic meters of water were displaced upward, arcing over the top of the dam in a 250 m tall wave. This wave did not destroy the dam but shot right over it and broke beyond it, forming a large crater, like you see on the Moon and Mars from bolide impact! This crater was 60 m deep by 80 m wide!

      • The tsunami swept down the Piave Valley, obliterating five villages downstream and killing their 2000 residents.

      • The dam is still standing! It is, however, no longer used to impound water and generate electricity.

      • All hell broke out in the media, with investigative reporting uncovering the geological concerns before the dam's construction and during the filling process and community opposition, which disrupted the government's favored narrative about it all being a completely unexpected and tragic natural event. The government accused the media of "fake news" and Communist agitprop, which is rather shameless, given that it had been suing reporters for their (prescient) work before the disaster.

      • Here is a 2.5 minute contemporary news clip. I don't know which news organization originally recorded it, but it looks like the kinds of news clips that used to be shown at the cinema between the two movies that were normally shown. It's been archived by Framepool: https://footage.framepool.com/en/shot/352552904-vajont-dam-disaster-longarone-alpine-infantry-belluno".

      • This is a particularly good animation of the landslide and tsunami. It's about three minutesd the early narration is in Italian but this simply introduces the animation that makes clear the sheer scale of the landslide and the reaction of the water: https://www.youtube.com/watch?v=4ebxtvL3ojE

• Still another cause of tsunami is volcanic activity. Volcanoes can act in a wide variety of ways to produce the necessary vertical displacement of water that sets off tsunami wave-series. Here are a few such mechanisms:

  • Major volcanic eruptions can raise megatsunami. An example was the 1883 eruption of Krakatau in the Indonesian Archipelago, west of Java and southeast of Sumatra. The explosion was so loud that it was heard nearly 5,000 km away!!! Virtually the entire island blew up, and a huge tsunami was raised. This entirely destroyed a town in northern Java (Merak) with a series of waves that attained as much as 46 m in runup height. The tsunami series is estimated to have killed most of the 36,417 known victims of the Krakatau eruptions. The tsunami affected ships as far away as South Africa.

  • Volcanic flank collapses occur when a steep-sided volcano island experiences a massive landslide. Depending on the size of the mass set in motion, the velocity of the movement, and the depth of the ocean it falls into, there is potential for a megatsunami, a tsunami of spectacular wave height but a limited radius of destruction. The only example of this happening in recorded history is the 13 March 1888 flank collapse of Ritter Island off Papua New Guinea in the South Pacific. This is believed to have killed several hundred people on a number of islands up to a few hundred kilometers away but, as with many highly concentrated source tsunami, it did not generate ocean-crossing behavior.

  • Pyroclastic flows or dense rock, ash, and dust flows suspended in extremely hot., glowing gas, race outward from a volcano at upwards of 200 km/hr. They may come from the collapse of the eruption column when the active upward flow phase ceases or eases or from flank eruptions blasting out laterally (as happened in Mt. St. Helens). These flows hit water bodies with such force that they can produce the vertical displacement necessary to raise tsunami.

    • Mt. St. Helens pyroclastic flow raised 250 m tall tsunami in Spirit Lake.

    • Tambora in Indonesia (east of Krakatau and Java) erupted in 1815, killing more than 71,000 people. Its pyroclastic flows struck the ocean and generated 10 m tall tsunami, which were responsible for 10,000 of those deaths along the Indonesian coast.

• Another potential cause of tsunami is bolide impact with an ocean. A bolide is an extraterrestrial object, such as an Earth-crossing asteroid or a comet, that smashes into the earth system. If the object strikes the ocean (and most will, because about 70% of the earth's surface is covered by oceans), the tsunami raised will be a function of the size of the impactor, the velocity at which it hits, the composition of the object, and the depth of the ocean at that point. The effect on coastlines will reflect distance from the impact site and the runup factor or the ratio between runup height and deepwater wave height. Here are some ballpark estimates derived from Paine, Michael. No date. Tsunami from asteroid/comet impacts. Australian Spaceguard Survey, available at: http://users.tpg.com.au/tsp-seti/spacegd7.html

  • Deepwater wave heights are an exponential function of bolide diameter. An object 100 m in diameter is estimated to raise a deepwater wave height of under 1 m; a 200 m object would raise up a 3 m wave out at sea; a 500 m object would create a wave 22 m high; a 1 km object would result in a 70 m high wave.

  • A common estimate for runup factor is about 10, though it can range from 5 to 40, depending on how far the coast is from the point of impact, the submarine topography, and the irregularity of the coastline. If we assume a runup factor of 10 (wave runup height is 10 times the height of the wave out at sea) and focussed on identifying how far a coast would have to be to receive some standard of tsunami danger, let's say a 10 m runup, you'd get: 100 m object could create a runup of 10 m on shore 70 km from the impact; 200 m would be 10 m runup at 250 km; 500 m would be 10 m runup at 1400 km; and a 1 km object would generate a 10 m runup as far as 5000 km away.

  • Bolides are pretty low probability sources for tsunami. We can expect 100 m impactors maybe once in 10,000 years (0.01% chance annually) and 1 km object perhaps once in a million years (0.0001% chance annually).

Social Impacts

The social impacts of tsunami are varied:

• Tsunami, no matter their source, kill human beings and other animals by sweeping them up in the inundation zone and drowning them or through blunt force trauma. Someone caught up in the fast moving and voluminous water will not be the only object in the water. The water contains a great deal of debris, from rocks borne in from the seafloor to uprooted vegetation to houses or their remnants to automobiles and boats ... and other people and animals. People are struck by these other objects, sometimes with enough force to kill them outright, but more commonly they are struck so hard they lose consciousness and then drown. Those who manage to stay alive and awake may be grievously wounded and succumb to their injuries or infections after their rescue.

• Tsunami destroy property.

  • Buildings are struck by the force of the water and the debris in it and may fail and join the debris. Others will withstand the force but be badly and expensively damaged. This will be a major obstacle to resuming economic activities and livelihoods.

  • Vehicles and boats will be destroyed or damaged by water infiltration. Even if operable or repairable, they will be subject to mold and mildew damage that may compromise their functionality. For many people in poorer areas, the loss of boats, cars, trucks, and carts may create enduring, perhaps even permanent inability to resume their livelihoods.

  • The debris left behind will be expensive and time-consuming to clear, hindering the ability to resume economic activity. For agricultural land, the damage may be essentially permanent with erosion of topsoil and deposition of salt and sand.

  • Infrastructure will be destroyed or damaged: roads, ports, electricity, water, gas, communications. These are quite expensive to rebuild or repair and their lack impairs the resumption of economic activities needed to maintain survivors' livelihoods.

  • The loss of animals can be economically devastating to subsistence farmers and limit their ability to resume making a living.

  • Coastlines are often tourist destinations and a tsunami will scare tourists off after the fact and the blighted appearance of the coast will wipe out the attractions to the area. The economic consequences for tourist-dependent local economies are devastating.

  • As in so many disasters, human loss, grief, and trauma are indescribable, both for those who have suffered personal tragedies and for those who try to help them, from the community or from the professional first-responders.

  • Personal loss is worsened even more by the economic ramifications of losing household labor power. Spouses lose partners, children lose parents, parents lose children, all of whom may have contributed to the labor needed to run a household. Death in the family may not be random, either. In the Sumatra tsunami, most coastal communities are focussed on fishing. In the gendered division of labor common in the Indian Ocean basin, men go out to fish and women stay on shore and process and sell the catch. All around the basin, men were out at sea when the earthquake struck and the tsunami crossed the open water. They were completely unaware of the quake or the tsunami, which, out at sea, was indistinguishable from the wind-generated waves. They came back to communities that no longer existed, their wives, parents, and children dead. Some of these coastal areas now have 90% men and maybe 10% women. Besides the incomprehensible trauma of their personal tragedies, most of these men will never get a chance to re-marry in their communities and, without female partners, they are unable to carry out the fishing that is their traditional way of making a living. The women died, but the men are scarcely "better off."

What can be done?

• Risk assessment is underway in many coastal communities as the salience of this hazard is being more widely appreciated. California is in the process of creating inundation maps to assist communities in specifying and planning around local risk. These can be combined with scenarios for distant source and near source tsunamis to estimate warning times and, from these, to estimate how far people can evacuate in the face of warning. In quite a few stretches of coast, the warning available and the distance that needs to be covered mean that many will simply not be able to get out of the way in time. Research is ongoing to identify sturdy buildings and other structures that may be suitable for vertical evacuation for those who cannot evacuate horizontally.

• In the developed world, planning and zoning are tools for mitigating potential tsunami hazard; in the developing world, this is far less feasible.

• Public education is critical. Residents and visitors need to know that particular stretches of coast are potentially susceptible to tsunami. This awareness can be promoted with signage. Signage should be standardized, densely placed, and clearly point out the best paths for evacuation from a particular location, whether that best path might be horizontal or vertical. Signage can become more salient if it is coupled with multimedia service announcements and social media campaigns. Owners and employees in buildings designated for vertical evacuation have to be educated about their critical röle and responsibility in admitting all evacuees into their structures in the event of a tsunami warning for a near-field source event. Such vertical evacuation should be at least up to the third floor.

• A major mitigation is a warning system. The Pacific basin has an elaborate system of sensors (tide gauges, Deep-ocean Assessment and Reporting of Tsunami [DART] buoys), to detect tsunami and mechanisms to communicate tsunami detections into a watch-and-warning system. This international warning system is operated by NOAA in Hawai'i and Alaska. After the 2004 Sumatra tsunami, the United Nations initiated an Indian Ocean Tsunami Warning System and another for the North Eastern Atlantic, the Mediterranean, and Connected Seas. The Caribbean is developing a similar system. Such systems can be invaluable in dealing with ocean-crossing tsunami but they cannot afford much warning for near-field source tsunami. What little warning can be provided has to be coupled with excellent public education and risk communication strategies so that people "instinctively" know what to do.

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