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Hazard: Tsunami!

This article originally appeared in the May/June 2016 issue.


Zhasmiin ignored the glittering beach a quarter-kilometer away and 30 meters down and kept her eyes glued to the horizon; she didn’t want to miss a thing. Dr. Terin, for his part, kept his eyes glued to his portable workstation console, occasionally reaching to adjust sensor controls without looking; almost unconsciously.

“Soon,” he announced, in acknowledgement of her palpable excitement. Years of formulation and testing of his seismic quake and tsunami prediction algorithm were about to bear fruit. The seaquake had already occurred, and matched exactly the location, time, and magnitude Terin predicted. They had now come to a cliff above the point at which he had predicted the worst tsunami would occur. This final prediction, if accurate, would allow Imperial scientists to begin predicting the worst seismic shocks and resulting deluges in advance, saving many lives. Zhasmiin, his most trusted assistant, had accompanied him on every trip to observe his theories in the field.

As Zhasmiin watched through the binoculars, the sea rapidly receded from the beach as though a giant drain had been opened somewhere. The sea floor lay bare as scores of unlucky fish flopped impotently on the newly expanded shoreline.

A sudden motion out of the left corner of the viewfinder caught her attention. A group of children ran out onto the beach, seeking out interesting, previously-hidden objects. They laughed and called to one another merrily as they took youthful advantage of the unexpected event.

Zhasmiin stared in shock. Terin must have happened to look up at that critical moment. She heard his voice, strained with horror. “Gods of Sharru! Those kids…”

The drawback meant the tsunami was due within minutes. Indeed, the far horizon darkened and swelled and a dull, faraway roar began. Terin had predicted it would be up to ten meters tall. Few structures could withstand such a wall of water; let alone fragile children.

Terin frantically tried to get the oblivious kids’ attention, shouting and waving his arms to no avail. Zhasmiin shook off her astonishment and sprinted for the air/raft. As she leaped into the pilot’s seat and thumbed the machine to life, she mentally calculated how long she had to reach the children, load them aboard, and gain enough altitude to top the wave. A glance at the sea told her the answer: not long, if she made it at all. She arrowed the air/raft forward, praying that she would.


Any world blessed with liquid water might have much of it collected into bodies of substantial size. Along with the abundance of water comes an abundance of dangers; usually indirect, such as cyclonic storms and marine wildlife. However, some dangers present a direct threat to land-based life; tsunamis are one such peril. They are among the most unassuming of natural disasters, many times speeding across the ocean completely unnoticed before delivering their deadly blow upon the land. This article provides background material, scientific explanation, and a game mechanic for use by the Classic Traveller referee in creating these events.

Disclaimer: while based on real-world science, the concepts herein are part of a game. Nothing presented here is intended to be used in real-life situations. Tsunamis are dangerous phenomena that cause a great deal of destruction, injuries, and deaths, and are best avoided. In the aftermath of severe marine disturbances, always observe authorities’ warnings and instructions regarding the potential for tsunamis and the areas where they are likely to strike.

Defining a Tsunami

A tsunami (from an ancient Terran Japanese word meaning “harbor wave”) is a series of giant waves originating from an event that causes a sudden large-scale displacement of water, usually seismic activity. Also commonly called “tidal waves” (a misnomer, since they have nothing to do with tides), tsunamis can occur in any substantial body of water, including inland seas, large lakes, bays, sounds, and harbors. They differ from normal wind-generated waves in their genesis and structure.

A common (but inaccurate) image of a tsunami is of a gigantic wave cresting above the tallest buildings. Tsunamis don’t crest—they more typically resemble a quickly-rising tide or surge, or a huge wall of water—and while they can become quite large, most are nowhere near the skyscraper-dwarfing monsters popularized in fiction. There have been exceptions, of course (see below). Tsunamis also typically form and strike in multiples. The initial wave that hits may not even be the most powerful.

The Making of a Tsunami

Tsunamis are created by abrupt, large-scale disruptions in the ocean. Large landslides, volcanic action, and even asteroid impacts are sometimes responsible, but the vast majority of tsunamis begin with underwater seismic activity. Strong seaquakes (7.0+ on the Richter scale) that cause the seabed along fault lines to move vertically up or down displace massive amounts of water, which in turn creates waves that ripple out in all directions at speeds of 500-1000 kph. Tsunamis are rarely single events, but a series of waves called a wave train. While moving across the ocean, tsunamis have a long interval, or period, between waves which may be measured in the hundreds of kilometers. Because their energy is distributed vertically all through the column of displaced water at maritime depths, at sea they’re generally less than a meter high. Indeed, travelers in watercraft may not even notice them.

Once near land, tsunamis display a totally different character. As the water gets shallower, the waves’ speed slows and the energy that was distributed into the depths now has nowhere to go but up. Additionally, the water behind the wave backs up. The result is a sharp buildup in the waves’ size and power. By the time they hit the shoreline they’ve become a deadly wall of water thousands of times more powerful than a normal wave, hitting with incredible force and flooding a large area. Unfortunately for the stricken areas, the destruction doesn’t end there. The powerful outflow as the wave recedes sweeps victims and debris into open water.

Each tsunami is a unique creation, so there is no “average” size and scope. They depend on where it originated, how powerful the trigger was, and the topography of the areas they hit. Some hit the coast as no more than a rushing tide a few meters high. Others are monsters tens of meters tall that scour the landscape. One of the worst tsunamis in Terran history struck Indonesia and countries on the Indian Ocean in −2512 (2004 CE). Created by a magnitude 9.0 quake, the resulting tsunamis averaged 15 meters in height, affected more than ten countries, and killed nearly 250,000 people. Another, classified as a megatsunami, struck Terra’s Lituya Bay, in the northwestern North American continent, in −2558 (1958 CE). Caused by a massive seismically-induced landslide, it had an initial wave height of over 500 meters and a secondary wave height (which traveled the length of the bay) of up to 30 meters. Likewise, the flooding can be extensive, and depends on the size of the tsunami and the topography. A tsunami that struck Terra’s Japan in −2505 (2011 CE) inundated about 561 square kilometers over a plain.

Tsunamis are measured according to a scale. One of the first was the Sieberg-Ambraseys tsunami intensity scale, which debuted in -2554 (1962 CE). This scale divided tsunami intensities and their effects into six categories ranging from Very Light to Disastrous. Other scales replaced it in subsequent years, but the Sieberg-Ambraseys lends itself to simplicity and the use of a six-sided die:

  1. Very light. Wave so weak as to be perceptible only on tide-gauge records.
  2. Light. Wave noticed by those living along the shore and familiar with the sea. On very flat shores generally noticed.
  3. Rather strong. Generally noticed. Flooding of gently sloping coasts. Light sailing vessels or small boats carried away on shore. Slight damage to light structures situated near the coast. In estuaries reversal of the river flow some distance upstream.
  4. Strong. Flooding of the shore to some depth. Light scouring on man-made ground. Embankments and dikes damaged. Light structures near the coasts damaged. Solid structures on the coast injured. Big sailing vessels and small ships carried inland or out to sea. Coasts littered with floating debris.
  5. Very strong. General flooding of the shore to some depth. Breakwater walls and solid structures near the sea damaged. Light structures destroyed. Severe scouring of cultivated land and littering of the coast with floating items and sea animals. With the exception of big ships all other type of vessels carried inland or out to sea. Big bores in estuary rivers. Harbour works damaged. People drowned. Wave accompanied by strong roar.
  6. Disastrous. Partial or complete destruction of man-made structures for some distance from the shore. Flooding of coasts to great depths. Big ships severely damaged. Trees uprooted or broken. Many casualties.

Unfortunately, since tsunamis occur in series, the danger may last for hours as one wave after another arrives. Each time, the sea retreats from the shoreline, serving as a warning that another wave is imminent. The interval between waves can be as little as 10 minutes or more than an hour. Worse, the energy of one wave can “bounce” off a landmass and add to the next wave, making it larger and deadlier.

Worlds with Hydrographics A (water worlds) aren’t spared the wrath of tsunamis. Although there are fewer landmasses for them to affect, underwater topography can still vary enough to allow them to build energy and create a recognizable wave. Also, since tsunamis retain their energy even across thousands of miles of ocean, a large enough quake could conceivably create a tsunami that would last, perhaps even for years or be more or less permanent. Even structures such as habitats and other installations on the seafloor aren’t immune. Recall that a tsunami’s energy is distributed throughout a vertical area of water, from the surface to the bottom. This energy is often more than enough to erode the seafloor, stir up silt and debris, and destabilize structures.

Detecting a Tsunami

Tsunami detection can be implemented on many worlds, with only the tech level dictating the degree of sophistication and accuracy. Primitive societies develop warnings based upon instinct and observation of local wildlife behavior. Worlds with advanced technology develop electronic methods of detecting tsunamis and can issue warnings immediately. Warnings can go out by voice (radio broadcast, runners and criers, etc.) or by simpler means such as sirens, whistles, or other agreed-upon sounds.

Surviving a Tsunami

The best way to survive a tsunami is preparation. This can mean simply being aware. Adventurers traveling near coastlines on geologically-active worlds should stay alert to the possibility of tsunamis and note their height above sea level, the distance to the nearest high ground, and potential escape routes. Acknowledgement of local history and natives’ skills are also invaluable preparation tools.

In coastal areas, the first sign that a tsunami is due is a strong seismic event. Adventurers should first protect themselves from the quake using standard survival techniques. Immediately upon the quake’s passing, travelers should head for high ground or take to the skies; a tsunami might be only minutes away. A sudden or unusual retreat of the water from the shoreline or other lowering of the water level means one is about to happen. Heroes that cannot escape to high ground or fly out of harm’s way should get into a sturdy building on the highest floor or (only as a last resort) climb the nearest, sturdiest tree they can find.

“Riding out” a tsunami is dangerous and foolish. The impact of a typical tsunami wave can flatten all but the sturdiest structures, and toss vehicles and other heavy objects like toys. Even though they slow down near the coast, they still travel far faster than heroes can run—if they’re still on the shoreline when they sight the tsunami, it’s already too late. Creatures caught in the wave are subject to being drowned, injured by floating debris, slammed against structures, and dragged out to sea.

Experienced sailors and coastal dwellers, however, with enough advance warning of a tsunami have been known to get as far out to sea in a watercraft as possible, where the tsunami’s energy is still at pelagic depths and any effect is not more than a gentle swell, as mentioned above.

After the Tsunami

In the aftermath, the shoreline and flood zone will be covered with debris, including watercraft thrown inland and the remains of buildings and infrastructure. Such debris carries the potential for secondary injury. More destruction can occur due to building collapse as tsunamis can erode structural foundations. Victims still ashore will need help, but the widespread destruction may stretch rescue services thin. While communications may still be possible after the disaster, chaos and confusion may well rule the day. Local authorities might also co-opt communications to coordinate relief efforts.

Responses to the disaster naturally include rescuing victims, getting them out of the danger zone (in case there are more waves forthcoming) and treating their injuries. Accounting must also be made of the missing, who may have been buried by debris or swept out to sea. The response to the disaster scales with its severity; a large or deadly enough tsunami may provoke aid responses from the entire world. Such was the case in the tsunami on Terra in -2512; a worldwide response was mobilized to help the affected countries and rebuild. While it’s rare that an Imperial response to a tsunami would be required, the empire might send aid if Imperial installations were widely affected, or upon request of a noble overseeing the world.

Socioeconomic Impact of a Tsunami

Tsunamis impact not only life and property, but a region’s economy, environment—even culture. A large tsunami can completely obliterate an area; such areas often must be rebuilt, fortified, and even resettled, at great cost. Replacing infrastructure and critical installations can take years, during which other areas must pick up the slack, putting a strain on their infrastructure as well. If the stricken area was considered a global economic engine, its sudden offlining and decline might have worldwide ramifications.

Along with economic decline, the impact of victims suddenly forced from their homes prompts homelessness and migrations. The destruction of businesses, financial institutions, and other economic engines means damage to the area economy. This is complicated by displaced multitudes seeking jobs and other forms of financial stability either where there is none, or in areas already straining under the weight of refugees. People already living there might welcome the migrants with open arms—or allow resentment to build. Either way, there is increased social tension. Worse yet is the possibility that disease and starvation runs rampant because of floodwater contamination.

Tsunamis can also cause great environmental disruption by scouring a landscape and rendering devoid of life. Food sources and habitation can be destroyed or swept away, forcing wildlife to establish new territories elsewhere, supplanting the previous animals. On Industrial worlds, the problem can be made worse by pollution as the tsunami damages or destroys industries and sanitation systems. The tsunami that hit Terra’s Japan in -2505 damaged a nuclear power plant, introducing to the Terrans the grim concept of radioactive contamination of the adjoining sea and water sources. Environmental damage may be years in the recovery, if ever.

Refereeing the Tsunami

The basic ingredients of a tsunami are simple: a causative event and liquid water (or other fluid) gathered into a sufficiently large body. We’ll skip the causative event since our focus is the tsunami itself. Remember that regardless of the cause the basic tsunami mechanism is the same: a sudden, massive displacement of water. In the case of a seismic disturbance, only those measuring 7.0 or greater on the Richter scale create tsunamis.

To determine the tsunami’s characteristics, first determine its Initial Power: throw 1D and apply the result (Tp) to the following calculations:

Creatures surviving the attack are swept up in powerful floodwaters. Throw the following:

Unconscious adventurers take an additional 1D drowning damage per combat round until rescue or death.

Reinforced structures and armored vehicles protect their occupants equal to (12 Tech Level of manufacture.) For example, an AFV (TL 6, from Book 3) offers 72 points of protection vs. the tsunami as long as it stays “buttoned up.” However, the force of the wave may move the vehicle uncontrollably; the total damage of the Attack / the object’s weight in tons = number of meters the object is moved. Triple this value for watercraft caught in the wave.

Example: the referee wants to complicate the PCs’ mission while they’re on Mercury (2624 Spinward Marches, (B658663-8) with a tsunami, presumably created by a strong seaquake. She begins by throwing 1D for Tp, getting a 4. Deciding to determine an inception point randomly, she throws 8 on 2D and uses the formula 2D 25(6 the planet’s UPP Size digit) = 8 25(6 8) = 9,600; the quake’s epicenter is 9,600 km away. The tsunami’s speed is 100(Tp + 4) = 100(4+4) = 800 kph. Dividing the total distance by this figure gives an ETA of 12 hours. When the time arrives, she determines the number of waves in the wave train = 2(Tp) = 2(4) = 8. Opting for simplicity, she throws a 1D for 2 ( 10), getting a period of 20 minutes; a wave will strike every 20 minutes until all 8 waves have hit. The wave height is Tp(wave train/2) + (6−period/10) = 4(4) + (6-2) = 20 meters; she rules that this will be the largest wave in the train, with the others somewhat smaller. The attack would do (Wave Height + Speed/100) = (20 + 800/100) = (20+8) = 28 dice of damage to anything in the area. Finally, she determines the area covered by the subsequent flooding: (Wave Height Speed) 100, or (20 800) 100 = 1.6 million square meters, or 1.6 square kilometers. Luckily, the PCs heeded the warnings and watch the devastation from the safety of their air/raft.

“I happen to know something about tsunamis…”

Since tsunamis are uniquely marine phenomena, only denizens of worlds with substantial hydrographics collected into large bodies of water will have even passing knowledge of them. Prior Traveller careers most suited as background for heroes with knowledge of tsunamis are Sailor, Scientists, and Barbarians (Supplement 4: Citizens of the Imperium). Sailors have especially useful knowledge not only of watercraft and nautical navigation, but marine phenomena such as tsunamis. Scientists who specialize in oceanography or water planets are able to impart knowledge behind their formation and mechanics. Coastal Barbarians often keep histories (oral or written) of prior tsunamis and the damage they do. They are also much attuned to even subtle changes in the sea and can tell when something is amiss long before the obvious warning signs.

Skills useful in dealing with tsunamis include Combat Engineering (for constructing protective structures such as seawalls); Electronics (for monitoring and maintaining tsunami detection equipment); and Survival (useful in post-tsunami recovery efforts). Water Craft gives knowledge of handling such vessels in rough seas and underwater topography conducive to creating the largest tsunamis.

The Tsunami’s Impact on the Game

Tsunamis are a game changer, for heroes and opponents. They also have widespread consequences beyond the initial impact. While they can be used as background (occurring before the PCs arrive or “off-stage” during the adventure); directly as a man-vs.-nature scenario; or even as a deus ex machina; the referee should give some thought as to the long-term effects of introducing such a widespread calamity.