# Chapter 4 Study Guide: Tsunamis

Prepare to define terms, explain how tsunamis form and change as they approach shore, identify high-risk regions, describe linked hazards, and evaluate warning, planning, and personal safety measures.

## Chapter Learning Objectives

The chapter states these objectives. Each section below identifies the objective or objectives it supports.

- **LO:1:** Explain the process of tsunami formation and development.
- **LO:2:** Locate on a map the geographic regions that are at risk for tsunamis.
- **LO:3:** Synthesize the effects of tsunamis and the hazards they pose to coastal regions.
- **LO:4:** Summarize the linkages between tsunamis and other natural hazards.
- **LO:5:** Understand that tsunamis are not caused by or affected by human activities, but damages are compounded as coastal populations increase.
- **LO:6:** Discuss what nations, communities, and individuals can do to minimize the tsunami hazard.
- **LO:7:** Identify the actions you should take and not take if a tsunami warning is issued.

## 1. Essential Definitions (supports LO:1 through LO:7)

- **Tsunami:** A series of long ocean waves produced by the sudden vertical displacement of a large volume of water.
- **Tsunami trigger:** An event that displaces water and starts a tsunami, such as a large earthquake, submarine landslide, volcanic collapse or explosion, or ocean impact.
- **Subduction zone:** A convergent plate boundary where one plate sinks beneath another. Large subduction-zone earthquakes are the most common source of damaging tsunamis.
- **Megathrust earthquake:** A very large earthquake, usually produced by rupture along a subduction-zone interface.
- **Local tsunami:** A tsunami that reaches a nearby coast soon after its source event, sometimes within minutes.
- **Distant tsunami:** A tsunami that travels across an ocean basin and may take hours to reach a distant coast.
- **Wave height:** The vertical size of a wave, commonly measured from trough to crest; tsunami descriptions may also report crest height relative to normal sea level.
- **Wavelength:** The horizontal distance between successive wave crests.
- **Runup:** The farthest horizontal and vertical distance that tsunami water travels inland.
- **Inundation:** Flooding of land by tsunami water.
- **Shoaling:** The slowing and increase in height of a tsunami as it enters shallow water.
- **Tsunameter:** A bottom-pressure sensor connected to a buoy that detects small changes in water pressure caused by a passing tsunami.
- **Tsunami warning:** An alert that a tsunami has been detected and may affect a particular area.
- **Tsunami watch:** A notification that an earthquake or other event capable of producing a tsunami has occurred and that a tsunami is possible.
- **Tsunami runup map:** A map showing the area and elevation likely to be reached by tsunami water.
- **Tsunami-ready community:** A community with the warning systems, emergency plans, communication, drills, and public education needed to respond to a tsunami.
- **Seawall:** A coastal barrier designed to reduce the effects of waves and flooding.
- **DART system:** A network of deep-ocean assessment and reporting buoys that detect tsunamis using bottom-pressure sensors.
- **Wave amplification:** An increase in tsunami height caused by shoaling, coastline shape, seafloor shape, bays, or interactions among waves.

## 2. How Tsunamis Form (LO:1)

Most damaging tsunamis begin with a large earthquake at a subduction zone. The earthquake must cause rapid vertical movement of the seafloor, usually by uplifting or lowering it. This sudden movement displaces the entire water column above the seafloor.

Other possible triggers include:

- submarine landslides;
- coastal or volcanic flank collapses;
- submarine volcanic explosions; and
- impacts from asteroids or comets.

Earthquakes are the most common source of large historic tsunamis. A damaging earthquake-generated tsunami generally requires a large earthquake, often about magnitude 7.5 or greater, with sufficient vertical seafloor displacement. Magnitude alone is not enough: the depth, direction, and amount of seafloor movement also matter.

### Four stages of tsunami development

1. A submarine earthquake rapidly uplifts or lowers the seafloor and displaces the overlying water.
2. Waves spread outward from the source in all directions. One part may travel toward a nearby coast as a local tsunami, while another travels across the ocean as a distant tsunami.
3. In deep water, the waves travel extremely fast, have very long wavelengths, and may be less than a meter high, so ships may barely notice them.
4. In shallow water, the waves slow down, their wavelengths shorten, and water piles up, increasing wave height. The water then moves inland as runup and flooding.

## 3. Tsunami Behavior (supports LO:1 and LO:3)

### Deep water versus shallow water

- In deep ocean water, tsunami speeds can approach 900 km/hr. A typical deep-water tsunami may be less than 1 m high with a wavelength greater than 100 km.
- As water depth decreases near shore, tsunami speed may fall to about 45 km/hr. This is still too fast to outrun, but the wave becomes much higher.
- Tsunami speed depends mainly on water depth. The chapter gives the relationship as the square root of gravitational acceleration multiplied by water depth.

### At the coast

A tsunami may not look like a normal breaking wave. It may appear as a rapidly rising flood or a turbulent vertical wall of water. The first arrival may be a crest or a trough. If a trough arrives first, the sea may suddenly withdraw and expose the seafloor.

Several waves usually arrive in a series. The second or third wave may be larger than the first because of edge-wave interactions and local amplification. The water returning to the ocean is also dangerous because it carries debris, erodes land, and can pull people and structures out to sea.

Runup varies greatly from place to place because of:

- offshore bathymetry, or seafloor shape;
- coastline shape and orientation;
- bays, harbors, and fjords;
- coastal elevation and topography;
- vegetation and land cover; and
- the size and direction of the incoming waves.

## 4. Local and Distant Tsunamis (supports LO:1 and LO:2)

### Local tsunami

A local tsunami is generated close to the coast it affects. It may arrive within minutes of the earthquake or earthquake-triggered landslide, leaving little time for an official warning. People must rely on natural warning signs, especially strong or prolonged ground shaking and sudden sea withdrawal.

### Distant tsunami

A distant tsunami travels across an ocean basin. It may take several hours to arrive, giving warning centers time to detect the source, model the wave, issue alerts, and organize evacuations. Even distant tsunamis can cause damage far from their source.

## 5. Geographic Regions at Risk (LO:2)

All ocean coastlines and some lake shorelines may experience tsunamis, but risk is not evenly distributed. The greatest risk occurs:

- adjacent to active subduction zones capable of magnitude 9 or greater earthquakes;
- directly across an ocean basin from a major subduction zone;
- near unstable submarine slopes or volcanic islands; and
- in low-lying coastal areas, bays, harbors, deltas, and fjords.

### Major risk regions

For a map question, be able to locate these regions and connect their risk to nearby or distant tsunami sources:

- The Pacific Rim has the greatest overall tsunami hazard because it contains many active subduction zones.
- Important areas include Alaska, Hawaii, Japan, Indonesia, the west coasts of North and South America, and parts of the Pacific Northwest.
- Other risk areas include the Indian Ocean, Mediterranean Sea, Caribbean Sea, Canary Islands, and coasts near possible submarine or volcanic landslides.
- The eastern United States and central California generally have lower tsunami risk, but they are not risk-free.

The size of a tsunami at one location does not guarantee the same size elsewhere. Local seafloor and coastline conditions can greatly amplify or reduce runup.

## 6. Subduction Zones and Tsunami Generation (supports LO:1 and LO:2)

At a locked subduction zone, the overriding plate is dragged downward near the trench while the landward part of the plate bends upward. Elastic strain accumulates for decades or centuries.

When the fault ruptures:

1. The overriding plate rebounds.
2. The seafloor rises or falls rapidly.
3. The overlying ocean water is displaced.
4. Tsunami waves travel toward nearby and distant coastlines.
5. Coastal land may subside, increasing flooding.

Subduction zones can produce the largest tsunamis because a locked fault can rupture for hundreds or thousands of kilometers and displace a very large volume of water. Drowned forests or "ghost forests" may preserve evidence of past coastal subsidence and tsunami-generating earthquakes.

## 7. Effects and Linked Hazards (LO:3 and LO:4)

### Primary effects

Primary effects result directly from tsunami inundation and currents:

- flooding and rapid currents;
- coastal erosion;
- sediment deposition;
- destruction of homes, buildings, roads, ports, bridges, and utilities;
- debris impact;
- destruction of vegetation and habitats; and
- loss of life and displacement.

Returning water can be as destructive as the incoming runup. It can carry people, vehicles, boats, buildings, trees, and other debris back toward the sea.

### Secondary effects

Secondary effects occur after the initial inundation and may include:

- fires from broken gas lines or released chemicals;
- polluted water from sewage systems, dead animals, plants, and damaged treatment facilities;
- disease outbreaks;
- loss of drinking water;
- damage to critical facilities;
- long-term displacement; and
- technological disasters caused by flooding of industrial or nuclear facilities.

### Linkages with other hazards

- An earthquake can produce both strong shaking and a tsunami.
- An earthquake can trigger a submarine landslide, which produces a larger local tsunami than the earthquake alone might have generated.
- Volcanic explosions and flank collapses can produce tsunamis.
- Tsunami currents cause coastal erosion and sediment deposition.
- Earthquake-related subsidence can increase tsunami inundation.
- Flooding can cause fires, pollution, disease, and infrastructure failure.

## 8. Human Interaction and Vulnerability (LO:5)

Human activities do not control the magnitude or frequency of tsunamis. However, human decisions strongly affect their consequences.

Risk increases when people:

- build homes, hotels, ports, power plants, and roads in low-lying coastal areas;
- remove mangroves and coastal forests;
- fail to educate residents and visitors;
- lack direct warning systems and evacuation routes; or
- underestimate rare events because no recent tsunami is remembered.

Coastal vegetation can reduce the speed and energy of small-to-moderate waves and trap some debris. Large waves can uproot trees and turn them into dangerous projectiles, so vegetation is not a substitute for evacuation.

## 9. Natural Service Functions (supports LO:3)

Tsunamis are highly destructive, but they may provide limited natural services:

- moving marine sediments inland;
- transporting nutrients from the ocean to coastal ecosystems;
- altering habitats and coastal landscapes; and
- contributing to long-term sediment and landscape development.

These possible benefits do not justify building in high-risk coastal areas.

## 10. Detection, Warning, and Risk Assessment (supports LO:2 and LO:6)

### Tsunami warning systems

A warning system uses several kinds of information:

- **Seismographs:** Locate offshore earthquakes and estimate their depth and magnitude.
- **Tide gauges:** Measure unusual rises and falls in sea level near coasts.
- **DART buoys and tsunameters:** Detect tiny changes in ocean-bottom pressure as a tsunami passes overhead.
- **Satellites and warning centers:** Relay data, model wave travel, estimate arrival times, and communicate alerts.

Distant tsunamis are easier to warn for because they take time to cross the ocean. Local tsunamis may arrive before a warning can be issued, so natural warning signs and education are especially important.

### Watch versus warning

- A **tsunami watch** means that an earthquake or other possible source event has occurred and a tsunami may develop or affect the area.
- A **tsunami warning** means that a tsunami has been detected or is considered likely and people should take protective action.

### Runup and inundation maps

Runup maps show how far inland and how high water may travel. They help communities:

- identify evacuation zones;
- design evacuation routes and shelters;
- regulate land use;
- locate critical facilities;
- plan building requirements; and
- estimate potential losses.

### Probabilistic analysis

Tsunami risk analysis considers:

1. Potential earthquake and landslide sources and their uncertainties.
2. How waves attenuate or change as they travel.
3. The probability, runup, arrival time, and likely damage for both local and distant sources.

Because tsunamis are rare at a particular location, scientists may use Monte Carlo simulations to model many possible earthquakes and tsunami outcomes.

## 11. Minimizing the Tsunami Hazard (LO:6)

### Community and national adjustments

- Build and maintain seismograph, tide-gauge, and deep-ocean sensor networks.
- Issue clear, timely warnings through sirens, mobile alerts, radio, television, and other systems.
- Prepare and update tsunami runup and inundation maps.
- Establish evacuation routes to higher ground or inland areas.
- Use land-use planning to avoid the most hazardous low-lying locations.
- Elevate, reinforce, or relocate critical facilities when possible.
- Use seawalls, floodgates, and other structural controls where appropriate.
- Preserve or restore coastal vegetation as a partial buffer.
- Educate residents, tourists, businesses, and emergency personnel.
- Conduct regular evacuation drills.
- Establish a continuously staffed emergency operations center.

Seawalls and other structures can delay or reduce inundation, but they may be overtopped or destroyed by an unusually large tsunami. Evacuation and education remain essential.

### Tsunami-ready status

A tsunami-ready community should have:

- a 24-hour emergency operations center;
- reliable ways to receive official warnings;
- methods to alert the public;
- a tsunami preparedness plan and emergency drills; and
- an ongoing public awareness and education program.

## 12. Personal Safety and Adjustment (LO:7)

### Natural warning signs

Immediately move away from the beach and toward higher ground if:

- you feel a strong or prolonged earthquake near the coast;
- the ocean suddenly withdraws and exposes the seafloor;
- you hear a loud roar like a train from the ocean; or
- officials issue a tsunami warning or sirens sound.

### What to do

- Move inland or uphill immediately; do not wait to watch the wave.
- Follow marked evacuation routes and official instructions.
- Stay away from beaches, low-lying areas, harbors, and river mouths.
- Expect multiple waves, and remember that the second or third may be larger.
- Stay away until officials issue an all-clear.
- If evacuation is impossible, move to the upper floors or roof of a strong, multistory reinforced building. If necessary, hold onto a strong floating object or climb a sturdy tree.
- If in a boat and conditions permit, move to deeper water; otherwise, leave the boat and evacuate to higher ground.

### What not to do

- Do not remain on the beach to observe a receding ocean or approaching wave.
- Do not assume that a small wave at one location means another location is safe.
- Do not attempt to outrun a tsunami along the shoreline.
- Do not return after the first wave.
- Do not rely only on a seawall, vegetation, or an official warning for a local tsunami.

## 13. Important Case Studies and Examples (supports LO:2 through LO:7)

- **2011 Tohoku tsunami, Japan:** A magnitude 9 earthquake vertically displaced the seafloor and generated waves up to about 40 m at the shore. Warnings and evacuation saved many lives, but seawalls were overtopped and flooding disabled backup generators at the Fukushima Daiichi nuclear plant, causing a nuclear disaster.
- **2004 Indian Ocean tsunami:** A magnitude 9.1 megathrust earthquake near Sumatra ruptured more than 1,500 km of fault. More than 200,000 people died across the Indian Ocean region. The absence of a regional warning system, direct communication, and public education greatly increased the catastrophe.
- **1700 Cascadia tsunami:** Geologic evidence in the Pacific Northwest and historical tsunami records in Japan revealed a large Cascadia earthquake and tsunami. This example shows how geologic history can identify events absent from written local records.
- **1998 Sissano Lagoon, New Guinea:** A magnitude 7.1 earthquake-triggered submarine landslide produced a large local tsunami with little warning. The earthquake's seafloor displacement alone probably would not have generated such a large tsunami.
- **1958 Lituya Bay, Alaska:** A massive earthquake-triggered rockfall entered a confined bay and produced an exceptionally high local splash, showing how landslides can create extreme local tsunamis.
- **Fudai, Japan:** A village survived the 2011 tsunami partly because its mayor insisted on constructing a much higher seawall based on knowledge of earlier tsunamis.
- **Maldives:** Very low-lying islands had little elevation for evacuation during the 2004 tsunami, illustrating how topography controls vulnerability.

## 14. Likely Comparison Questions

Be ready to answer these directly:

- How does a tsunami differ from an ordinary wind-generated wave?
- How do local and distant tsunamis differ?
- Why do subduction zones produce the largest tsunamis?
- How do tsunami behavior and wave height differ in deep and shallow water?
- How do a tsunami watch and a tsunami warning differ?
- How do primary and secondary tsunami effects differ?
- How do earthquake-generated and landslide-generated tsunamis differ?
- How do tsunami magnitude, runup, inundation, and wave height differ?
- How do warning systems, seawalls, vegetation, land-use planning, and evacuation reduce risk?
- Why can the second or third wave be larger than the first?
- Why are local tsunamis harder to warn for than distant tsunamis?
- How can human population growth increase tsunami losses without changing tsunami occurrence?

## 15. Practice Questions

1. Define a tsunami and list several possible trigger events.
2. Explain how vertical seafloor displacement generates tsunami waves.
3. Describe the four stages of tsunami development from the source to the shore.
4. Why are tsunamis difficult for ships to notice in deep water?
5. Explain why a tsunami becomes taller as it approaches land.
6. What is runup, and what factors control its height and inland extent?
7. Compare local and distant tsunamis.
8. Why do subduction zones produce the largest and most destructive tsunamis?
9. Identify the areas of the world with the greatest tsunami risk and explain why.
10. Describe the primary and secondary effects of a tsunami.
11. Explain how earthquakes, landslides, volcanoes, flooding, erosion, fire, and disease can be linked in a tsunami disaster.
12. Why can a submarine landslide produce a large tsunami after a comparatively small earthquake?
13. Explain why tsunami occurrence is not controlled by human activity but tsunami damage is strongly affected by human decisions.
14. What natural service functions can tsunamis provide?
15. Describe the roles of seismographs, tide gauges, DART buoys, satellites, and warning centers.
16. Distinguish between a tsunami watch and a tsunami warning.
17. How do runup maps and probability analysis help communities prepare?
18. List the requirements for tsunami-ready status.
19. What natural warning signs indicate that you should evacuate immediately?
20. What should you do if you receive a tsunami warning, and what should you avoid doing?
21. Use the 2004 Indian Ocean tsunami and 2011 Tohoku tsunami to compare how warning systems and preparedness affect losses.
22. Explain how the 1700 Cascadia tsunami was identified using geologic and historical evidence.

## Quick Review Checklist

Before the quiz or test, make sure you can:

- define tsunami, runup, inundation, local tsunami, distant tsunami, tsunami watch, and tsunami warning;
- identify earthquake, landslide, volcanic, and extraterrestrial tsunami triggers;
- explain why vertical seafloor movement is important;
- describe the four stages of tsunami formation and development;
- compare tsunami speed, wavelength, and height in deep and shallow water;
- explain shoaling and wave amplification;
- identify factors that control runup and inundation;
- locate the Pacific Rim and other important tsunami-risk regions;
- distinguish primary from secondary effects;
- trace tsunami linkages to earthquakes, landslides, fires, pollution, disease, and technological disasters;
- explain why human activity changes consequences but not tsunami frequency or magnitude;
- describe tsunami detection and warning systems;
- explain the purpose of runup maps and probabilistic analysis;
- list the requirements for tsunami-ready status;
- explain the limits of seawalls and vegetation;
- respond correctly to natural warning signs and official warnings; and
- use the 2004 Indian Ocean, 2011 Tohoku, 1700 Cascadia, Sissano Lagoon, and Lituya Bay examples.