Get Ready & Prepare

Tsunamis

Overview

A tsunami is a wave or series of waves, caused by a displacement of the ocean floor, generated by an earthquake, landslide, volcanic eruption, or even large meteor hitting the ocean. What typically happens is a large, submarine earthquake (magnitude 8 or higher) creates a significant upward movement of the sea floor resulting in a rise or mounding of water at the ocean surface. The mound of water moves away from this center in all directions as a tsunami. These events are also often referred to as tidal waves or seismic sea waves.

A tsunami wave is not a tall breaking wave or a tidal wave, although that term is a common misnomer for a tsunami. A tsunami actually resembles a flood or surge, consisting of several waves or surges. The first tsunami surge is often not the highest and the largest surge may occur hours after the first wave. It is not possible to predict how many surges or how much time will elapse between waves for a particular tsunami. This will become a major issue for public safety officials responding to a tsunami. The configuration of the coastline, the shape of the ocean floor, and the characteristics of advancing waves play important roles in the destructiveness of the waves. Offshore canyons can focus tsunami wave energy and islands can filter the energy. The orientation of the coastline determines whether the waves strike head-on or are refracted from other parts of the coastline. A wave may be small at one point on a coast and much larger at other points. Bays, sounds, inlets, rivers, streams, offshore canyons, islands, and flood control channels may cause various effects that alter the level of damage. It has been estimated, for example, that a tsunami wave entering a flood control channel could reach a mile or more inland, especially if it enters at high tide.

The first visible indication of an approaching tsunami may be recession of water caused by the trough preceding the advancing, large inbound wave crest. Rapid draw down can create strong currents in harbor inlets and channels that can severely damage coastal structures due to erosive scour around piers and pilings. As the water’s surface drops, piers can be damaged by boats or ships straining at or breaking their mooring lines. The vessels can overturn or sink due to strong currents, collisions with other objects, or impact with the harbor bottom. Conversely, the first indication of a tsunami may be a rise in water level. The advancing tsunami may initially resemble a strong surge increasing the sea level like the rising tide, but the tsunami surge rises faster and does not stop at the shoreline. Even if the wave height appears to be small, 3 to 6 feet for example, the strength of the accompanying surge can be deadly. Waist-high surges can cause strong currents that float cars, small structures, and other debris. Boats and debris are often carried inland by the surge and left stranded when the water recedes.

At some locations, the advancing turbulent wave front will be the most destructive part of the wave. In other situations, the greatest damage will be caused by the outflow of water back to the sea between crests, sweeping all before it and undermining roads, buildings, bulkheads, and other structures. This outflow action can carry enormous amounts of highly damaging debris with it, resulting in further destruction. Ships and boats, unless moved away from shore, may be dashed against breakwaters, wharves, and other craft, or be washed ashore and left grounded after the withdrawal of the seawater.

Tsunamis are typically classified as local or distant. Locally generated tsunamis have minimal warning times, leaving few options except to run to high ground. They may be accompanied by damage resulting from the triggering earthquake due to ground shaking, surface faulting, liquefaction, or landslides. Distant tsunamis may travel for hours before striking a coastline, giving a community a chance to implement evacuation plans.

History

Ten observed tsunamis generated waves in Monterey County. Almost all of the tsunamis were produced by earthquakes and resulted in wave run-ups of 1 meter or less. In Monterey County, two earthquakes have caused recorded tsunami run-up heights that exceeded 1 meter. The 1960 Chilean Earthquake of estimated magnitude 9.5 (largest ever recorded) caused a 1.1-meter run-up-and killed one person, while the slightly smaller but nearer 1964 Alaska earthquake of magnitude 9.2 caused a 1.4-meter run up and significant boat damage at harbors in the Monterey Bay area.

In 1989, the magnitude 6.9 Loma Prieta earthquake, which was centered inland on the San Andreas fault in the Santa Cruz Mountains, triggered a small to moderate landslide offshore in Monterey Canyon. This caused a 1-foot tsunami measured at the tide gauge in Monterey. In 2011, a tsunami caused by an earthquake off the coast of Japan created large and rapid changes in water level (up to 6 feet) at Moss Landing, causing large volumes of water to rush in and out of the north and south harbor areas. This ebbing and flowing combined with large sediment transport resulted in shear stresses on dock structures in the harbor, causing approximately $1.5 million in damages to 220 timber dock piles.

Tsunami Risk

Tsunamis are a threat to life and property to anyone living near the ocean, but the impact of a tsunami can vary widely. The entire coastal area of Monterey County is susceptible to a tsunami. The Big Sur coast is less susceptible to significant tsunami run-up due to the rugged and steep cliffs of the coastal mountains. However, the coastal low-lying areas and riverine valleys to the north are highly susceptible to tsunamis.

The tsunami’s size and speed, as well as the coastal area’s form and depth, affect the impact of a tsunami. A small tsunami may result in unusual tides or currents that can be dangerous to swimmers or cause damage to berthed boats. Powerful tsunamis can level structures, and result in significant loss of human life. Tsunami waves can persist for many hours because of complex interactions with the coast. Large tsunamis cause strong rips and currents in oceans around the world for up to a few days after the initiating earthquake. When they strike land, most tsunamis are less than 10 feet high, but in extreme cases, they can exceed 100 feet near their source. A tsunami may come onshore like a fast-rising flood or a wall of turbulent water, and a large tsunami can flood low-lying coastal areas more than a mile inland. Rushing water from waves, floods, and rivers is incredibly powerful. Just six inches of fast-moving water can knock adults off their feet, and twelve inches can carry away a small car. Tsunamis can be particularly destructive because of their speed and volume. They are also dangerous as they return to the sea, carrying debris and people with them. Harbors in particular are vulnerable to damage from strong tsunami currents.

Learn if You Live in a Tsunami Inundation Zone

Prepare for a Tsunami

Preparing for a tsunami is very similar to preparing for any emergency, make a plan, have emergency supplies at home, stay alert, and sign up for alerts.

• Learn the signs of a potential tsunami, such as an earthquake, a loud roar from the ocean, or unusual ocean behavior, such as a sudden rise or wall of water or sudden draining of water showing the ocean floor.
• Know and practice community evacuation plans. Some at-risk communities have maps with evacuation zones and routes. Map out your routes from home, work and play. Pick shelters 100 feet or more above sea level, or at least one mile inland.
• Consider earthquake insurance and a flood insurance policy through the National Flood Insurance Program (NFIP). Standard homeowner’s insurance does not cover flood or earthquake damage.

Stay Safe During a Tsunami

If a large earthquake occurs, you receive a tsunami warning, or you see water receding rapidly. Leave the coast immediately moving to higher ground and as far inland as possible. Ideally 2 miles inland and 100 feet above sea level. Evacuation routes often are marked by a wave with an arrow in the direction of higher ground. Do not return until officials say it is safe, remember a tsunami is a SERIES of waves and the first wave may not be the largest

• If you cant evacuate, go to the upper floor of a sturdy building such as a hotel or hospital or climb a tree as a last resort.
• If you are in the water, then grab onto something that floats, such as a raft or tree trunk.
• If you are in a boat, contact the coast guard (831-647-7300), Harbor Master or Port Captain immediately for advice, as what to do will vary based on the current tsunami risk created by the earthquake. If there is an tsunami advisory and you are in deep water (180+ feet/ at least 600 feet from the coast) you may need to stay there until the tsunami threat is gone. If you are near land, you will either be advised to go to deep water or quickly dock the boat and move inland, depending on tsunami size, estimated time of arrival, and weather conditions.
• If you are swept up by the water, grab onto something that floats, such as a raft, tree trunk, or door.



Stay Safe After a Tsunami

• Listen to local alerts and authorities for information on areas to avoid and shelter locations.
• Save phone calls for emergencies. Phone systems often are down or busy after a disaster. Use text messages or social media to communicate with family and friends.
• Avoid wading in floodwater, which can contain dangerous debris. Water may be deeper than it appears.
• Be aware of the risk of electrocution. Underground or downed power lines can electrically charge water. Do not touch electrical equipment if it is wet or if you are standing in water.
• Stay away from damaged buildings, roads and bridges.
• If you become injured or sick and need medical attention, contact your healthcare provider and shelter in place, if possible. Call 9-1-1 if you are experiencing a medical emergency.
• Document property damage with photographs. Conduct an inventory and contact your insurance company for assistance.



Additional Resources

• Tsunami Information Sheet (PDF)
• Protective Actions Research for Tsunami
• NOAA Tsunami program
• American Red Cross Tsunami Preparedness
• Tsunami Safety (weather.gov)