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It’s Not a Matter of If a Tsunami is Coming, it’s When and How Large?

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NOAA video demonstrating tsunami formation and landing

More than two dozen tsunamis have been recorded in San Diego since 1806, seven of which have caused damage, according to National Oceanic and Atmospheric Administration (NOAA) tsunami research (see list).

“Tsunami” comes from the Japanese words for harbor (“tsu”) and wave (“nami”), although these waves are more like a swiftly rising tide than a wave.

Tsunami waves can travel 500 miles per hour in the open sea, but slow to as little as 20 mph in shallower waters, increasing wave heights and including the full force of the water behind them.

Like the sets surfers wait for, the first tsunami wave is seldom the highest or the last. Additionally, the waves in these sets can arrive hours apart, depending on the distance at which they formed.

Danger to San Diego

Tsunamis that affect San Diego County communities can originate from local incidents or distant incidents.

Recently discovered underwater landslides may have caused large past tsunamis

USGS map shows large submarine “land”slides that could have created tsunamis in southern California (outlined in small white dots).

Local incidents:

The idea of underwater earthquake/landslides causing tsunamis is not new. They may well have been the cause of disastrous tsunamis in Santa Barbara (1812) and Point Arguello (1927). But only recently have scientists begun to discover areas off the San Diego coast where potential tsunami generating landslides  have occurred in the past and could occur in the future. For example, according to U.S. Geological Survey (USGS), new seafloor data has revealed huge landslide scars along the steep slopes of the continental shelf off Huntington beach, Palos Verdes, and five miles off the coast of Del Mar. Researchers estimate slides could recur every 10,000 years.

In the case of a local underwater landslide, residents would have only several minutes to evacuate and little time for emergency services to assist.

Distant incidents:

Distant incidents include large earthquakes in areas like Alaska, Chile, or Japan, and their tsunami waves take hours to arrive in San Diego. Each earthquake of magnitude 7.1 or larger is monitored by NOAA’s tsunami warning centers, which issue warnings within 15 minutes of the quake. Depending on the alert, San Diego emergency services will activate emergency local alerts for affected areas.

Worst Case Scenario? Emergency Services in San Diego County Have It Covered

1 a.m. People are sleeping and their electronics are off. A 9.1 magnitude earthquake occurs off Alaska and large tsunami waves begin rolling towards San Diego.

“We have a thoroughly vetted Tsunami evacuation plan,” says Del Mar Community Services Director & Chief Lifeguard Pat Vergne. “It doesn’t matter what time it is or where we are, because it is an ocean issue our lifeguards will get the alert, come in to assess the situation, and take appropriate action.”

As in all San Diego County beach areas, emergency services – firefighters, sheriff, lifeguards – City and County offices, and military if (San Diego Bay) together to assure residents are safe. In close contact, they watch the NOAA alerts and use their extended networks to assess tsunami size and strength in real time.

“After the 2011 Japan quake, we were in contact with our counterparts on the beach in Hawaii when the waves hit,” said Verne. “They were small enough that we knew with certainty we did not need to order an evacuation. If the waves had been larger, we would have worked in tandem with our firefighters, driving through the streets, announcing evacuation procedures over our loudspeakers, and helping those who needed it.”

What to do in case of a tsunami

According to Holly Crawford, San Diego County’s Office of Emergency Services Director, “If the earth shakes for 20 seconds, that’s a really significant earthquake. You need to leave the coast, go to a place that’s at least 100 feet above sea level or two miles inland if you can’t evacuate vertically.”

If surfing:

Do not try to surf tsunami waves. These waves are not like normal San Diego waves-you can’t duck dive under them. The current will drag you along with it and smash you into whatever is in its path. Get out of the water and follow the general guidelines below.

If boating:

According to the NOAA, if you are on the water, less than 100 fathoms (600 ft) deep, and you can’t get to shore and safety within 10 minutes of a local quake, pull up your anchor, cut your lines (if necessary), and head to an area deeper than 100 fathoms.

  • Proceed perpendicular to the shore
  • Sail directly into wind waves, keeping in mind that wind waves opposed by tsunami currents will be greatly amplified.
  • Maintain as much separation as possible from other vessels.
  • Synchronize movements with other vessels to avoid collisions.

If on shore

  1. Do not go to watch the waves from the beach, or piers. Multiple people have been swept into the sea and drowned.
  2. Know your evacuation routes. Signs have been posted all over San Diego County to help direct traffic, however it’s best to have a plan ahead of time.  (Click maps below to see larger version)
  3. Encinitas-tsunami-evacuation-map tsunami_flood_map_carlsbad tsunami_flood_map_oside tsunami_flood_map_solana Coronado MapSan-diego-tsunami-evacuation-map
    http://www.conservation.ca.gov/cgs/geologic_hazards/Tsunami/Inundation_Maps/SanDiego)
  4. Listen to the emergency alerts issued on television, radio, reverse 911 calls, social media, etc. They will provide instructions and updates for local communities. (Register your mobile phones at readysandiego.org to receive emergency notifications. Landlines are automatically included in the reverse 911 outreach.)
  5. Obey firefighters, lifeguards & police. Every beachfront community in San Diego has trained for tsunami evacuation.
  6. Take your emergency go bag
  7. Have a designated safe location
  8. Have a contact out of the tsunami zone that all family members can check in with.
  9. Do not return to your location until emergency responders give the all clear. Tsunami waves can come over several hours.
  10. Do not try to “ride it out”
  11. Help any neighbors who are unable to evacuate due to physical or health restrictions
  12. Call 211 for information. Do not call 911 unless you have an emergency. Keep the lines clear for those who need immediate help.

Improving Tsunami Detection

Along the Pacific Coast, Scripps Institution of Oceanography, NASA Jet Propulsion Laboratory, and NOAA have collaborated to developed a way to measure earthquake size, intensity, and potential damage through GPS, pressure, temperature, and seismic data in real-time.

This new technology, called  Seismogeodesy, “… will provide a more accurate magnitude estimate, within two to three minutes of the earthquake’s initiation, as well as the type of earthquake fault mechanism – two critical elements for local tsunami warning.” Says Dr. Yehuda Bock from the Scripps Institution of Oceanography. The initial estimates of larger quakes are often inaccurate, and can take up to 25 minutes to refine – Precious minutes needed by first responders to assess risk and implement evacuation procedures.

NASA is working with NOAA‘s to implement this technology in the Tsunami Warning Centers. There are also plans to expand the geographic reach of these technologies so that they will one day span the Pacific Rim.

Tsunamis that damaged San Diego:

May 27, 1862, San Diego, California. 5.9
Earthquake caused landslides into bay. Waves three-to-four feet higher.

August 13, 1868, Chile. 8.5
Wave 2.6 feet. Loading dock submerged and a residence flooded.

March 9, 1957, Aleutian Islands. 8.3
30 mph currents ripped out docks and boat slips on Shelter Island. Five large vessels damaged. 82-foot Coast Guard cutter broke its 1.5 inch cable and crashed into a yacht. Ocean dropped three feet and rose again in three minutes.

May 22, 1960, Chile. 8.6
Boats broke moorings and docks destroyed. Coast Guard cutter broke its mooring. barges broke in half, rammed bridges. Three 50 to 100 foot concrete sections were overturned.
Five harbor crewmen on boat swept out to sea. (Finally docked at Navy station.)
Reported waves eight feet above normal.

1964, March 28, Alaska. 8.4
Water rose 6.5 feet in ten minutes. one Surge broke mooring at the Bali Ha’i restaurant. Boats torn from their moorings. Moved two sections of a concrete pier at the Navy Amphibious Base anchored by 5,000 pound anchors. One dragged 100 yards.

2010 Feb. 27, Chile. 8.8
Minor damage to docks, boats, and concrete pillars off north Shelter Island.

2011, March 11, Japan. 9.0
Boat sunk and damage to dock in south Shelter Island.

See full NOAA list.