Oceanography Field Trip Pre-Lab:

Tsunami Hazards for the San Diego Coast

Tsunamis have struck the San Diego coastline in the past and will strike again. The map below shows major earthquake faults in Southern California, including known faults offshore. Over geologic time, movements on these faults have generated millions of earthquakes, creating a rugged offshore topography of deep basins, shallow banks, and islands. Tsunamis could be generated by sea floor movement during earthquakes on these faults, particularly where strike-slip (side-by-side moving) faults bend to create compression and up-down motion of the sea floor. Tsunamis could also be triggered by undersea landslides on the steep slopes in the offshore region.

Source: Surf, Sand, and Stone by Keith Meldahl (University to California Press)

Tsunami Causes and Measurement

Tsunamis are waves triggered by sudden displacements of ocean water. Their causes, from most common to least, are:

1. Shifting of undersea faults during earthquakes
2. Undersea landslides
3. Undersea volcanic eruptions
4. Large manmade explosions (such as underwater nuclear bomb tests)
5. Impacts of large meteorites

Of those causes, the first two—undersea earthquakes and undersea landslides—are the main ones of concern for Southern California.

When tsunami waves arrive at a coastline, one way to measure their impact is by run-up: the vertical elevation above normal sea level reached by the tsunami. During the great Japan tsunami of March 2011, parts of the coastline of Japan experienced run-ups of more than 30 meters (100 feet)! Evidence of run-up is usually obvious, as this photograph shows:

Run-up at the city of Ostuchi, Japan, March 2011 (Associated Press)

The Pacific Ring of Fire

The map below shows why most of the world’s tsunamis happen in the Pacific Ocean. The reason comes down to subduction—the process in which an oceanic plate dives into the planet at an ocean trench. Ocean trenches (the blue lines on the map) ring the Pacific. Each trench marks a place where a plate of oceanic lithosphere is subducting underneath the plate next door. As subducting plates dive into the mantle, they generate magma that rises to the surface to form active volcanic arcs. These volcanoes represent the “fire” of the Pacific Ring of Fire. Active volcanoes ring the Pacific because the Pacific is surrounded by subduction zones.

Pacific trenches and the Pacific Ring of Fire: Source: USGS

Subduction also generates earthquakes, and sometimes tsunamis. Most of the world’s most powerful earthquakes occur in subduction zones. How a subduction-generated earthquake can trigger a tsunami is illustrated by the figure below. The process, we think, goes like this (look at block diagrams 1, 2, 3, and 4 below):

1. As a plate of oceanic lithosphere subducts, it sticks to the edge of the plate above it.

2. The plate above is slowly bent as the subducting plate pushes relentlessly underneath.

3. The down-bent plate eventually snaps free and pops back it former shape and position.

4. This popping-back displaces ocean water above the plate, making a tsunami that travels outward in all directions at high speed.

A mechanism for how subduction can generate tsunamis. Source: USGS

Tsunami Hazards in San Diego

Southern California is, of course, on the edge of the Pacific Ocean: home to the Ring of Fire and most of the world’s tsunamis. So what is our tsunami risk here? A piece of good news is that no subduction is happening close to Southern California. As the map below shows, the nearest ocean trenches are the Cascadia Trench to the north and the Central America Trench to the south. Both are hundreds of miles from Southern California.

Source: USGS

Tsunamis generated by earthquakes in Pacific subduction zones have traveled across the ocean to our shores a number of times. The table below lists tsunamis that have hit San Diego with run-ups of 0.2 meters or more (8 inches or more) in historic time. Note that these have all been far-traveled tsunamis generated by distant earthquakes, with one exception: a tsunami from a local earthquake on May 27, 1862. (See the reference list at the end for data sources for this table.)

Date	Run-up	Tsunami source	Date	Run-up	Tsunami source
7-24-1854	0.33 m	Unknown source	3-9-1957	0.45 m	Rat Is, South Pacific
5-27-1862	1.0 m?	Local earthquake	5-22-1960	1.5 m	Central Chile
8-13-1868	0.8 m	Chile	3-27-1964	1.1 m	Southern Alaska
11-10-1922	0.2 m	Central Chile	12-26-2004	0.32 m	Indonesia
2-4-1923	0.2 m	Kamchatka, Siberia	2-27-2010	0.4 m	Chile
4-1-1946	0.37 m	Southern Alaska	4-25-2017*	0.3 m	Chile
11-5-1952	0.7 m	Kamchatka, Siberia

* This most recent tsunami was witnessed by Mira Costa Oceanography instructor Phil Stoffer and his students on April 24, 2017. The waves surged as high as 30 cm in San Elijo Lagoon, causing strong currents and turning the water cloudy with mud.

Note in the table above that the largest run up experienced historically in San Diego was from the 1960 Chile earthquake—the most powerful earthquake ever measured by modern instruments (magnitude 9.5). The tsunami waves arrived in Southern California 13 hours after the earthquake, with run-ups of about 1.5 meters (4 to 5 feet). The back-and-forth surge of the tsunami waves generated fierce currents that ripped boats from their moorings and smashed piers in San Diego and Los Angeles harbors. No fatalities were reported.

Risk from Locally Generated Tsunamis

We have seen above that far-traveled tsunamis—waves that arrive in Southern California after crossing the Pacific Ocean—are generally little threat. With few exceptions, far-traveled tsunamis have small run-ups (usually less than one meter) and cause minimal damage. But there are two causes that could produce local tsunamis—tsunamis triggered within a few miles of our coastline—that might be larger and more destructive: 1) undersea earthquakes on offshore faults, and 2) undersea landslides. A major local tsunami has not happened in Southern California in historic time, so oceanographers have turned to computer modeling to try to estimate how big tsunamis from these two local sources might be.

The table below lists estimated run-ups for four possible locally generated tsunamis: two from undersea earthquakes on specific offshore faults, and two from undersea landslides in areas that have steep slopes. (The reference list at the end of this document lists the technical papers from which these estimates are derived.) You can see the estimated run ups are larger than for the historic tsunamis listed in the previous table. The largest estimated run up is 6 meters—nearly 20 feet!

Potential run-up	Tsunami source
3.6 m	Computer modeling of a 7.6 magnitude earthquake on the Catalina Fault by Catalina Island.
3.5 m	Computer modeling of a 7.5 magnitude earthquake on the Coronado Bank Fault west of San Diego.
4.0 m	Computer modeling of a large undersea landslide in Coronado Canyon or Thirty-Mile Bank.
6.0 m	Computer modeling of a large undersea landslide on the San Pedro Escarpment offshore of Palos Verdes.

Scientists from NOAA and the USGS are continually working with insurance companies and coastal city and harbor managers to attempt to estimate potential damage from tsunamis. The good news is that nearly three quarters of the Southern California coastline has high sea cliffs, so there would be minor impacts in those areas. The bad news is that the remaining quarter of the coastline is lies close to sea level and is highly developed. Tsunami inundation maps (see below) show that large portions of these low-lying coastal areas would experience significant damage. Damage estimates from other regions of the world that have experienced major tsunamis—such as Japan, Alaska, Chile, and Hawaii—provide a basis for estimated potential damages in San Diego. One published estimate for the San Diego region projects costs of 40 billion dollars and 2000 people killed by a possible magnitude 6.9 earthquake on the partially submerged Rose Canyon Fault. (This damage estimate takes into account both the potential tsunami and land shaking from the earthquake.) This estimate is based on a published in a preliminary report sponsored by the Earthquake Engineering Research Institute and reported by the San Diego Union-Tribune (see Robbins 2017 in the reference list).

Tsunami Inundation Maps

One way that coastal communities can prepare for tsunamis is to construct tsunami inundation maps. A tsunami inundation map for Oceanside is shown below. The map shows estimated inundation from a 4-meter (about 13 feet) run-up. Note that this inundation map was produced before lessons learned from the great tsunami that struck Japan in March, 2011. The high damage and death toll of that tsunami occurred, in part, because maximum tsunami run-up estimates used in coastal planning and development had been underestimated. As we saw above, run-ups of up to 6 meters may be possible along parts of the San Diego coastline.

Tsunami Inundation Map for Emergency Planning, Oceanside Quadrangle/San Luis Rey Quadrangle

Pacific Tsunami Warning Center

On April 1, 1946, a magnitude 7.8 earthquake in the Aleutian Islands generated a 30-meter tsunami on Unimak Island, destroying Scotch Cap lighthouse and killing five Coast Guardsmen. The tsunami was up to 56 feet high in Hawaii, killing 173 people and causing over $26 million in damage. This tsunami was observed throughout the Pacific Basin, including in San Diego (see date 4-1-1946 in the table under “Tsunami Hazards in San Diego” above).

This may have been the most important tsunami in recent history because it resulted in the creation of the Pacific Tsunami Warning Center, the development of tsunami travel time charts, and the promotion of research and international cooperation. Today the Pacific Tsunami Warning Center is part of an international tsunami warning system run cooperatively by several countries. The system uses data from earthquakes, tide gauges, and pressure recorders on the seabed to forecast and track potential tsunamis.

References

Agnew, D., 1979. Tsunami history of San Diego. In: P Abbott and W Elliott, eds, Earthquakes and Other Perils: San Diego Region. San Diego Association of Geologists (1979), p117-122.

Barberopoulou, A., M.R. Legg, B. Uslu, and C.E. Synolakis, 2011. Reassessing the tsunami risk in major ports and harbors of California I: San Diego. Natural Hazards (2011), v. 58, p479-496.

Bohannon, R.G., and J.V. Gardner, 2004. Submarine landslides of San Pedro Escarpment, southwest of Long Beach, California. Marine Geology, v. 203, no. 3–4, p261-268.

Borrero, J. C., M. R. Legg, and C. E. Synolakis 2004, Tsunami sources in the southern California bight, Geophysical Research Letters, 31, L13211.

Lander, JF, Lockridge, PA, and Kozuch, MJ, 1993, Tsunamis Affecting the West Coast of the United States 1806-1992. (NOAA) NGDC Key to Geophysical Record Documentation No. 29. URL:
ftp://ftp.ngdc.noaa.gov/hazards/publications/Kgrd-29.pdf

Lee, H.J., H.G. Greene, B.D. Edwards, M.A. Fisher, and W.R. Normark, 2009, Submarine landslides of the Southern California Borderland. In: Lee, H.J., and W.R. Normark, editors, Earth Science in the Urban Ocean: The Southern California Continental Borderland. Geological Society of America Special Paper 454, 251-270.

Legg, M.R., J.C. Borrero, and C.E. Synolakis 2002. Evaluation of tsunami risk to Southern California coastal cities. 2002 NEHRP Professional Fellowship Report, funded by the Federal Emergency Management Agency and administered by the Earthquake Engineering Research Institute.

Moffatt and Nichol (consulting firm), 2007. Tsunami Hazard Assessment for the Ports of Long Beach and Los Angeles: Final Report. April 2007, M&N File 4839-169.

National Weather Service (2005), Historical Tsunami Information (recompiled by “SurfDog” website, accessed 2017)

Robbins, G., 2017. San Diego quake could kill 2,000, inflict $40B in property damage. San Diego Union Tribune, Sept 12, 2017.