Department of Conservation
The earliest efforts to explain great sea waves were made in legend and story by people who lived on land that bordered the ocean. Early Greek philosophers tried to explain the huge sea waves in various ways. The Greek historian Thucydides (fifth century B. C.) attributed the waves to earthquake forces. Aristotle believed the "air element" of Greek cosmology caused earthquakes which created the huge waves. Strabo, who contributed one of the last Greek explanations, came closest to modern theory when he wrote: "For the floor [of the sea] that is saturated with water is far more easily moved and is liable to undergo more sudden changes. . . . Deluges, as we have seen, are caused by upheavals of the bed of the sea; and earthquakes, volcanic eruptions, and upheavals of the submarine floor raises the sea, whereas the settling of the sea lowers the sea . . ." (Ambraseys, 1962).
At this point inquiry stopped. The origin of the great sea waves was considered again in the late Renaissance period and has been of interest and concern to scientists to the present day.
The terms "tsunami" and "tidal wave" are often used in the same context. Tsunami is a Japanese word meaning "harbor wave" and is the commonly accepted name for the phenomenon. Seismic sea wave is a synonym for tsunami. The term "tidal wave" is incorrect because the origin of tsunamis is not related to moon-caused tides (Hauge, 1972).
Tsunamis can be generated "by any large-scale, short-duration disturbance of the ocean floor, principally by a shallow submarine earthquake, but also by submarine earth movement, subsidence, or volcanic eruption" (Glossary of Geology, 1980). Submarine earthquakes generally occur in the trench areas that rim the Pacific Ocean (figure 1). Undersea earthquakes of Richter magnitude greater than 6.5 and focal depth of less than 50 kilometers are capable of producing tsunamis (Houston and others, 1974). However, earthquakes in this category do not always cause tsunamis (see article on p. 53???).
SEE FIGURE 1
Figure 1. Major trench areas, circum-Pacific belt of seismic activity. The contours represent the time in hours that it would take a tsunami generated at an earthquake epicenter in the Aleutian trench to reach various areas in the Pacific Ocean basin. A tsunami would take approximately 6 to 7 hours to reach California. Adapted from U. S. Department of Commerce, 1966.
Within the Pacific Ocean basin there are six principal regions with prominent trenches where sea floor movement occurs. The origin of earthquakes in trench areas is explained by the plate tectonic theory, which is beyond the scope of this article (CALIFORNIA GEOLOGY, October 1978 and September 1979). These undersea trenches border on Indonesia, Japan, Kamchatka (USSR), the Aleutian Islands (USA), Acapulco (Mexico), and Chile. A tsunami which originates at one of these areas and travels across the open ocean is called a "distant wave" (Gross, 1972). Five tsunamis from distant sources have damaged the California coast. These distant waves occurred in 1946 (Aleutian Islands), 1952 (Kamchatka), 1957 (Aleutian Islands), 1960 (Chile), and 1964 (Aleutian Islands) (Welday, 1980).
Distant waves (tsunamis) can also be generated at the site of volcanic eruption. The tsunamis caused by the great 1883 Krakatoa volcanic explosion caused advance of water (runup) onto foreshores in distant areas. Generally, however, tsunamis generated by volcanic activity are not likely to be destructive outside the local area in which the eruption occurs (Houston, 1974).
A local tsunami can be produced in the same manner as a distant wave tsunami; however, the travel time for the waves is obviously short. Often only a matter of minutes elapse before arrival of the first wave.
Along the California coast, the principal submarine fault alignments trend approximately southeast-northwest and are parallel to major faults on land that principally have had strike-slip (horizontal) movement. Therefore, earthquakes which produce horizontal displacement off the California coast are not considered likely to produce tsunamis. However, damaging local tsunamis have originated in the offshore areas of the Transverse Range province of California in 1812 and 1927 (Bolt, 1975).
Nearshore submarine landslides can generate local tsunamis that reach the shoreline within minutes. The magnitude of a landslide-caused tsunami is proportional to the volume of the slide material, rate of movement, and distance of slide movement. A slide of millions of cubic meters of material would be necessary to produce a damaging wave. Tsunamis generated in this way are known to have wave heights which reached 9 to 12 meters and to have a shoreward wave velocity of 120 km per hour.
It is possible for a massive landslide into a bay to generate a tsunami. A landslide-induced tsunami occurred on July 9, 1958 in Lituya Bay, Alaska, after a M 7.0 earthquake. The amplitude of the surge was reported to be 60 m and the wave stretched completely across the bay. Fishing boats were carried over 25 m high trees on a peninsula and out to sea (Bolt, 1975).
A tsunami consists of a series, or train, of waves. The number of waves and variations in wave heights depends on the energy force of the sea floor disturbance and on the configuration and depth of the ocean bottom. The sudden causative movement creates a series of waves that travel across the open ocean almost unnoticed until they approach a shallow coastal area. On the ocean surface the waves have low height (one meter or less), a long wavelength (up to 200 km), varying wave periods (five minutes to several hours between wavecrests), and a velocity of up to 60 km per hour.
Tsunami characteristics (such as wave heights, wave period, and velocity) can vary due to offshore topographic irregularities. As the tsunami approaches a coastal region where the water depth decreases, the waves are restricted by the shallow bottom. The wave velocity decreases to about 32 km per hour, wave height increases (up to 9 meters or more), and the tremendous wave energy strikes the shoreline.
Waves that are generated by seismic forces are stronger and retain their height longer than waves generated by wind. The destructive power of tsunamis is due to the speed and force with which they strike a coastal area.
EFFECT OF WAVES
The physical configuration of a particular harbor or bay is an important factor in governing the height of a tsunami in that particular bay. The reflection of wave energy by the shape and topographic changes of a harbor or bay can lead to resonance (seiche effect). When this happens the amplitude (wave height) of the resulting wave is often greater than the original tsunami.
Runup (flooding of shoreline) is equal to the wave amplitude at the shoreline. On the west coast of the United States, tsunami action is generally manifested in the form of a relatively slow change in water level; that is, the tsunami appears essentially as a rapidly rising and falling tide and, like a tide, the runup equals the wave amplitude. During the 1964 tsunami at Crescent City, California (photo 1), the wave amplitude (height) was estimated to be approximately 6 meters based on estimates from tide gauge records (water level exceeded the range of the tide gauge recorder).
SEE PHOTO 1
Photo 1. Tsunami damage at Crescent City, California, 1964. U. S. Army Corps of Engineers photo.
PUBLIC WARNING SYSTEM
The U. S. Environmental Science Services Administration (ESSA) operates warning systems for tornadoes, hurricanes, floods, severe storms, and tsunamis. The Pacific Tsunami Warning Center of ESSA, located in Hawaii, monitors a network of seismological and tidal instruments around the Pacific Ocean region (U. S. Department of Commerce, 1966).
The first indication of a tsunami usually comes from the monitoring station nearest to the disturbance. Because tsunamis move through the ocean in accordance with known physical laws, an estimated time of arrival (ETA) for the initial wave can be calculated and given to each coastal station in the warning system (figure 1).
When an earthquake of sufficient magnitude to generate a tsunami occurs in the Pacific Ocean area, Pacific Tsunami Warning Center personnel determine the location of the earthquake epicenter. If the epicenter is under or near the ocean, tsunami generation is assumed to be possible and the center issues a tsunami WATCH to emergency units. The tsunami WATCH is the center's first warning advisory.
When existence of a tsunami is confirmed, the center issues a WARNING and gives each station the tsunami ETA. The WARNING advises of the approach of a potentially destructive seismic sea wave.
Within California, these advisories are sent to the Department of the Army, Office of the Civil Defense Warning Center, and are forwarded to the California Office of Emergency Services (OES), Sacramento or, if OES cannot be reached, to the California Highway Patrol, Sacramento. The State Warning Control Officer evaluates the significance of the tsunami messages and alerts local police, sheriffs, and Civil Defense Directors.
Evacuation of the coastal area where tsunami waves may strike is essential to avoid loss of life. When a local tsunami wave is generated, even with fast communications there generally is not enough time to warn and evacuate local residents. Feeling an earthquake may be the only warning for coastal residents. The time interval between waves may last for extended periods (6 to 12 hours), but many people have the tendency to return after the first wave passes regardless of prior warning. This error in judgment can cause loss of human life. The casualties, major damage (photo 1), and fires which occurred in Crescent City, California, after the 1964 Alaskan earthquake, resulted from the fourth wave, which arrived 1 hour and 16 minutes after the first wave.
Tsunami damage to onshore structures can be very extensive and costly. The powerful wave-action can (1) sweep structures off foundations, (2) batter buildings with accumulated shoreline debris carried on the advancing wave front, (3) undercut foundations and pilings, and (4) overturn structures by suction of receding waves.
To calculate potential damage to structures several factors must be considered including tsunami characteristics, exposure of coastline, configuration of local bays and harbors, area of inundation of the coastal zone, and the value of the property in the affected coastal zones.
One of the major causes of tsunami damage is surge-carried debris piled onto the shore. Elimination of debris from the shore areas can reduce tsunami damage. Seawalls, dikes, or breakwaters may be installed to shield low-lying areas, if the cost can be justified. Trees, which are deep-rooted and grow with branches high off the ground, are very resistant to tsunamis. They can be used as effective barriers to partially dissipate the tsunami and catch the debris carried in the wave.
During the 1964 Alaska earthquake and tsunami event, oil tanks at Whittier, Alaska were a source of another type of damage. Small oil tanks located at shore line oil tank farms were crushed against larger tanks during the tsunami surge. The impact caused the oil tanks to rupture; the oil ignited and fire damage was extensive.
At the present time, the only factor concerning seismic sea waves that can be predicted by general rule is the time of arrival of the initial wave on the coast (figure 1). Factors concerning wave height, runup, resonance, and wave refraction have to be established by researchers by means of numerical models for each unique coastal region. Two areas of California (Crescent City and San Francisco) have enough historical data to confirm numerical models (Houston and Garcia, 1978).
Tsunamis are natural, uncontrollable phenomena, and can cause extensive loss of life and damage to property in coastal areas. Most tsunamis that have affected California were caused by distant earthquakes in the circum-Pacific belt of seismic activity (figure 1).
The public warning system can provide some measure of protection by alerting the coastal residents to evacuate the area.
Tsunami hazard zones on the California coast have been identified (Alfors and others, 1973; figure 2). Land-use planning to avoid construction in the most vulnerable coastal areas would eliminate some property damage.
SEE FIGURE 2
Figure 2. Tsunami hazards in California. From Alfors and others, 1973.
Alfors, J. T., Burnett, John L., and Gay, Thomas E. Jr., 1973, Urban geology master plan for California: California Division of Mines and Geology, Bulletin 198, p. 41-43, p. 63-67.
Ambraseys, N. N., 1962, Data for investigation of the seismic sea-waves in the eastern Mediterranean; Bulletin of the Seismological Society of America, v. 52, no. 4, p. 895-913.
Bolt, B. A., Horn, W. L., Macdonald, G. A., and Scott, F. A., 1975. Geological Hazards: Springer-Verlag.
Camfield, F. E., 1977, Tsunami-structure interaction, Symposium on Tsunamis: Department of Fisheries and the Environment, Ottawa, Canada, p. 194-195,
Glossary of Geology, 1980; American Geological Institute.
Gross, M. G., 1972, Oceanography: A view of the earth: Prentice-Hall, Inc., Englewood Cliffs, New Jersey, 581 p.
Hauge, Carl J., 1972, Tides, currents, and waves: CALIFORNIA GEOLOGY, v. 25, no. 7, p. 147-160.
Houston, J. R., and others, 1974, Type 16 flood insurance study: Tsunami Predictions for Pacific Coastal Communities; U. S. Army Corps of Engineers, Waterways Experimental Station, Technical Report AEWES-TR-H-74-3, p. 139.
Houston, J. R. and Garcia, A. W., 1974, Tsunami runup predictions for southern California coastal communities, U. S. A.: U. S. Army Corps of Engineers, Waterways Experimental Station, 17 p.
Houston, J. R. and Garcia, A. W., 1978, Type 16 flood insurance study: Tsunami prediction for the West Coast of the Continental United States, Technical Report H-78-26; U. S. Army Corps of Engineers Waterways Experiment Station, P. O. Box 631, Vicksburg, MS 39180.
Svyatlovski, A. E., 1977, Tsunami-structure interaction, Symposium on Tsunamis: Department of Fisheries and the Environment, Ottawa, Canada, p. 194-195.
U. S. Department of Commerce, 1966, Tsunami: Watch and warning: Environmental Science Services Administration, ESSA/P1 660026.
Welday, E. E., 1980, Tsunami hazards along the Orange County coastal zone, Appendix D, in Morton P. K. and others, 1980, Environmental geology of Orange County California: California Division of Mines and Geology, Bulletin 204 (in press).