The History of Tsunami Hazard Mitigation

Aerial view of the coast of Valzez AK after the 1964 tsunami

Figure 1: Valdez AK after the 1964 tsunami (Credit NOAA)

The history of tsunami hazard mitigation tracks well with the history of destructive tsunamis in the United States. Following the 1946 Alaska-generated tsunami that killed 173 people in Hawaii, the Pacific Tsunami Warning Center was established in Hawaii in 1949 by a predecessor agency to the National Oceanic and Atmospheric Administration (NOAA). Following the 1960 Chilean tsunami that killed 1000 people in Chile, 61 in Hawaii, and 199 in Japan, the United States formed the Joint Tsunami Research Effort (JTRE) and staffed the International Tsunami Information Center (ITIC) in Hawaii. JTRE was formed to conduct research on tsunamis while ITIC, sponsored by the United Nations, was formed to coordinate tsunami warning efforts of the Pacific Countries. Many research and mitigation efforts were focused on the distant tsunami problem. Following the 1964 Alaskan tsunami that killed 117 in Alaska, 11 in California, and 4 in Oregon, the U.S. was confronted with the local tsunami problem. In response, the U.S. established the Alaska Tsunami Warning Center in Palmer, Alaska in 1969.

In 1992, a Ms 7.2 earthquake in California generated a tsunami that killed no one. However, the earthquake was the first subduction zone earthquake recorded on the U.S. west coast with modern instruments. The earthquake triggered concern that larger earthquakes could generate large local tsunamis along the heavily populated west coast. In response to the local tsunami threat, the National Tsunami Hazard Mitigation Program (NTHMP) was formed in 1997 and the Alaska Tsunami Warning Center was renamed the West Coast and Alaska Tsunami Warning Center.

In 1994, Congress asked NOAA, responsible for issuing tsunami warnings to the U.S., to assess tsunami awareness and preparedness of the west coast for local tsunamis. Congress asked NOAA to lead a group of representatives from the U.S. Geological Survey (USGS), the Federal Emergency Management Agency (FEMA), and from emergency management agencies in the states of Alaska, California, Hawaii, Oregon, and Washington to formulate a plan of action. This group formed a State/Federal partnership, the National Tsunami Hazard Mitigation Program, which developed a 5-year implementation plan, including a budget. Congress funded the implementation plan beginning in 1997.

The 2004 Sumatra tsunami, which killed over 237,000 people along the Indian Ocean coastline, reminded the world of the tsunami hazard. The United States contributed about $1 billion to rescue, relief, and rehabilitation in the Indian Ocean countries. The United States also expanded its warning services to include the East and Gulf Coasts of the U.S. Congress also provided funds to significantly upgrade the tsunami warning system by incorporating new technology into the tsunami warning operation and establishing 24 x 7 office staffing at each warning center.

DART™ buoy development and creation of the Tsunameter

Over the past 20 years, NOAA's Pacific Marine Environmental Laboratory (PMEL) has identified the requirements of the tsunami measurement system through evolution in both technology and knowledge of deep ocean tsunami dynamics.

Warning Systems

A modern tide gauge used in harbors to provide direct measurements of tsunamis

Figure 2: Tide Gauge (Credit NOAA)

Since 1946, the tsunami warning system has provided warnings of potential tsunami danger in the Pacific basin by monitoring earthquake activity and the passage of tsunami waves at tide gauges. However, neither seismometers nor coastal tide gauges provide data that allow accurate prediction of the impact of a tsunami at a particular coastal location. Monitoring earthquakes gives a good estimate of the potential for tsunami generation, based on earthquake size and location, but gives no direct information about the tsunami itself. Tide gauges (Figure 2) in harbors provide direct measurements of the tsunami, but the tsunami is significantly altered by local bathymetry and harbor shapes, which severely limits their use in forecasting tsunami impact at other locations. Partly because of these data limitations, 15 of 20 tsunami warnings issued since 1946 were considered false alarms because the tsunami that arrived was too weak to cause damage.

The theory of ocean waves guided the solution to forecasting tsunamis to avoid over warning. By measuring the tsunami in the deep ocean, the tsunami measurements were free of coastal influences and could be used as input to forecast models. The direct measurement of a tsunami in the deep ocean required a highly sensitive pressure gauge. Instruments used in the oil services industry were adapted for use in the deep ocean. The main problem with this approach was the instrument had to be placed on the ocean floor to detect the minute changes in pressure as the tsunami passed over the instrument. The technical feat of transmitting data from an instrument on the sea floor to a tsunami warning center in real time required exceptionally creative engineering The new tsunami measuring technology has given science a new instrument—the tsunameter—that provides tsunami researchers and practitioners with the basic information to understand and predict tsunamis. In 2003, a real-time tsunameter detected a non-destructive tsunami which led to the early cancellation of a tsunami warning and averted an unnecessary evacuation in Hawaii. For this significant feat, the Department of Commerce awarded NOAA its highest award, the Gold Medal.

The first-generation tsunameter was named Deep-ocean Assessment and Reporting of Tsunamis (DART). The DART™ design featured an automatic detection and reporting algorithm triggered by a threshold wave-height value. The DART™ II design (Figure 3) incorporated two-way communications that enables tsunami data transmission on demand, independently of the automatic algorithm; this capability ensures the measurement and reporting of tsunamis with amplitude below the auto-reporting threshold. The next generation DART™ ETD (Easy To Deploy) buoy is presently under development at PMEL.

DART II System schematic shows a surfacec buoy, tsunameter and communications and control methods

Figure 3: Larger image (Credit NOAA/PMEL)

The tsunami forecasting technology developed at PMEL is based on the integration of real-time tsunameter measurements and modeling technologies, a well-tested approach used in most hazard forecast systems.

Forecasting impacts

Recently developed real-time, deep ocean tsunameter provides the data necessary to make tsunami forecasts. The November 17, 2003, Rat Island tsunami in Alaska provided the first comprehensive test for the forecast methodology. The Mw 7.8 earthquake on the shelf near Rat Islands, Alaska, generated a tsunami that was detected by three tsunameters located along the Aleutian Trench. These real-time data combined with the model database were then used to produce the real-time model tsunami forecast. For the first time, tsunami model predictions were obtained during the tsunami propagation, before the waves had reached many coastlines. The initial offshore forecast was obtained immediately after preliminary earthquake parameters (location and magnitude Ms = 7.5) became available from the West Coast/Alaska TWC (about 15-20 minutes after the earthquake). The model estimates provided expected tsunami time series at tsunameter locations. When the closest tsunameter recorded the first tsunami wave, about 80 minutes after the tsunami, the model predictions were compared with the deep-ocean data and the updated forecast was adjusted immediately.

graph of Rat Island AK tsunami showing water depth as measured at the tsunameter

Figure 4: Rat Island, Alaska Tsunami of November 17, 2003, as measured at the tsunameter located at 50 N 171 W in 4700 m water depth. (Credit NOAA/PMEL)

comparison of forecast and measured gauge data for coastal Hilo HI for Rat Island tsunami

Figure 5: Coastal forecast at Hilo, HI for 2003 Rat island, showing comparison of the forecasted (red line) and measured (blue line) gauge data. (Credit NOAA/PMEL)

These offshore model scenarios were then used as input for the high-resolution inundation model for Hilo Bay. The model computed tsunami dynamics on several nested grids, with the highest spatial resolution of 30 meters inside the Hilo Bay. None of the tsunamis produced inundation at Hilo, but all of them recorded nearly half a meter (peak-to-trough) signal at Hilo gauge. Model forecast predictions for this tide gauge are compared with observed data in Figure 5. The comparison demonstrates that amplitudes, arrival time and periods of several first waves of the tsunami wave train were correctly forecasted. More tests are required to ensure that the inundation forecast will work for every likely-to-occur tsunami. When implemented, such forecast will be obtained even faster and would provide enough lead time for potential evacuation or warning cancellation for Hawaii and the U.S. West Coast.

Reduction of impacts

The recent development of real-time deep ocean tsunami detectors and tsunami inundation models has given coastal communities the tools they need to reduce the impact of future tsunamis. If these tools are used in conjunction with a continuing educational program at the community level, at least 25% of the tsunami-related deaths might be averted. By contrasting the casualties from the 1993 Sea of Japan tsunami with that of the 1998 Papua New Guinea tsunami, we can conclude that these tools work. For the Aonae, Japan case about 15% of the population at risk died from a tsunami that struck within 10 minutes of the earthquake because the population was educated about tsunamis, evacuation plans had been developed, and a warning was issued. For the Warapa, Papua New Guinea case about 40% of the at risk population died from a tsunami that arrived within 15 minutes of the earthquake because the population was not educated, no evacuation plan was available, and no warning system existed.