Multi-Hazard Mitigation Plan: 09. Tsunamis

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Chapter 9:Hazard Analysis—Tsunamis CHAPTER 9 - TSUNAMIS 9.1 Description of Hazard A tsunami is a series of great waves most commonly caused by violent movement of the sea floor, usually a fault resulting in an earthquake, but also caused by nearshore or underwater landslides or volcanic eruptions. In contrast, regular ocean waves are generated from the wind. Tsunami are characterized by exceptional speed(up to 590 mph), long wave length(up to 120 miles), long period between successive crests (varying from 5 minutes to a few hours, generally 10 to 60 minutes), and low height in the open sea. Often the first wave of a tsunami may not be the largest. The danger from a tsunami can last for several hours after the arrival of the first wave. Sometimes a tsunami causes the water near the shore to recede, exposing the ocean floor. Tsunamis can be very large. At the shoreline, their height can be initially as great as 30 feet or more (100 feet in extreme cases), and they can move inland several hundred feet. Tsunamis can travel up rivers and streams that lead to the ocean. The speed of onset depends upon whether the tsunami is a distant tsunami or a local tsunami. Distant tsunamis result from earthquakes in Alaska, Japan, Chile, and other Pacific Rim loca- tions (see Figure 9-1). A warning is almost certain and ranges from three hours to 12 hours. Local tsunamis result from earthquakes or underwater landslides in the vicinity of the Hawai- ian Islands, most frequently near the south coast of this island with the probability much higher for the Ka'u-Puna districts than for South Kona. Adequate advance warning for local tsunamis is not presently technologically possible. Therefore, the public must be educated that the occurrence of strong ground shaking in a coastal area should be considered as a local tsunami"warning"and an immediate coastal evacuation should follow. 9-1 Hawaii County Multi-Hazard Mitigation Plan