Revisiting tsunami survival strategies

The tragic story of the Indian Ocean earthquake and tsunami is still unfolding, and a full accounting of the effects of the destructive sea waves is sure to be made in the coming months. For residents and visitors of Hawai`i, the startling eyewitness accounts and stunning video of deafening waves racing onshore and rapidly retreating serve as a reminder of the destructive power of tsunamis and the action we must take to reduce risks to ourselves and others.

With great sorrow and shock, the world is bearing witness to the horrendous effects of the earthquake and tsunami that occurred in the Indian Ocean two weeks ago. Even as humanitarian aid and assistance pour into the countries impacted, the full scope of destruction and human suffering caused by the waves and earthquake are difficult to grasp.

The magnitude-9.0 earthquake occurred on December 26 beneath the ocean off the west coast of northern Sumatra Island, Indonesia. It was located at a depth of about 30 km (18.6 miles) along one of Earth’s subduction zones, where the India Plate is diving beneath the Burma Plate.

During the earthquake, scientists surmised that the sea floor overlying the subduction zone was suddenly uplifted by several meters (yards) along the zone for hundreds of kilometers (miles). In that same instant, the overlying water was also pushed upwards, spawning the massive tsunami that raced toward nearby Sumatra and across the Indian Ocean.

Within a few hours of the earthquake, tsunamis swept onto the shorelines of 11 countries from west Asia to east Africa. Survivor’s stories and video depict a frightening series of waves or rising tides that rushed onshore, each followed by the retreat of fast-moving currents carrying a great mixture of debris—smashed homes, trees, cars, and wreckage—and people.

Along the west coast of Thailand, lined with hotels and resorts, Japanese scientists described waves 10.5 m (34 feet) high moving at speeds of nearly 30 km (18 miles) per hour—a speed too fast to outrun. Across the Indian Ocean the largest wave struck after the first few waves came ashore. They kept coming for several hours – typical of tsunami behavior.

More than 150,000 people were killed by the tsunamis. The death toll in Indonesia alone is estimated at more than 94,000. In Sri Lanka, more than 46,000 were swept to their deaths. News reports describe tens of thousands more people still missing and hundreds of thousands homeless. The World Health Organization estimates that more than 500,000 people are injured and in need of medical care and warns that 150,000 are at extreme risk if a major disease outbreak occurs in the affected areas of Indonesia.

In Hawai`i, potentially damaging tsunamis may originate from massive subduction-zone earthquakes located around the margins of the Pacific Ocean or from earthquakes on the Island of Hawai`i.

Depending on the location of a distant great earthquake, the resulting tsunami may take 5 to 15 hours to reach the Hawaiian Islands. The Pacific Tsunami Warning Center will issue warnings hours before the first wave arrives, and Hawai`i Civil Defense will oversee the evacuation of areas at risk from these distant sea waves.

On Hawai`i Island, a locally generated tsunami is perhaps the most hazardous type, because the time interval between a large local earthquake and the arrival of the first wave may be too short to warn people to evacuate. A magnitude-8 earthquake in 1868 and a 7.2-earthquake in 1975 both generated a tsunami that struck in minutes and sent waves as high as 14 m (45 feet) on the southeast shore of Hawai`i and nearly 3 m (9 feet) in Hilo.

A local earthquake strong enough to make it difficult to stand or walk should be regarded as a tsunami warning by people in coastal areas. When the ground shakes violently from a strong earthquake or the sea suddenly recedes, go to high ground immediately.

For information on surviving a tsunami in Hawai`i, see the disaster preparedness information and tsunami evacuation maps inside a Hawaiian telephone book and visit the following Web sites:

Activity Update

Eruptive activity at Pu`u `O`o continues. Spatter cones in the crater of Pu`u `O`o glow brightly on clear nights but have not produced any lava flows for several months. The MLK vent area, at the southwest base of the cone, intermittently erupts small pahoehoe flows that stack up close to the vent.

The PKK flow continues to host substantial breakouts from above the top of Pulama pali to the coastal plain. Lava is not entering the ocean. As of January 5, sluggish breakouts were active on the coastal plain, about 200 m (220 yd) inland of the shore at Lae`apuki. The area of breakouts is about 3.2 km (2 mi) from the end of the pavement on Chain of Craters Road in Hawai`i Volcanoes National Park. Expect a 2-hour walk each way and remember to bring lots of water. Stay well back from the sea cliff, regardless of whether there is an active ocean entry or not. Heed the National Park warning signs.

During the week ending January 5, one earthquake was felt on our Big Island. A magnitude-3.2 quake was reported felt at Papa`aloa and Volcano. It was located 15 km (9 miles) east of Laupahoehoe at a depth of 28 km (18 miles).

Mauna Loa is not erupting. The summit region continues to inflate. Since July 2004, the rate of inflation and number of deep earthquakes has increased. Weekly earthquake counts have varied from 5 to over 150. During the week ending January 5, only 8 earthquakes were recorded beneath the summit area. This is a significant decrease from the past week. Nearly all are 30 km (18 mi) or more deep and are the long-period type, with magnitudes less than 3.

This article was written by scientists at the U.S. Geological Survey’s Hawaii Volcano Observatory and is republished by with permission.

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