Categorized | Sci-Tech

Volcano Watch: Be akamai — when in doubt, head mauka

U.S. Geological Survey Hawaiian Volcano Observatory staff and volunteers map run-up elevations and inundation distances of tsunami waves that reached the shores around Hawaii Island following the magnitude-9.0 earthquake in Japan. (Photo courtesy Andrew R. Hara | USGS)

(Volcano Watch is a weekly article written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory.)

By now, everyone is familiar with the magnitude-9.0 (M9.0) earthquake and tsunami that devastated Japan on March 11, 2011, and arrived at Hawaii Island early that same day.

During the first few days, most residents were aware of the damage in Kona and thought that the tsunami hit only the west side of the island. However, wave heights on the east side were comparable to most locations on the west side!

Geologists from HVO have traversed the island, measuring wave inundation distances and run-up. On average, water levels reached 1.8 m (6 ft) high across the island, with a few locations where heights exceeded 3 m (10 ft) at Kealakekua, Keauhou, North Kona, and a few isolated areas in South Kona.

“Tsunami” is a Japanese word meaning “harbor wave” even though most are generated by earthquakes that occur in submarine or coastal environments. Furthermore, tsunamis can be triggered by whole-scale uplift or subsidence of the ocean floor. Tsunamis can also be generated by slope failures or landslides that displace water and generate a “wave.”

The Hawaiian Islands, situated in the middle of the Pacific Ocean, is no stranger to tsunamis. Distant earthquakes have generated tsunamis that have inundated our shores on 29 occasions. Roughly half of these tsunamis resulted in damage here.

Two memorable tsunamis occurred in 1946 and in 1960. Each originated from different regions around the Pacific: the 1946 tsunami from the Aleutian Islands of Alaska and the 1960 tsunami from Chile, South America.

The April 1, 1946, magnitude-7.9 earthquake in Alaska generated a tsunami that killed 159 people in the state of Hawaii. In Hilo, wave heights, or run-up, averaged 8.1 m (26.5 ft), with a maximum height of 12 m (39.4 ft). The school at Laupahoehoe was destroyed by waves that reached 9.1 m (29.9 ft) above sea level.

It was after this catastrophic event that the forerunner to the Pacific Tsunami Warning Center was created and a tsunami early-warning system implemented.

Yet, even with this early warning system in place, 66 lives were lost in Hilo from the devastation caused by the May 22, 1960, tsunami, generated by the magnitude-9.5 Chilean earthquake. Wave heights reached up to 10.6 m (35 ft) and wiped away communities that were situated where the soccer fields and open spaces now line Hilo’s bay front.

The loss of property is understandable, but the fatalities demonstrated people’s lack of hazard awareness.

Improved technologies, better education, advances in science, and enhanced communication have almost negated risks to human life caused by distant earthquakes and associated tsunamis.

The technology to locate earthquakes and forecast tsunamis has improved significantly. Distal events provide us with sufficient time — on the order of hours — to evacuate low-lying areas and get out of harm’s way.

One scenario regarding tsunami generation that is largely overlooked is a tsunami created by a local earthquake. This has happened on at least two occasions: in 1868 from an estimated magnitude-7.9 earthquake beneath Ka‘u and in 1975 from a magnitude-7.2 earthquake beneath Kalapana.

Both of these events generated local tsunamis resulting in loss of life. Like the recent tsunami in Japan, the time to respond in areas close to the earthquake was brief.

When such an event occurs locally, it is only be a matter of minutes before the resultant tsunami inundates the coastline around the island. If these types of earthquakes were to occur on the southwest side of Hawaii Island, the tsunami impact could be felt statewide. In less than an hour, waves would reach the outer islands. We all need to be vigilant and aware of our surroundings.

Do you know where to evacuate to in the event of a tsunami?

April is Tsunami Awareness Month, and we need to be our own best defense against being swept away. Here are a few basic facts about tsunamis:

* Tsunamis can occur at anytime
* Tsunamis consist of multiple surges that can occur over several hours
* Tsunamis do not produce surfing waves
* Tsunamis can wrap around the islands

All major Hawaii coastal areas have been struck by tsunami. If you feel a large earthquake, be akamai and head mauka—in other words, head for the hills!

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