Be akamai—when in doubt, head mauka

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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 Hawai‘i Island early that same day.

this is a photo of staff and volunteers mapping run-up elevations and inundation distances of tsunami waves.

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 Hawai‘i Island following the magnitude-9.0 earthquake in Japan. USGS Photo by Andrew R. Hara.

(Public domain.)

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 Laupāhoehoe 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‘ū 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 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 Hawai‘i Island, the tsunami impact could be felt State-wide. 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 Hawai‘i 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!


Volcano Activity Update

Lava erupted sporadically within the crater at Pu‘u ‘Ō‘ō over the past week, with activity increasing during the inflation phase of deflation–inflation cycles (DI events) and decreasing during the deflation phase. The lava is more than 95 m (about 300 ft) below the east rim of Pu‘u ‘Ō‘ō, and no lava is erupting outside of the crater.

A small lava lake was also present deep within the Halema‘uma‘u Overlook vent over the past week during periods of DI inflation. During periods of DI deflation, the lava lake dropped to a lower level and crusted over. Several small rock falls within the vent cavity also occurred during the week. Volcanic gas emissions remain elevated, resulting in relatively high concentrations of sulfur dioxide downwind.

No earthquakes beneath Hawai‘i Island were reported felt this past week.