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Volcanowatch

March 4, 2010

A weekly feature provided by scientists at the Hawaiian Volcano Observatory.


Golden anniversary of Chilean earthquake comes early

Destruction in Hilo after the May 22-23, 1960 tsunami. (source USGS)
Destruction in Hilo after the May 22-23, 1960 tsunami. (source USGS)

May 22, 2010, marks the 50th anniversary of the 1960 magnitude-9.5 Chilean earthquake, which was the largest earthquake worldwide in the last 200 years or more. But, for Hawai`i residents, this great earthquake is rarely far from memory, due to the destructive tsunami it triggered, which killed 61 people in Hawai`i and 122 in Japan.

An early reminder of this anniversary came in the form of a magnitude-8.8 earthquake that struck Chile in the early morning hours of Saturday, February 27, 2010 (local Chilean time). Located approximately 230 km north of the 1960 earthquake, this event produced a modest tsunami that crossed the Pacific Ocean. Luckily, in Hawai`i, we experienced wave heights of only a few feet. However, in Japan, waves were high enough to flood several coastal towns.

Both the 1960 and the 2010 earthquakes occurred at the boundary between the Nazca and the South American tectonic plates. The two plates are converging at a rate of 70 mm (3 inches) per year as the Nazca plate is subducted beneath the South American plate. The relatively fast convergence is the reason why this region has a long history of large earthquakes and can expect to have more in the future.

From Chile, it takes about 15 hours for tsunami waves to reach Hawai`i and about 22 hours to reach Japan. Thus, local authorities have ample time to warn residents and prepare for the approaching waves. If used wisely, this lead time allows for orderly evacuations of low-lying areas, as demonstrated by the smooth response to Saturday's (February 27, 2010) tsunami warning.

There was also ample time for evacuation in Hawai`i on May 22, 1960, as the tsunami traveled across the Pacific Ocean. At 6:47 p.m., Hawaiian standard time, the U.S. Coast and Geodetic Survey issued an official warning that waves were expected to reach Hilo around midnight. At 8:30 p.m., coastal sirens in Hilo sounded and continued to sound intermittently for 20 minutes.

Many people heeded the warning and evacuated, but some did not, and when the first wave arrived just after midnight, hundreds of people were still at home on low ground in Hilo. Because the first waves were only a few feet (1-2 m) high, many people returned to Hilo, thinking that the danger had passed. But the highest wave of the tsunami struck shortly after that, at 1:04 a.m. on May 23, H.s.t.

Just before midnight on May 22, 1960, Jerry Eaton, a seismologist at the U.S. Geological Survey's Hawaiian Volcano Observatory, and four companions set up instruments in order to measure the tsunami wave heights from the Wailuku River Bridge on Hilo's bay front. Their measurements show that the tsunami was a series of waves that occurred over a span of more than two hours, with the highest wave of about 4.3 m (14 ft) arriving just after 1:00 a.m.

These wave heights sound modest compared to the 15-m (50 ft) waves that surfers tackle on the north shores of Hawai`i. Tsunami waves, however, are much different, and more dangerous, in that each wave can raise sea level for tens of minutes and can push an incredible amount of debris-filled water on land.

Tsunami size is controlled by how much an earthquake displaces the sea floor. So, small differences in earthquake location (closer or farther from the coast) and earthquake depth can have big effects on whether or not a tsunami will be generated-and how big it will be. Magnitude can also make a difference. The 1960 earthquake released over 10 times more energy than the 2010 earthquake, which may have contributed to the size of the 1960 tsunami.

The February 27, 2010, Chilean earthquake was well-recorded by scientists around the globe and will likely be studied for years to come. This should lead to a greater understanding of how these large earthquakes occur and under what conditions they produce widespread, destructive tsunamis.

Kīlauea Activity Update

Surface flows have been active on Pulama pali within the Royal Gardens subdivision and on the coastal plain along the west edge of the subdivision. The flows have advanced very little across the coastal plain over the past week and are about 1.7 km (1 mile) from the ocean.

At Kīlauea's summit, a spattering and roiling lava surface, deep within the collapse pit inset within the floor of Halema`uma`u Crater, was visible via Webcam. The lava surface rose to a slightly higher level early in the week as the volcano reinflated following last week's deflation. With the higher lava level, the lava surface was occasionally seen to cyclically rise and fall over periods of several minutes. Volcanic gas emissions remain elevated, resulting in high concentrations of sulfur dioxide downwind.

No earthquakes beneath Hawai`i Island were reported felt during the past week.

Visit our Web site (http://hvo.wr.usgs.gov) for detailed Kīlauea and Mauna Loa activity updates, recent volcano photos, recent earthquakes, and more; call (808) 967-8862 for a Kīlauea activity summary; email questions to askHVO@usgs.gov.

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Updated: March 8, 2010 (pnf)