# Volcano Watch — Worrying realistically

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Whether the issue is big, little, real, or imagined, we all worry. Most of us would agree, though, that it is better to worry about things we can do something about, not things out of our control. Giant submarine landslides and island-topping tsunami fall in the latter category.

Whether the issue is big, little, real, or imagined, we all worry. Most of us would agree, though, that it is better to worry about things we can do something about, not things out of our control. Giant submarine landslides and island-topping tsunami fall in the latter category.

Recently, much attention has been paid to the instability of the Big Island and its catastrophic slides into the sea. Giant tsunami, hundreds of meters high, are visualized as lethal byproducts of such a slide. Interpretations of seafloor topography suggest that submarine landslides have occurred repeatedly in the past around all of Hawaii's major islands. One scientifically controversial interpretation is that a giant tsunami 330 m (1,000 ft) high, triggered by a submarine landslide off the Kona coast, swept across Lanai about 100,000 years ago.

These interpretations have captured the fancy of the public and have led to magazine articles, television documentaries, and countless interviews with the media, all with varying degrees of accuracy. The report last March of Kīlauea's south flank moving 7.1 cm/sec (2.8 inches/second) generated nationwide interest but exaggerated the rate by 22.4 million times; the actual maximum rate is about 10 cm/year (4 inches/year).

Interest in submarine landslides among Big Island residents is high. Two questions are almost always asked following public lectures by HVO volcanologists: when is the vog going to go away, and when is the volcano going to fall into the sea. The bottom-line answer to the first question is "not until the eruption has ended." The answer to the second is "almost certainly not in our lifetimes."

Large submarine landslides are infrequent events in human terms, occurring perhaps once every few tens of thousands of years. When they do occur they will almost certainly devastate coastal areas. However, we can do virtually nothing to mitigate such gigantic slides and tsunami--except to follow evacuation routes predetermined by Civil Defense and hope that the tsunami is not too high.

The Big Island is, however, faced with far more frequent events about which residents can and should do something. Large earthquakes in 1975, 1868, and probably in 1823 caused Kīlauea's south coast to drop several meters and generated tsunami that reached as high as 15 m (50 ft), killing people and destroying property. These earthquakes and other lesser, but still damaging, ones in this century caused serious destruction on the island--most notably beneath Hualālai in 1929, on the Kealakakua fault in 1951, near Kalapana in 1954, under Honomu in 1973, and in the Kaoiki fault system on Mauna Loa in 1983. Such earthquakes and local tsunami will happen again, probably within our lifetimes.

We can do something to minimize the effects of these frequent and large earthquakes. We can, and should, construct according to strict building codes--the Big Island is now in Seismic Zone 4, the same as California. Improved detection devices can, and should, be installed for locally generated tsunami of 1868-1975 size to maximize the short time available to evacuate coastal areas. We can, and should, be taught in school to prepare for a big earthquake. For example, secure your propane tanks, prepare emergency caches of food and water, and head for high ground if you're at the beach during an earthquake.

These are the issues that we need to worry about--not the rare (in human terms) gigantic slides and tsunami of supermarket tabloids and television documentaries. Worrying realistically means directing our limited resources to the hazardous events most likely to occur—the ones whose effects we now have the ability to mitigate through education and foresight.

### Volcano Activity Update

During the early morning hours of Thursday, November 6, lava came within 10 meters of topping the rim of Puu Oo crater. Except for this activity, the column of lava within Puu Oo has remained out of view for the entire week. Lava continued to flow through a network of tubes from the vent to the seacoast where it entered the ocean at two locations - Waha`ula and Kamokuna. Conditions at the coastal entries are unstable with frequent collapses resulting in explosive activity. Lava viewers are reminded that the area is extremely hazardous.

One earthquake was reported felt during the past week. A magnitude 2.1 earthquake was felt by a resident of Pahala at 7:09 p.m. on Wednesday evening. The epicenter was 2 km northwest of Pahala at a very shallow depth.