Is Kilauea falling into the ocean?

The question, typical of e-mail that has been pouring in in the last few weeks:

"Geologists are monitoring anxiously a huge chunk of Hawaiian mountainside -- 12 miles long, 6 miles wide and 5.4 miles deep--which is creeping out to sea at a rate of 2.8 inches a second.

There's a remote risk that the Kilauea volcano, one of the most active in the world, will slump into the ocean, triggering a gigantic tsunami, a wave up to 990 feet high, which could devastate coastlines around the Pacific from California to Australia.

Although no tsunami on that scale has been recorded during historic times, there is scientific evidence for mega-slumps and gigantic tsunamis in the Pacific within the past 100,000 years.

Sonar images of the ocean floor show landslides involving hundreds of square miles of rock. And geologists have found deposits of crushed coral, lumps of pumice stone and other wave-borne debris up to 1,000 feet above sea level in Hawaii, Australia and New Zealand which they say could only have been carried there by a tsunami.

Kilauea is bristling with high-tech monitoring equipment installed by geologists to detect changes in the shape of the mountain. The idea is to give advance warning of hazardous volcanic activity from new eruptions of gas and lava to catastrophic landslides."

The answer:

The article gives the wrong impression because the rate quoted, 2.8 inches/second, is 30 million times to large.

The original article appeared in the Financial Times (London), March 8. That article was a bit sensational, but it was fundamentally sound. Unfortunately, in being transcribed for the wire services, "inches/year" somehow changed to "inches/second," making the whole thing sound fantastically dangerous. The article was further garbled as various editors rewrote the piece for their local papers.

In both the Financial Times article and an earlier article in Honolulu Magazine, the case was made that if the south flank of Kilauea collapses, Hawai`i will suffer a giant tsunami. Honolulu Magazine said "If the worst happened, the tsunami would take only 20 minutes to devastate the most densely populated parts of Hawai`i."

While it's true that a collapse of the entire south flank would be devastating for Hawai`i, the suggestion that such a collapse might occur misses the point: nobody is predicting that the entire south flank of Kilauea is going to fail any time soon. The Hilina Block of Kilauea's south flank has been moving at better than two inches/year ever since people started measuring it, but that block is only about 80 square miles; it is far from the entire south flank of Kilauea. If the Hilina Block went, it would produce a tsunami which would damage the Puna and Ka`u coasts of the Big Island and would create damaging waves in Hilo. But it wouldn't destroy Hilo, or Honolulu, and it wouldn't "devastate coastlines around the Pacific from California to Australia." Even if the entire flank of Kilauea collapsed (something no scientist is seriously suggesting), the waves would die away too rapidly to cause much damage beyond Hawai`i. At most locations the waves would be much smaller than those of the Chile tsunami of 1960. For example, waves along the California coast might be several feet high--enough to damage boats, wash out a few bridges, and cause troublesome surges in harbors--but no more than that.

There is no question that huge landslides have occurred in Hawaii's past. The seafloor evidence is inescapable. When the last one occurred, however, is a big question. The "100,000 years" figure is now being seriously questioned, and the "tsunami" deposits at 1000' elevation (on the island of Lana`i) are very equivocal (the corals in question may just be field markers used by the ancient Hawaiians in their dryland agriculture). What is found in Australia is not deposits but great scars cut into a sandstone platform several feet above sea level. No waves from a Hawaiian landslide, even a giant one, would have been large enough to carve the scars (the waves from a landslide source in Hawai`i would have decayed too rapidly with distance to have caused that damage). Further, the erosion rate of the sandstone platform is so high that any 100,000-year-old surface would by now have been completely lost. If the scars in Australia are tsunami scars, that tsunami did not come from Hawai`i and was more recent than 100,000 years ago.

We have no reason to believe that the south flank of Kilauea will fail as a catastrophic landslide soon, or indeed within the lifetime of anyone now living. If a landslide were about to happen, we would see a marked increase in the number of earthquakes and we would see the current sliding accelerate. With the huge concentration of geophysical instruments all over Kilauea, such action could not be hidden from the many watchful eyes.

What is much more likely than a major collapse of Kilauea's south flank is an earthquake in which the Hilina Block, or a larger part of the south flank, lurches seaward by twenty feet or so. The Kalapana Earthquake of 1975 was such an event; the tsunami it generated killed two people. The Great Ka`u Earthquake of 1868 was similar but larger. The tsunami from the Ka`u earthquake killed 47 people. The moral should be obvious: if you are at the shoreline and you feel severe shaking, get away from the coast immediately.