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Slow-motion slip may drive tsunami surprise.

Because sudden jarring of the ocean floor or underwater landslides can trigger devastating waves, or tsunamis, people in earthquake-prone coastal regions have learned to run for high ground when the Earth begins to shake violently. Unfortunately, many people living on Nicaragua's Pacific coast barely felt the effects of a September 1992 earthquake. But just about 45 minutes after the main shock, say survivors, a tsunami reaching 10 meters high crashed onto a 300'kilometer-long stretch of the coastline.

More than 20 years ago, seismologist Hiroo Kanamori coined the term "tsunami earthquake" to describe such deceptively mild quakes that seem to spawn disproportionately large waves. Kanamori also proposed a possible explanation for the phenomenon, but the technical limitations of 1970s seismometers denied him compelling proof.

Now, based on seismic measurements taken during the Nicaraguan earthquake -- the first event of its kind monitored with sensitive modern instruments - Kanamori and colleague Masayuki Kikuchi offer the clearest evidence to date that tsunami earthquakes stem from a "slow-slip" motion between oceanic plates. The seismic waves from this relatively gradual movement of the seafloor may scarcely be noticed by people on land.

Kanamori, of the California Institute of Technology in Pasadena, and Kikuchi, of Yokohama City University in Japan, report their findings in the Feb. 25 NATURE.

Judging from the seismic record of the Nicaraguan disaster, the researchers believe that the motion of oceanic plates in a deep trench off the Nicaraguan coast jiggled a 100-kilometer-long section of the ocean floor, moving it about 1 meter in a period of 2 minutes. The water set in motion by this event caused the surprise tsunami that destroyed 13,000 dwellings and killed more than 150 people.

Kanamori believes this slow slip -- an event that releases great energy, but in a way that can mask its true magnitudemay prove the hallmark of the tsunami earthquake. During a slow-slip earthquake, he suggests, an oceanic plate lubricated by soft ocean sediments slides under an adjoining plate relatively slowly, This produces a seismic spectrum rich in long-period waves, like a piece of music full of cellos and bass drums. Seismic readings show clearly that long-period waves dominated the Nicaraguan earthquake, the researchers report.

Unfortunately, humans are only sensitive to the electric-guitar thrash at the higher end of the seismic spectrum - the kind of short-period seismic waves that shake buildings to their foundations. Thus, during a tsunami earthquake, people on land fail to notice the malevolent bass drone of impending inundation.

Until recently, seismometers were largely insensitive to long-period waves, a technical problem solved in the early 1980s. However, calculations of earthquake magnitude - the famous Richter scale - are still based on relatively shortperiod waves and thus may underestimate the destructive potential of tsunami earthquakes, Kanamori says.

Based on short-period waves, the Nicaraguan quake comes out as a magnitude 7.0 event. However, by using long-period waves to calculate the earthquake's "seismic moment," a quantity that reflects the actual motion of the plates rather than their ground-level effects, the researchers peg the Nicaraguan earthquake at a significantly larger magnitude 7.6.

Kanamori and others argue that use of long-period waves to calculate seismic moment, a task made quick and easy by modern technology, can provide an accurate determination of an earthquake's tsunami potential and perhaps offer sufficient warning to coastal communities at risk. Without such warning, the chances of surviving a tsunami traveling at freeway speed remain slim. "Once you see a tsunami coming towards you, it's usually too late," Kanamori notes.
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Title Annotation:earthquakes
Author:Pendick, Daniel
Publication:Science News
Date:Feb 27, 1993
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