NU Online News Service, Oct. 17, 2:30 p.m.EDT

Scientists believe there is evidence that great earthquakesoccur in clusters over a period of years and that a significantevent is on the horizon, according to a report fromcatastrophe modeler Eqecat.

The report, “Spatial and Temporal Earthquake Clustering: Part 1,Global Earthquake Clustering,” lays out evidence that giantearthquakes of magnitude 8 or higher may occur in clusters over adecade or more.

The authors of the report, Paul C. Thenhaus, Kenneth W. Campbelland Mahmoud M. Khater, say the occurrences of great and giantearthquakes “on a global scale cannot be attributed to chance.”

They say the giant Andaman-Nicobar (Sumatra, Indonesia)earthquake of 2004 of magnitude 9.1 began “a new cycle of globalgreat earthquake activity.”

If this cycle follows the cycle of earthquakes in the 1950-1965timeframe, “we may be only about halfway through the cycle, and thelargest earthquake in the current cluster may not have yetoccurred.

During that period starting 1950, there were three earthquakesof magnitude 9 or higher:

• Kamchatka, magnitude 9 that struck in 1952.
• Prince William Sound, Alaska, magnitude 9.2 that struck in1964.
• Chile, magnitude 9.5 that struck in 1960.

Why this is happening is unknown, they say, but it may havesomething to do with “post-seismic relaxations and strain transfermechanisms in the deep ductile layers of the Earth.”

If earthquakes follow the pattern of the 1900s then a thirdmajor earthquake is a very real possibility.

So far this century, there has been the Indonesia 9.1 earthquakethat struck in 2004 and then the Tohoku-oki earthquake and tsunamiin 2011 which was a magnitude 9 earthquake.

Regarding how destructive an earthquake is, whether it isdevastating or not has less to do with magnitude than other factorssuch as proximity of “occurrence to population centers,construction vulnerability to shaking, soil stability, and localsoil, basin and topographic amplification.

“This distinction between earthquake consequences and magnitudeis important,” the report notes. '“Devastating earthquakes' as aclass have only an obscure relationship to magnitude.”

The report is the first in a series of three reports Eqecatplans to produce on this subject of global clusters of damagingearthquakes.