New theory on earthquakes

Press, Volume CXI, Issue 32803, 31 December 1971, Page 11

New theory on earthquakes

Since July 14 10 severe earthquakes have shaken the Solomon Islands. The sequence of tremors, the last of them on October 25, has given new weight to a theory linking total earthquake activity to wobbles in the earth’s spin.

The earthquake record, plus recent astronomical observations, have convinced a Government specialist who promulgated the theory, that this intense activity will continue into early 1972.

The scientist is Charles A. Whitten, chief geodesist of the National Ocean Survey. The survey is an agency of the National Oceanic and Atmospheric Administration, which also runs the National Earthquake Information Centre. It was the latter that announced the most recent Solomon Islands earthquake. It occurred near the west coast of Bougainville in an area where two earthquakes, each of magnitude 7.9, occurred on July 14 and 26. On the Richter scale of intensities, these were major earthquakes. The latest earthquake was

of less magnitude and, like the seven other earthquakes that have shaken the area since July, was probably an aftershock of the two big events.

As explained recently by Mr Whitten the wobbles of the earth seem to control the total amount of energy released by earthquakes, during a given period, rather than the number of’ very large earthquakes. Two wobbles of the spin axis are involved, he said.

“In phase” One completes its wobble in 12 months and the other does so in 14. For one phase of each seven-year period these two wobbles are “in phase.” That is, they work together to produce a rapid shift in the spin axis. Midway between these times of maximum motion the two wobbles work at crosspurposes and the spin axis hardly moves at all.

The combined effect is known as the Chandler wobble. Because of it the North and South Poles wander about within a circle some 50 feet in radius. It is

monitored by the International Polar Motion Service whose five observatories, around the world, are in the same longitude and sight on the same stars each clear night.

In addition some 35 other observatories also make observations. The data are then processed by the Polar Motion Service in Japan. Observations through July—the month of the big Solomon Island earthquakes—showed that the axis was, in fact, moving rapidly. Among the year’s major earthquakes was one in New Guinea on January 10 with a magnitude of 8.1, in a class with the most severe on record. On May 12 and 22 Turkey was struck by two earthquakes that killed about 800 people, more because rural, mud-brick housing is highly vulnerable than because of unusually severe tremors.

A major earthquake killed some 20 people in Chile, July 9. No information is available on casualties from the Solomon earthquakes, whch occurred in a region of the South Pacific where communications are rudimentary.

According to Carl Von Hake of the. Earthquake Information Centre, there have not been three “great” earthquakes in a single year since 1950. Subsequent years of maximum activity were 1957 and 1964 (the year of the great Alaskan earthquake).

Peak activity The period of peak activity, in this seven-year cycle, lasts somewhat more than a year and for this reason Mr Whitten believes it will extend into 1972. Early this year he made public a graph showing an apparent relationship between the Chandler wobble over the last 70 years and the total release of earthquake energy in each year.

He does not argue that the wobble is the sole cause of earthquakes. The energy producing an earthquake derives from accumulated strain within active areas of the earth’s interior. But when the earth’s spin axis shifts rapidly, some believe, the swift redistribution of forces within the earth determines when such strain is released. —Copyright.-' "New York Times” News Service.