Earthquake prediction may be possible

Press, Volume CXIII, Issue 33413, 20 December 1973, Page 20

Earthquake prediction may be possible

(By WALTER SULLIVAN)

Of all the natural catastrophes, none is more deeply terrifying than an earthquake. Great storms do not strike without warning. Winds and floods tear at the land, but underneath, the earth remains steadfast.

Not so in an earthquake. The land is riven and sometimes its surface rolls like a sea. Buildings collapse. The world seems to be coming apart.

Just a few weeks ago an earthquake struck SouthCentral Mexico, tearing open the surface of the land, shaking down roofs, bringing down adobe walls and small apartment houses from the region south-east of Mexico City to the gulf coast at Vera Cruz. More than 700 were killed and 4000 were injured in what was termed the country’s greatest disaster. Was such an event completely unpredictable? Are earthquakes, beyond all other disasters, impossible of control? Was the loss of life inevitable?

A few years ago the answers to these questions would have been grimly in the affirmative. But recent findings on the cause of earthquakes, and on ways to predict and even regulate some of them, have begun to change the picture.

The science of earthquake prediction and control is in its infancy. Its ultimate capabilities may be applicable only in special regions. Pressure waves It has been found that in the Adirondacks and in one region of California and in parts of the Soviet Union, an observable change occurs within the earth before an earthquake takes place. The change causes pressure waves, such as those produced by an explosion or small regional tremors, to travel through the deep rock more slowly than usual. When the travel velocity climbs back to normal the earthquake occurs. The more prolonged this premonitory effect, the greater the earthquake that follows. The current explanation is that the travel velocity of such waves slows down when the deep rock is distended by stress, opening up

pores and cracks. Only when water under the high pressure typical of such depths has filtered into the pores does the rock fracture readily and an earthquake occur.

This thesis has been supported by a variety of laboratory and field tests. In the Rangely oil field of Colorado earthquakes have been “turned on” by pumping water down wells at high pressure and they have been [“turned off” by withdrawing water from the wells.

This type of control may :prove practical along the ; California faults, where earthquakes occur at relatively shallow depths and where, perhaps, many small earthquakes could be generated to relieve the stress that causes big ones. But it seems questionable whether this will ever be possible where earthquakes occur at intermediate depths, as below Central Mexico. Complex activity The Mexican earthquake manifests a pattern of activity within the earth that has begun to explain the very structure of Central America. The activity is complex and not fully understood. But its broad outlines are evident.

Along the West Coast of Mexico, from Mazatlan south-east almost to Costa Rica, is a series of deep trenches in the sea floor, the primary one being the Middle America Trench. Two sections, or plates, of the Pacific floor are apparently descending beneath the rim of the continent, producing these trenches. The northern one—the socalled Rivera Plate—is barely under the Mexican coast from Mazatlan to Manzanillo. The southern section of descending sea floor from Manzanillo to the south-east! is the Cocos Plate.

These plates push down until they are hundreds of miles beneath the surface landscape of Mexico, and their descent generates earthquakes. Near the coast the earthquakes are at shallow depths, but further inland they become progressively deeper. Under the high pressures

and temperatures at depths of 60 miles or more some of the material forming these oceanic plates melts and forces its way up as lava. Greatest volcanoes Consequently this zone in Mexico boasts that country’s greatest volcanoes, such as Popocatepetl, Ixtacihautl and Orizaba, the last-named at 18,700 ft, being the highest summit in North America, apart from Mount McKinley. It is in this volcanic region, spanning Mexico from Manzanillo on the Pacific to Vera Cruz on the gulf, that earthquakes of intermediate depth occur.

The recent one levelled homes from Puebla, southeast of Mexico City, to Vera Cruz. One of the hardest-hit cities was the one bearing the name of the great volcano, Orizaba. Diverse and persuasive evidence indicates that these earthquakes are a by-product of the descent of the oceanic plates, but their precise cause is still uncertain.

Although control of these earthquakes, at intermediate depths, is believed to be unlikely, prediction may some day be possible. Advance warning would certainly have saved many lives in the Mexico disaster and the same was true of the earthquake a few months ago that levelled much of Managua in Nicaragua. As with the Mexican earthquake, that one apparently occurred when an oceanic plate is descending beneath the continent.

As noted recently by Dr John H. Healy, of the United States National Centre for Earthquake Research, who has been responsible for some of the experiments in eathquake control in Colorado, the application of such techniques to regions of high earthquake risk may prove too expensive—or too 'dangerous. i Nevertheless, he said, “the ■ fundamental understanding that will be gained in the research will more than justify the expense and will almost certainly lead to new application that we do not now envisage.”—Copyright, “New York Times” News Service.