Sure, the Los Angeles Earthquake Was Predicted

January 23, 1994|By LEONARDO SEEBER

Earthquakes can be beautiful. Like stars in the sky, they are a window into some of the wonders of the universe -- they offer a view of current rock deformation in the upper crust, for example.

Excitement is probably the best way to characterize the reaction to a series of small earthquakes that started early this year in Columbia. It is a common and correct perception that the occurrence of small earthquakes raises the chance of a large one, but apprehension did not seem to interfere with the generally positive exercise of curiosity that characterized the public awareness of that earthquake sequence in Maryland.

Unfortunately, earthquakes have and will continue to bring about human tragedy all over the world. Visions of disjointed freeway overpasses, collapsed apartment buildings and displaced people in Los Angeles are fresh in our minds.

But the latest example of a catastrophic loss of life is the magnitude 6.4, earthquake of Sept. 30, 1993, in central India, which left about 10,000 people dead -- an earthquake similar in size to the one in Los Angeles, but responsible for about 200 times more fatalities.

I visited the epicentral area in the gently undulating hills of the Puna Plateau in central India two weeks after the main shock and had a chance to study the effects of that earthquake. Little had changed since the main shock in the 25 or so villages that had been reduced to seas of rubble. Intimate household memorabilia mixed with stones from the walls of the houses and charred remains from improvised cremations could still project the horror of the night when those villages had been wiped out.

Excavations revealed two to three feet of fault slip, but they also revealed no evidence of previous faulting in the exposed rocks, which are 65 million years old. Furthermore, available records showed no historic epicenters in a large portion of India that includes the 1993 epicenter. These data offered no hint that a large earthquake as about to occur there; hazard maps placed the 1993 epicenter in an area of least inferred hazard. Was this earthquake surprising?

The 1993 Indian earthquake is not a freak. All stable continents, including North America, offer examples of large earthquakes centered in areas with little historic seismicity and with no evidence of recent geologic deformation -- centered, therefore, in areas where they were "not expected" to occur. Not expected, that is, according to hazard maps.

Many seismologists, including myself, are not surprised by the Indian earthquake, suggesting that these maps should be taken with a grain of salt.

Yet, earthquake-hazard maps for stable continents are still based on the distribution of historic earthquakes and faults.

One reason is that those kinds of data do provide the basis for useful hazard zonation in California and other regions of high movement along plate boundaries. The Los Angeles Basin had been targeted as a zone of high earthquake hazard because it is riddled with active faults and it is affected by a high rate of seismic activity with recurrent damaging earthquakes.

Without viable alternatives, the same approach to infer earthquake hazard based on seismicity and active faults is applied to stable continents. The 1993 Indian earthquake is a dramatic indication that the traditional approach to mapping future seismic sources in stable continents needs to be revised. Furthermore, circumstances leading to this earthquake suggest possible improvements to earthquake hazard forecasting.

First, an artificial reservoir is very close to the 1993 epicenter; like many other recent damaging earthquakes in India and elsewhere, this earthquake might have been triggered by mechanical changes in the upper crust.

Second, the 1993 India earthquake, like many other large earthquakes in stable continents, was preceded by a burst of seismicity in 1992. These precursory earthquakes did some damage and caused alarm in the same villages that were to be destroyed a year later. Plans to relocate the population were proposed, but, unfortunately, were not implemented.

Closer to home, we are in the midst of an earthquake sequence in the Wyomissing Hills near Reading, Pa. -- a swarm of earthquakes characterized by many small events but also by two events large enough to cause significant damage. In this case, the general area of southeastern Pennsylvania has been relatively active during recorded history. Particularly significant, however, is a swarm of small events that was felt during late spring 1993 in the same area where damage occurred with this month's quakes.

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