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.
A burst of unprecedented seismicity, a recently created reservoir, or their combination offer the kind of signals that need to be incorporated into procedures to determine the distribution of hazard. Such signals would not "predict" a large earthquake; the chance of this earthquake's occurring may still be much less than the chance of its not occurring. Yet, this kind of signal does raise temporarily the probability of a large earthquake.
Is there a useful seismological comparison between the 1993 epicentral area in central India, the Wyomissing Hills in southeastern Pennsylvania and Howard County in Maryland? All ZTC of these areas are in stable parts of the continents, that is, they are not along plate boundaries characterized by high rates of tectonic activity, such as California or the Himalayas.
In both Maryland and central India significant historic earthquakes are particularly scarce, unlike other areas within "stable" continents, such as New Madrid, Mo., Charleston, S.C., or New York City, where damaging earthquakes have occurred. Southeastern Pennsylvania has been relatively active, with at least four earthquakes with magnitude larger than 4.0 during recorded history, but without reports of damage prior to the 1994 event.
All three areas experienced a recent swarm of small, shallow earthquakes. Relatively few small earthquakes have characterized the swarm in Howard County, but an event Nov. 17 suggests that the sequence is continuing. The possibility of an artificial environmental change like the one brought about by the reservoir near the 1993 epicenter in India has not yet been investigated in the Wyomissing Hills and seems unlikely in the case of the swarm in Howard County.
Even after the examples from central India and the Wyomissing Hills, a large destructive earthquake is still very unlikely in Howard County. However, the chance of such an earthquake is probably substantially higher after the onset of the Columbia sequence than before it. How much higher? We do not know. Unfortunately, little research effort is devoted to addressing this question by the National Earthquake Hazard Reduction Program. Our study of the Howard County earthquakes last spring was a limited effort that received little encouragement.
Seismological research is concentrated in areas of high historic activity. Agencies that allocate resources for earthquake hazard reduction will, it is hoped, take notice that the largest earthquake disaster in India during the last 50 years occurred in the stable continental part of that country, not along the Himalayan plate boundary; and that, furthermore, this earthquake disaster, which also the largest in a stable continent during this century, was centered in an area where the hazard was estimated to be the lowest.
So, my advice to the Howard County community is: Have fun with your small earthquakes, but also take steps to reduce your risk. Such steps should include working on your house to reduce the possibility of earthquake damage, but also demanding a fair share of the benefits from the National Earthquake Hazard Reduction Program. Your current share is likely to be disproportionately low compared to the level of risk in your area.
Leonardo Seeber is a seismologist at the Lamont-Doherty Earth Observatory of Columbia University.