Lessons from Kutch

By C. P. Rajendran

THE JANUARY 26 Day earthquake, centred around Bhuj in Gujarat, is yet another reminder of the shaky ground on which we all live. It is one of the worst earthquakes to have hit independent India, and has devastated modern towns and cities in Gujarat. This takes us back to 1819 when a major earthquake hit this region. An earthquake discussed in geology textbooks as a classic example for ground effects, it killed nearly 2000 people. It was felt as far away as Calcutta in the east and Malabar in the south, and was probably an illustration of how future large earthquakes in this region would behave.

The region was rocked again by a large earthquake in 1956, at Anjar. This one also caused severe damage in the adjoining areas and reiterated the fact that the crust beneath Gujarat is far from calm. The occurrence of such large earthquakes has brought this region in zone V, which marks the highest seismic potential, comparable to the Himalayas and the Andamans.

Study of earthquakes and assessing the seismic hazard in a region has gone far beyond broad zonation. The seismogenic characteristics of a region are evaluated not only on the basis of historical and instrumentally-recorded data, but also using the geological evidences of recent movements preserved near earthquake source regions. Paleoseismology, an emerging branch of study, has been successfully used to dig up details about past earthquakes. It has been a favourite preoccupation of many earthquake geologists to dig through fault segments that have essentially remained quiet for several centuries, in the hope that details of their past movements may become useful in predicting their future behavior. This data has become extremely valuable in long-term planning and seismic hazard mitigation.

There are several examples of how the lack of knowledge of active tectonic processes has resulted in cost overruns, delays and even cancellation of expensive projects. The experience of the Pacific Gas and Electric Company which wanted to establish a nuclear reactor in northern California back in the 1960s is particularly interesting. The site selected was near the notorious San Andreas fault that caused the great 1906 San Francisco earthquake. As the excavations for the site progressed, many potential faults were unearthed and the possible threat to the reactor could not be underestimated. With the limited knowledge of earthquake processes available at that time, no one could suggest if this fault would break in the future. Whether this region was getting ready for an earthquake during the life-time of the critical facility was the real concern. After several years of debates, the whole project was abandoned. Today, in the U.S., it is mandatory to assess the seismic history of faults in the early stages of planning for critical facilities.

If there is one good place other than the Himalayas to successfully date past earthquakes, it is the Rann of Kutch. The reasons are many: geological as well as seismological. This region is part of an ancient rift that closed sometime during the collision that threw up the Himalayas. It has been documented that large earthquakes in the continental interior regions can occur by the reactivation of faults that are related to ancient rift systems. Most of the faults have been mapped in the Rann of Kutch, but their seismic histories remain unknown.

The 1819 earthquake of magnitude 7.5 produced a 90-km-long scarp popularly known as the Allah Bund, stretching across the international border, about one third of it running into Pakistan. Although several parallel faults are known in this region, only one has broken during the last century. This happened at Anjar, in 1956, when an earthquake of magnitude around 7 occurred here. Assessing the seismogenic potential of the exposed faults and the recurrence interval of earthquakes is the major challenge before the earthquake geologist. The error margins in such estimates may be large, but these may be the only estimates before us to assessing the seismic risk.

The initial success story in dating past earthquakes comes from central Idaho in the United States where a 36-km-long spectacular fault scarp was formed during an earthquake of magnitude 7.3 in 1983. Geologists had mapped this region and had concluded that a large earthquake had occurred about 5000 years ago. The 1983 earthquake occurred exactly where it was expected, and in fact it mimicked the displacement of its predecessor in size and sense of movement. The knowledge that recent breaks occur along pre- existing breaks and the ability to predict how much a fault is likely to slip are important in assessing future displacements.

If indeed faults have a rhythm and history of breakage, how about the Allah Bund scarp? When did it break before 1819? This was the question that we were primarily trying to address through our detailed investigation in the area. The complexity of the issue was compounded by the logistical problems and toughness of the terrain. Despite these limitations, we have been able to survey much of the scarp in the Indian side and also make several excavations to study the signatures of past earthquakes. Based on a variety of data including tidbits from history, we concluded that the last earthquake prior to 1819 occurred about 800 years ago. This ancient event occurred right at the epicenter of the 1819 earthquake. What about the eastern terminus of the bund? We found very little evidence for major earthquakes in the eastern segment of the fault running into Rajasthan. So was this a potential segment for the next quake, was the question raised by one of our earliest publications on this study.

The Republic Day earthquake, however, seems to have occurred on a fault that runs parallel to the Allah Bund. This was a fault whose existence was noted before, but its history was yet to be investigated. But earthquakes come before we know they are coming. There are many faults that cut across the Indian crust. How many of them will break in the near future, is difficult to assess unless we take up extensive study of these faults. This may not be an easily achievable target for a large country like ours. However, not knowing the existence of a fault and its rupture history make it difficult to assign seismic risk to a region with acceptable errors of margin. Societal implications of such observations are another matter to be tackled. Unless our the society matures and learns to accept and accommodate scientific findings of this nature, these efforts will be futile.

(The writer is a geologist with the Centre for Earth Science Studies, Thiruvananthapuram.)

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