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Scientists Warn of a Great Himalayan Quake

[WTN-L World Tibet Network News. Published by The Canada Tibet Committee. Issue ID: 01/08/24; August 24, 2001.]

CHIDANAND RAJGHATTA
The Times of India
SATURDAY, AUGUST 25, 2001.

WASHINGTON: Several lines of evidence show that one or more great earthquakes may be overdue in the Gangetic belt below the Himalayas threatening millions of people in the thickly populated region, Science magazine is reporting in its forthcoming issue.

The capital cities of Bangladesh, Bhutan, India, Nepal, and Pakistan and several other cities with more than a million inhabitants are vulnerable to damage from some of these future earthquakes. "Today, about 50 million people are at risk from great Himalayan earthquakes, many of them in towns and villages in the Ganges plain," the magazine warns in a paper written by three researchers.

Prefacing their report with a reference to the Bhuj earthquake, the researchers say "it focused the eyes of the public away from a part of India where even worse damage and loss of life should be expected." The areas below the Himalayas are just grinding with strain and even a repeat of the relatively small 1905 Kangra quake could cause 200,000 predictable fatalities.

"Such an estimate may be too low by an order of magnitude should a great earthquake occur near one of the megacities in the Ganges Plain (where the urban population alone exceeds 40 million)," the paper warns. It gives no timeframe of when such a mega quake can occur.

According to the researchers, Global Positioning System (GPS) measurements show that India and southern Tibet converge at 20 mm ( 3 mm) per year. A 50-km-wide region centered on the southern edge of the Tibetan Plateau strains to absorb about 80% of this convergence. The surrounding Himalaya accommodates the remaining 20%. Two meters of potential slip in earthquakes thus accumulate each century (in contrast, control points in southern India approach each other no faster than a few millimeters per year.)

"Earthquakes must therefore release most, if not all, of India's 2 m per century convergence with southern Tibet," the paper concludes.

According to the researchers, although the major earthquakes that have occurred along the Himalayas since 1800 differed in dimensions, less than half of the Himalaya has ruptured in that period. When they divided the central Himalaya into 10 regions, with lengths roughly corresponding to those of great Himalayan ruptures (~220 km), they found six of these regions currently have a slip potential of at least 4m - equivalent to the slip inferred for the 1934 earthquake.

This implies that each of these regions now stores the strain necessary for such an earthquake. Moreover, the historic record has no great earthquake throughout most of the Himalaya since 1700, suggesting that the slip potential may exceed 6 m in some places.

Even if only one segment has stored potential slip comparable to that of the 1950 Assam earthquake, the largest intracontinental earthquake in recorded history, a replication of that earthquake along the more populous segments of the Himalaya would be devastating.

"We cannot rule out the possibility that parts of the Himalaya have not ruptured in major earthquakes for 500 to 700 years and will be associated with slip exceeding 10 m," they warn. In that case, the mid-Himalayan 20th century earthquakes, not to speak of the Bhuj upheaval, would then have been "atypically small" - just a small hiccup compared to the Big One on the cards.


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