Let us not forget that Assam and Arunachal are wrapped by the
shallow fault-line that forms the northern edge of the Indian
Plate this article refers to. Movements of shallow faults are
especially devastating. The Sunbansiri dam, and the proposed Middle
Siyang dam sit right on this fault line or very close by.
cm
Evidence
Scientists Renew a Warning About a Himalayan Danger Zone
By KENNETH CHANG
Published: October 10, 2005
Great earthquakes - magnitude 8 or larger - occurred in the Himalayas in 1803, 1833, 1897, 1905, 1934 and 1950. But in the last half century, the region has been relatively quiet, with no earthquakes anywhere near the one with a magnitude of 7.6 that struck northern Pakistan on Saturday.
That calm may have given a false sense of security to growing populations living there.
"Those of us in the business knew we were overdue," said Peter Molnar, a professor of geological sciences at the University of Colorado. Dr. Molnar was a co-author of a 2001 article in the journal Science that looked at the history of Himalayan earthquakes and how much tectonic stress is building up as the Indian subcontinent crashes into Asia.
A Vulnerable Region
The Science article warned, "Several lines of evidence show that one or more great earthquakes may be overdue in a large fraction of the Himalaya, threatening millions of people in that region."
Steven G. Wesnousky, director of the Center for Neotectonic Studies at the University of Nevada, Reno, said earthquakes with magnitudes similar to the one on Saturday could occur at almost any time along any part of the Himalayas. "It might not occur for 10 to 20 years, but if it occurred tomorrow, it wouldn't be a surprise," he said.
Furthermore, quakes like the one on Saturday are "really relatively small compared to the largest earthquakes that zone is capable of producing," Dr. Wesnousky said. "That's the scary part."
The Indian subcontinent slides northward about 1.6 inches a year as part of the natural movement of continental plates. Half of that motion is absorbed farther to the north in Asia, but the other half goes to pushing up the Himalayan mountains, continually building up strain in the rocks.
Dr. Wesnousky and his colleagues have found that one or two mega-earthquakes, magnitude 8.5 to 9, struck sometime in the 1400's or 1500's. He said it was not yet clear whether the faults all broke at once or whether two sections broke at different times to cause two quakes.
For a quake with the magnitude of the one on Saturday - or the one of about the same magnitude in San Francisco in 1906 - the fault slips perhaps 3 to 6 yards. In a mega-earthquakes, the fault slips up to five times as much. "We see displacements on the order of 15 or 20 meters or more that occurred during these earthquakes," Dr. Wesnousky said.
Geologists' understanding of the earthquakes is inadequate for predicting whether the earthquake on Saturday will lead to a larger, more catastrophic one. "There isn't really anything we can say," Dr. Wesnousky said. "It's difficult to say whether they are a foreshock or not."
Dr. Wesnousky said indications so far were that it took a fault 1,000 years to build up enough strain to unleash a mega-earthquake. Because the last mega-earthquakes occurred about 500 years ago, the next mega-earthquakes could be several centuries in the future. On the other hand, a section of the fault near Nepal may not have broken since about the year 1100, so it may be due.
The Asian continent - not including Japan - has also been quiet in recent decades. Dr. Molnar said that from 1897 to 1957, there were 10 to 15 earthquakes as large as or larger than the one on Saturday. "Then, nothing of comparable magnitude until 2001," Dr. Molnar said, though there were two of magnitude 7.5 in China in the 1970's.
Published: October 10, 2005
Great earthquakes - magnitude 8 or larger - occurred in the Himalayas in 1803, 1833, 1897, 1905, 1934 and 1950. But in the last half century, the region has been relatively quiet, with no earthquakes anywhere near the one with a magnitude of 7.6 that struck northern Pakistan on Saturday.
That calm may have given a false sense of security to growing populations living there.
"Those of us in the business knew we were overdue," said Peter Molnar, a professor of geological sciences at the University of Colorado. Dr. Molnar was a co-author of a 2001 article in the journal Science that looked at the history of Himalayan earthquakes and how much tectonic stress is building up as the Indian subcontinent crashes into Asia.
A Vulnerable Region
The Science article warned, "Several lines of evidence show that one or more great earthquakes may be overdue in a large fraction of the Himalaya, threatening millions of people in that region."
Steven G. Wesnousky, director of the Center for Neotectonic Studies at the University of Nevada, Reno, said earthquakes with magnitudes similar to the one on Saturday could occur at almost any time along any part of the Himalayas. "It might not occur for 10 to 20 years, but if it occurred tomorrow, it wouldn't be a surprise," he said.
Furthermore, quakes like the one on Saturday are "really relatively small compared to the largest earthquakes that zone is capable of producing," Dr. Wesnousky said. "That's the scary part."
The Indian subcontinent slides northward about 1.6 inches a year as part of the natural movement of continental plates. Half of that motion is absorbed farther to the north in Asia, but the other half goes to pushing up the Himalayan mountains, continually building up strain in the rocks.
Dr. Wesnousky and his colleagues have found that one or two mega-earthquakes, magnitude 8.5 to 9, struck sometime in the 1400's or 1500's. He said it was not yet clear whether the faults all broke at once or whether two sections broke at different times to cause two quakes.
For a quake with the magnitude of the one on Saturday - or the one of about the same magnitude in San Francisco in 1906 - the fault slips perhaps 3 to 6 yards. In a mega-earthquakes, the fault slips up to five times as much. "We see displacements on the order of 15 or 20 meters or more that occurred during these earthquakes," Dr. Wesnousky said.
Geologists' understanding of the earthquakes is inadequate for predicting whether the earthquake on Saturday will lead to a larger, more catastrophic one. "There isn't really anything we can say," Dr. Wesnousky said. "It's difficult to say whether they are a foreshock or not."
Dr. Wesnousky said indications so far were that it took a fault 1,000 years to build up enough strain to unleash a mega-earthquake. Because the last mega-earthquakes occurred about 500 years ago, the next mega-earthquakes could be several centuries in the future. On the other hand, a section of the fault near Nepal may not have broken since about the year 1100, so it may be due.
The Asian continent - not including Japan - has also been quiet in recent decades. Dr. Molnar said that from 1897 to 1957, there were 10 to 15 earthquakes as large as or larger than the one on Saturday. "Then, nothing of comparable magnitude until 2001," Dr. Molnar said, though there were two of magnitude 7.5 in China in the 1970's.
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