Seismologists worried about the prospect
of a massive earthquake in the
shadow of the Himalayas, where it
could devastate cities such as Kathmandu
and Delhi, have long cast a
wary glance at an eerily calm region
called the central seismic gap (CSG). A massive
earthquake in southwestern Tibet in
1505 C.E., researchers proposed a decade
ago, relieved enough strain to quiet that
stretch of the restive Himalayas. But new
findings now suggest that the 1505 temblor
was smaller than thought and was just one of
a cluster of potent quakes to rattle the region
within a few centuries. If so, major quakes
in the Himalayas, unlike in many other seismic
hot spots, may not relieve enough strain
to forestall later quakes—meaning that authorities
must gird for a mega earthquake
anywhere at any time.
Thrust up by the continuing collision of the Indian subcontinent with Asia, the Himalayas are frequently rattled by major earthquakes. But for several centuries, the CSG, a 600-kilometer-long region extending northeast of Delhi, has been quiet, even though it straddles major faults. In 2003, the late Greek geologist Nicholas Ambraseys and Roger Bilham, a geophysicist at the University of Colorado, Boulder, proposed that a large earthquake on 6 June 1505, known from Tibetan annals and the Akbarnama, a chronicle of the 16th century Mughal emperor Akbar’s reign, could have relieved some of the strain building up at the CSG.
Based on severe structural damage to Tibetan monasteries located nearly 700 kilometers apart, Ambraseys and Bilham estimated that the 1505 quake would have registered at 8.2 or so on the moment magnitude scale, which measures the energy released during an earthquake. The geological smoking gun for the quake seemed to materialize a few years later. After digging six trenches at points along a 250- kilometer stretch of the CSG in 2006, geologist Senthil Kumar, who was then at the University of Nevada, Reno, and colleagues uncovered faults that could be radiocarbon dated using charcoal in the sediments. They attributed the faults to an earthquake between 1400 and 1422 C.E. But written records do not mention a major quake during this period, and because the charcoal dating’s error bars encompassed 1505 C.E., Kumar’s group chalked up the findings to the 1505 Tibetan quake.
Also cited as evidence for that quake was a trench in western Nepal described only in an abstract in 2006. Other experts, however, have doubted the 1505 earthquake’s potency. Many medieval monasteries in the region are built from rock masonry without mortar, making them vulnerable to even moderate earthquakes, says Chittenipattu Rajendran, a paleo seismologist at the Jawaharlal Nehru Centre for Advanced Scientific Research here. Such damage, therefore, “is not realistic to use as an indicator of magnitude,” says Rajendran, who adds that there is no historical record of extensive damage in Indian cities like Agra and Delhi in 1505.
To get a fresh perspective on the CSG’s seismic history, Rajendran’s group dug a new trench in the Uttarakhand district of Ramnagar, adjacent to a trench Kumar’s team had excavated. They got lucky, uncovering a colluvial wedge of sediment— the geological signature of any quake that ruptures the earth’s surface. The rupture briefly pushes up a scarp, which crumbles to form a wedge. Rajendran’s team found that the material in the wedge had been ruptured not once, but twice, which could only mean two earthquakes, they say. Carbon dating linked both ruptures to earthquakes in 1255 C.E. and 1344 C.E. that are known from historical records, the team reports in an article posted online last month in the Journal of Geophysical Research: Solid Earth. Yet the trench showed no sign of a later, large earthquake.
R. Jayangondaperumal, a geologist at the Wadia Institute of Himalayan Geology in Dehradun and a member of Kumar’s team in 2006, says a reanalysis he has conducted on their 2006 trenches also suggests a pair of earthquakes rather than a single one. The finding not only casts doubt on the extent of the strain-relieving quake in 1505 but also “confirms an irregular cycle for the earthquakes in Himalaya,” says Jean-Louis Mugnier, a geologist at the University of Savoy, Bourget-du-Lac, in France who wasn’t part of the study. Bilham declined to comment on the new findings.
But Laurent Bollinger, a geologist at France’s Alternative Energies and Atomic Energy Commission, argues that the evidence so far is not persuasive enough to verify the clustering hypothesis or rule out the big 1505 quake. “There is a risk that there are some very big earthquakes that are being missed in the historical chronicles,” he says. What’s needed, he says, are more data from more trenches. In the meantime, Mugnier says, disaster management authorities need to recognize that a massive temblor can strike anywhere in the Himalayas, at any time. “The level of risk is stable: always high.” In 2013, India’s National Disaster Management Authority estimated that an earthquake of magnitude 8 or greater just about anywhere in the rapidly urbanizing Himalayas would kill, on average, about 800,000 people. ■
This story appeared in Science Magazine in February 2015
Thrust up by the continuing collision of the Indian subcontinent with Asia, the Himalayas are frequently rattled by major earthquakes. But for several centuries, the CSG, a 600-kilometer-long region extending northeast of Delhi, has been quiet, even though it straddles major faults. In 2003, the late Greek geologist Nicholas Ambraseys and Roger Bilham, a geophysicist at the University of Colorado, Boulder, proposed that a large earthquake on 6 June 1505, known from Tibetan annals and the Akbarnama, a chronicle of the 16th century Mughal emperor Akbar’s reign, could have relieved some of the strain building up at the CSG.
Based on severe structural damage to Tibetan monasteries located nearly 700 kilometers apart, Ambraseys and Bilham estimated that the 1505 quake would have registered at 8.2 or so on the moment magnitude scale, which measures the energy released during an earthquake. The geological smoking gun for the quake seemed to materialize a few years later. After digging six trenches at points along a 250- kilometer stretch of the CSG in 2006, geologist Senthil Kumar, who was then at the University of Nevada, Reno, and colleagues uncovered faults that could be radiocarbon dated using charcoal in the sediments. They attributed the faults to an earthquake between 1400 and 1422 C.E. But written records do not mention a major quake during this period, and because the charcoal dating’s error bars encompassed 1505 C.E., Kumar’s group chalked up the findings to the 1505 Tibetan quake.
Also cited as evidence for that quake was a trench in western Nepal described only in an abstract in 2006. Other experts, however, have doubted the 1505 earthquake’s potency. Many medieval monasteries in the region are built from rock masonry without mortar, making them vulnerable to even moderate earthquakes, says Chittenipattu Rajendran, a paleo seismologist at the Jawaharlal Nehru Centre for Advanced Scientific Research here. Such damage, therefore, “is not realistic to use as an indicator of magnitude,” says Rajendran, who adds that there is no historical record of extensive damage in Indian cities like Agra and Delhi in 1505.
To get a fresh perspective on the CSG’s seismic history, Rajendran’s group dug a new trench in the Uttarakhand district of Ramnagar, adjacent to a trench Kumar’s team had excavated. They got lucky, uncovering a colluvial wedge of sediment— the geological signature of any quake that ruptures the earth’s surface. The rupture briefly pushes up a scarp, which crumbles to form a wedge. Rajendran’s team found that the material in the wedge had been ruptured not once, but twice, which could only mean two earthquakes, they say. Carbon dating linked both ruptures to earthquakes in 1255 C.E. and 1344 C.E. that are known from historical records, the team reports in an article posted online last month in the Journal of Geophysical Research: Solid Earth. Yet the trench showed no sign of a later, large earthquake.
R. Jayangondaperumal, a geologist at the Wadia Institute of Himalayan Geology in Dehradun and a member of Kumar’s team in 2006, says a reanalysis he has conducted on their 2006 trenches also suggests a pair of earthquakes rather than a single one. The finding not only casts doubt on the extent of the strain-relieving quake in 1505 but also “confirms an irregular cycle for the earthquakes in Himalaya,” says Jean-Louis Mugnier, a geologist at the University of Savoy, Bourget-du-Lac, in France who wasn’t part of the study. Bilham declined to comment on the new findings.
But Laurent Bollinger, a geologist at France’s Alternative Energies and Atomic Energy Commission, argues that the evidence so far is not persuasive enough to verify the clustering hypothesis or rule out the big 1505 quake. “There is a risk that there are some very big earthquakes that are being missed in the historical chronicles,” he says. What’s needed, he says, are more data from more trenches. In the meantime, Mugnier says, disaster management authorities need to recognize that a massive temblor can strike anywhere in the Himalayas, at any time. “The level of risk is stable: always high.” In 2013, India’s National Disaster Management Authority estimated that an earthquake of magnitude 8 or greater just about anywhere in the rapidly urbanizing Himalayas would kill, on average, about 800,000 people. ■
This story appeared in Science Magazine in February 2015
