Study finds earthquake threat from Riasi fault in Himalayas

The Riasi fault in the Himalayas could pose a major earthquake threat to 1 million people in parts of Kashmir currently controlled by Pakistan and India, researchers at a U.S. university have found.

In a study accepted for publication by the Geological Society of America Bulletin and published online, the researchers with Oregon State University (OSU) said the Riasi fault has been building up pressure for some time and when it does release or "slip," the resulting earthquake may be large, as much as magnitude 8.0 or greater.

The Riasi fault in the part of Kashmir now controlled by India has been known to researchers, but was not thought to be as much as a threat as other more active fault systems.

However, following a magnitude 7.6 earthquake in 2005 on the nearby Balakot-Bagh fault in the part of Kashmir controlled by Pakistan, which killed a total of about 80,000 people, researchers began scrutinizing other fault systems in the region.

"What we set out to learn was how much the fault has moved in the last tens of thousands of years, when it moved, and how different segments of the fault move," said Yann Gavillot, lead author on the study who did much of the work as a doctoral student at OSU. "What we found was that the Riasi fault is one of the main active faults in Kashmir."

"The fault hasn't slipped for a long time, which means the potential for a large earthquake is strong," he said. "It's not a question of if it's going to happen. It's a matter of when."

There is direct evidence of some seismic activity on the fault, where the researchers could see displacement of the Earth where an earthquake lifted one section of the fault five or more meters, possibly about 4,000 years ago.

Nevertheless, the researchers do not have much evidence as to how frequent major earthquakes occur on the fault, or when it may happen again.

In that region, the India tectonic plate is being subducted beneath the Asia plate at a rate of 14 millimeters a year. The Riasi fault accounts for half of that but has no records of major earthquakes since about 4,000 years ago, indicating a major slip, and earthquake, is due.

"In the last 4,000 years, there has only been one major event on the Riasi fault, so there is considerable slip deficit," said Andrew Meigs, a co-author on the study and a geology professor in OSU's College of Earth, Ocean, and Atmospheric Sciences.

"When there is a long gap in earthquakes," he explained, "they have the potential to be bigger unless earthquakes on other faults release the pressure valve. We haven't seen that."

A major earthquake at the Riasi fault could have a major impact on cities and towns near or along the fault, which have a combined population of about 2.2 million people, said Gavillot, adding that "the potential for destruction is much greater than the 2005 earthquake." Endi