Geologic clues to Himalayan earthquakes
Kusala Rajendran, Centre for Earth Sciences
How soon can the next great earthquake occur along the Himalayan front, and how large is it likely to be? A new paper, published in Journal of Geophysical Research, grapples with these difficult questions about South Asia's seismic risk.
The earthquakes at issue result from the well-known collision of the India Plate with the Eurasian Plate. The southern edge of the collision zone follows the front of the Himalaya for some 2500 km. The zone has produced four great (M> 8) earthquakes, since AD1905, but none since then.
As the story of earthquakes goes far beyond instrumentally recorded and historically documented details, the job of extending earthquake history thousands of years into the past falls on Earth scientists who try to read the geologic clues that earthquakes have left behind. Previous studies have pointed at the missing slip and an imminent great earthquake in the central segment of the Himalaya that remains unruptured, as against the adjoining ones that ruptured during the 1905 Kanga and 1934 Bihar, great earthquakes. With one seventh of the entire global population living in the outlying areas and with the country’s capital located just about 100 km away from the suspected earthquake sources, it is important to know the earthquake history of this segment of the Himalaya.
The new paper focuses on geological evidence in the youngest strata that have been deformed as the plates collide. This evidence, exposed by trenching, suggests that pulses of previous earthquake activity occurred within about hundred years, on adjacent segments of the central Himalaya. The last such pulse was followed by a long quiescence of about 700 years.
The trench sections studied at Ramnagar, on the foothills of the Himalaya (see Figure),preserve geological evidence of two times of surface rupture in 13th and 14th centuries AD. The quiescence since that time has been interrupted only by an M ~7.5 earthquake in 1803, which reportedly affected large areas of the Indo-Gangetic plain, including Delhi. The new findings add to concern that a central section of the fault has been locked for a long time, building up pressure for a future earthquake far larger than the one in 1803.
Rajendran, C.P., John, B. and Rajendran, K. 2015. Medieval pulse of great earthquakes in the central Himalaya: Viewing past activities on the frontal thrust. Jour. Geophys. Res., DOI: 10.1002/2014JB011015
New jitters over megaquakes in Himalayas ( an article appearing in Science News)
Important earthquakes along the Himalaya arc, also showing the
Central Segment, considered due for a great earthquake.