Geological archives lead to past earthquakes in the Himalaya
- Kusala Rajendran, CEaS
Large earthquakes often leave imprints of deformation on local geology, their preservation in favorable conditions lead to the recognition of past events that might have affected the area. Where ancient civilizations go back in time longer than the interval between large earthquakes, signatures of destructive earthquakes evident on ancient monuments may also add credence to the geological inferences. The science of paleo- and archeo-seismology is essentially about reconstructing the earthquake history beyond the instrumental period, using such archives. The Himalaya has witnessed four great earthquakes during the past two centuries and the current estimates from GPS-based deformation models suggest that a great earthquake (M > 8.2) is imminent in the Central Himalaya. The devastation from such an earthquake could be unprecedented.
A key issue to seismic hazard evaluation of a region concerns reconstructing its earthquake history. Trenching excavations in the rupture zones of earthquakes often lead to evidence of past slip on the same fault, which can be used to estimate the size and timing of paleoearthquakes. The known great earthquakes of the Himalaya have not caused any surface ruptures, placing constraints on the search for past events. However, trenches excavated in the foothills of the Himalaya in India and Nepal by earlier workers exposed slip of 17 and 28 m, respectively, which they relate to earthquakes during AD 1100 and 1505. These are considered to be great earthquakes of magnitude > 8.0.
A few pertinent questions remain. One, why do some earthquakes generate surface slip whereas some others do not? Two, how could the 11-12th century structures in the Garhwal Himalaya and Delhi escape damage from the great 1505 earthquake? Three, is the magnitude estimated from the slip real? Our studies suggest that dual slip mechanisms are possible. One in which the earthquake nucleates on the gently dipping basal detachment surface along which the Eurasian plate is advancing towards the Indian plate. In this case rupture could be inhibited by the crustal ramps on the leading edge of the plate. In the other, the earthquake originates on the shallow and relatively steeper ramps and the rupture propagates upward, leading to large surface slips. Given these geometrical constraints, the slip does not directly reflect the magnitude. On the timing of the past earthquakes, our studies suggest that the last great earthquake in Garhwal Himalaya occurred during AD 1119-1292, an inference derived from geological, historical and archeological evidence. The predecessor of the great 1897 event in the vicinity of Shillong is about 1000 yr old; the timing of the predecessor of the 1950 earthquake in the Upper Assam is less certain, but it could be prior to AD 1400.
K. Rajendran and C.P. Rajendran, 2011, Revisiting the earthquake sources in the Himalaya: Perspectives on past seismicity, Tectonophysics, in Press. doi:10.1016/j.tecto.2011.03.001