Thursday, December 8, 2011

Insights Into Plate Interior Earthquakes Of Peninsular India

A paper in Current Science (open access) by Roger Bilham and Vinod K Gaur gives interesting insights into the patterns of occurrences and causes of earthquakes in the plate interior regions of Peninsular India. In south Maharashtra for instance there have been moderate size earthquake of 6.3 - 6.4 magnitude in recent decades at Koyna and Latur. Earthquake risk assessment has shot to prominence recently due to a proposal to site a large nuclear power station in Jaitapur, which is close to Koyna.

The general understanding of earthquakes in Peninsular India is that the Precambrian terrain  is heterogeneous in strength, criss-crossed with rifts, shear zones and old orogenic belts and these ancient zones of weak crust get reactivated from time to time and rupture.

But what is building up strain along these old faults? The graphic below from the paper is a good way to conceptualize the tectonic and stress situation.

The collision of the Indian plate with Tibet has resulted in the bending of the Indian plate underneath Tibet and the flexural buckling into a long waveform of the rigid Indian plate. At the northern end i.e. at the crest of the flexure the plate experiences tensional forces at shallow depths and compressional forces at the base of the plate. Farther south in the trough of the flexure known as the outer moat, the situation is reversed. The shallow part of the plate experiences compressional forces and the base of the plate experiences tensional forces.

 Imagine now that this waveform is static in the sense that the stress fields described remain fixed in space. The rocks of the Indian plate are however moving northwards. They pass through the compressional stresses in the trough and many millions of years later pass through the tensional stresses of the crest. At the base of the plate its the other way around. Geodetic studies using GPS i.e. studies aimed at understanding slip along faults to estimate how quickly strain is building up, suggest a very low rate of strain buildup in Peninsular India. Calculations suggest that replenishment of strain is on the order of 300,000 years, meaning on average faults won't slip more than 3 times per million years. But given the millions of years these rocks have remained in either compressional or tensional stress fields and given the rarity of historical and recent earthquakes, the authors thesis is that there would be many many faults reaching a state of critical stress i.e. close to rupturing.

One practical implication of this is that the seismic hazard maps issued by various agencies don't always portray future risk accurately. These are based on a very short historical record and since the cycle of strain buildup is much longer, faults that may have slipped thousands of years ago and have been building up strain since, will remain unnoticed. That is what likely happened at Latur, an area considered to be at low seismic risk based on history, until the sudden large earthquake. 

Koyna and Latur fall in the trough region of the flexure. So does Jaitapur. The faults at Koyna and Latur have slipped recently and given the low rate of strain buildup are unlikely to rupture for some time to come. But there is no record of recent or historical seismicity near Jaitapur. There is a lot of discussion in the paper on earthquake catalogs for India and their usefulness. The reliable historical record doesn't extend beyond a couple of hundred years or so. And further, its not known whether there is a subsurface fault underneath Jaitapur, but the regional picture tells us that Jaitapur would be subject to the same stress regime as Koyna and Latur and there is a possibility - albeit low - of a moderate size earthquake occurring underneath the proposed power plant site.

One quibble I have regarding the graphic above is the placing of Bhuj in the flexural trough. It is in fact located north of the flexural trough, in the crest domain. Bhuj refers to the town in Gujarat state which suffered a large 7. 6 magnitude earthquake in 2001. Bhuj had suffered an earlier earthquake of 7.9 magnitude in 1819.  The area falls within the Kutch rift which was initiated during the late Triassic breakup of Gondwanaland. Rifting was aborted during the late Cretaceous pre collisional stage of the Indian plate. Since then under a compressional stress regime, the rift has developed strike slip faults with local transpressional zones i.e. the stress is oriented is such a way so as to cause local reverse faulting and strike slip faulting. Both the Bhuj earthquakes were located in this transpressional zone in the eastern part of the rift. So Bhuj is something of a special case. A relatively younger crustal structure with complex local stress fields and faults reactivated by the ongoing collision of India with Tibet.

 Coming back to Maharashtra, it is difficult to get a handle on future seismic risk. This is because as I mentioned the historical record goes back only a couple of centuries and is patchy. Therefore, reliable statistics on the rate of earthquakes can't be developed so as to be used as a guide to future risk. Also because the Deccan volcanics have covered the Precambrian over most of the region, and because many of these ancient faults don't reach the surface,  examining these faults so as to understand their slip history is not possible. So only broad zones of weaknesses are interpreted by extrapolating the structural grain of Precambrian terrains at the margins of the Deccan volcanics or by using geophysical methods like mapping gravity lows (which might point to the presence of a sedimentary rift basin underneath the Deccan volcanics).

Under these constraints, site specific earthquake risk assessment is not done. But the broad picture developed in this paper does point out that much of Peninsular India may be under a high incipient state of stress and there are possibly many faults within the flexural trough between latitudes 16 deg N and 19 deg N which are not exposed at the surface that represent seismic hazards.

1 comment:

  1. Great diagram, thanks. I might use that to explain things for the region.