Koyna earthquakes may become more frequent

WITH RECURRING earthquakes in Maharashtra, Indian geologists warn the fault zone under the Koyna Dam has been weakened by annual reservoir filling and that with the passage of time the region will experience earthquakes even with slight fluctuations in reservoir level.

Kusala Rajendran and C. M. Harish at the Center for Earth Science in Thiruvananthapuram claim that the spatial and temporal patterns of the recurring earthquakes in Koyna region suggest the possibility of the fault zone becoming further unstable, progressively lowering the stress needed to trigger an earthquake.

Their conclusion is based on analysis of earthquakes that had occurred in the region since Koyna experienced the first earthquake in 1967 five years after water was impounded in the reservoir.

Reporting their findings in Current Science they say that seismicity associated with Koyna reservoir is unique in the world because it is one of the few sites where earthquakes of magnitude greater than five continue to occur three decades after the initial spurt of activity.

Reservoir induced earthquakes have occurred in other parts of the world too, but seismicity in these regions had tapered off with time, the scientists said.

Persistence of moderate seismicity at Koyna is enigmatic considering also the fact that the Indian peninsular shield is relatively stable region, they said.

Most earthquakes in the last three decades occurred in an area of about 25 kilometre long and 10 kilometre wide extending south of the Koyna reservoir.

According to the geologists, the ongoing seismicity at Koyna suggests that the annual loading cycles in the reservoir, that was impounded in 1962, continue to influence the overall stability of the region.

Scientists said earthquakes in Koyna region have been occurring with striking regularity two to three months after reservoir filling. According to the scientists, the annual filling cycles of the reservoir continue to weaken this fault.

``With the passage of time the fault zone will become further unstable, progressively lowering the threshold changes required to initiate a failure,'' they said.

- PTI

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