Discussion of seismicity near Jaitapur

We thank Rastogi for pointing out a number of factual errors in our article, and welcome this opportunity to correct several scientific misunderstandings that have crept into his manuscript. Rastogi is in agreement with our statements and conclusions concerning the seismicity in the Jaitapur region that: (a) no significant M > 6 earthquakes have been recorded by seismographs located within 50 km of Jaitapur, and no historical reports of intense shaking can be attributed to such earthquakes near Jaitapur, and (b) shaking intensity from distant historical earthquakes has not exceeded MSK Intensity VI. We conclude, as we assume Rastogi does, that these observations do not place a reliable upper bound on shaking due to future seismicity in the Jaitapur region. This was the thrust of our article. Some of Rastogi’s comments are easily addressed with minor corrections or clarifications, but his main counterargument requires a more thorough analysis. We provide the former first. Rastogi incorrectly ascribes our analysis to data shown in figure 2 b. As stated in our text, it was calculated using a 16°35′N latitude for Jaitapur from data in figure 2 a as listed in our appendix. Figure 2 b shows a map of recent data, with one of the measurement sites labelled ‘Jaitapur’. The source of figure 2 b is cited. We mentioned incorrectly that several minor earthquakes have occurred within 50 km of Jaitapur. Rastogi asserts that ‘no such earthquake has been located by the Koyna network operating since 1962...’. The Executive Director of the Nuclear Corporation of India, however, reports an earthquake on ‘10 October 2008 of 3.1 magnitude at 44.9 km distance’. The closest globally recorded earthquake is 52 km from Jaitapur. It was an Mw = 4.1 earthquake that occurred at 16.82°N, 73.28°E on 26 March 2005, which resulted in a calculated intensity ≈ IV at Jaitapur. With deference we correct Rastogi’s arithmetic: a strain of 10 is 100, not 10, microstrain. Also he presumably agrees with most seismologists that estimates of earthquake magnitudes, Mw, from seismic data are typically uncertain to 0.2 Mw units. The values asserted by him depart from those used by us for the Koyna and Latur earthquakes by a lesser margin. He writes that post-glacial sea-level rise results in stresses too small to fracture rocks. This is unwittingly deceptive, because in a region that is already critically stressed, the marginal stresses that he computes may suffice to trigger an earthquake, a central theme of his wellknown, reservoir-induced seismicity articles. We cite as an example of present-day triggering by minor hydraulic loading, the increase of micro-seismicity attending the annual stressing of the Himalaya that accompanies monsoon flooding of the Ganga Plain. Our rebuttal, however, mostly concerns his substantial commentary on geological observations, which he proposes as evidence for the absence of seismicity south of Koyna. The most important considerations include: (i) the presence or absence of possible active faults in the study area; (ii) constraints on timing of past ruptures on these structures that are provided by regional geomorphology and (iii) the existence, likely magnitude and rate of change of regional stresses related to lithospheric flexure.