Special Section: Intraplate Seismicity

Re-evaluated intensity distributions for the 1897 Mw 8.0 Shillong and the 1905 Mw 7.8 Kangra earthquakes, combined with geodetic constraints on rupture geometries, allow us to compare observed distributions of intensity with theoretically predicted shaking. The difference between predicted and observed shaking is interpreted in terms of the site response of the Ganges and Brahmaputra basins. These comparisons identify regions of enhanced shaking along main rivers, revealing amplifications of 1–2 intensity units, roughly an amplification of 2–4 in acceleration. We also find two unexpected results in our analysis of the Kangra earthquake: (i) The epicentral region is surrounded by a halo of enhanced intensity at 150–200 km radius and (ii) The Dehra Dun region was the locus of a broad region of anomalously high intensities. We interpret the former result as the signal from post-critical Moho reflections and the latter observation as a probable second large earthquake (M > 7) at 30–50 km depth triggered within minutes of the 1905 main shock. These results have important consequences from future earthquakes in the Himalaya.