GNGTS 2015 - Atti del 34° Convegno Nazionale

56 GNGTS 2015 S essione 1.1 algorithm in the direct mode and the information available on April 26, 2015 (see Tab. 3). We consider the suspected faults with their hypothesized mechanisms, and related uncertainties, and treat them parametrically. In the present post-event case, all hypocentral determinations and focal mechanisms, calculated by several international agencies shortly after the earthquake, were used. The discrepancies between the various data were treated as uncertainties. The combination between Tabs. 3 and 4 gave 37,800 sources. Conclusions. The inversion of the Shillong, 1897 earthquake seems reliable because the synthetic match is striking and the source found is fully compatible with the seismotectonic situation of southern Assam presented in a fundamental paper (Sapkota et al. , 2013). The M S 8.2 Bihar-Nepal, 1934 earthquake offered the opportunity of one more verification of the automatic KF inversion technique. Its application to the intensities of this earthquake retrieved a source which is compatible with that obtained by authors who used instrumental data, field observations, palaeo-seismological logging of river-cut cliffs and trenches. Then, the KF inversion results are far better than those obtained by Singh and Gupta (1980) from instrumental data. In general, our source inversions provide some constraints on our ideas of how the seismotectonics of Himalaya condition the present seismic hazard in the area within a tectonic process lasting 60-80 millions years. The verification of the fast KF parametric scenario of the M7.8 April 25, 2015 Gorkha earthquake will be feasible when reliable site intensities will be available. Acknowledgements. The inversion of the Shillong, 1897 earthquake was performed in cooperation with NORSAR. Susan Hough called our attention on Sapkota et. al. (2013) and provided a figure from Dixit et al. (2015, in press). References Ader, T., J. P.Avouac, J. Liu-Zeng, H. Lyon-Caen, L. Bollinger, J. Galetzka, J. Genrich,Amatya, K. M., and B. M. Jnawali (2006), Geological Map of Nepal (1994) , Department of Mine and Geology, Lainchaur, Kathmandu, Nepal. Ambraseys, N. N., and J. Douglas (2004), Magnitude calibration of north Indian earthquakes, Geophys. J. Int., 159:165-206. doi: 10.1111/j.1365-246X.2004.02323.x Argand, É (1924), La Tectonique de l’Asie, Congrès Géologique International, Comptes rendus de la XIIIe session, en Belgique 1922 , Liège, 1924, 171-372. Bilham R. and P. England (2001), Plateau ‘pop-up’ in the great 1897 Assam earthquake, Nature, 410, 806-809. Chen, W.-P., and P. Molnar (1977), Seismic moments of major earthquakes and the average rate of slip in Central Asia, Geophys. Res., 82, 2945–2969. Dixit, A.M., A. Ringler, D. Sumy, E. Cochran, S.E. Hough, S.S. Martin, S. Gibbons, J. Luetgert, J. Galetzka, S.N. Shrestha, S. Rajaure, and D. McNamara (2015), Strong motion recordings of the M7.8 Gorkha, Nepal, earthquake sequence from low-cost, Quake Catcher Network accelerometers, Seism. Res. Lett., in press. Evans, P. (1964), The tectonic framework of Assam, J. Geol. Soc. India, 5, 88-96. Feldl, N., and R. Bilham (2006), Great Himalayan earthquakes and the Tibetan plateau, Nature , 444 9, 165-170 doi:10.1038/nature05199. Hough, S. E., and R. Bilham (2008), Site response of the Ganges basin inferred from re-evaluated macroseismic observations from the 1897 Shillong, 1905 Kangra, and 1934 Nepal earthquakes, J. Earth Syst. Sci., 117, S2, 773-782. Lyon-Caen, H., and P. Molnar (1984), Gravity anomalies and the structure of western Tibet and the southern Tarim Basin, Geophys. Res. Lett., 11, 1251-54. Molnar, P, and P. Tapponier (1975), Cenozoic Tectonics of Asia: Effects of a Continental Collision, Science, New Series, 189, 4201, 419-426. Pettenati, F., L. Sirovich, and D. Sandron (2011), Rapid simulation of seismic intensity for civil protection purposes; two recent cases in Italy, Seism. Res. Lett., 82, 3, 420-430. Doi: 10.1785/gssrl.82.3.420. Raghukanth, S. T. G. (2008), Simulation of Strong Ground Motion During the 1950 Great Assam Earthquake, Pure & Applied Geophysics , 165, 9/10, 1761-1787. Doi: 10.1007/s00024-008-04-3-z. Sapkota, S. N., L. Bollinger, Y. Klinger, P. Tapponnier, Y. Gaudemer, and D. Tiwari (2013), Primary surface ruptures of the great Himalayan earthquakes in 1934 and 1255, Nature Geosci., 6, 71-76 (http://dx.doi.org/10.1038/ ngeo1669). Sarma, K. P. (2014), Shillong Supergroup: a New Lithostratigraphic Unit in the Basement-Cover Precambrian Rocks of the Shillong Plateau, Northeast India, Int. J. Geol. Earth & Environ. Sci., 4 (2), 158-171 (http://www.cibtech. org/jgee.htm; last accessed 5 Oct., 2015).