GNGTS 2013 - Atti del 32° Convegno Nazionale

Pliocene ages. The lesser Kumaun Himalaya exposes a thick pile of highly folded Proterozoic sedimentary strata together with a few outcrops of older crystalline rocks. It is bounded by the MBT to the south and the MCT to the north. The Great Himalaya exposes a massive pile of high grade metamorphic rocks and the Tethys Himalaya includes a thick pile of sedimentary rocks of Cambrian to Lower Eocene ages. The extension of Aravalli structures into the Himalayan regions has played a role in the tectonics of the Kumaun Himalaya, and probably is the cause of the complex nature of seismicity of the region. In Kumaun Himalaya region the groups of rocks are known as Vaikriti group (Valdiya, 1980). According to the model of Srivastava and Mitra (1994), the Kumaun Himalaya evolved by an overall forelandward progression of thrusting, with some reactivation along the Munsiari thrust (MT), the Main Boundary thrust (MBT), and the Main Central thrust (MCT). In this part the maximum strain-energy release is related to the Main Central Thrust (MCT). It is among the least understood parts of the Himalayan fold-and- thrust belt. Valdiya (1980) gives the most comprehensive account yet published on the geology of the region. It has been found that the large scale thrusts recognized in the Kumaun lesser Himalaya are boundary thrusts defining the limit of the various litho-tectonic units. There are large numbers of local thrusts less than 50 km in length, which have severed the tightly folded rock formations along the axial plane and brought the older rocks over the younger. This sector evidenced reactivation of some of the faults and thrusts during Quaternary times. This is amply evident by the recurrent seismicity patterns, geomorphic developments and by geodetic surveys (Valdiya, 1999). A generalized tectonic sequence for the Lesser Kumaun Himalaya (Valdiya, 1978) is tabulated below and shown in the Fig. 1 (after Célérier et al. , 2009). Vaikrita Group .......................Vaikrita Thrust...................... Munsiari Formation ................................Main Central Thrust............................... Almora-Dudhatoli Nappe (with Askot, Baijnath, Chiplakot and Satpuli Klinne) ........................Almora Thrust.......................... Outer Sedimentary Belt Inner sedimentary Belt ...........................Main Boundary Fault............................. Siwalik Group Methodology. In this work we used coda normalization method which provides a reliable way to estimate the frequency dependence of important parameters quantifying the seismic source radiation and receiver site amplification, both of which are used in seismic risk assessment. It also allows the investigation of propagation effects. Most of seismology is focused on characterizing one of these three influences, (1) source radiation propagation, (2) site amplification, on seismograms and (3) propagation effect. The coda normalization method is based on the idea that at lapse time, the seismic energy is uniformly distributed in some volume surrounding the source (Sato and Fehler, 1998). In this method spectral amplitude of the earthquake source is normalized by coda waves at a fixed lapse time. It is based on the idea that coda waves consist of scattered S waves from random heterogeneities in the Earth (Aki, 1969; Aki and Chouet, 1975; Sato, 1977). Roughly lapse time is taken twice of the direct S-wave travel time and spectral amplitude of coda at a lapse time t c , A c ( f , t c )is independent of hypocentral distance r in the regional distance range, and can be described (Aki, 1980) 140 GNGTS 2013 S essione 1.1