GNGTS 2015 - Atti del 34° Convegno Nazionale

GNGTS 2015 S essione 1.1 51 located in the Himalayas. Lyon-Caen and Molnar (1984) suggested 50 to 80 km or more of underthrusting beneath northern Tibet, Brandon and Romanowicz (1986) measured only 50 to 60 km underthrusting. Indian crust surely was underthrusted beneath southernmost Tibet, and one of the troubling uncertainties still is - however - the magnitude of this underthrusting. Then, according to Molnar (1989), crustal thickening accounts for only part of Tibet’s high altitude; its high uniform elevation would be supported by buoyancy and also by a hot upper mantle. The Himalayan convergence is a unique case of continental subduction and both the Himalaya and the Assam earthquakes are driven by the convergence of approximately 20 mm/ yr between India and the Himalaya. One part of the India’s penetration is absorbed by local crustal thickening, while a significant fraction of the India-Eurasia convergence is absorbed by extrusion of material out of its way (e .g., Molnar and Tapponnier, 1975). This last mechanism is mostly relevant for the region of the M S 8.1, 1897 Shillong earthquake. The rest of the northward movement of the Indian subcontinent (approximately 20 mm/yr more) mainly causes the contraction of the Tibetan plateau (e.g.: Feldl and Bilham, 2006; Ader et al., 2012). The principal tectonic structure of underthrusting is the Main Himalayan Thrust (MHT), a sub-horizontal plane directly connected with the subduction of India beneath the Asian continent. The shallow-dipping MHT detachment is responsible of the Himalaya growth with a series of crustal ramps from the base of the detachment to the surface; from south, from the youngest to the oldest these ramps are: the Main Frontal Thrust (MFT, formed at the beginning of Pleistocene, still the most active), the Main Boundary Thrust (MBT) and the Main Central Thrust (MCT). odels of seismic coupling suggest that the MHT is fully locked up to the surface portion called the ain Frontal Thrust (MFT) (Ader et al. , 2012; Sapkota et al. , 2013). Four strong earthquakes struck the Himalayan seismic belt approximately in one hundred years: the moment magnitude M8 Kangra, 1905 and the M S 8.2 Bihar-Nepal, 1934 (Hough and Bilham, 2008; Ambraseys and Douglas, 2004); the M8.5 Assam, 1950 (Raghukanth, 2008), and the M7.6 Kashmir, 2005. The Kangra, 1905 and Bihar-Nepal, 1934 are related to the main thrust of Himalaya growth MHT. In 1505, a huge earthquake of M8.8 struck the Himalayan belt between the meizoseismal areas of the Kangra, 1905 and Gorkha, 2015 events (Feldl and Bilham , 2006). Fig. 1 shows a tectonic sketch of the area of study with: (A) the surface rupture of MFT found by Sapkota et al. (2013), with the hypothesis by Dixit et al. (2015, in press); (B) the best-fitting source of the 1934 earthquake obtained from the present inversions; (C) the central source used for the fast KF scenario of the M7.8 Gorkha, 2015 earthquake (see Fig. 3). The positions of the MFT, the MBT and the MCT Thrusts in Fig. 1 are taken from Stephenson et Fig. 1 – Tectonic sketch of the area of study. The star is the epicenter of the M S 8.2, 1934 Bihar-Nepal earthquake (Chen and Molnar, 1977); black cross: same, by Singh and Gupta (1980). The polygons/rectangles are the projections of: A) the source of the 1934 earthquake according to the surface rupture of MFT found by Sapkota et al. (2013) and to the hypothesis by Dixit et al. (2015, in press); B) the best-fitting source of the 1934 earthquake obtained from the present inversions; C) the central source used for the fast KF scenario of the M7.8 Gorkha, 2015 earthquake (asterisk: epicenter); D) as (C) for the strong aftershock (M7.3) of 12 May, 2015 (not shown in this paper).