SEISMIC HAZARD, STRAIN ACCUMULATION, AND TOPOGRAPHY WITHIN THE CENTRAL HIMALAYAN SEISMIC GAP, NW NEPAL

Broad regions of over-steepened river channels have been documented in areas later found to have under estimated seismic hazards, like Wenchuan China. Channel steepness analysis is applied to the Dolpo and Mugu districts of Nepal, in the Central Himalayan seismic gap, and finds a 200 x 50 km swath of over-steepened channels. The most over-steepened channels are coincident with the forelimb of the Dolpo anticlinorium, and the epicenter of the Mw 8.2 1505 earthquake, suggesting a significant seismic hazard. The zone of over-steepened channels lies ca. 10 km north, downdip, of a cluster of micro-seismicity interpreted to mark the location of a ramp along the Main Himalayan Thrust (MHT). Models integrating Himalayan strain accumulation, GPS observations, and long term average slip-rates predict shortening/uplift above the transition from stable sliding beneath Tibet to stick-slip behavior at the Main Frontal Thrust (MFT). The transition (dislon) is likely controlled by the intersection of the 350°C isotherm with the MHT. Models of MHT locking depth suggest that the dislon is located ca. 150 km from the range front at the position of our zone of over-steepened channels. Persistent shortening/uplift above the dislon is thought to create a storehouse of elastic strain contributing to great Himalayan earthquakes. Active uplift coincident with the High Himalaya was recognized in early studies of vertical GPS motions, however the significance of these 10¹ year observations to long term processes is open to question. We extend these 10¹ year observations to at least 10⁴ and possibly 10⁶ years by combining normalized channel steepness index (k_sn) analysis with the earthquake record, and the Mid-Miocene generation of the Dolpo folds. The short timescale of GPS observations compared with great Himalayan earthquake recurrence intervals (10³ – 10⁴ years) has made it impossible to constrain whether hinterland uplift induced by elastic strain is entirely released in earthquakes, or whether some portion becomes permanent deformation. Our observations of over-steepened bedrock river channels provide the first evidence that over 10⁴ years timescales inelastic strain is recorded in bedrock channels overlying the MHT dislon. Suggesting that not all of the strain induced by the creeping Tibetan plateau is transferred to MFT.