RESUMEN
According to paleoseismological studies, the last earthquake that ruptured the Main Frontal Thrust in western Nepal occurred in 1505 AD. No evidence of large earthquakes has been documented since, giving rise to the concept of a seismic gap in the central Himalaya. Here, we report on a new record of earthquake-triggered turbidites from Lake Rara, western Nepal. Our lake-sediment record contains eight possibly moderate-to-large earthquake-triggered turbidites during the last 800 years, three of which overlap in age with previously reported Mw ≥ 7 events in western Nepal. Shaking intensity modelling, together with instrumental records, suggests that near-field earthquakes (≤15 km) should have a minimum Mw 5.6, and regional earthquakes (≤80 km) a Mw > ~ 6.5, to trigger turbidites. We present a likely scenario that western Nepal may be as seismically active as central Nepal; however, more data are needed to revaluate the seismic risk in the central Himalaya.
RESUMEN
Collisional mountain belts grow as a consequence of continental plate convergence and eventually disappear under the combined effects of gravitational collapse and erosion. Using a decade of GPS data, we show that the western Alps are currently characterized by zero horizontal velocity boundary conditions, offering the opportunity to investigate orogen evolution at the time of cessation of plate convergence. We find no significant horizontal motion within the belt, but GPS and levelling measurements independently show a regional pattern of uplift reaching ~2.5 mm/yr in the northwestern Alps. Unless a low viscosity crustal root under the northwestern Alps locally enhances the vertical response to surface unloading, the summed effects of isostatic responses to erosion and glaciation explain at most 60% of the observed uplift rates. Rock-uplift rates corrected from transient glacial isostatic adjustment contributions likely exceed erosion rates in the northwestern Alps. In the absence of active convergence, the observed surface uplift must result from deep-seated processes.