Abstract
In 1255, 1344, and 1408 AD, then again in 1833, 1934, and 2015, large earthquakes, devastated Kathmandu. The 1255 and 1934 surface ruptures have been identified east of the city, along comparable segments of the Main Frontal Thrust (MFT). Whether the other two pairs of events were similar is unclear. Taking into account charcoal’s age inheritance, we revisit the timing of terrace offsets at key sites to compare them with the seismic record since 1200 AD. The location, extent, and moment of the 1833 and 2015 events imply that they released only a small part of the regional slip deficit on a deep thrust segment that stopped north of the Siwaliks. By contrast, the 1344 or 1408 AD earthquake may have ruptured the MFT up to the surface in central Nepal between Kathmandu and Pokhara, east of the surface trace of the great 1505 AD earthquake which affected western Nepal. If so, the whole megathrust system in Nepal broke in a sequence of earthquakes that lasted less than three centuries, with ruptures that propagated up to the surface from east to west. Today’s situation in the Himalayan seismic sequence might be close to that of the fourteenth century.
Highlights
The deadly Mw 7.8 Gorkha earthquake of April 25, 2015, is the latest of a long series of earthquakes which has affected Kathmandu valley, partially releasing the strain accumulated along the Main Himalayan Thrust fault (e.g., Ader et al, 2012; Stevens and Avouac, 2015; Grandin et al, 2015; Kobayashi et al, 2015)
The results show that both Kathmandu and Pokhara, the two largest cities of Nepal, may be exposed to earthquakes rupturing the Main Frontal Thrust (MFT), as it likely last did in medieval times
The first conclusion to be drawn from this work is that the surface ruptures described along strike of the active frontal thrusts in Nepal can be tied to historical earthquakes, taking into account detrital charcoal age inheritance of a few decades
Summary
The deadly Mw 7.8 Gorkha earthquake of April 25, 2015, is the latest of a long series of earthquakes which has affected Kathmandu valley, partially releasing the strain accumulated along the Main Himalayan Thrust fault (e.g., Ader et al, 2012; Stevens and Avouac, 2015; Grandin et al, 2015; Kobayashi et al, 2015). The unfaulted unit above yielded late thirteenth and fifteenth to sixteenth century detrital charcoal ages (Upreti et al, 2000) (Fig. 2), consistent with rupture by the great 1255 earthquake or, less likely because it would require larger inbuilt ages, by the 1344 event.
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