Earthquake size, recurrence and rupture mechanics of large surface-rupture earthquakes along the Himalayan Frontal Thrust of India

Plate motion models and GPS measurements indicate that the India-Eurasia convergence continues today at a rate of ∼40--50 mm/year. Between ∼10 and 20 mm/year of the total ∼40--50 mm/year is taken up by thrusting along the Himalayan Frontal Thrust (HFT). This ongoing India-Eurasia convergence has produced three major historical earthquakes (1905 Kangra, 1934 Bihar-Nepal, and 1950 Assam Earthquake) during the past century. None of the events are reported to have produced coseismic surface ruptures. Lack of primary surface rupture during these historical earthquakes has led previous workers to interpret the HFT as a blind thrust, or more strictly, as characterized by a fault plane that does not reach the surface. Towards understanding the relationship between strain accumulation and strain release in the context of mechanics of the earthquake and mountain building process and quantifying the seismic hazard associated with the globes largest continental thrust system, I describe the late Quaternary expression and paleoseismic evidence for great surface-rupture earthquakes at six sites along the Himalayan Frontal Thrust (HFT) system of India. Our observations span a distance of ∼250 km along strike of the HFT. Uplifted and truncated fluvial strath terraces resulting from the Holocene displacements on the HFT are preserved along canyons of the Ghaggar, Markanda, Shajahanpur, and Kosi Rivers. Dividing the elevation of the bedrock straths by the maximum radiocarbon ages of the terrace surface abandonments yield an estimate of the vertical uplift rate equal to ∼4--6 mm/year, which when assumed to be the result of slip on a thrust dipping at ∼20°--45° is equivalent to a fault slip rate of ∼6--18 mm/year or shortening rate of ∼4--16 mm/year. Trench exposures reveal the HFT to break surface sediments near the cities and villages of Chandigarh, Kala Amb, Rampur Ganda, Lal Dhang, and Ramnagar. Radiocarbon ages of samples obtained from displaced sediments indicate surface rupture at each sites took place after ∼AD 1200 and before ∼AD 1700. Uncertainties attendant to the radiocarbon dating currently do not allow an unambiguous definition of a capping bound on the age of the displacement at each site or whether or not the displacements at all sites were contemporaneous. Trench exposures and vertical separations measured across scarps at Rampur Ganda, Lal Dhang, and Ramnagar are interpreted to indicate single-event displacements of ∼11--38 m. Dividing the observed single-event vertical displacements by the estimated longer-term uplift rates indicate{09}∼1330--3250 years would be required to accumulate the slip sufficient to produce similar sized displacements. (Abstract shortened by UMI.)...