Long recurrence interval of faulting beyond the 2005 Kashmir earthquake around the northwestern margin of the Indo-Asian collision zone

Abstract

The 2005 Kashmir earthquake in Pakistan occurred on a previously mapped active fault around the northwest margin of the Indo-Asian collision zone. To address the quantitative contribution of the earthquake to plate convergence, we performed paleoseismological trench excavations at Nisar Camp site near Muzaffarabad across the middle section of the 2005 surface rupture. The fault strands exposed in the trench cut late Holocene fluvial deposits and record evidence of both the 2005 and a penultimate event, supported by the presence of colluvial deposits and a downdip increase in displacement along the fault strands. The 2005 event produced a net slip of 5.4 m, and the penultimate earthquake exhibits a similar amount of slip. Radiocarbon ages and historical accounts loosely constrain the timing of the penultimate event between 500 and 2200 yr B.P.; however, the exposed section encompasses ~4 k.y. of stratigraphy, suggesting an average interevent interval of ~2 k.y. for the 2005 type events. We thus conclude that the 2005 event did not occur on the plate boundary megathrusts, but on intraplate active faults within the Sub-Himalaya. Consequently, the accumulated elastic strain around the complex northwestern margin of the Indo-Asian collision zone has not been significantly released by the 2005 earthquake.