Holocene paleoeseismology of the Fodongmiao-Hongyazi Fault along the Northern Tibetan margin (Western China) and implication to intraplate earthquake rupturing pattern

Abstract

The margins of the Tibetan Plateau are the most recent evolving part of the orogen, where distributed fault systems accumulate intraplate crustal deformation induced by the Indo-Asian collision, and many great earthquakes occur. Understanding the paleoearthquake history of the faults along the Tibetan margins will increase our knowledge of the process of outward growth of the Tibetan Plateau, seismic hazard assessments, and intraplate earthquake patterns. However, until now, few paleoseismic studies were conducted along the northern Tibetan margin, which hampers our understanding of fault activity and seismic hazards posed to the densely populated cities in the region. In this study, we excavated paleoseismic trenches across the Fodongmiao-Hongyazi fault (FHF) along the northern Tibetan margin. Five Holocene events were documented from five trenches based on OSL and radiocarbon-14 dating of trench horizons and careful analysis of stratigraphy and fault structures. These events are FE5 (9.6–7.7 ka), FE4 (7.6–7.0 ka), FE3 (4.4–3.5 ka), FE2 (3.2–1.8 ka), and the 1609 M 71/4 Hongyapu earthquake. The average recurrence interval of the five events is ~2.0 ka. Additionally, combined with morphotectonic features and trench observations, the 1609 Hongyapu earthquake most likely caused the whole fault to rupture with a length of ~95 km. Its moment magnitude was estimated to be 7.2 ± 0.2 based on empirical statistical relationships between the magnitude, surface rupture length, and coseismic offset.