Timing of Holocene Surface-Ruptures Across Adjacent Rupture-Segments Where the Jinsha River Crosses the Yushu Fault, Qinghai Province, China

Abstract

The Yushu fault, part of the Yushu–Ganzi fault zone, is one of several E-W-trending left-lateral, strike-slip fault zones that extend across the Tibetan Plateau accommodating lateral transfer of crustal material out of the collision zone between the Indian and Eurasian plates. Discontinuous left-lateral surface rupture occurred along WNW-ESE-striking, near-vertical faults during two Mw 6.9 14 April 2010 Yushu earthquakes. Geomorphic and surface-rupture characteristics change at the base of a red cliff separating the Guoqiong and Buqionggei segments. Excavations across these segments near the Jinsha River show: (a) more continuous deposition on the down-dropped side; and (b) how frequently these surface-rupture histories were linked or sequential (major to great earthquakes) versus not linked (moderate/strong to large earthquakes). Trench CUG-2011-1 and roadcut CUG-2012-1 (Guoqiong segment) were on older fan surfaces and Trench CUG-2012-2 (Buqionggei segment) crossed a narrow graben. On the Guoqiong segment, using OxCal v.4.4 that works with IntCal20 database for Bayesian-ordered ages from four ¹⁴C and nine OSL ages, surface ruptures occurred during earthquakes at 2010CE, ∼200BCE, ∼2400BCE, ∼4250BCE, ∼6750BCE, ∼7400BCE and ∼10400BCE consistent with an ∼2200-year Holocene recurrence interval. For the Buqionggei segment, Bayesian-ordered ages from six OSL ages indicate three Holocene surface ruptures occurred at 2010CE, ∼4600BCE and ∼6750BCE. Surface ruptures on both segments (suggesting linked or sequential major to great earthquakes) only occurred two to three times at 2010CE, ∼4500BCE and/or ∼6750BCE. Thus, risk of infrequent major to great Holocene earthquakes is ∼2400–∼8800 years along the Yushu fault.