Paleoseismic events inferred from soft sediment deformation structure of varved lake sediment in Lake Kusai of northern Tibetan Plateau during the past 2000 years

Abstract

The 1500-km-long Kunlun strike-slip fault is one of the largest and most active faults in the Tibetan Plateau. The eastward extrusion of Tibet has generated the 2001 Mw 7.8 Kunlunshan earthquake along the eastern Kunlun fault (elevation ≥ 4500 m) that produced the longest surface rupture yet recorded on land, ~400 km. The knowledge of past earthquake history and long-term fault behavior along the eastern Kunlun fault is still limited by short seismological records. Moreover, suitable sites for paleoseismological trenching are rarely available due to the active freeze-thaw action in such an extreme environment. Lacustrine paleoseismology, exploits lacustrine sediments to retrieve a much longer record of paleoseismic shaking, may shed light on earthquake occurrence and fault behavior along the eastern Kunlun fault.Lake Kusai is bounded by the eastern Kunlun fault in the north, and its north end was once ruptured by the 2001 Mw 7.8 Kunlunshan earthquake. Sedimentary sequence from the lake is characterized by layered sediments, thus an ideal site for lacustrine paleoseismology investigation. A 8.2 m-long core has been drilled from the lake in 2006. 210Pb/137Cs and AMS14C dating, and detailed varve counting confirm that the upper 165 cm of the drilled sedimentary sequence is varved and spans the last 2000 years. This sedimentary sequence is therefore ideal for event deposits recognition and high-resolution seismic record reconstruction. We identify sedimentary imprint of the 2001 Mw 7.8 earthquake and another 13 deeper horizons of in situ soft sediment deformations and micro-faults from the sedimentary sequence. The primary data reveals a clustered earthquake recurrence pattern with a mean recurrence interval of ~150 years on the central part of the eastern Kunlun fault. Keywords: Eastern Kunlun fault; Northern Tibetan Plateau; Lake Kusai; Varved lake sediment; Soft sediment deformation structure; Paleoseismic events.