THE MW 6.9 14 APRIL 2010 YUSHU EARTHQUAKE AND A RECORD OF HOLOCENE-LATE PLEISTOCENE PALEOSEISMICITY AT THE RED CLIFF ALONG THE ZUOERLA SEGMENT OF THE YUSHU FAULT, QINGHAI PROVINCE, CHINA

Our ongoing paleoseismic studies of segmentation along the Yushu fault during the M_W 6.9 14 April 2010 Yushu earthquake (Bartholomew et al., 2010, 2013; Li et al., 2010; Feng et al., 2012) suggested that the adjacent Guoqiong and Zuoerla segments experienced very different rupture histories prior to the 2010 Yushu earthquake. As we reported previously (Armstrong et al., 2012, 2013), our results using OSL and ¹⁴C ages from trench CUG2011-1 demonstrated that the 2010 event and the 3 Holocene events (~2.5, ~6.0, and ~8.0ka) could account for all the ~6m left-lateral offset of the nearby stream channel along the Guoqiong segment during the Holocene. Because the 2010 surface displacement near the trench progressively diminished to the SE toward the base of the red cliff, we suspected a segment break occurred there. In 2012, we new selected a trench-location (CUG2012-2) above the red cliff to ensure excellent preservation of the sedimentary record there. At this trench location, a linear, 4-5m deep trough is present along the fault and the 2010 surface rupture occurred along its NE-flank, suggesting a significant vertical component of movement. We then used both OSL and ¹⁴C dating techniques to determine the rupture history of the Zuoerla segment above the red cliff. In the trench, prior to the 2010 event, only one Holocene event occurred at ~5.5ka. By contrast, at least 5 Late Pleistocene events occurred ~10-18ka. A large crevasse (~1m wide) along the scarp extended to an excavated depth in excess of 3m. Multiple times of crevasse-filling occurred, with the deepest fill-material having an age of ~64ka. The significent differences in rupture histories, fault geometries, and displacement gradients indicate that a segment boundary occurs at the base of the red cliff between the Guoqiong and Zuoerla segments.