Morphologic patterns of stream channels on the active Yishi Fault, southern Shandong Province, Eastern China: Implications for repeated great earthquakes in the Holocene

Abstract

Stream channels, totaling 427, on the active Yishu fault were systematically measured in the field. Three categories of deflected stream channels were identified. These were the R, L and U groups, which correspond to channels that are right-laterally deflected, left-laterally deflected, and those from which fault displacement cannot be determined, respectively. Of the measured channels, 48.7% are group U, 37.7% group R, and 13.6% group L. Each group has four different patterns that are strongly affected by local conditions such as channel density, width of crush zone, slope angle, and stream incision and sedimentation. By introducing a parameter, the deflection angle, which is defined as the angle between the fault slip direction (on the downstream block) and the downstream flow direction, at least three paleoslip events preceding the 1668 M = 8.5 earthquake at Zuoshan were identified and their occurrence times estimated. They were all within Holocene time, but the recurrence intervals seem to have become longer over the past 10,000 years. The recurrence intervals appeared to be doubled, from 2000 years to about 4000 years over the same period. Each faulting event was associated with 4–9 m right-lateral displacement on the fault. If the typical maximum displacement associated with each event is 8 m, then the slip rate has decreased from 4 mm/yr to about 2 mm/yr. However, considering the possibility that the maximum displacement during each event may not be located at the same place, then the recurrence interval on the whole rupture section could be underestimated because, in that case, the recurrence interval would not be a simple inverse relationship of the average slip rate.