Field study of a highly active fault zone: The Xianshuihe fault of southwestern China

Abstract

The Xianshuihe fault of western Sichuan Province, China, is one of the world's most active faults, having produced 4 earthquakes during this century of magnitude ≥7 along a 350-km length of the fault. At least 8 such events have occurred since 1725. In the more limited 150-km-long segment including Luhuo and Daofu, major earthquakes in 1904, 1923, 1973, and 1981 (M = 7, 7½, 7.6, 6.9) were associated with overlapping surficial fault ruptures and with individual left-lateral displacements as large as 3.6 m. Field studies indicate that this high degree of activity is typical of the fault's longer-term history. The Holocene left-lateral slip rate on the north-western segment of the fault has been 15 ± 5 mm/yr, decreasing to about 5 mm/yr on its southeastern segment, based on radiometrically dated offset stream-channel and terrace deposits and on offset glacial moraines. Physiographic features of active faulting are fully as diagrammatic as those of California's San Andreas fault, mainly because of high-altitude preservation and the absence of cultural modification on this eastern margin of the Tibetan Plateau. Detailed en echelon tensional and pushup features resulting from surface ruptures in 1973, 1955, 1923, and 1893 can still be recognized today, and new data have been collected bearing on the offsets and fault-rupture lengths during these and other events. The locations and magnitudes of historic earthquakes suggest that the characteristic earthquake model may apply to the Xianshuihe fault. Obvious geometric segmentation of the fault has controlled the initiation and termination of ruptures in some events, whereas segmentation control for others remains obscure. Based on the historic record, repeat times estimated from slip rates, and current seismic gaps, two segments are particularly likely sites for M = 7+ earthquakes in the near future: the 65-km-long segment between Daofu and Qianning, and the 135-km-long segment bracketing Kangding. Continuing creep has been documented along some segments of the fault, and this, together with the high degree of activity and other unique attributes, makes the Xianshuihe fault one of the most promising sites in the world for earthquake prediction and hazard-evaluation studies.