Field study of a highly active fault zone: The Xianshuihe fault of southwestern China CLARENCE R. ALLEN Seismological Laboratory, California Institute of Technology, Pasadena, California 91125 LUO ZHUOLI \ QIAN HONG > Seismological Bureau of Sichuan Province, Chengdu, China WEN XUEZE ) ZHOU HUA WEI Department of Geosciences, University of Houston, Houston, Texas 77004 HUANG WEISHI Seismological Laboratory, California Institute of Technology, Pasadena, California 91125 ABSTRACT ord, repeat times estimated from slip rates, climatic environments? Added impetus was and current seismic gaps, two segments are given to the study because the Xianshuihe fault The Xianshuihe fault of western Sichuan particularly likely sites for M = 7+ earth- is the only major fault worldwide, other than the Province, China, is one of the world's most quakes in the near future: the 65-km-long San Andreas, where continuing fault creep has active faults, having produced 4 earthquakes segment between Daofu and Qianning, and been reported over significant lengths of the during this century of magnitude >1 along a the 135-km-long segment bracketing Kang- fault. 350-km length of the fault. At least 8 such ding. Continuing creep has been documented The Xianshuihe fault zone is part of a much events have occurred since 1725. In the more along some segments of the fault, and this, more extensive left-lateral fault system of at least limited 150-km-Dong segment including Lu- together with the high degree of activity and 1,400-km length, extending from southern Yun- huo and Daofu, major earthquakes in 1904, other unique attributes, makes the Xian- nan Province northwest through Sichuan into 1923, 1973, and 1981 (M = 7, IVi, 7.6, 6.9) shuihe fault one of the most promising sites in Qinghai (Fig. 1, insert) (Ding, 1984). Almost all were associated with overlapping surficial the world for earthquake prediction and segments of the system have been the locus of fault ruptures and with individual left-lateral hazard-evaluation studies. major earthquakes within the historic record displacements as large as 3.6 m. Field studies (Fig. 2), and it currently constitutes probably the indicate that this high degree of activity is INTRODUCTION most active fault system within China. It clearly typical of the fault's longer-term history. The deserves comparison with other highly active Holocene left-lateral slip rate on the north- The purpose of this study was to examine in strike-slip fault systems worldwide, such as the western segment of the fault has been 15 ± 5 the field a highly active fault zone—one of the San Andreas fault of California, the North mm/yr, decreasing to about 5 mm/yr on its most active in the world during this century. Anatolian fault of Turkey, and the Alpine fault southeastern segment, based on radiometri- During the present century, four earthquakes of of New Zealand. cally dated offset stream-channel and terrace magnitude 7 or greater have occurred along a Following recent Chinese practice, we differ- deposits and on offset glacial moraines. 350-km segment of the Xianshuihe fault, and at entiate herein between the Xianshuihe fault Physiographic features of active faulting least eight such events have occurred here since zone, extending some 350 km northwest from are fully as diagrammatic as those of Califor- 1725 (Figs. 1, 2; Table 1). Four events exceed- near Shimian to Kasu (Fig. 1), and the more nia's San Andreas fault, mainly because of ing magnitude 6.8 have occurred during this restricted Xianshuihe fault itself, which is only high-altitude preservation and the absence of century along a single 150-km segment of the one of five individual segments within the over- cultural modification on this eastern margin fault, with overlapping surface ruptures. At the all zone. These five segments (Fig. 3) are the of the Tibetan Plateau. Detailed en echelon commencement of the field work, we had sev- Moxi fault, Selaha (Kangding) fault, Zheduo- tensional and pushup features resulting from eral questions in mind. (1) Could the very high tang fault, Yalahe fault, and the restricted Xian- surface ruptures in 19*73, 1955, 1923, and degree of activity have been recognized by phys- shuihe fault, extending northwest from Laoqian- 1893 can still be recognized today, and new iographic features alone, even in the absence of ning. The nature and significance of the data have been collected bearing on the the historic record? (2) Is the high level of cur- segmentation between these different members offsets and fault-rupture lengths during these rent activity representative of the late Quater- are important questions that will be addressed in and other events. nary history, or is it merely a temporal burst of a section below. The type area of the restricted The locations and magnitudes of historic activity such as that along the North Anatolian Xianshuihe fault is the 150-km-long segment be- earthquakes suggest that the characteristic fault of Turkey since 1939? (3) Can the source tween Songlinkou Pass and Kasu, where its earthquake model may apply to the Xian- areas of recent earthquakes be related to seg- trace is relatively simple and continuous, and shuihe fault. Obvious geometric segmentation mentation of the fault? (4) In this high-altitude, where it controls the course of the Xianshui of the fault has controlled the initiation and alpine environment along the eastern margin of River and its tributaries. termination of ruptures in some events, the Tibetan Plateau, how are fault features pre- At its northwestern end, the Xianshuihe fault whereas segmentation control for others served as compared with those along other ac- overlaps the southeastern end of the Ganzi- remains obscure. Based on the historic rec- tive faults in China and elsewhere in more usual Yushu fault, with a left en echelon stepover of Geological Society of America Bulletin, v. 103, p. 1178-1199, 29 figs., 3 tables, September 1991. 1178 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/103/9/1178/3381303/i0016-7606-103-9-1178.pdf by guest on 01 October 2021 Figure 1. Map of Xianshuihe fault in area of this study. Boxes within map indicate areas of more detailed sketch maps herein. Inset at lower left shows location of study area and regional relationship between Xianshuihe fault and some other major active faults of China and adjacent areas. All elevations are in meters. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/103/9/1178/3381303/i0016-7606-103-9-1178.pdf by guest on 01 October 2021 1180 ALLEN AND OTHERS 1960 6| M^ol Figure 2. Map showing epicenters of earthquakes of magnitude (Mj) 6% and greater in the Chinese 7.2 1976 catalog (Luo, 1980), with relation to major faults in the Sichuan-Yunnan region. Box shows location of Figure 1, the area of this study. 1933 about 40 km. At its southeastern extremity, the Xianshuihe fault zone connects with the An- ninghe fault in a complex and little-studied area near Shimian. Still farther southeast, a signifi- cant en echelon offset occurs between the An- ninghe and Xiaojiang faults (Fig. 2), with the very active Zemuhe fault in the transition area. All of these individual fault zones, on the other hand, must be considered parts of the same overall left-lateral fault system. The entire sys- tem lies along the northeast boundary of a large rhomb-shaped block, or miniplate, that is bounded on the opposite southwest side by the right-lateral Red River fault system (Allen and others, 1984), and the apparent southeastward movement of this relatively stable block relative to its neighbors has been the subject of much tectonic discussion (for example, Li and Wang, 1975; Kan and others, 1977). It is admittedly somewhat surprising that two major fault sys- tems that are almost parallel and only 200 km apart should have opposing current senses of lateral slip, although this has been explained, at least in principle, by the mechanics of the im- pingement of India into Eurasia and the conse- quent block rotations (Molnar and Tapponnier, 1975; Tapponnier and Molnar, 1976; Tapponn- ier and others, 1986). HISTORIC EARTHQUAKES AND SEISMICITY Regional Activity. Figure 2 shows historic large earthquakes on the Xianshuihe and asso- ciated faults of southwestern China. It should be noted that the historical record in this part of China, which is very sparsely populated (primar- ily by Tibetan people), is very short as compared to those of the more heavily populated areas to TABLE 1. HISTORIC EARTHQUAKES OF M i 6.9 ON THE XIANSHUIHE FAULT ZONE the north, east, and south; the record of large events on the Ganzi-Yushu fault system to the Date Location Magnitude Fault Rupture Maximum west is even more limited (Wen and others, length displacement (km) (m) 1985). The earliest large earthquake of record on the Xianshuihe fault zone itself is that of 1725 N. Kangding >7 Selaba? 1 1 1725, although some large earthquakes that oc- 1786 S. Kangdiog 1* Moxi-Selaha >70 ? 1816 Luhuo >n Xianshuihe 1 7 curred as early as 1515 A.D. are documented 1893 Qiaoning >7 Xianshuihe >40 ? 1904 Daofu 7 Xianshuihe ? 7 along southern extensions of the fault system in 1923 Renda 714 Xianshuihe >60 3± Yunnan (Fig. 2). The overall pattern, nonethe- 1955 Kangding n Zheduotang 27 1- 1973 Luhuo 7.6 Xianshuihe 90 3.6 less, clearly indicates that major earthquakes 1981 Daofti 6.9 Xianshuihe 44 1- have tended to occur on the major faults of the Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/103/9/1178/3381303/i0016-7606-103-9-1178.pdf by guest on 01 October 2021 Figure 3. Sketch map of Xianshuihe fault zone in Laoqianning-Kangding area.