The Daliangshan fault zone(DF)constitutes an important part of the large-scale strike-slip Xianshuihe-Xiaojiang fault system(XXFS). Affected by the channel flow of the middle-lower crust in the western Sichuan region, the XXFS is strongly active, and large earthquakes occur frequently. On average, there is an earthquake of magnitude 7 or more every 34 years. However, the DF, as an important part of the middle segment of the XXFS, has only recorded several earthquakes with magnitude 5-6, and no earthquakes with magnitude over 6 have been recorded. The reason for the lack of strong earthquake records may be related to the lack of historical records in remote mountainous areas, but the main reason may be attributed to the active behavior of the faults. He et al.(2008)hold that the DF is a new fault, resulting from straightening of the middle section of the XXFS, and its activity gradually changes from weak to strong, and will probably replace the Anninghe-Zemuhe Fault. However, this view lacks evidence of strong earthquakes. In recent years, some scholars have studied the paleoearthquakes on the DF, and found the signs of strong earthquake activity, and considered that the fault has the seismogenic capacity of earthquakes with magnitude more than 7. These studies are mainly concentrated in the middle and southern segments of the DF. Although there are scattered activity data and individual trench profiles, direct evidence of Holocene activity and paleoearthquake data are very scarce in the northern part of DF. On the basis of the previous studies, combined with our detailed field geomorphological surveys, we excavated a set of two trenches at Lianhe village in Shimian Fault to reveal the direct evidence of fault activity in Holocene. From paleoseismic analysis and radiocarbon samples accelerated mass spectrometry(AMS)dating, four paleoseismic events are identified, which are E1 between 20925—16850BC, E2 between 15265—1785BC, E3 between 360—1475AD, and E4 between 1655—1815AD. The results of the latest two events should be relatively reliable, and the latest event may be related to the Moxi earthquake of magnitude 7^3/4 on June 1, 1786 or the Dalu earthquake of magnitude ≥7 on June 10, 1786. Among the four events revealed, three are since the Holocene, and the recurrence interval of the latest two events is about 800 years. Compared with other active faults at the triple junction, the recurrence interval is slightly longer than that at the northern segment of the Anninghe fault zone, but close to that at the Moxi segment of the Xianshuihe fault zone. Compared with the western segment of Xianshuihe Fault and the northern segment of Anninghe Fault, the Shimian Fault also has a higher seismic risk, which needs further attention.

Complex geometrical structures on strike-slip faults would likely affect fault behavior such as strain accumulation and distribution, seismic rupture process, etc. The Xianshuihe Fault has been considered to be a Holocene active strike-slip fault with a high horizontal slip rate along the eastern margin of the Tibetan plateau. During the past 300 years, the Xianshuihe Fault produced 8 earthquakes with magnitude≥7 along the whole fault and showed strong activities of large earthquakes. Taking the Huiyuansi Basin as a structure boundary, the northwestern and southeastern segments of the Xianshuihe Fault show different characteristics. The northwestern segment, consisting of the Luhuo, Daofu and Qianning sections, shows a left-stepping en echelon pattern by simple fault strands. However, the southeastern segment(Huiyuansi-Kangding segment)has a complex structure and is divided into three sub-faults: the Yalahe, Selaha and Zheduotang Faults. To the south of Kangding County, the Moxi segment of the Xianshuihe Fault shows a simple structure. The previous studies suggest that the three sub-faults(the Yalahe, Selaha and Zheduotang Faults of the Huiyuansi-Kangding segment)unevenly distribute the strain of the northwestern segment of the Xianshuihe Fault. However, the disagreement of the new activity of the Yalahe Fault limits the understanding of the strain distribution model of the Huiyuansi-Kangding segment. Most scholars believed that the Yalahe Fault is a Holocene active fault. However, Zhang et al.(2017)used low-temperature thermochronology to study the cooling history of the Gongga rock mass, and suggested that the Yalahe Fault is now inactive and the latest activity of the Xianshuihe Fault has moved westward over the Selaha Fault. The Yalahe Fault is the only segment of the Xianshuihe Fault that lacks records of the strong historical earthquakes. Moreover, the Yalahe Fault is located in the alpine valley area, and the previous traffic conditions were very bad. Thus, the previous research on fault activity of the fault relied mainly on the interpretation of remote sensing, and the uncertainty was relatively large. Through remote sensing and field investigation, we found the geological and geomorphological evidence for Holocene activity of the Yalahe Fault. Moreover, we found a well-preserved seismic surface rupture zone with a length of about 10km near the Yariacuo and the co-seismic offsets of the earthquake are about 2.5~3.5m. In addition, we also advance the new active fault track of the Yalahe Fault to Yala Town near Kangding County. In Wangmu and Yala Town, we found the geological evidence for the latest fault activity that the Holocene alluvial fans were dislocated by the fault. These evidences suggest that the Yalahe Fault is a Holocene active fault, and has the seismogenic tectonic condition to produce a large earthquake, just like the Selaha and Zheduotang Faults. These also provide seismic geological evidence for the strain distribution model of the Kangding-Huiyuansi segment of the Xianshuihe Fault.