The Kunlunshan M_s 8.1 earthquake of 2001 is a large earthquake produced by a large-scale intraplate strike slip faulting. Field observation shows that the surface rupture of this earthquake is about 426km long,and the maximum sinistral displacement is about 6m. As compared with the other similar intraplate large earthquakes,its rupture length is extraordinarily longer but its horizontal displacement is relatively small. The distribution of horizontal displacement along the surface ruptures is markedly controlled by fault structure. At the section where the horizontal displacement is small,the vertical displacement is also smaller,and no transformation of horizontal and vertical displacements is observed. This feature is also different from that of the other earthquakes. The relationship among rupture length,displacement and seismic magnitude follows a certain empirical equation,in which the ratio between the average displacement amount and rupture length is called "Ultimate linear strain",and is constantly within the range of 10^-5 for the rupture in the earth's crust. This feature can be used as criterion for testing the independency and integrality of the rupture segments. In this paper,we calculated the "ultimate linear strain" of the entire rupture zones and sub-segments of several large earthquakes,including the 1920 Haiyuan earthquake,1951 Bong Co earthquake,1932 Changma earthquake and 2001 Kunlunshan earthquake. The results show that average values of ultimate linear strain of the Haiyuan,Bong Co and Changma earthquakes are approximately close to the statistic value,but the values of ultimate linear strain of their sub-segments are significantly higher. In contrast,the value of ultimate linear strain for the entire surface rupture of the Kunlunshan earthquake is much lower than the statistic value,but the values of ultimate linear strain of its four sub-segments,except the west Taiyanghu segments,are close to the statistic value. Therefore,the Kunlunshan earthquake (M_s 8.1) should be produced by four relatively independent faulting events,instead of a uniform faulting. These characters provide geological evidence supporting the deduction that the Kunlunshan earthquake is successively triggered multiple earthquake events,rather than a single earthquake.

The 14 November,2001 M_S8.1 West Kunlun Pass Earthquake is the largest event associated with the longest surface rupture that has occurred in the Tibetan Plateau since 1951. We made 291 surficial left lateral slip measurements and 111 net vertical slip measurements along the main fault zone. The displacement on the main fault strand is dominated by left lateral strike slip of 2.7m in average,with vertical slip component of mostly less than 1m. The maximum left lateral slip is 6.4m,with as much as 5.1m of vertical slip component. Sinistral surficial slip is quite variable along the main strand of the rupture at distance scales ranging from a few tens of meters to a few hundreds of kilometers,with slip gradient ranging between 10^-1～10^-4. The slip variations over short length scales (tens of meters to a few kilometers) might be caused by variations in thickness of unconsolidated sediments,fault strike and slip of the previous earthquake,distributed non brittle deformation and secondary fractures,complexities in fault geometry,and perhaps by measurement error. Despite this short wavelength variability,there is fairly regular long wavelength (tens to hundreds of kilometers) behavior to the east of the Buka Daban Peak. One notable characteristic of slip distribution along the faults is that very large surficial slips (as large as 5～6 meters) were observed at 5～6 sites located at different surface rupture segments in asymmetry to their left lateral slip functions. Slip on each of these rupture segments diminishes away from the highest slip site to its terminations with different slip gradients. This asymmetric distribution of slips may indicate the propagation direction of the rupture along the faults. This long wavelength variation in slip might be influenced by fault geometry,while the segmentation of the surface rupture zone might play a key role. It should be pointed out that the surficial slip (at both short and long length scales) is only a near field slip measured in the field by using tape measure. Therefore,it should be considered as a minimum value,and may represent the real variations in the amount of brittle slip on visible fractures at the surface,but it potentially underestimates the actual slip produced by the earthquake and slip distribution over the whole surface rupture due to the difficulty in identifying distributed non brittle deformation. This calls for caution in discriminating between one or multiple discrete events and in estimating the size of past and future earthquakes by using displaced deposits in trenches or offset geomorphologic features along strike slip prehistoric fault ruptures.

On 14 November 2001, an extraordinarily large earthquake occurred on the Hoh Sai Hu segment of the Eastern Kunlun Fault, northern Tibetan Plateau. This event, named as Hoh Sai Hu (Kunlanshan) Earthquake, is the largest earthquake occurred in China continent for the past 50 years. The moment magnitude of this earthquake reaches 7.7 to 7.9 (USGS National Earthquake Information Center, 2001; Harvard CMT Catalog, 2001) and the surface wave magnitude reaches 8.1 (China Digital Seismic Network, 2001). Field investigation indicates that the surface rupture zone produced by this earthquake is striking N80°±10°W with a length of 350 km, which initiates from 91°E in the west nearby the east of Buka Daban Feng, a snow-capped summit with an altitude of 6 800m, extends eastwards along the fault traces of the Hoh Sai Hu segment, and terminates at the 94.8°E in the east. The surface ruptures of this earthquake consist of shear fractures, transtensional fractures, tension gashes and mole tracks arranged in en echelon. The shear fractures are N80°～90°W trending and dominated by left-lateral slip. Transtenssional fractures are several to tens meters long, the strike of which varies from N62°E to N65°E or from N70°E to N75°E, and are dominated by left-lateral slip with a component of tensile opening, the width of which decreases with depth. The shear and/or transtensional fractures are arranged in left-stepping or right-stepping to form releasing or restraining steps, on which tension gashes or mole tracks are developed. Tension gashes strike N45°～50°E and are developed at a releasing step to connect with the boundary shear or transtensional fractures which constrain the step in most cases. The tension gashes may also be arranged in en echelon pattern along the surface rupture zone, and especially at the termination of the surface ruptures. The mole tracks of 1.5 to 3 m height are trending 295°to 330°, which are well developed at the right-steps of the shear and/or transtensional fractures of different scales along the surface rupture zone. This surface rupture pattern appears to be purely strike-slip characterized by several meters of left-lateral offset. The maximum left-lateral offset we observed reaches 6 m at a site (93°05.384’E, 35°47.623’N), where a shallow channel bed was left-laterally offset by a single pure shear fracture. The macroscopic epicenter of the Hoh Sai Hu (Kunlunshan) earthquake is then inferred to be located at the piedmont area to the northeast of Hoh Sai Hu Lake, about 80 to 90km west of Kunlunshan Pass, in terms of the features of surface ruptures. It is postulated that this earthquake may trigger the occurrence of future large earthquake on the Dongdatan-Xidatan segment to the east of the Hoh Sai Hu segment of the Eastern Kunlunshan Fault,reflecting the eastward motion or flowing of the Tibetan Plateau along the fault.