PALEOSEISMOLOGY ALONG THE RANGE-FRONT FAULT OF HELAN-MOUNTAINS, NORTHCENTRAL CHINA

The NNE trending range-front fault of the Helan Mountains along the we stern margin of Yinchuan graben, northcentral China, is a 13-km-long h igh-angle normal and right-lateral strike-slip fault. It cuts late Ple istocene to Holocene alluvial fans, forming a SE facing scarp, and off sets the Great Wall of the Ming Dynasty (about 400 years B.P.) right l aterally for 1.45 m and vertically for 0.95 m. The offset of the Great Wall is probably a result of the M = 8 Yinchuan-Pingluo earthquake of 1739. The 1739 earthquake formed an 88-km-long discontinuous surface rupture zone along the fault scarps. Both the length and height of the fault scarps are largest at Hongguozi in the north and Suyukou in the south. We have prepared large-scale topographic and geologic maps alo ng the Hongguozi and Suyukou scarps, surveyed 88 profiles across the s carps, and excavated 14 trenches across the fault, in order to study H olocene paleoseismicity and recurrence intervals of large earthquakes along the fault. Four levels of terraces were developed on the upthrow n block of the fault along some gullies that dissected the alluvial fa ns. The older terraces, such as T-4 and T-3, are tilted to the west, w hereas the younger terraces are less tilted. This indicates that the t erraces may have been tilted and rotated by multiple faulting events. The height difference between the two adjacent terraces is about 1-2.7 m. The elevation differences between the lower original surface and t he T-3, T-2, and T-1 terraces were measured in 64 topographic profiles across the scarps. The results show that the elevation of T-3 is high est, averaging 8.4+/-1.9 m and 5.5+/-1.1 m along the Suyukou and Hongg uozi fault scarps, respectively. The elevations of T-3 at the two site s are 5.8+/-1.4 m and 3.4+/-0.4 m, respectively, whereas elevations of T-1 are 3.1+/-1.2 m and 1.2+/-0.3 m, respectively. Ages of terrace ri sers between T-3 and T-2, as well as between T-2 and T-1, are 3600-460 0 years and 2000-2800 years, respectively, as calculated using a diffu sion model of scarp development. The morphology of the fault scarps is complicated. Two or three bevels were developed on some fault scarps that were active in an earlier period, and the upper and the lower slo pe sections correspond to T-3 and T-2 alluvial terraces, respectively. A free face exists only on T-1 terrace. The slope angles of the upper and lower slope sections of the scarp at Suyukou are 6-22 degrees and 17-38 degrees, respectively. The difference between the two in the sa me profiles is consistently in the range of 9-21 degrees. In the place where the fault scarp is high, the free face, with a slope angle up t o 60 degrees, may be a result of the M = 8 earthquake of 1739. We infe r that the other two bevels represent two paleoearthquake events. Eigh t and six trenches have been excavated across the fault scarps at Suyu kou and Hongguozi, respectively. All the trenches reveal normal faults , multiple offsets, and colluvial and filled wedges along the scarps. In these trenches, we found colluvial wedges formed in four different periods indicating multiple faulting events. On the basis of this evid ence, four Holocene seismic events have been identified. The timing of the events was determined through C-14 dating samples collected from the trenches. On the basis of terrace offset, scarp morphology, and pa leoseismic trenching, we conclude that the four large earthquakes alon g the fault occurred 8400, 4600-6300 (or 5700), 2600, and 256 years B. P. The latest event was the M = 8 Yinchuan-Pingluo earthquake of 1739. The recurrence interval of these earthquakes is 2300-3000 years. We i nfer that the magnitudes of these events were probably similar to 8, b ecause the thickness of each colluvial wedge and the height of terrace risers produced by these events are similar to those produced by the 1739 earthquake.