THE EXTRACTION OF KNICKPOINT SERIES BASED ON THE HIGH RESOLUTION DEM DATA AND THE IDENTIFICATION OF PALEO-EARTHQUAKE SERIES —A CASE STUDY OF THE HUOSHAN MTS. PIEDMONT FAULT

When a reach of a stream is steepened with respect to the adjoining reach, it defines a topographic knickpoint. A knickpoint is supposed to be a response to the base-level changes,and the base-level of a drainage basin is influenced by the fault movement. The formation of a knickpoint on a gully long-profile,whose base-level is the footslope of the fault scarp, is associated very closely with thevertical movement of a fault,therefore,the ages of paleo-earthquake events can be estimated by the knickpoint series along the longitudinal profile of a gully. We have made a case study of the Huoshan Mts. Piedmont Fault,and extracted tens of gullies across the fault based on the high-resolution DEM data and identified out knickpoints in 23 gullies. There are 5 gullies with only one knickpoint which are laid on the fault. And there are two gullies having two knickpoints with the latest one laid on the fault. The positions of these knickpoints and their higher height ranging from 4～9m imply that there are several knickpoints superposed together and the knickpoints have not migrated upstream. The other 16 gullies respectively have 2 -3 knickpoints. The latest knickpoints have been migrated upstream to a distance of 40 - 70m from the fault. The knickpoints of intermediate ages are at a distance of 150 - 150m upstream from the fault and the oldest ones at a distance of 300～500m. Under the conditions that the latest knickpoints are associated with the 1303 Mw8.0 Hongdong earthquake(Event Ⅱ)and that the gullies keep the same rate of headward erosion during the Holocene,Event Ⅱ is estimated to take place during 3336 - 2269a B.P. and Event Ⅰ is estimated to take place during 3336 - 2269 a B. P., respectively. The recurrence of events is about 1500 -2600a. These results are consistent with those obtained through the trench investigations.