基于高分辨率DEM的裂点序列提取和古地震序列的识别--以霍山山前断裂为实验区

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

以霍山山前断裂为实验区,基于高分辨率IRS-P5DEM数据提取了横穿断裂的冲沟,在23条冲沟纵剖面上识别出断裂活动诱发裂点。在其中5条冲沟上识别出1级裂点,裂点几乎位于断裂上;在2条冲沟上识别出2级裂点,最新裂点也位于断裂上。这些位于断裂上的裂点,高度达4～9m,推测它们形成之后并未向上游明显迁移,并可能接受了多次事件的叠加。在其他16条冲沟上识别出2～3级裂点,最新一级裂点自断裂向上游迁移了40～70m,次新一级裂点主要分布在距断裂150～250m的河段内,第3级裂点距断裂300～500m。设定最新裂点形成于1303年洪洞8级地震(事件Ⅲ)发生时,再假定全新世中晚期以来研究区内河流溯源侵蚀速率基本不变,则推算出次新裂点对应的古地震事件(事件Ⅱ)发生在距今3336～2269a之间,最早一次事件(事件I)发生在距今5618～4504a之间,霍山山前断裂强震重复周期为1500～2600a。这与前人通过探槽揭露的古地震事件序列和强震重复周期基本一致。

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.