断层面高精度形貌对比研究:基岩区古地震研究的选址

Comparative study of high-precision morphology of fault surfaces:A discussion of site selection for paleo-seismological study in bedrock area

随着断层面形貌测量、宇宙成因核素测年、物理和化学指标测定等一系列先进技术的发展,已有大量的研究案例成功地从基岩断层面上提取了古地震信息,基岩断层面在古地震研究方面的优势被不断发掘。然而,沟谷等地表过程的侵蚀作用,也会使得基岩断层面出露。这种非构造成因的断层面,很容易与构造成因的断层面混淆,却不记录古地震信息。已有的研究工作,缺乏对这两种断层面出露方式的系统研究,更缺少对适合于古地震研究的基岩断层面特征的归纳总结,这将不利于在基岩区开展古地震研究工作。为了解决这一问题,选取了山西地堑系的马家窑和南马庄两处基岩断层面,采用小型无人机航测(s-UAV)结合陆基三维激光扫描(t-lidar)技术,获取了基岩断层面高精度形貌及周围的三维地形数据,将断层面形貌特征的定量分析结果与断层坎精细的地貌解译结合,研究基岩断层面的不同暴露方式下的形貌特征。结果显示,马家窑基岩断层面受沟谷侵蚀而逐渐暴露,表面形貌呈现出连续平滑的特征,不具有地震活动信息。南马庄基岩断层面的形貌显示出明显的阶跃式分带特征,具有分段出露的特点。结合精细地貌解译的结果,这种出露特征可能与周期性的地震活动有关,并识别了两次同震倾滑位移量为3.1 m和3.3 m的古地震事件。最后,本研究强调了在开展基岩断层面古地震研究时,选择合适的研究地点的重要性,对比了构造活动和地表侵蚀两种成因断层坎的特点,提出了适合提取古地震信息的基岩断层面的特征和识别原则。

The reconstruction of paleo-earthquake history from bedrock fault scarps by a combination of quantitative morphological analysis,terrestrial cosmogenic nuclides(TCNs)dating and other physical/chemical index has been proven feasible by a number of previous studies.However,this success heavily depends on a suitable site selection along the bedrock fault scarp since the geomorphic processes are also capable to lead to the exhumation of the fault surface,i.e.,the erosion-origin fault surface.To distinguish the tectonic-origin fault surface from the erosion-origin one,two bedrock fault surfaces called Majiayao(MJY)and Nanmazhuang(NMZ)in the northern Shanxi Rift are selected as research objects to carry out a comparative study using quantitative morphology analysis derived from t-LiDAR scanning combined with s-UAV survey.For the MJY fault surface,the quantitative morphology shows a gradually-change characteristic without segmentation.This gradually-change characteristic indicates an erosional exposure mode from geomorphic processes,and therefore the paleo-seismic information cannot be extracted,which matches study locations(in the gullies)interpreted from s-UAV aerial survey.For the NMZ fault surface,which are at a highland away from gullies in the s-UAV aerial interpretation,the quantitative morphology analysis shows that both bedrock fault surfaces have the characteristics of vertical segmentation.This kind of segmentation may indicate that the fault surfaces are exhumed by repeated seismic events,and the corresponding co-seismic slips can be determined by the height of these segments.Accordingly,two seismic events at NMZ on the Yuguang Basin southern marginal fault,with a co-seismic dip slip of 3.1 m and 3.3 m are identified.Thus,this study highlights the importance of a suitable study site selection and the necessity of a reasonable planning when conducting paleo-earthquake research on bedrock faults.Based on experience obtained in this study combined with previous cases,we propose the characteristics of bedrock fault surfaces suitable for extracting paleo-seismic information and corresponding identification principles.