活断层与工程应用Ⅰ:定义与分类

中国大陆在新构造时期发育了复杂的活断层体系,工程建设经常面临活断层灾害及其相关风险问题。采用合理的、有针对性的活断层定义和分类,有助于科学应对或防范工程建设中的活断层相关危害。通过系统梳理总结和分析相关领域的国内外研究现状与进展,以及工程领域的应用情况与存在问题等,并结合中国活动构造环境和活断层特点,针对工程应用的实际需求,形成了3点主要结论和建议。①活断层定义的本质是“在人类关心的未来断层发生活动的可能性”,建议工程领域采用“工程活断层”定义,即“在工程寿命期内(未来约100年内)可能出现地表活动或发生M≥5.0破坏性地震的断层,尤其是那些在距今约800 ka或约130 ka以来(即中更新世或晚更新世以来)多次活动的断层”,并注意区分“地表活断层”与“地震断层”和重点关注工程寿命期内相对危险的地表断层。②工程领域的活断层分类适合采用最新活动时代和活动强度方案。结合中国活动构造环境和活断层发育特点,建议在工程评价中可将场地区的断层按照活动时代区分为“历史的、晚第四纪(晚更新世和全新世两个亚类)、早—中更新世、新构造和不活动断层”5类,按照活动强度分为三大类5个级别:强、中等(包含3个亚类:中—强、中等和中—弱)和弱活动。③工程评价中需正确处理活断层的活动性与危险性关系。活动性强的断层或全新世断层并不意味着在工程寿命期内危险性高。因此,工程勘察中需对近场区及场址内的地表活断层进行危险度分级,其中的高、中危险度活断层才是工程绕避或防范的重点,而非所有活断层。

Characteristics of Late Quaternary Activity of the Gadê Segment in the Madoi-Gadê Fault Zone

Madoi-Gade fault is an active fault in the Bayan Har block. According to field investigation, there is an earthquake surface rupture fairly well preserved on the Gade segment of the Madoi-Gade fault zone. The length of the rupture is approximately 50km, with a general strike of NW. The maximum horizontal sinistral displacement is about 7.6m and the maximum vertical displacement is about 4m. A large number of earthquake traces are to be found along the rupture zone, and the phenomena on the surface rupture are also various. Field investigation and analysis on the geological and geomorphological phenomena show that the formation age of the surface rupture is relatively young. A series of linear arranged, triangular facets, fault scarps, fault springs, dislocated gullies, twisted mountain ridges, sag-ponds, dislocated ridges, etc. exist along the fault. Based on the analysis of field investigation and the data available, we believe that the surface rupture is due to a strong earthquake in the history of this area. And it is inferred that the Madoi- Gade fault within the Bayan Har block has been highly active since Late Quaternary and may still be active nowadays.