Abstract:

It is difficult to use traditional trenching and field geological investigation to yield the age of paleoseismic events along active fault in western mountainous areas of China where the geomorphic trace mark and sediments are often eroded or altered by human activities. The recurrence interval of paleoearthquake possesses greater uncertainty. It is necessary to yield ages of paleoearthquake event from different ways and examine the reliability of paleoearthquake results. In these regions, an earthquake with magnitude greater than 7 can produce rock avalanches around 200~400km away from the epicenter, such as the Wenchuan earthquake in 2008, due to their structure setting of strong neotectonic activity and the higher topographic relief. Therefore, the seismic bedrock landslide and rock avalanche can record the occurrence time, intensity and damage of strong earthquake in the mountainous area. This provides a new way to assess the frequency and intensity of paleoearthquake occurring in the intraplate continental areas(such as the north-south seismic zone)where strong earthquakes recurred for hundreds to thousands of years based on the seismic landslide records. Identifying ancient earthquake bedrock collapse relics in Quaternary deposits and accurately determining their ages will not only help broaden the study on the recurrence history of active fault, but also assess the earthquake risk in mountainous area.
As shown by previous studies, the Schmidt-hammer exposure-age dating(SHD)method is a relatively simple, rapid, cheap and non-destructive in-situ exposure age dating method. In this study, ancient earthquake bedrock landslides and rock avalanches with known historical records distributed on the Qinling northern piedmont fault and the Huashan piedmont fault are used to preliminarily establish the rock weathering factor with age calibration curve. The rebound values of rock surface at dozens of sampling sites of each rock avalanche and landslide are measured by Schmidt hammer and analyzed statistically. The weathering factor of the exposed rock of each rock avalanche and landslide is calculated and the solution of SHD method is discussed. The reliability of SHD is evaluated according to the measured data and the records of historical age. The main conclusions are as follows:
(1)The Schmidt hammer rebound value of rock surface at three ancient earthquake bedrock landslides and rock avalanches is negatively correlated with their historical ages. The older the historical record age, the lower the average rebound value of the rock, and vice versa. Based on the statistical analysis of weathering factors of rocks of bedrock landslides and rock avalanches, a preliminary age calibration curve is obtained as T=(19 723±888)×f_w-(2 145±166). This curve can be used to infer the bedrock landslides and rock avalanches of more than 5×10² a BP, and it provides a new relative dating method for the ancient bedrock landslide and rock avalanches within the age of 3 000a BP in the northern margin of the Qinling Mountains.
(2)Under the climatic and lithological conditions of the northern margin of the Qinling Mountains, the relative ages of bedrock landslide and rock avalanches can discriminate the interval of millennium scale according to the rock rebound value measured by Schmidt hammer. However, it cannot distinguish the difference in weathering degree of the bedrock landslide and rock avalanches with the interval of less than 500 years.
(3)The Schmidt hammer rebound value measured repeatedly on fresh rocks shows that the fluctuation range of the rebound values is small, within the value of 0-3, which is helpful to rapidly select qualified sampling sites for terrestrial in-situ cosmogenic nuclide dating(TCND). Thus, the Schmidt hammer value can be used to evaluate whether the rock samples have the problem of nuclide inheritance induced by complex exposure history such as post-exposure and secondary transportation. This would introduce greater objectivity to the sample selection and possibly require less samples, thus reducing the costs; meanwhile, it will improve the dating efficiency and ensure the reliability of TCND. Therefore, SHD method is a valuable complementary method to TCND.
(4)Under the climatic and lithological conditions in the northern margin of the Qinling Mountains, the rebound value decreases by (25%±1%) for rocks after weathering for 2ka, by (16%±1%) for 1ka, and by (15%±1%) for 0.5ka.

Key words: paleoearthquake, rock avalanche and landslide, Schmidt hammer, northern margin of Qinling Mountains, relative dating

摘要：

地震崩塌、 滑坡是较为严重的地震次生灾害, 研究地震崩塌、 滑坡发生的时代及其活动规律, 从而恢复断裂带古地震活动历史、 地震复发周期, 有助于地震危险性评价及地震灾害防御工作。文中选择秦岭北缘断裂带和华山山前断裂中段被历史文献记录的地震基岩崩塌、 滑坡为研究对象, 使用施密特锤测量416个岩块的反弹值, 并对其进行统计分析, 计算了崩塌、 滑坡体暴露岩石的风化因子, 讨论了施密特锤暴露测年法的分辨率, 评估了施密特锤测年的可靠性, 同时建立了古地震基岩崩塌、 滑坡历史记录年代与反弹值参数之间的关系曲线。结果表明: 施密特锤暴露测年方法提供了一种简单、 快速、 廉价、 无破坏的原位相对测年方法, 初步建立的岩石风化因子-年龄校正曲线为T=(19 723±888)×f_w-(2 145±166), 基于该校正曲线, 可以对形成年龄>500a的基岩崩塌和滑坡进行分期, 为确定秦岭北缘古地震基岩崩塌、 滑坡形成时代提供一种新的相对测年手段。施密特锤岩石反弹值还可用来评估宇宙成因核素暴露测年样品是否存在先期暴露、 翻滚、 二次埋藏等复杂暴露历史带来的核素继承浓度问题, 帮助选择合格的宇宙成因核素测年样品的采集地点, 保证测年数据的可靠性, 提高测年效率。在秦岭北缘的气候和岩性条件下, 岩石经历2ka风化后, 表面强度减小约(25%±1%), 约1ka后减小(16%±1%), 约0.5ka后减小(15%±1%)。

关键词: 古地震, 基岩崩塌和滑坡, 施密特锤, 秦岭北缘, 相对测年