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海原断裂是青藏高原东北缘一条重要的陆内活动左旋走滑断裂,于1920年发生过里氏8?级特大地震,形成约230km的地表破裂带和高达10.2m的同震左旋位移。该断裂的大地震复发行为特征一直是地震地质学家关注的重点,然而现有的认识需要更多以精细沉积地层约束的古地震数据的验证。基于此,在海原断裂中段干盐池盆地成功开挖了数个大型三维探槽,揭露了清晰的韵律性、面状展布地层和丰富的古地震事件证据。在探槽上部2.5m厚的最新细粒沉积层序记录了AD 1500以来的3次地震事件。基于地层中~(14)C样品的结果和历史地震史料的考证,限定这3次地震事件分别对应于AD 1920年、AD 1760年(或1709年)和AD 1638年的地震,但其震级差别很大。除了最新一次地震,即1920年海原大地震的震级为8?级,其他2次地震事件的震级较小,均小于7级,说明海原断裂上伴生有地表破裂的地震不全是特征型地震事件。结果表明,古地震探槽中揭示的地震强度不一定相同,而且中等震级地震也可以产生地表破裂,其地层证据在合适的条件下,如无沉积间断、沉积速率大等环境能在地层中得到保存。
Haiyuan fault is an important intracontinental left-lateral strike-slip fault on the northeastern margin of the Qinghai-Tibet Plateau. In 1920, a magnitude 8 magnitude earthquake occurred, forming about 230 km of surface rupture zone and up to 10.2 m of coseismic left-lateral strike. The characteristics of the recurrence of the large earthquake fault have always been the focus of the seismologists. However, the current understanding needs more verification of paleoseismic data constrained by fine sedimentary strata. Based on this, several large three-dimensional trenches have been successfully excavated in the Qianyanchi basin in the middle of the Haiyuan fault, revealing clear rhythms, surface distribution of strata and abundant evidence of paleoearthquakes. The latest fine-grained sedimentary sequence 2.5 m thick in the upper part of the trench records 3 earthquakes since the AD 1500. Based on the results of ~ (14) C samples in the stratum and the historical records of historical seismology, we can define that these three earthquakes correspond to the earthquakes of AD 1920, AD 1760 (or 1709) and AD 1638, respectively, but their magnitude differences Great. Except for the latest earthquakes, that is, the magnitude of the 1920 Haiyuan earthquake was 8 ?, the magnitude of the other 2 earthquakes was smaller than 7, indicating that the earthquakes on the Haiyuan fault associated with surface rupture are characteristic earthquake events. The results show that the seismic intensity revealed in paleoseismic trenches is not necessarily the same, and the earthquakes may also produce surface rupture in moderate magnitude earthquakes. The evidence of its formation can be obtained in the strata under suitable conditions, such as no sedimentary interval and large deposition rate save.