海原断裂干盐池探槽揭示非特征性古地震序列
Research on various magnitudes of paleoearthquakes: A case study of non-characteristic earthquakes from the Salt Lake site of Haiyuan fault
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摘要: 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(1/2)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14C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(1/2)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。Abstract: The active left-lateral Haiyuan fault is one of the major continental strike-slip faults in the Tibetan Plateau. The last large earthquake that occurred on the fault was the great 1920 Ms 8(1/2) Haiyuan earthquake with a 230-km-long surface rupture and maximum left-lateral slip of 10.2m. Much less known is its earthquake recurrence behavior in spite of much focused studies. The current understanding is still preliminary and requires validation of paleoseismic data based on fine stratigraphy in trenching. In this study, the authors present results of a paleoseismic study at the Salt Lake site in a shortcut pull-apart basin within the section that broke in 1920. 3D excavation at the site exposed fine-grained and layered stratigraphy and ample evidence of multiple paleoseismic events. Charcoal fragments are abundant in the trenches. AMS dating of charcoal fragments shows that multiple events occurred during the past 3600 years. Of these, the youngest three and possibly four events were recorded in the top 2.5m section of distinctive thinly-layered stratigraphy. A comparison of paleoseismic with historical earthquake records suggests that these three events are correlative to the AD 1920, 1760 (or 1709) and 1638 earthquakes. Historical accounts of earthquake damage suggest that earthquakes exposed in the trenches are markedly different in magnitude. With the exception of the most recent M8 earthquake in AD 1920, two earlier events were considerably smaller, with magnitude M<7, and more likely M6 or less. Thus, the Haiyuan fault could produce surface-rupturing earthquakes with a variety of magnitudes, not just characteristic earthquakes. This study indicates that paleo-earthquakes exposed in trenches are not necessarily similar in size, and moderate magnitude events might produce surface ruptures, which can be preserved in stratigraphy and exposed in a paleoseismic trenching under some conditions, for instance, the sedimentation is fast enough and there exists no hiatus in deposition.
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Key words:
- Haiyuan fault /
- Salt Lake /
- paleoearthquake /
- historical earthquake /
- non-characteristic earthquake
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