Research on various magnitudes of paleoearthquakes: A case study of non-characteristic earthquakes from the Salt Lake site of Haiyuan fault

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.