Palaeo-earthquake and palaeo-mudflow events at the Machangyuan Ruins in the Huangshui River valley, northeastern margin of the Tibetan Plateau

摘要

Palaeo-earthquake event recorded by loess rapture fissures (N30 degrees-40 degrees W and N40 degrees-50 degrees E trending) and palaeo-mudflow event recorded by red clay deposits were identified at the Machangyuan Ruins in the Huangshui River valley, at the foot of the northeastern Tibetan Plateau. Comparative analysis of grain-size distributions and geochemical elements of local outcrops indicates that the red mudflows were sourced from the gullies on the valley side where unconsolidated Neogene red clay formation was exposed The palaeo-earthquake was associated with regional tectonic structures including a NNW-trending left-lateral strike-slip fault (Lajishan fault) and a NE-trending local fault (Bazhougou fault). Analysis combining optically stimulated luminescence ages and radiocarbon ages of archaeological record dates the palaeo-earthquake and palaeo-mudflow events to ca. 4.80 ka BP. During that stage, many Neolithic settlements distributed around the Machangyuan Ruins. Enhanced human activities of the Majiayao Culture disturbed the landscape of the Minhe Basin causing widespread soil erosion. Contemporaneous storm rain and earthquake mobilized the loosened sediments in the upper stream gullies forming mudflows. The hollow ground around the Machangyuan Ruins was covered by mudflow and the earthquake fissures were filled in by the mudflow at the same time. This study provides important insights into early human impact during climatic and tectonic events in the environmentally vulnerable zones over the world.