【摘要】 在西藏安多地区进行活动断裂研究过程中,进行地表调查和探槽开挖,证实错那-安多地堑北缘主边界断裂上的最新地表断层陡坎实际是该区史前大地震的地表遗迹。根据地表观察和实地测量结果可知,该区最新古地震断坎的延伸范围可达26～36km,平均垂直位移量达1.0～1.4m。相关地层-地貌体的年代学测试结果和古地震破裂参数表明,最新的古地震发生在距今约10.0～8.1ka期间,估计当时的古地震震级介于6.9～7.3级之间,当时的极震区烈度可能≥Ⅸ度。此古地震破裂的发现表明,青藏高原中部正断层型大地震之后的沉寂时间可以长达近万年,明显长于藏南裂谷带上的正断层型古地震活动间隔。由于安多地区最新大地震之后的离逝时间已足以积累类似强度的大地震,因此,该区未来的大地震危险性较高。更多还原

【Abstract】 Based on surface surveying and excavated trench investigation, we discovered evidences for the most recent palaeo-earthquake event at Tsona - Amdo Basin. The NE normal fault scarps along the normal faults at the north margin of the NE Tsona - Amdo Graben was likely generated by a paleo-earthquake. The surface measuring results indicate that the maximum and minimum vertical displacements of the fault scarps are 1.4 ± 0.1 m and 0.6 ± 0.2m respectively. The small fault scarp intermittent extends southwestward from the north of Amdo Town to around the Zhasha Village along the north margin of the Tsona -Amdo Basin,and is about 26km or 36km long. The trenching profile crossing the surface rupture reveals that there is only a set of colluviums along the small fault scarp, and demonstrates the deduction that the surface normal faulting expressed as small fault scarps on surface was resulted from a severe palaeo-earthquake. The optically stimulated luminescence （OSL） dating results of the youngest terrace （ Tt ） deposits offset by the surface rupture indicate that the most recent palaeo-earthquake occurred after about 13.8 ～ 17.8ka before present,and the OSL age of the soil layer overlying the youngest terrace deposits infers that the most recent palaeo-earthquake occurred before about 6.2ka before present. In trench profile,three OSL ages（including one terrace deposits and two fault colluviums） suggest that the recent paleo-earthquake occurred during 10.0 ～ 8. lka before present. Based on empirical formula between earthquake magnitude and surface rupture length and displacementse, the estimated magnitude and maximum intensity of the most recent palaeo-earthquake occurred at Amdo is probable of Ms6.9 ～ 7.3 and ≥IX degree. Regional research shows that similar surface ruptures of palaeo-earthquake are also distributed along the normal faults at the eastern margin of the Peng Co-Dung Co Graben that constitutes the southern segment of the Peng Co -Amdo Graben system. The oldest surface rupture is located at the southeastern edge of the Dung Co Graben,and occurred probably at about 28.2ka before present, but the youngest surface normal faulting runs along the northeastern margin of the Dung Co Graben,occurring at about 1320± 60A. D. The new discovers show that the quiescence period between two severe earthquake events resulted from normal faulting may last for ten thousands years or even longer along the Peng Co -Amdo Graben system in Central Tibet. The quiescence period after severe earthquake is obviously longer than that along the SN trending rift in Southern Tibet. It implies that the magnitude of extension deformation in Southern Tibet should be greater than that in Central Tibet. The displacements of T₂ formed during 39.9 ～ 42. 3kaB. P. indicate an average vertical slip rate of 0. 14 ± 0.03mm/a for the normal fault at the northern margin of the Amdo Basin. Based on the slip rate,the quiescence period has been long enough to cumulate a similar magnitude severe earthquake along the northern margin fault of the Tsona-Amdo Basin.更多还原