Study On Earthquake-generated Regional Rockfall And U-series Dating For Its Calcareous Coats Along Yushu Fault Zone, In Eastern Qinghai-Xizang Plateau

In 2010,the Ms7.1 earthquake in Yushu, Qinghai Provence triggered a series of secondary geological disasters, in addition to the earthquake landslide, the phenomenon of earthquake-generated regional rockfall distributed along the fault zone and its adjacent side is extremely common. In the course of surface investigation after the earthquake, it is found that there is a number of typical earthquake-generated regional rockfall zones. In addition to the latest rockfalls formed in the 2010 earthquake, there are still a lot of early earthquake-generated regional rockfalls scattered, which indicates that the development of rockfall is closely related to the paleo-earthquake activity. Based on remote sensing interpretation, this article emphasizes on the research of typical earthquake-generated regional rockfall in Yushu through a regional surface investigation. The U-series dating method is used to the dating of calcareous coats on the surface of early rockfalls. From the comparison between U-series dating results of earthquake-generated regional rockfalls and the AMS 14 C dating results of seismic events reflected from paleo-earthquake trench and paleo-earthquake landslide it is preliminary confirmed that the multi-period rockfalls is in close genetic relationship with paleo-earthquake activities. Further discussion about the signification of earthquake-generated regional rockfall may show the preliminary conclusion, as follows:(1) Synthesizing the surface investigation results the development characteristic of earthquake-generated regional rockfall in Yushu is mainly: generally concentrated distribution below steep rock slopes near the active fault belt minority in intermountain river valley, scattered distribution, rolling far away, and often accompanied by paleo-earthquake landslide. The types of rockfalls mainly include landslide-generated rockfalls, collapse-generated rockfalls, slide-generated rockfalls and proluvial rockfalls distributed in mountain front. The development of rock fall is of multi-period and a certain number of in-situ recurrent.(2) After systematic sampling for the calcareous coats on the surface of earthquake-generated rock fall in several typical distribution areas near the Yushu active fault belt the author made an attempt of U-series dating for the calcareousii coats by using dilute acid leaching method. The U-series ages are mainly in the range of about 69980 a 36300 a 14900 12700 a, 6030 a 4720 a, 3560 3530 a 2010 1090 a about 760 a and 230 a before present, indicating that most of the rockfalls in Yushu area formed in the late of Holocene period. This results tally well with the seismic events reflected from paleo-earthquake trench and paleo-earthquake landslide. It preliminary confirmed that the multi-period rockfall is in close genetic relationship with paleo-earthquake activities.(3)Combining the surface investigation and dating results the seismic geological significance reflected from the earthquake-generated regional rock fall is mainly in the following aspects:①In the same area with the same terrain conditions The formation age and the period of earthquake-generated regional rock fall could reveal the age and the period of paleo-earthquake events. The development scale of earthquake-generated regional rock fall with different periods of time is likely to reflect the intensity of paleo-earthquake events.②The distribution range of massive earthquake-generated regional rockfalls with the same period can indicate the magistoseismic area of relevant paleo-earthquake events.③ The forming time interval of multiphase paleo-earthquake-generated regional rockfalls can reflect the fault activity to some extent.In conclusion, as a phenomenon of the secondary geological disaster of earthquake, the research of earthquake-generated regional rockfall has the vital significance. Choosing the earthquake-generated regional rockfall in Yushu as the carrier, this article make an attempt to reflect the age period and intensity of paleo-earthquake events from the distribution characteristic and U chronology analysis. It is possible to provide a new way for the future study of paleo-earthquake activity in active tectonic region.