Loess is widely distributed in China and the Loess Plateau is one of the major areas where strong earthquakes often take place. The seismic amplification effects were discovered in the Plateau during the Wenchuan Ms8.0 earthquake and some other strong events. Based on earth tremor observation, borehole exploration and site seismic response analysis, the site effects of topography of Loess Yuan on ground motion were investigated in details. The earth tremor investigation shows that predominant frequencies at the bottom sites of Loess Yuan are greater than those at the top obviously. The sites seismic response analysis shows that the Loess Yuan may amplify peak ground acceleration (PGA) by 1.44 2.0 times. Therefore, site effects of mountains and loess topography on ground motion should be taken account into in seismic design in loess regions.
In early morning of Aug 8th, 2010, the rain-triggered tremendous debris flows broke out simultaneously at the Sanyanyu ravine and Luojiayu ravine, which locate in the north part of Zhouqu County town. The debris flow is the most severe event of the same kind of disasters in the past sixty years in China, which caused great losses of people's lives and properties. Based on field investigation, remote sensing image interpretation and analysis of local climatological data, the local topographical conditions, active tectonic movement, massive debris source and torrential rains were the main formation causes which induced the catastrophic debris flows. Moreover, detailed geological surveys were carried out following the disaster, the other geological potential hazard sites were found out, and the geological and seismic hazard assessment has been put into practice. At last, scientific and appropriate countermeasures have been suggested to prevent and mitigate the extraordinarily serious debris flow.
