Microtremor Measurements at Mogao Grottoes, Dunhuang, China for Preliminary Dynamic Characterization for Performance During Earthquake

Abstract

Mogao Grottoes, Dunhuang, China, is a world heritage of UNESCO created in steep cliff with 50–60 m in height. Micro-tremor measurements and seismic refraction surveys have been carried out as a preliminary study to obtain dynamic characteristics. The site surface geology consists of young sediment of conglomerate in Pleistocene period above granite rock. A velocity sensor with a natural period of 0.5 s was used to monitor the micro-tremors at several points on the top terrace of the cliff and the lower plain in front of the cliff. Two survey lines of some 40 m for refraction survey were performed along which P and S waves were monitored. The velocities obtained from the both refraction lines are in the range of 500–700 m/s. and 1.6–4.0 km/s. for S and P waves respectively. The monitored micro-tremors were in less than a few m kines and the intensities of the motions were changed with time. Particle motions at the most observed sites shows the perpendicular direction to the cliff of EW is the predominant direction irrespective the intensity of the motions. Predominant period of spectral ratio of SW to UD direction shows about 2 s at the top terrace of the cliff and 1 s at the lower plain in front of the cliff. The thickness of the top surface layer of conglomerates is estimated about 125–180 m below the lower plain and 250 m or more. Another characteristic period was found at a period of 0.3–0.4 s. that might be caused by cliff-edge vibration.