A Study On The Effect Of Thick Sediment On The Long-period Ground Motion

The921Chi-Chi, Taiwan, Earthquake and several large aftershocks produced strong surface waves on the western coastal plain(WCP) in Taiwan. The long-period surface waves with high amplitude and long duration caused severe damage to high-rise buildings on the WCP. The generation mechanism of the surface waves and the effect to constructions are still uncertain since seldom researches have been done on this phenomenon. We started from analyzing the phenomenon, and then fully studied the characters, effect coefficients, the cause of generation, effect to the construction seismic design code and varying pattern of response spectrum on the sediment layer.The quaternary sediments on the WCP are about1.5-2.Okm thick, and become thicker from west to east slowly, and they are nearly horizontal stratification. The values of both normalized amplitude spectrum and normalized response spectrum of long-periods (3-6s and1-7s respectively) from station inside the plain are much higher than that from station outside the plain, and the long-period waves are typical Rayleigh waves. There is a big difference between surface waves produced by different events or in different areas. Peak values of both displacement and long-period response spectrum located in the center of the plain, and the locations of peak response spectrum of different periods are different.By using1D layer structure ground motion numerical simulation program, we found that: duration of the surface waves are longer and the phenomenon appears to be more obviously when the thickness is bigger or the velocity is lower of the sediment layer; the surface waves became more intensity when the ratio of the fault plane distributed on the shallow part is higher.1D sediment layer which is horizontal stratified can produce strong surface waves, and the comparison results between the simulated and real records are well after the numerical simulation of the real condition.Severe damage has been caused to the long-period construction on the WCP and Taipei Basin. The thick sediment on the WCP can obviously amplify the long-period ground motion, and the displacement amplification coefficient is considerably larger than that from the seismic design code of mainland of our country.The variation of the amplification coefficient with thickness of sediment layer present to be oscillation rising. The peak value of the response spectrum locates in the center of the sediment plain, and the locations of different periods are different.The nearly horizontal stratified low velocity sediment on the WCP is the main factor which generated the strong surface waves during the Chi-Chi earthquake, and the boundary effect of the plain is little. Improvement should be done to the seismic design on some plains especially those with low velocity and large thickness of the sediments in both mainland of China and Taiwan.1D sediment layer can produce similar basin focusing effect. There are also many similar deep sedimentary plains like North China plain in our country, we should pay much more attention to the efficiency of the seismic design codes for high-rise buildings in the plains especially the middle area.