Preliminary Study On Rock Attenuation Relationship And Site Factor For Western China

The attenuation relationship plays a key role in the seismic safetyevaluation and risk analysis for earthquake.Currently, the evaluation of groundmotion is accomplished through two steps, in the first place, the value isobtained by the rock attenuation relationship, and then estimates the effect ofsite condition on the predicted value. However, the present rock attenuation wasbased on the seismic intensity, but the theoretical basis for the result is notconvincing, besides, the course of the work is complex. To improve the currentsituation, this paper aimed to establish a new rock attenuation relationship on thebasic of domestic record and NGA data. The model in this paper can reflectmagnitude saturation and the coefficients for the model was obtained throughrandom effect method. On the previous work this paper studied the effect of soillayer on the ground motion. Finally, this paper presented a model of siteamplification factor. The main work and conclusions from this paper are asfollow.1. Collecting the information of Wenchuan after-shocks and site conditionfor the stations in Wenchuan earthquake. The information on Wenchuanearthquake and NGA data comprise the data sets in this paper.2. This paper founded a model for the rock attenuation on the basic ofrecords of Wenchuan earthquake and foreign records. Performing the analysis onthe magnitude saturation for the short period motion, and the outcomedemonstrated the distance within which the saturation occurs is different forindividual period, meanwhile, the value of distance reduced with the increasingperiod.3. On the basic of the avaible rock attenuation relationship, this paperestimated the soil amplification factor, and got insight into the affecting factorson the value. Finally a model for the amplification factor was presented, theunknown factors for which was established through regession. Through theanalysis on the model, three conclusions are as follow.(1) For PGA and acceleration spectrum, if the motion input was less than30gal, the amplification factor has no relationship with the motion on thereference condition, merely depends on the Vs30. If the motion input exceeds20-50gal, the soil sites made an nonlinearity amplification for the motion on thereference motion-----amplification factor reduced with the increasing referencemotion.(2) For the short period motion, the soil site had a remarkable effect on the amplification. For the long period motion and the rock sites on whichnonlinearity is hard to occur, the motion input has little effect on theamplification factor.4. The predicted values of the model in this paper are close to the valuesfrom NGA models.