Abstract:The Minxian MS6.6 earthquake,the largest earthquake to occur in Gansu province during the past decade,devastated areas in Minxian and Zhangxian and affected counties in Longnan and the Gannan Tibetan Autonomous Prefecture more than 100 km from the epicenter.During the tenth five-year plan,20 strong-motion stations have been erected in southeastern Gansu,most of which recorded the main shock.In addition,several stations in Lanzhou also recorded strong motion.After preliminary processing of the strong motion data,we determined that the majority of stations that recorded the main shock are broadly distributed with same strike; therefore,a simplified mathematical model was developed to analyze the attenuation of these strong motion records,The epicentral distances of the strong motion stations recording the main shock are 18 km to more than 200 km.The largest peak ground acceleration was recorded by the Minxian strong motion station at 172.5 cm/s2 with an epicentral distance of 18 km.For the free-field soil observation conditions,these values essentially reflect the local ground acceleration; therefore,the extent of damage reached Ⅷ°.Stations near Lanzhou with epicentral distances close to 200 km also recorded the motion,with peak acceleration between 1 cm/s2 and 8 cm/s2.Chinese ground motion attenuation data originally used ground motion data observed in the United States as a reference before such information was recorded in China.In recent years,a Chinese digital strong motion observation network was been created to provide important data in earthquakeprone regions for in-depth research of Chinese ground motion attenuation.Therefore,the study of Minxian MS6.6 earthquake ground motion attenuation also has practical significance in areas outside of southeast Gansu.In the study of peak ground acceleration attenuation,the following formula is often used：lg y =c1 ＋ c2 M＋ c3M2＋ c4lg [R ＋ c5exp （c6 M）],…………………… （1）where y is the peak ground acceleration,M is the magnitude,R is the epicentral distance,and c1,c2,c3,c4,c5,and c6 are regression coefficients.When the magnitude is determined,equation （1）can be simplified to：lg y=A ＋ B＊lg （R ＋ R0）.…………………………………… （2）The correspondence of the parameters is as follows：A =c1 ＋ c2 M ＋ c3 M2; B =c4; R0 =c5exp （c6 M）.We fit the peak ground acceleration attenuation curve by using formula （2）.Because the peak ground acceleration from the station on a thick layer （bedrock） would be significantly greater （smaller）,data from five stations were removed during the attenuation curve fitting.We took R0 as 30 for lg y and lg （R ＋ R0） to obtain the best linearity.The results show that the peak ground acceleration attenuation curves of the three components had the same shape.However,the value of limit acceleration,or the value of A ＋ B＊lg （R0） differed.The formulas of the peak ground acceleration attenuation of three components are：EW：lg y=-2.847 7＊lg（R ＋ 30） ＋ 7.056 9; R2=0.846 6NS：lg y =-2.793 5＊lg（R ＋ 30） ＋ 6.984 4; R2 =0.860 3UD：lg y =-2.796 9＊lg（R ＋ 30） ＋ 6.751 8;R2 =0.849 7.We compared the results with the disaster assessment intensity map to determine that the ranges of Ⅷ and Ⅶ degrees given by our results are slightly larger because of fewer near-field strong motion records were used.
