Strong Ground Motion Attenuation Relationship For Sichuan-yunnan Region From Small Earthquake Records By China Earthquake Networks

Ground motion attenuation relationship is a vital field in research ofengineering seismology. Up to now, the main achievements are mostly theempirical relations based on a large number of observed data. For the regionswithout enough strong motion data, it is a difficult problem to build therelation. On the basis of the progresses of random vibration theory based orseismology based relationship studies, the possibility to build strong groundmotion attenuation relationship from regional small event records byearthquake observation network is dealt with in this dissertation. In generalregions, small earthquake records from regional observation network are allenough, therefore if the degree of accuracy to estimate acceleration spectrafrom velocity time histories of small event records, the restriction from thelack of ground motion record will be overcome effectively, and a very broaddevelopment road of motion attenuation research will be opened up.The possibility to transfer velocity spectra to acceleration spectra isdealt with by process and analysis of small event records from regionalearthquake observation network in Sichuan-Yunnan region. Totally147velocity time histories recorded at29regional digital observation stations ofSichuan in82small quakes with moment magnitude3.5to4.5and depthless than30km, and863recorded at26stations of Yunnan in154smallearthquakes, are collected and processed. With the object function from theenvelops of Fourier spectra of the small records,5regional parameters ofsource spectrum and the crust medium, stress drop Δσ, quality factorparameters Q0and η, geometric attenuation parameters R1and R2areinversed by Micro-Genetic algorithm.For a magnitude-distance pair, the Fourier amplitude spectrum of thecorresponding ground motion is estimated, then a time history is inversetransformed from it with a random phase spectrum together, a peak groundmotion acceleration, PGA, is finally read from the history. To manage allPGAs in order, a two dimensional table is designed, in which each line for agiven magnitude, each column for a given distance, each intersection of thetwo for the corresponding PGA. If the well-distributed magnitude intervaland the distance interval are both small enough, the organized values candescribe the regional attenuation feature very well, without any pre-restrain,and the accuracy loss in further fit a attenuation relation can be avoided.The ground motion attenuation relationships for Sichuan and Yunnanregions are built by this way, and it is pointed out that a higher efficiencecan be achieved by the table in calculation of seismic hazard assessment.In order to validate the result of this dissertation by observed data ofregional ground motion,1237accelerograms recorded at69stations inSichuan in118strong earthquakes with magnitude no less than4.5and depth no deeper than30km,66accelerograms at36stations in Yunnan in27strongearthquakes are collected and processed. Three manners are adopted in thevalidation, result data points in curves each for a magnitude of5,6and7arecompared with observed data points in interval of the magnitude minus andplus0.5, distribution of errors defined as logarithm of the ratio of observedPGA to the calculated PGA from the table, the mean and standard deviationof of the errors. The comparison of the result with the observed data and alsothe empirical attenuation relations,6sets for Sichuan,5sets for Yunnan,shows that the most attenuation curves pass through ideally the data pointplexus, and the data points are distributed relatively uniform at two sides ofup and down, except the curve for magnitude7in Sichuan shows an obviousslow attenuation. The result errors of this dissertation are mostly at up anddown of zero, that means there is no systematic bias, and the attenuationseems faster at low magnitude and slower at high magnitude the thecorrespond data. The mean and standard deviation of the result error arebetter in general, even better for magnitude6and7in Sichuan. On the whole,they both are relatively low. The validation by the three comparisons allshow that the idea and the approach developed in this paper to build strongground motion attenuation relations from small event records by the regionalobservation network are feasible.The detail solution to adopt the result motion attenuation relation inseismic hazard assessment is showed by a case of the assessment for aengineering site in Sichuan. It is demonstrated the practical feasibility of theresult of the paper in seismic hazard assessment of engineering site andregional seismic zoning. It is also pointed out that the regional parametersinversed and method to estimate the PGA in this paper are acceptable andindispensable to estimate PGA in near field of large earthquake with quitegood accuracy from a finite fault based hybrid source model, by means of theanalysis of the synthesized ground motions at two rock station where themotions have been recorded during the Wenchuan earthquake.Finally, four suggestions for the further study is mentioned from theproblems the author met in the research of this paper.