Study On Attenuation Relationships Of Long Period Ground Motions

The goal of this paper is to develop the long period attenuation relationships for horizontal acceleration response spectrum of rock site in China. The study is carried out both in database and methodology aspects.In the database aspect,the digital broad-band records originally used by seismological studies are used. These data mainly come from the digital broad-band records which are used by seismologists. The recordings include the CDSN records,the Wushi station records of the Geoscope,the TERRAScope records of southern California. The analog low-gain displacement-type records of China are also used.The reliability of the observation data in long period range is particularly inspected. This is done "by verifying the instrument response characteristic,examining the signal-to-noise ratio,comparing the recovered ground motions and response spectra from the velocity type and acceleration type digital broad-band records. The processing method for the digital broad-band records is developed. For the analog low-gain displacement-type records,two horizontal-component records of a M7.1 aftershock of Tangshan earthquake,July 28,1976,in Tai'an seismic station are used as the example to show the procedures of digitizing,arc-shape correction,baseline correction,equal-interval interpolation and instrument response correction. The reliability of the analog low-gain displacement-type records for long period study is also discussed.In order the database to have a better magnitude-distance distribution,a method to estimate the long period response spectrum values from earthquake magnitude definition is developed.The empirical attenuation relationship for horizontal acceleration response spectrum in reference region is developed by regression,and the relationships for eastern China and western China are obtained by transform method and the data from China. The western North America is set as the reference region. 754 horizontal-component digital broad-band records in western North America are processed. The acceleration response spectra are calculated and used as part of the database. The response spectrum values are estimated by the earthquake magnitude definition and are used as another part of the database. The long period attenuation relationship of acceleration response spectrum is developed using this database. The short-period attenuation equation isdeveloped by using the analog strong motion records of western North America. By connecting the short period and the long period attenuation relationships together,the wide period (7"=0.04-20sec) attenuation relationships for horizontal acceleration response spectrum of western North America are obtained. Then the transform method and the digital broad-band data from China is applied to obtain the wide period attenuation relationships for horizontal acceleration response spectra of eastern China and western China in rock sites (C,=5%).We have got the following knowledge and conclusions:(1) The records of low-gain analog displacement-type seismograph can be used to study long period ground motion. After proper correction procedures the usable period can be up to 10 sec. The processing procedures are showed in the paper.(2) The digital broad-band seismographs used by seismologists have high quality long period characteristics and the records are suitable for studying long period ground motions. We find that the noise level of the record is very low. Even in weak ground motion condition the digital broad-band record has high signal-to-noise ratio when the period is as long as 20 sec. The signal-to-noise ratio of the velocity-type records is lower than that of the acceleration-type records. From the velocity-type records and the acceleration-type records in one station the same ground motions can be recovered,and the calculated response spectra are the same in a specific period range. This indirectly proves the reliability of the two types of records. When in the weak ground motion condition,the velocity-type digital broad-band records are more suitable fo...