【摘要】 采用地脉动噪声法和b值曲线拟合法对四川大岗山水电站地震台网的最小监测能力进行了研究和对比,结果表明在重点监视区理论监测能力达到了ML0.5的要求,实际记录重点监视区完整性震级下限为ML0.5,达到了地震台网设计目标。更多还原

【Abstract】 Located at the northern section of the NS-trending seismic tectonic zone at the border between Sichuan and Yunnan,the reservoir of the Dagangshan hydropower station is at the intersection of three seismic belts,Xianshuihe,Longmenshan,and Anninghe,which present a complex geological environment.The earthquake seismic network at the Dagangshan hydropower station is a dedicated small diameter minor earthquake monitoring network for hydropower station projects and consists of eight stationary monitoring sites located across the Luding county of Ganzi and the Shimian county of Yaan,Sichuan province.The monitoring sites are located at both sides of the Dadu river,around the dam,at the main reservoir area,and at the fault zones,in accordance with the identified key fault zones to monitor reservoir induced earthquakes,increase the precision of locating seismic events,and decrease the lower limit of seismic magnitudes detectable in the area.The theoretical minimum monitoring capability of the designed seismic network in the focused monitoring area is ML0.5.Based on the velocity records,the displacements recorded by the W-A short period seismometer are emulated,and then the max.P-P and the corresponding period on the displacement waveform emulated are substituted into the formula for near earthquake magnitude to calculate the earthquake magnitude.For validation of the background noise of earth motion,the PSD per hour at each station and the RMS values in the range of 1~20Hz are calculated based on the noise data randomly selected from the background noise of earth motion recorded in a 48-hr interval at the Dagangshan earthquake monitoring network and the mean RMS value is taken as the average background noise of earth motion.Taking the ratio of the maximum magnitude of the S wave to the background noise as the minimum lower limit for seismic velocity detection by the system,the calibration function value R(Δ)is calculated using the earthquake magnitude formula.The minimum earthquake magnitude recordable at each site for various epicentral distances is determined based on the R(Δ).For validation using the bvalue curve fitting,the data for the earthquakes,recorded in the monitoring area for which the seismic sources have been determined,and the Gutenberg-Richter law are used,and the monitoring capability of the earthquake seismic network is validated using the deviation from the linear relationship between lg N(M)and M.Ab-value curve is plotted on the basis of 585 located earthquakes in the range of ML=0.0~4.0recorded from Mar to Sep,2013 in the specified area.When the magnitude is below a certain value,the actual number of earthquakes recorded significantly deviates from the reference line,i.e,the total number of earthquakes recorded is less than the number indicated by the fitting curve,implying that the system doesn’t record all the minor earthquakes that have a magnitude below that value,and the deviation value is established as the lower limit of the magnitude detectable by the monitoring system.The statistical analysis of the seismic data recorded indicates that when ML< 0.5,the recorded value is significantly below the fitting line.Therefore,the lower limit of detection at the earthquake seismic network at the Dagangshan hydropower station is ML=0.5.The theoretical monitoring capability of the earthquake seismic network at the Dagangshan hydropower station is calculated based on the near earthquake magnitude formula and the Gutenberg-Richter law.The calculations show that the theoretical monitoring capability in the focused monitoring areas is in compliance with the ML0.5requirements.The practical minimum magnitudes of completeness at ML0.5are validated by the actual record,which fulfills the design objectives of the reservoir induced earthquake seismic network at the Dagangshan hydropower station.更多还原