Taking into account of the frequency bands of microseismographs and the timefrequency characteristics of microseismic waves,we selected 218 microearthquakes recorded by a microseismic array deployed near the third borehole of the Wenchuan earthquake Fault Scientific Drilling (WFSD-3) with arrival time differences tx-tp ＜ 1 s.We located the hypocenters of microeathquakes by joint use of Geiger and Wadati method.The result shows that all these microearthquakes are distributed in a NE-SW belt,which is roughly parallel to the strike of the Longmenshan fault.Local magnitudes based on the calibration functions of the specifications of earthquake observatories (Calibration function GF) and Li et al.(2003) (Calibration function LXZ) have good linear relationships with moment magnitudes.Besides,the relationships show that the trend ML＜Mw holds when the magnitudes decrease,especially when the calibration function LXZ is used.Two source spectral attenuation models,the Brune model and the Boatwright model,were all used to estimate the corner frequencies of the microearthquakes.The results show that the corner frequencies are dependent on selected models:the corner frequencies based on the Brune model are larger than those based on the Boatwright model.The linearity between moment magnitude and the logarithm of corner frequency and rupture radius is not good,while it is relatively good between that and the logarithm of stress drop or apparent stress.All these relationships indicate that stress drop,reduced energy,and apparent stress are not constants,which may suggest that microearthquakes and larger earthquakes have different source physical processes.Apparent stress is in proportion to stress drop and the proportional coefficient is less than 0.5,which may suggest that the rupture dynamic model is consistent with the Savage-Wood model.The b value in fitting relation Mw =a + bML is related to the relationship between stress drop and seismic moment,that is Aσ ∝ Mγ0 is corresponding to b =1/(1 + γ).Therefore,we are able to roughly deduce the relationship between stress drop and seismic moment from the value of b.In this study,γ≈1,which is in agreement with b=0.53 in the fitting relation between local magnitude based on the calibration function LXZ and moment magnitude,and also suggest the calibration function LXZ is more accurate than GF from the point of view of seismic radiated energy.%基于微震监测仪器频带及波形记录的时频特征,我们筛选出汶川地震科学钻探3号井孔周围微震台阵2012年记录的218个ts-tp＜1 s的微震.通过盖戈法与和达法相结合确定微震震源的几何参数,发现这些微震分布大体呈NE-SW展布,与龙门山断裂带的走向基本一致.无论是采用《地震台站观测规范》中的量规函数(量规函数GF)还是李学政等(2003)的量规函数(量规函数LXZ),近震震级均与矩震级呈现出较好的线性关系,向震级小的一端延伸时都表现为ML＜MW,但采用李学政等的量规函数时该趋势更加明显.同时采用Brune和Boatwright震源谱衰减模型对观测震源谱的拟合表明拐角频率具有模型依赖性:基于Brune模型拟合得到的拐角频率大于基于Boatwright模型得到的拐角频率.无论基于哪一种模型,矩震级与拐角频率、破裂半径的对数线性关系均较弱,与应力降、视应力的对数则存在较好的线性关系.这些关系不支持应力降、折合能量、视应力为常数的观点,表明微震与大地震的震源物理过程存在差异.视应力与应力降成比例,比例系数小于0.5,表明破裂动力学模式符合Savage-Wood模式.近震震级与矩震级拟合关系Mw=a+bML中b的大小与应力降和地震矩的关系有关,△σ∝Mγ0,则b=1/(l+γ),因此从b的大小可以粗略地判断应力降与地震矩的关系.本文对应于y≈1的情况,与基于量规函数LXZ得到的近震震级与矩震级关系中系数b=0.53吻合,这说明仅从辐射能量的角度考虑量规函数LXZ较量规函数GF准确.
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