龙门山断裂带科学钻探3号井孔附近微震近震震级与矩震级的关系及意义

The relationship between local magnitude and moment magnitude of microearthquakes near the third bore hole of the Wenchuan Earthquake Fault Scientific Drilling(WFSD-3)and its implications

摘要:

通过震源谱拟合得到龙门山断裂带科学钻探3号井孔附近218个微震的矩震级，并分别基于《地震台站监测规范》的量规函数（量规函数GF）和李学政等（2003）的量规函数（量规函数LXZ）计算了这些微震的两种近震震级。基于回归分析得到了两种近震震级与矩震级的关系，并讨论了此关系可能隐含的意义。近震震级与矩震级拟合关系M_W=a+bM_L中系数b的取值与应力降Δσ和地震矩M₀的关系相关，b=1/（1+γ） 等价于Δσ∝M₀^γ，本文对应于γ=1的情形。由于动态应力降在数值上与静态应力降差别不大，这种关系同样适用于折合能量和视应力，因此仅根据拟合关系M_W=a+bM_L中b的大小就可以判断地震矩与应力降、折合能量及视应力的关系。基于量规函数LXZ得到的近震震级与能量震级更为接近，且两者在与矩震级的拟合关系M_W=a+bM_L中有相同的b，接近于0.5，这既印证了从b的大小来判断应力降与地震矩关系的论断，也说明从能量的角度来看量规函数LXZ优于量规函数GF。

Abstract:

We obtained the moment magnitude of 218 microearthquakes recorded by a microseismic network deployed near the third bore hole of Wenchuan Earthquake Fault Scientific Drilling(WFSD-3)by fitting the displacement spectra. Meanwhile,we derived the local magnitude of these microearthquakes based on the calibration function of the Specifications of Earthquake Observatories(Calibration function GF)and the calibration function of Li Xuezheng et al(Calibration function LXZ,2003),respectively. Then,we extracted the relationship between the two types of local magnitude and moment magnitude of microearthquakes with linear regression,and discussed its implications. The b-value in the relation M_W=a+bM_L reflects the relation between the stress drop Δσ and the seismic moment M₀ of microearthquakes,that is,b=1/(1+γ) equivalent to the relationship between the static stress drop Δσ and the seismic moment M₀ is Δσ∝M₀^γ. The result in this study is corresponding to b=1. Due to the differences between the static and the dynamic stress drop are not too large,we can also deduce the same relationship between the seismic moment and the reduced energy,apparent stress as that between the seismic moment and the stress drop. Therefore,we are able to infer the relation between the seismic moment and the stress drop,reduced energy,as well as apparent stress from the value of b in the formula M_W=a+bM_L. The local magnitude based on the calibration function LXZ is more close to the energy magnitude,and all of the two in the fitting relation M_W=a+bM_L have the same value of b, almost equal to 0.5,which is not only corresponding to the relationship between the stress drop and the seismic moment,but also suggests that the calibration function LXZ is superior to the calibration function GF from the viewpoint of energy.