丽江7.0级地震的余震环境应力τ_0值研究

Study on the τ_0 Value of the Environment S tress for Aftershock Sequence of M7.0 Lijiang Earthquake

利用 1996年丽江 7 0级余震序列资料 ,通过波谱分析 ,求出了余震的地震矩M0 ,以断裂力学和地震标定律的新观点 ,导出了用峰值速度νp 求震源处环境应力的关系 ,估算了环境应力τ0 值。震源参数测定的结果给出 ,震级ML2 0～ 3 5之间求出的地震矩M0 在 10 19～ 10 2 2 (dyne·cm)之间 ,每次地震平均环境应力 τ0 值在 1 0～ 9 0MPa的范围内变化 ,绝大多数为 2 0～ 5 0MPa。高τ0 值主要分布在 7 0级主震断裂构造及几次 5 0～ 6 0级强余震的周围地区。其次 ,本文研究的另一个重要结果是 ,发现场地放大对地震波记录影响十分明显 ,通过基岩台站与沉积层台站记录的数字地震波谱场地影响分析 ,结果给出频率f在 1 0～ 3 0Hz的范围内 ,沉积层水平向放大约为基岩的 3 5倍 ,理论关系给出二种场地测定的τ0 值沉积场地比基岩场地大1 5倍 ,本文用峰值速度实测求出的震源环境应力τ0 值也相应增大约 1 3倍 ,与理论关系结果基本一致。由此 ,本文详细研究了丽江峡谷的场地放大对震源应力参数测定的影响。

By spectrum analysis using data of sequence of aftershocks of M7 0 Lijiang Earthquake in 1996, the seismic moment M 0 is obtained. According to a new viewpoi nt of fractural mechanics and the seismic scaling law, a formula for the environ ment stress at source area is derived by using peak velocity V P and the τ 0value s of the environment stress is calculated too. The results of measured source pa rameters show that the seismic moment M 0 is between 10 19 dyne.cm and 10 22 dyne cm while magnitude M L is in the range from 2 0 to 3 5 a nd the average τ 0 value s are in the range from 1 0MPa to 9 0Mpa for each earthquake. For most earthq uak es, the average τ 0 values are in the range from 2 0MPa to 5 0MPa. High τ 0 value mainly distribute in the area of seismogenic fault and the strong aftershocks wi th the magnitude of 5 0 to the magnitude of 6 0. Another important result of the study is that the influence of the site amplification upon the recordings of ea rthquakes is very obvious. The analysis of site response for the seismic spectru m of the digital recordings of stations for bedrock and deposit bring forward a conclusion that in the frequency range of 1 0 Hz to 3 0Hz, the horizontal amp lif ication of deposit is 3 5 times as large as that of bedrock. The τ 0 values calcu lated by theoretical relation of two site show that the τ 0 value of deposit is 1 5 times as large as that of bedrock. In this paper, it is found that the calcul ated τ 0 values have increased by 1 3 times and they are a lmost the same with the result get from the theoretical relation. Therefore, the influence of site ampl ification in Lijiang Valley upon the measurement of source-stress parameters i s studied in detail.