We calculate source parameters for fluid-injection induced earthquakes near Guthrie, Oklahoma, guided by synthetic tests to quantify uncertainties. The average stress drop during an earthquake is a parameter fundamental to ground motion prediction and earthquake source physics, but it has proved hard to measure accurately. This has limited our understanding of earthquake rupture, as well as the spatio-temporal variations of fault strength. We use synthetic tests based on a joint spectral-fitting method to define the resolution limit of the corner frequency as a function of the maximum frequency of usable signal, for both individual spectra and the average from multiple stations. Synthetic tests based on stacking analysis find that an improved stacking approach can recover the true input stress drop if the corner frequencies are within the resolution limit defined by joint spectral-fitting. We apply the improved approach to the Guthrie sequence, using different wave types and signal-to-noise criteria to understand the stability of the calculated stress drop values. The results suggest no systematic scaling relationship of stress drop for M ≤ 3.1 earthquakes, but larger events (M ≥ 3.5) tend to have higher average stress drops. Some robust spatio-temporal variations can be linked to the triggering processes and indicate possible stress heterogeneity within the fault zone. Tight clustering of low stress drop events at the beginning stage of the sequence suggests that pore pressure influences earthquake source processes. Events at shallow depth have lower stress drop compared to deeper events. The largest earthquake occurred within a cluster of high stress drop events, likely rupturing a strong asperity.

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改进的应力降估算方法及其在俄克拉荷马州诱发地震序列中的应用

我们在综合测试的指导下，计算了俄克拉荷马州古斯里附近的流体注入诱发地震的震源参数，以量化不确定性。地震期间的平均应力降是预测地面运动和地震震源物理的基本参数，但事实证明很难准确测量。这限制了我们对地震破裂以及断层强度的时空变化的理解。我们使用基于联合频谱拟合方法的综合测试来定义转折频率的分辨率极限，该极限频率是可用信号最大频率的函数，适用于单个频谱和多个站点的平均值。基于堆叠分析的综合测试发现，如果转折频率在联合频谱拟合定义的分辨率极限之内，则改进的堆叠方法可以恢复真正的输入应力降。我们将改进的方法应用于Guthrie序列，使用不同的波类型和信噪比标准来了解计算得出的应力降值的稳定性。结果表明，应力下降没有系统的比例关系。中号≤3.1地震，但较大的事件（中号≥3.5）倾向于具有更高的平均应力下降。一些鲁棒的时空变化可以与触发过程联系起来，并指出断层带内部可能存在应力异质性。在该序列的开始阶段，低应力下降事件的紧密聚类表明孔隙压力会影响震源过程。与较深的事件相比，浅深度的事件具有较低的应力降。最大的地震发生在一系列高应力下降事件中，可能会破坏强烈的凹凸不平。