A new strategy for earthquake focal mechanisms using waveform‐correlation‐derived relative polarities and cluster analysis: Application to the 2014 Long Valley Caldera earthquake swarm
A new strategy for earthquake focal mechanisms using waveform‐correlation‐derived relative polarities and cluster analysis: Application to the 2014 Long Valley Caldera earthquake swarm
复制标题
使用波形相关性相对极性和聚类分析的地震震源机制新策略：在 2014 年长谷火山口地震群中的应用
DOI：
--
复制
发表时间：
2016
期刊：
影响因子：
--
通讯作者：
D. Hill
中科院分区：
文献类型：
--
作者：
D. Shelly;J. Hardebeck;W. Ellsworth;D. Hill
在微震活动分析中，可靠的震源机制通常只能获得定位事件的一小部分。我们在这里解决了这一限制，提出了一个框架，用于确定非常小事件的整个群体的稳健震源机制。为了实现这一点，我们通过使用它们的符号相关系数来确定两个波形之间的相对P波和S波极性--这是以前进行精确地震定位的副产品。然后，我们使用聚类分析来对网络中具有相似极性模式的事件进行分组。最后，我们使用目录或相关派生的P波极性数据集，对分组数据应用标准机制反演。这种方法在加强对震群、主震-余震序列、工业油藏刺激或注入诱发地震序列等空间集中微震活动的分析方面具有很大潜力。为了证明它的实用性，我们将这一技术应用于2014年长谷火山口震群。在我们的分析中，85%的事件(由Shelly等人定位的8494个事件中的7212个)。[])属于五个受到良好约束的机制群组，是由网络决定的机制群的12倍多。在我们描述的地震中，3023次(42%)的震级小于0.0级。我们发现，机制变异与相应的震源结构密切相关，但机制异质性也出现在不能通过震源模式解决的地方，通常局限于小震级事件。当我们应用三维速度模型时，机制方向和地震位置趋势之间的小(5-20°)旋转持续存在，可能反映了一种梯形、连锁剪切和扩张断层的几何形状。
In microseismicity analyses, reliable focal mechanisms can typically be obtained for only a small subset of located events. We address this limitation here, presenting a framework for determining robust focal mechanisms for entire populations of very small events. To achieve this, we resolve relative P and S wave polarities between pairs of waveforms by using their signed correlation coefficients—a by‐product of previously performed precise earthquake relocation. We then use cluster analysis to group events with similar patterns of polarities across the network. Finally, we apply a standard mechanism inversion to the grouped data, using either catalog or correlation‐derived P wave polarity data sets. This approach has great potential for enhancing analyses of spatially concentrated microseismicity such as earthquake swarms, mainshock‐aftershock sequences, and industrial reservoir stimulation or injection‐induced seismic sequences. To demonstrate its utility, we apply this technique to the 2014 Long Valley Caldera earthquake swarm. In our analysis, 85% of the events (7212 out of 8494 located by Shelly et al. [ ]) fall within five well‐constrained mechanism clusters, more than 12 times the number with network‐determined mechanisms. Of the earthquakes we characterize, 3023 (42%) have magnitudes smaller than 0.0. We find that mechanism variations are strongly associated with corresponding hypocentral structure, yet mechanism heterogeneity also occurs where it cannot be resolved by hypocentral patterns, often confined to small‐magnitude events. Small (5–20°) rotations between mechanism orientations and earthquake location trends persist when we apply 3‐D velocity models and might reflect a geometry of en echelon, interlinked shear, and dilational faulting.