We examine localization processes of low magnitude seismicity in relation to the occurrence of large earthquakes using three complementary analyses: (i) estimated production of rock damage by background events, (ii) evolving occupied fractional area of background seismicity, and (iii) progressive coalescence of individual earthquakes into clusters. The different techniques provide information on different time scales and on the spatial extent of weakened damaged regions. Techniques (i) and (ii) employ declustered catalogs to avoid the occasional strong fluctuations associated with aftershock sequences, while technique (iii) examines developing clusters in entire catalog data. We analyze primarily earthquakes around large faults that are locked in the interseismic periods, and examine also as a contrasting example seismicity from the creeping Parkfield section of the San Andreas fault. Results of analysis (i) show that the M > 7 Landers 1992, Hector Mine 1999, El Mayor-Cucapah 2010 and Ridgecrest 2019 mainshocks in Southern and Baja California were preceded in the previous decades by generation of rock damage around the eventual rupture zones. Analysis (ii) reveals localization (reduced fractional area) 2–3 yr before these mainshocks and before the M > 7 Düzce 1999 earthquake in Turkey. Results with technique (iii) indicate that individual events tend to coalesce rapidly to clusters in the final 1–2 yr before the mainshocks. Corresponding analyses of data from the Parkfield region show opposite delocalization patterns and decreasing clustering before the 2004 M6 earthquake. Continuing studies with these techniques, combined with analysis of geodetic data and insights from laboratory experiments and model simulations, might improve the ability to track preparation processes leading to large earthquakes.

中文翻译：

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大地震前地震活动的定位与合并

我们使用三个补充分析来检查与大地震发生有关的低震级地震的定位过程：（i）背景事件对岩石破坏的估计产量，（ii）背景地震活动性的占据分数区域的演变，以及（iii）渐进合并个别地震成簇。不同的技术可提供有关不同时间尺度和弱化损坏区域的空间范围的信息。技术（i）和（ii）使用分散的目录，以避免与余震序列相关的偶然强烈波动，而技术（iii）则检查整个目录数据中正在形成的聚类。我们主要分析在地震期间锁定的大型断层周围的地震，并以圣安德烈亚斯断层蠕动的帕克菲尔德断层的地震活动作为对比示例。分析结果（i）显示M  > 7 Landers 1992年，Hector Mine 1999年，El Mayor-Cucapah 2010年和Ridgecrest 2019年在南加利福尼亚和下加利福尼亚州发生的主震，在过去的几十年中，最终破裂区周围产生了岩石破坏。分析（ii）显示了这些主震之前和M之前2-3年的局部化（减小的分数面积） > 1999年土耳其杜兹7地震。技术（iii）的结果表明，在主震之前的最后1-2年中，单个事件趋于迅速合并为簇。在2004年M6地震之前，对Parkfield地区数据的相应分析显示出相反的离域模式和聚类的减少。继续使用这些技术进行研究，再结合对大地测量数据的分析以及实验室实验和模型模拟的见解，可能会提高跟踪导致大地震的准备过程的能力。