We analyze the spatiotemporal evolution of seismicity during a sequence of moderate (an Mw 4.7 foreshock and Mw 5.8 mainshock) earthquakes occurring in September 2019 at the transition between a creeping and a locked segment of the North Anatolian fault in the central Sea of Marmara, northwest Turkey. To investigate in detail the seismicity evolution, we apply a matched‐filter technique to continuous waveforms, thus reducing the magnitude threshold for detection. Sequences of foreshocks preceding the two largest events are clearly seen, exhibiting two different behaviors: a long‐term activation of the seismicity along the entire fault segment and a short‐term concentration around the epicenters of the large events. We suggest a two‐scale preparation phase, with aseismic slip preparing the mainshock final rupture a few days before, and a cascade mechanism leading to the nucleation of the mainshock. Thus, our study shows a combination of seismic and aseismic slip during the foreshock sequence changing the strength of the fault, bringing it closer to failure.

中文翻译：

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在土耳其伊斯坦布尔的马尔马拉海进行的两级准备阶段之前，发生了5.8兆瓦地震。

我们分析了2019年9月在西北部马尔马拉海中部北安那托利亚断层的蠕变和锁定段之间的过渡期间发生的一系列中等地震（4.7级前震和5.8级主震）地震期间地震活动的时空演变。火鸡。为了详细研究地震活动的演化，我们将匹配滤波器技术应用于连续波形，从而降低了检测的幅度阈值。可以清楚地看到两个最大事件之前的前震序列，表现出两种不同的行为：沿整个断层段的长期地震活动激活和大事件震中附近的短期集中。我们建议采用两个阶段的准备阶段，前几天通过抗震滑移来准备主震最终破裂，和导致主冲击成核的级联机制。因此，我们的研究表明，在前震序列中地震和地震滑动的组合改变了断层的强度，使其更接近于破坏。