Exploring the influence of tidal stress changes on the generation of secondary slip fronts during slow slip events in Cascadia

地球内部的能量在断层上释放，这些断层可能突然破裂，引发危险的地震，或者在主要无地震的事件中缓慢滑动，这些事件可能持续几个小时到几年。在卡斯卡迪亚，慢滑现象频繁发生，响应并改变了下一次大推覆地震发生的锁定区域下沿板块界面的条件。主宰慢滑行为的特定物理过程也决定了距离太平洋西北部主要人口中心最近的潜在破裂面的状态。区分不同的流变模型和已提出的解释慢滑的机械处理方法是具有挑战性的，因为它们中的许多都可以重现慢滑事件的主要特征，如传播速度、应力降和复发间隔。这使得所有能够复制这些特征的模型都同样可信。最近发现的与慢滑同时出现的“二次”滑移前锋，为区分相互竞争的模型提供了新的机会。因此，该项目将通过确定二次滑动前锋的空间范围、滑动速度和应力降来表征其特征。然后，这些诊断将与模拟的次生滑动前锋进行比较，以确定哪种相互竞争的模型公式能够重现观测结果，并提供一个窗口，了解太平洋西北缘下不断变化的断层条件。慢滑现象要求滑移率增加到观测速度，通常是板块速度的10到100倍，但不要加速到足以产生地震波的程度。实施这一速度限制的具体物理机制存在争议。我们的研究结合了观测和理论部分来考察潮汐应力变化对慢滑过程的影响，从而提供了对几种竞争的模型处理方法的客观测试，这些处理方法已经成功地再现了慢滑的一阶特征(例如，滑移速度、应力降等)。拟议的观测工作将在卡斯卡迪亚的低频地震数据集上使用主成分分析，以系统地量化紧随主滑移锋通过后的次要前锋的长度尺度、时间尺度、传播速度和传播方向。理论工作将把剪切和法向应力振荡纳入慢滑模拟，其中包括速率和状态公式以及膨胀硬化。我们收集的观测目录将用于测试和改进模型处理，这些模型处理寻求沿着卡斯卡迪亚巨型逆冲传播的次生锋的预测和实际特征之间的一致性。

Energy within the Earth is released on faults that can either rupture abruptly, causing hazardous earthquakes, or slip slowly in largely aseismic events that can last from hours to years. Slow-slip phenomena occur frequently in Cascadia, responding to and modifying conditions along the plate interface beneath the locked region where the next mega-thrust earthquake will occur. The specific physical processes that govern slow-slip behavior also determine the state of the potential rupture surface nearest to the major population centers in the Pacific Northwest. Discriminating between the different rheological models and mechanical treatments that have been proposed to explain slow slip is challenging because many of them can reproduce the primary characteristics of slow slip events, such as propagation speeds, stress drops, and recurrence intervals. This makes all models that can reproduce these features equally plausible. The recent discovery of "secondary" slip fronts that occur in conjunction with slow slip provides a new opportunity to distinguish between competing models. Accordingly, this project will characterize secondary slip fronts by determining their spatial extents, slip velocities, and stress drops. These diagnostics will then be compared with modeled secondary slip fronts to determine which of the competing model formulations is able to reproduce the observations and provide a window into changing fault conditions beneath the Pacific Northwest. Slow-slip phenomena require the slip rate to increase to observed speeds, typically 10 to 100 times the plate rate, but refrain from accelerating fast enough to generate seismic waves. The specific physical mechanisms responsible for imposing this speed limit are in dispute. Our research combines observational and theoretical components to examine the influence of tidal stress changes on slow-slip processes, thereby offering an objective test of several competing model treatments that have succeeded in reproducing the first-order characteristics of slow slip (e.g. slip speeds, stress drops, etc.). The proposed observational effort will use Principle Component Analysis on a low-frequency earthquake dataset from Cascadia to systematically quantify the length scales, time scales, propagation speeds, and propagation directions of secondary fronts that immediately follow passage of the main slip front. The theoretical effort will incorporate shear and normal stress oscillations into slow-slip simulations that include rate-and-state formulations and dilatancy hardening. The observational catalog that we assemble will be used to test and refine model treatments that seek agreement between predicted and actual characteristics of secondary fronts that propagate along the Cascadia mega-thrust.