Developing high-resolution tremor catalogs to constrain numerical models of slow slip

在过去的十年中，地球物理学的一个重大发现是“间歇性的慢滑和地震”。在世界上许多地方？S俯冲带。大多数已知的断层要么以板块构造速率(每年几厘米)稳步滑动，要么大部分时间处于？锁定？仅在短暂的地震中滑动，滑动速度约为1米/秒，传播速度约为3公里/秒(岩石的声速)。另一方面，慢滑事件的平均滑动速度仅为0.1微米/S，并以约10公里/天的显著可重复性速度沿走向传播至300公里。与大地测量观测到的缓慢滑动在时间和空间上一致的是一种被称为构造震动的地震信号。与典型的地震不同，地震波的到达是脉冲的，震颤是一种低幅度的信号，可以持续几个小时，而且通常缺乏清晰可识别的地震波到达。现在，在几乎所有的俯冲区都发现了缓慢滑动，如果存在的话，有足够的仪器可以看到它。地震在空间和时间上与可能的缓慢滑动事件重合，在加利福尼亚州圣安德烈亚斯断裂的深部伸展处也发现了地震。除了代表一种以前未被认识到的断层滑动类型外，幕式慢滑还与地震灾害有关，因为它增加了能够产生9级地震的断层锁定部分的应力率。也有人提出，它可以在这些地震期间划定滑动的下倾程度(特别是，强烈的地面震动比之前认为的更接近西雅图市中心)。由于深断层的大地测量数据的分辨率相当低，目前地震位置提供了我们最详细的慢滑时空历史图像。但由于地震缺乏清晰的波到达，并且可以同时在断层的多个区域活跃，因此无法使用标准技术进行定位。我们正在开发一种新的地震检测算法，应用于太平洋西北海岸外的俯冲带，目前正在产生世界上最准确的地震位置。它不是比较同一台站不同时间窗口的地震记录的更传统方法，而是比较不同台站的相同时间窗口。相对位置误差通常小于1公里，这使我们能够更详细地成像出现在慢滑事件主锋后面的次生震锋，并比主锋传播快12个数量级。通过高保真地成像这些次级锋面以及它们与主锋面的关系，我们期望能更多地了解慢滑背后的过程。此外，关于震颤的研究在世界范围内激增，但到目前为止还没有单一的、普遍同意的方法来定位它。通过将我们的定位方法与更传统的方法进行比较并结合各自的方面，我们将学习到更多关于如何改进地震定位算法以及如何在其他地区应用这些知识的知识。

One of the major discoveries in geophysics in the past decade has been that of ?episodic slow slip and tremor? in many of the world?s subduction zones. Most known faults either slip steadily at the plate tectonic rate (a few centimeters per year), or spend most of the time ?locked? and slip only during short-lived earthquakes, with slip speeds of order 1 m/sec and propagation speeds of order 3 km/sec (the sound speed of rock). Slow slip events, on the other hand, have average slip speeds of only ~0.1 micron/s and propagate up to 300 km along strike at remarkably reproducible speeds of roughly 10 km/day. Coincident in time and space with the geodetically-observed slow slip is a seismic signal termed ?tectonic tremor?. Unlike typical earthquakes, which have impulsive seismic wave arrivals, tremor is a low-amplitude signal that can last for hours and that most often lacks clearly identifiable seismic wave arrivals. Slow slip has now been discovered in nearly all subduction regions with sufficient instrumentation to see it, if it were present. Tremor is coincident in space and time with may slow slip events, and has also been discovered on the deep extension of the San Andreas fault in California. In addition to representing a previously unrecognized style of fault slip, episodic slow slip is relevant to seismic hazards because it increases the stressing rate on the locked portions of faults capable of producing magnitude 9 earthquakes. It has also been proposed that it may delimit the down-dip extent of slip during those earthquakes (bringing, notably, strong ground shaking considerably closer to downtown Seattle than had previously been thought).Because the resolution of geodetic data for deep faults is quite poor, tremor locations currently provide our most detailed images of the space-time history of slow slip. But because tremor lacks clear wave arrivals and can be active on multiple regions of the fault simultaneously, it cannot be located using standard techniques. We are developing a new tremor detection algorithm that, applied to the subduction zone off the coast of the Pacific Northwest, is currently producing the most accurate tremor locations in the world. Rather than the more traditional method of comparing seismograms from different time windows at the same station, it compares the same time windows at different stations. Relative location errors are often less than 1 km, allowing us to image in great detail secondary tremor fronts that arise behind, and propagate 1?2 orders of magnitude faster than, the main front of the slow slip event. By imaging these secondary fronts and their relation to the main with high fidelity, we expect to learn more about the processes underlying slow slip. In addition, studies of tremor are proliferating worldwide, but as yet there is no single, generally-agreed-upon method for locating it. By comparing our location method with more traditional methods and combining aspects of each, we stand to learn much about how to improve tremor location algorithms and how to apply this knowledge in other regions.