Comprehensive Analyses of Cascadia Tremor and Slow Slip

慢滑和震颤是俯冲带构造板块运动的关键过程，其中一个俯冲带位于太平洋西北海岸，从门多西诺角到加拿大温哥华岛。为了了解俯冲带的基本机制，必须进行慢滑以揭示其秘密。在过去十年中，这个谜题的进展已经并将在未来继续具有变革性。为了了解这些过程，该项目正在分析过去十年收集的丰富的地震和大地测量数据。目前正在整合不同的数据类型和分析方法，以更精确地确定应变积聚的地点和时间，以及慢滑在加载和释放板界面应力中的作用。慢滑与俯冲带的危险锁定部分的潜在相互作用要求更好地理解慢滑现象。例如，下一代的太平洋西北地震危险图将使用我们对地震的上倾极限的估计作为估计大型逆冲断裂的下倾极限的一种方法，从而改进对下一次美国大地震中横跨西海岸的震动的估计。此外，观察和解释的缓慢滑动可能会随着运动模式的改变、加速和减速而揭示地震危险的变化。调查人员参与区域地震台网的运作，并积极开展各种形式的外联活动，向公众宣传地震危害。透过博客、电视、报纸访谈和NOVA纪录片，偶发的震颤和滑动事件提供了一个绝佳的机会，让公众提高对区域地质环境和地震危害的认识。自2004年以来，华盛顿大学已经用地震阵列捕捉到了华盛顿北部7次缓慢滑动，其中两次是用地球范围阵列（8个阵列），积累了丰富的数据集。我们正在整合几种方法，从堆积如山的连续录音和不断增长的目录中筛选关键关系。本研究的重点是：(1)利用波束反投影和包络相关方法描述地震的时空进展；(2)利用LFE位置和重复精确跟踪慢滑；(3)系统地确定地震振幅及其与潮汐应力和滑动量的关系；(4)结合地震时空约束的GPS和应变仪时间序列；(5)将象牙塔慢滑模型与观测结果进行比较。这些方法允许改进地震传播模式（如快速地震反转和条纹）与滑动前沿传播和低频地震产生的关系，从而产生对滑动速度、传播速度和慢滑动脉冲上升时间的限制。此外，本研究还梳理了慢滑从地震中延伸到上倾的程度，这对地震危险性有影响，并评估了地震振幅在将地震持续时间与慢滑力矩释放的比例关系中的重要性。目前掌握的是越来越多的几种类型的地球物理数据缓存：从卡斯卡迪亚范围内的21万个地震位置到阵列阵列收集的高分辨率位置的多种地震目录，以及新兴的低频地震目录。这些提供了从1000公里到100米尺度的视野。此外，GPS、应变和倾斜仪时间序列更直接地记录变形。将上述数据集联系起来，可以在一定的空间和时间尺度上推断出慢滑锋的传播特征。

Slow slip and tremor are key processes in tectonic plate motion in subduction zones, one of which is positioned along the Pacific Northwest coast from Cape Mendocino up to Vancouver Island in Canada. For the basic mechanics of subduction zones to be understood, slow slip must be made to yield its secrets. Progress in this puzzle in the past decade has been and will continue in the future to be transformative. To understand these processes, this project is analyzing the copious seismic and geodetic data that has been gathered over the past decade. Different data types and analysis approaches are being integrated to determine more precisely where and when strain is accumulating and the role of slow slip in loading and releasing plate-interface stress.The potential interaction of slow slip with the hazardous locked portions of subduction zones mandates better understanding of slow slip phenomena. For example, the next generation of Pacific Northwest seismic hazard maps will use our estimates of the updip limit of tremor as one way to estimate the downdip limit of megathrust rupture, refining estimates of the shaking across the west coast in the next great American earthquake. In addition, it is possible that the slow slip being observed and interpret may reveal changes in the seismic hazard over time as motion changes pattern, accelerates and decelerates.The investigators are involved in the operation of the regional seismic network and are active in various forms of outreach to educate the public about seismic hazard. Episodic tremor and slip events is providing a golden opportunity to advance public awareness of the regional geological environment and seismic hazard through blogs, TV and newspaper interviews and NOVA documentaries. The University of Washington has captured seven episodes of slow slip under northern Washington since 2004 with seismic arrays two with the EarthScope Array of (eight) Arrays, accumulating a rich dataset.Several approaches are being integrated to sift through the mountain of continuous recordings and our growing catalogs in search of key relationships. The foci in this work is (1) delineation of the space-time progression of tremor by both beam backprojection and envelope correlation methods, (2) precise tracking of slow slip with LFE locations and recurrence, (3) systematic determination of tremor amplitude and its relation to tidal stressing and amount of slip, (4) integration of GPS and strainmeter timeseries with spatio-temporal constraints from tremor, and (5) comparison of emerging ivory-tower slow-slip models with observation. These approaches allow refinement of the relation of tremor propagation patterns such as rapid tremor reversals and streaks to slip front propagation and low frequency earthquake generation, and thus yield constraints on the slip velocities, propagation velocities, and rise times of slow slip pulses. As well, this study is teasing out how much slow slip extends updip from tremor, with implications for seismic hazard, and to assess the importance of tremor amplitude in scaling tremor duration to slow slip moment release.Currently in hand are growing caches of several types of geophysical data: multiple catalogs of tremor ranging from Cascadia-wide with 210,000 tremor locations to high-resolution locations collected by the Array of Arrays, as well as a burgeoning catalog of low-frequency earthquakes. These provide views from the 1000 km to 100 m scales. In addition, GPS, strain, and tilt-meter time series record deformation more directly. Linking the above datasets is leading to characterization of the inferred propagation of slow slip fronts over a range of spatial and temporal scales.