Subduction zones host some of Earth's most damaging natural hazards, including megathrust earthquakes and earthquake‐induced tsunamis. A major control on the initiation and rupture characteristics of subduction megathrust earthquakes is how the coupled zone along the subduction interface accumulates elastic strain between events. We present results from observations of slow slip events (SSEs) in Cascadia occurring during the interseismic period downdip of the fully coupled zone, which imply that the orientation of strain accumulation within the coupled zone can vary with depth. Interseismic GPS motions suggest that forces derived from relative plate motions across a shallow, offshore locked plate interface dominate over decadal timescales. Deeper on the plate interface, below the locked (seismogenic) patch, slip during SSEs dominantly occurs in the updip direction, reflecting a dip‐parallel force acting on the slab, such as slab pull. This implies that in subduction zones with obliquely convergent plate motions, the seismogenic zone of the megathrust is loaded by forces acting in two discrete directions, leading to a depth‐varying orientation of strain accumulation on the plate interface.

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俯冲界面的双向加载：来自慢滑事件运动学的证据

俯冲带承载着地球上最具破坏力的自然灾害，包括大推力地震和地震引起的海啸。俯冲巨推力地震起裂和破裂特征的主要控制是沿俯冲界面的耦合带如何在事件之间累积弹性应变。我们提供了从卡斯卡迪亚慢滑事件（SSE）的观察结果，该事件发生在完全耦合区域的地震间隔下倾期间，这暗示耦合区域内应变累积的方向可能随深度而变化。GPS的间震运动表明，在相对浅的海上锁定板块界面上，相对板块运动所产生的力在十年时间尺度上占主导地位。在板界面的更深处，在锁定的（抗震）斑块下方，SSE期间的滑移主要发生在上倾方向，反映了作用在平板上的平行倾斜力，例如平板拉力。这意味着在板块斜交运动的俯冲带中，大推力的发震带受到作用在两个离散方向上的力的作用，从而导致板界面上应变累积的深度变化方向。