Most destructive tsunamis are caused by seismic slip on the shallow part of offshore megathrusts. The likelihood of this behaviour is partly determined by the interseismic slip rate deficit, which is often assumed to be low based on frictional studies of shallow fault material. Here, we present a new method for inferring the slip rate deficit from geodetic data that accounts for the stress shadow cast by frictionally locked patches, and show that this approach greatly improves our offshore resolution. We apply this technique to the Cascadia and Japan Trench megathrusts and find that, wherever locked patches are present, the shallow fault generally has a slip rate deficit between 80 and 100% of the plate convergence rate, irrespective of its frictional properties. This finding rules out areas of low kinematic coupling at the trench considered by previous studies. If these areas of the shallow fault can slip seismically, the global tsunami hazard could be higher than currently recognized. Our method identifies critical locations where seafloor observations could yield information about frictional properties of these faults so as to better understand their slip behaviour.

大多数破坏性海啸是由近海巨型逆冲断层浅部的地震滑动引起的。这种行为的可能性部分取决于震间滑移率不足，根据对浅层断层材料的摩擦研究，通常假设该滑移率不足。在这里，我们提出了一种从大地测量数据中推断滑移率赤字的新方法，该方法解释了摩擦锁定斑块投射的应力阴影，并表明这种方法大大提高了我们的海上分辨率。我们将这项技术应用于卡斯卡迪亚和日本海沟巨型逆冲断层，发现无论哪里存在锁定斑块，无论其摩擦特性如何，浅层断层的滑移率通常都在板块收敛率的 80% 到 100% 之间。这一发现排除了先前研究考虑的沟槽中的低运动耦合区域。如果浅层断层的这些区域可以发生地震滑动，那么全球海啸危险可能会高于目前的认识。我们的方法确定了海底观测可以产生有关这些断层的摩擦特性信息的关键位置，以便更好地了解它们的滑动行为。