The unknown onshore extent of megathrust earthquake rupture in the Cascadia subduction zone represents a key uncertainty in earthquake hazard for the Pacific Northwest that is governed by the physical state and mechanical properties of the plate interface. The Cascadia plate interface is segmented into an interseismically locked zone located primarily offshore that is expected to rupture in large earthquakes, a region of aseismic slow slip at greater depth, and an intervening transition zone of uncertain rupture potential. Here we image the evolution of the ratio of seismic compressional to shear wave velocities from the locked zone to the transition zone, which is related to changes in fluid content of the plate boundary zone, using a dense onshore–offshore seismic dataset from southernmost Cascadia. The locked zone shows evidence of high fluid content implying a high porosity, yet the downdip transition zone shows an order of magnitude lower porosity. This strong variation is consistent with models that contain a ductile region between the earthquake rupture and slow slip zones that would inhibit onshore propagation of future large earthquake ruptures and hence reduce seismic hazard.

卡斯卡迪亚俯冲带中未知的陆上大逆冲地震破裂范围代表了太平洋西北地区地震危险的关键不确定性，该不确定性受板块界面的物理状态和力学特性的控制。卡斯卡迪亚板块界面被分割成一个主要位于近海的震间锁定带，预计将在大地震中破裂，一个在更深处发生抗震缓慢滑动的区域，以及一个具有不确定破裂潜力的中间过渡带。在这里，我们使用来自卡斯卡迪亚最南端的密集陆上 - 海上地震数据集，对锁定区到过渡区的地震纵波与横波速度比的演变进行了成像，这与板块边界带流体含量的变化有关。锁定带显示出高流体含量意味着高孔隙度的证据，但下倾过渡带显示出低一个数量级的孔隙度。这种强烈的变化与在地震破裂和缓慢滑移带之间包含延性区域的模型一致，该区域将抑制未来大地震破裂的陆上传播，从而降低地震危险。