P ‐to‐S converted waves observed in controlled‐source, multicomponent ocean bottom seismometer (OBS) records were used to derive the Vp/Vs structure of Cascadia Basin sediments. We used P ‐to‐S waves converted at the basement to derive an empirical function describing the average Vp/Vs of Cascadia sediments as a function of sediment thickness. We derived one‐dimensional interval Vp/Vs functions from semblance velocity analysis of S ‐converted intra‐sediment and basement reflections, which we used to define an empirical Vp/Vs vs. burial depth compaction trend. We find that seaward from the Cascadia deformation front, Vp/Vs structure offshore northern Oregon and Washington shows little variability along strike, while the structure of incoming sediments offshore central Oregon is more heterogeneous and includes intermediate‐to‐deep sediment layers of anomalously elevated Vp/Vs . These zones with elevated Vp/Vs are likely due to elevated pore fluid pressures, although layers of high sand content intercalated within a more clayey sedimentary sequence, and/or a higher content of coarser‐grained clay minerals relative to finer‐grained smectite could be contributing factors. We find that the proto‐décollement offshore central Oregon develops within the incoming sediments at a low‐permeability boundary that traps fluids in a stratigraphic level where fluid overpressure exceeds 50% of the differential pressure between the hydrostatic and the lithostatic pressure. Incoming sediments with the highest estimated fluid overpressures occur offshore central Oregon where deformation of the accretionary prism is seaward vergent. Conversely, landward vergence offshore northern Oregon and Washington correlates with more moderate pore pressures and laterally homogeneous Vp/Vs functions of Cascadia Basin sediments.

中文翻译：

---

卡萨迪亚俯冲带外来沉积物的Vp / Vs比，来自受控源多组分OBS记录的分析

在受控源多组分海底地震仪（OBS）记录中观察到的P到S转换波被用来推导卡斯卡迪亚盆地沉积物的Vp / Vs结构。我们使用P -到-小号波转换在地下室导出描述平均经验函数VP / Vs的卡斯卡迪亚沉积物作为沉积厚度的函数。我们从S转换后的沉积物和基底反射的相似速度分析中得出了一维间隔Vp / Vs函数，我们用它来定义经验Vp / Vs与埋深压实趋势。我们发现，从卡斯卡迪亚形变前缘向海，俄勒冈北部和华盛顿近海的Vp / Vs结构沿走向显示出很小的变化，而俄勒冈州中部近海的进入沉积物结构则更加不均一，包括异常高Vp的中深层沉积层。/ Vs的。这些区域的Vp / Vs升高可能是由于孔隙流体压力升高造成的，尽管相对于细粒蒙脱石而言，高砂含量的层插在更黏性的沉积层序中，和/或较粗粒的粘土矿物含量更高。我们发现，俄勒冈州中部的原始褶皱沉积物在进入的沉积物中在低渗透率边界内发育，该边界将流体截留在地层水平，其中流体超压超过静水压与岩石静压差的50％。估计的流体超压最高的进入沉积物发生在俄勒冈州中部近海，增生棱镜的变形是向海的。相反，俄勒冈州北部和华盛顿州近海陆缘聚散与较适中的孔隙压力和横向均一性相关卡斯卡迪亚盆地沉积物的Vp / Vs函数。