The occurrence of intermediate depth seismicity (70–300 km) is commonly attributed to the dehydration of hydrous phases within the downgoing oceanic plate. While some water is incorporated into the oceanic crust at formation, a significant amount of water is introduced into the plate immediately before subduction along outer-rise faults. These faults have been shown to extend to depths of over 30 km and can channel water to depths of 20 km or more beneath the seafloor. However, the amount of water introduced into the oceanic mantle lithosphere, and the role of that water in the formation of intermediate depth seismicity, has been the topic of ongoing research. Here we compile evidence from areas where the subducted oceanic crust is likely thicker than the penetration depth of water into the downgoing plate. These regions comprise aseismic plateaus and ridges (hot spot tracks) that can be compared directly to adjacent segments of the oceanic plate where oceanic crust of normal thickness is subducted. Regions with thick oceanic crust show little to no seismicity at intermediate depths, whereas adjacent regions with normal oceanic crust (∼6–8 km thick) have significant seismicity at similar depths and distances from the trench. We hypothesize that intermediate depth earthquakes observed in regions with thinner oceanic crust are caused by mantle dehydration reactions that are not possible in regions where the oceanic mantle was never hydrated because the thickness of the oceanic crust exceeded the penetration depth of water into the plate. We compare our observations to phase diagrams of hydrous basalt and hydrated depleted peridotite to determine pressures and temperatures that would be consistent with our observations. These can provide valuable constraints, not only on the degree of hydration and dehydration in the downgoing plate, but also as ground-truth for thermal models of these regions, all of which have complex, three-dimensional, time-variant subduction geometries and thermal histories.

中等深度地震活动的发生（70-300 km）通常归因于下沉洋洋板块内含水相的脱水。虽然一些水在形成时被掺入了大洋地壳中，但大量的水在沿外层断层俯冲之前即被引入板中。这些断层已经显示出延伸到30公里以上的深度，并且可以将水引导到海底以下20公里或更长的深度。然而，引入海洋地幔岩石圈的水量以及该水在中深度地震活动形成中的作用一直是正在进行的研究主题。在这里，我们从俯冲的洋壳可能比水对下沉板的渗透深度更厚的地区收集证据。这些区域包括抗震的高原和山脊（热点轨迹），可以直接与俯冲的正常厚度的洋壳相比较的洋盘的相邻部分进行比较。地壳较厚的地区在中间深度处几乎没有地震活动，而正常地壳（约6-8公里厚）的邻近地区在相似的深度和距海沟的距离处具有明显的地震活动。我们假设在地壳较薄的区域中观察到的中深度地震是由地幔脱水反应引起的，而在地幔从未水化的区域是不可能发生的，因为洋地壳的厚度超过了水对板的渗透深度。我们将我们的观察结果与含水玄武岩和水合贫化橄榄岩的相图进行比较，以确定与我们的观察结果一致的压力和温度。这些不仅对下板的水合和脱水程度提供了有价值的约束，而且还为这些区域的热模型提供了依据，所有这些模型都具有复杂的，三维的，时变的俯冲几何结构和热力。历史。