Improving subduction zone thermal models by including hydrothermal circulation in subducting crust

俯冲带是一个构造板块移动到另一个构造板块之下的地方，是地球上最大地震的发生地。 在俯冲带中，分隔两个构造板块的断层的物理和化学条件沿着主要受温度控制;断层温度随着地表以下深度的增加而增加。 对岩石间摩擦力的研究表明，只有在断层温度在150到350摄氏度之间的地方才能发生地震。 这些温度限制与特定地点的断层温度估计相结合，以概述俯冲带地震的潜在地区。 这些潜在的地震区域被用来估计俯冲带上的沿海地区（例如，华盛顿、俄勒冈州和北方加州）的地面震动和海啸危险。大多数现有的俯冲带温度估计不包括流经海洋地壳含水层的水的热效应。 这些流动的水可以将大量的热量从俯冲带的一部分转移到另一部分，从而影响断层带的温度。 拟议的研究将通过考虑流体流动的影响来改善俯冲带的温度估计。这项研究的结果将用于改善俯冲带（包括美国太平洋西北部）的地震危险性估计-这是一个直接的社会效益。精确的俯冲带热模型对于了解关键的变质和孕震过程是必要的。最近对南海边缘（日本南部）的一项研究表明，以前的热模型忽略了一个对俯冲带温度产生巨大影响的过程-俯冲地壳玄武岩基底含水层内的热液循环。对于南开边缘，热液循环解释了长期存在的，大的，神秘的热异常，并降低了孕震区温度高达100摄氏度相对于没有热液循环的情况下。这项研究将测试的假设，在俯冲地壳类似的流体循环是一个重要的控制俯冲带温度的四个边缘能够产生M9+地震：卡斯卡迪亚，阿拉斯加，智利和苏门答腊。我们将开发这些俯冲带的热模型，使用传导代理来模拟海洋地壳含水层中剧烈流体循环的影响[例如，Spinelli和Wang，2008; 2009]。 数值模型将受到表面热通量观测和主要板块蚀变位置的限制，这项研究将改进俯冲带的热模型。这将使我们能够检验板块边界断层孕震带从~150-350 ℃延伸的假设。南海边缘孕震带与~150-350 ℃温度的重合已被证明，其中热模型包括流体循环的影响。确定这种关系是否适用于众多的俯冲带，将推动热定义孕震区的概念超越猜测，或证明无法根据俯冲带温度来描绘孕震区。拟议的研究有可能改变俯冲带研究的一些途径，因为变质反应进展的预测和断裂带过程的解释，随后从早期的热模型，没有考虑俯冲地壳中的流体循环可能需要重新审视。

Subduction zones, where one tectonic plate moves under another, are the locations of the Earth's largest earthquakes. The physical and chemical conditions along a fault separating two tectonic plates in a subduction zone are controlled largely by temperature; fault temperatures increase with depth below the surface of the Earth. Studies of friction between rocks suggest that earthquakes can be generated only where fault temperatures are between 150 and 350 C. These temperature limits have been combined with site-specific estimates of fault temperatures to outline potential areas of subduction zone earthquakes. These potential earthquake areas are used to estimate the ground-shaking and tsunami hazards for coastal regions overlying subduction zones (for example, Washington, Oregon, and northern California).Most existing subduction zone temperature estimates do not include the thermal effects of water flowing through an aquifer in the ocean crust. This flowing water can move large amounts of heat from one part of a subduction zone to another, affecting fault zone temperatures. The proposed study will improve temperature estimates in subduction zones by accounting for the effects of fluid flow. Results of this research will be used to improve seismic hazard estimates for subduction zones (including the U.S. Pacific Northwest) - a direct societal benefit.Accurate subduction zone thermal models are necessary to understand key metamorphic and seismogenic processes. A recent study of the Nankai margin (southern Japan) shows that previous thermal models had neglected a process that dramatically influences subduction zone temperatures - hydrothermal circulation within the basaltic basement aquifer of subducting crust. For the Nankai margin, hydrothermal circulation explains longstanding, large, enigmatic thermal anomalies and reduces seismogenic zone temperatures by up to 100 C relative to a case without hydrothermal circulation. This study will test the hypothesis that similar fluid circulation in subducting crust is an important control on subduction zone temperatures for four margins capable of producing M9+ earthquakes: Cascadia, Alaska, Chile, and Sumatra. We will develop thermal models for these subduction zones using a conductive proxy to simulate the effects of vigorous fluid circulation in an ocean crust aquifer [e.g., Spinelli and Wang, 2008; 2009]. The numerical models will be constrained by surface heat flux observations and the location of major slab alteration.This study will result in improved thermal models for subduction zones. This will allow us to test the hypothesis that the seismogenic zone of the plate boundary fault extends from ~150-350 C. The coincidence of the seismogenic zone with temperatures of ~150-350 C has been demonstrated for the Nankai margin, where thermal models include the effects of fluid circulation. Determining if this relationship applies to numerous subduction zones will either advance the concept of a thermally defined seismogenic zone beyond conjecture or demonstrate the inability to delineate a seismogenic zone based on subduction zone temperatures. The proposed research has the potential to transform a number of avenues of subduction zone research, as the predictions of metamorphic reaction progress and interpretation of fault zone processes that followed from earlier thermal models that did not account for fluid circulation in subducting crust may need to be revisited.This award was supported by the Geophysics Program and EPSCoR.