Testing the role of metastable olivine in subduction dynamics and deep earthquakes

地球表面的板块构造是由地幔中固体岩石的对流运动驱动的.地幔热对流是由于化学成分、矿物结构（相态）和温度的差异引起的密度差异。对流的主要驱动力来自于俯冲带内冷的、因此是致密的构造板块的重力下沉。以前有人提出，在下沉的构造板块（称为俯冲板块）内，寒冷的温度可以延迟随着压力增加而发生的矿物结构的正常变化。 这导致板的密度低于仅基于温度的预期。也有人提出，这个密度较低的区域-称为亚稳橄榄石楔-是触发400公里深处发生地震的必要条件。 这些地震被称为深源地震。然而，是否在所有观测到深震的板片中都存在亚稳橄榄石，仍是未知数。 还有其他可能的机制可能引发深源地震。在这里，研究人员使用最先进的数值模拟来测试亚稳橄榄石的存在如何影响俯冲板片动力学。他们在建模中集成了各种实验和观测参数，并测试了不同的场景。他们将模型输出与地球仪周围现有俯冲板块的观测结果进行比较。 这项研究的结果直接告知不同的板块动力学和深源地震的潜在起源。 新改进的计算代码与科学界共享。 该项目对了解俯冲带的动力学具有重要意义，俯冲带是威胁人类社会的最大地震的起源。该项目还为加州-戴维斯大学的几名研究生和本科生提供支持和培训。地幔动力学最突出的问题之一是，为什么有些板块似乎停滞在过渡带内或以下，而另一些板块似乎直接沉入下地幔。答案与相变和海沟运动有关，但目前尚不清楚这些反馈与其他材料特性和大规模地幔流如何产生板行为的明显变化。该项目测试了亚稳橄榄石楔（MOW）在俯冲动力学中的作用及其作为深源地震的潜力。该团队首先通过将平衡相变的HeFESTo（包括密度和潜在热效应，以及粒度和水依赖性亚稳橄榄石转变（MOT）模型）整合到具有粘塑性的完全动态俯冲模拟中，克服了之前模型的几个简化包括Peierls蠕变在内的流变学。模拟在软件Aspect中运行。二维模型测试了不同年龄和俯冲速率（由板块边界剪切带控制; PBSZ）的板块如何导致不同的俯冲动力学。板片形态与板块和沟槽运动的观测结果进行比较，以约束参数，包括PBSZ粘度，下地幔的粘度梯度，和水含量的板片的不确定性的模型输出。此外，研究人员还比较了MOW的分布及其与板块强烈变形区域的重叠;目的是预测预期的地震活动模式。将该图案与观察结果进行比较并考虑板的热结构（例如，是指示热板对冷板的模式）。该团队使用3D模型来量化沟槽宽度的影响以及俯冲板块年龄和PBSZ属性的沿走向变化。虽然这些都是通用的模型，初始条件选择代表三个不同的俯冲带：日本伊豆小笠原，汤加Kermadec和南美洲。利用在日本、伊豆-小笠原和马里亚纳群岛的MOW范围的观测结果，该团队校准了MOT模型，并使用它来预测其他两个地区的MOW范围。该模型的结果进行了比较，观察使用沿走向的变化，板块形态，板块运动，地震活动作为进一步的约束模型parameters.This奖项反映了NSF的法定使命，并已被认为是值得通过评估使用基金会的智力价值和更广泛的影响审查标准的支持。

Plate tectonics on the Earth’s surface is driven by convective motion of solid rocks the mantle. Mantle thermal convection is due to density differences arising from differences in chemical composition, mineral structure (phase), and temperature. The primary driving force for convection arises from gravitational sinking of cold, and therefore dense, tectonic plates within subduction zones. It has previously been proposed that within the sinking tectonic plate (called subducting slab), the cold temperature can delay the normal changes in mineral structure that occur with increasing pressure. This causes the plate to be less dense than expected based on temperature alone. It has also been proposed that this less dense region - called a metastable olivine wedge - is necessary to trigger earthquakes that occur at depths of 400 km. These earthquakes are referred to as deep earthquakes. However, it is still unknown whether metastable olivine exists in all slabs where deep earthquakes are observed. There are also other possible mechanisms that may trigger deep earthquakes. Here, the researchers use state-of-the-art numerical simulations to test how the presence of metastable olivine affects subducting-slab dynamics. They integrate in the modeling various experimental and observational parameters and test different scenarios. They compare the model outputs with observations in existing subducting slabs around the Globe. Results of this study directly inform potential origins of the different slab dynamics and that of deep earthquakes. The new improved computational codes are shared with a scientific community. The project has strong implications for the understanding the dynamics of subduction zones, at the origin of the largest earthquakes threatening human societies. The project also provides support and training for several graduate and undergraduate students at University of California - Davis.One of the biggest outstanding questions of mantle dynamics is why some slabs appear to stagnate in or below the transition zone, while others appear to sink directly into the lower mantle. The answer is related to phase transitions and trench motion, but it is unclear how these feedback with other material properties and larger-scale mantle flow to generate the apparent variability in slab behavior. This project tests the role of a metastable olivine wedge (MOW) in subduction dynamics and its potential to serve as a source for deep earthquakes. The team first overcome several simplifications of previous models through integration of HeFESTo for equilibrium phase transitions - including density and latent-heat effects, and a grain-size and water-dependent metastable olivine transformation (MOT) model- into a fully dynamic subduction simulations with a visco-plastic rheology including Peierls creep. The simulations run in the software Aspect. 2D models test how slabs with different ages and rates of subduction (controlled by the plate boundary shear zone; PBSZ) lead to different subduction dynamics. Observations of slab morphology together with plate and trench motion are compared to model output to constrain uncertainty in parameters including PBSZ viscosity, the gradient of viscosity into the lower mantle, and the water content of the slab. In addition, the researchers compare the distribution of the MOW and its overlap with strongly deforming regions of the slab; the aim is to predict the expected pattern of seismicity. This pattern is compared to observations and analyzed accounting for the thermal structure of the slab (e.g., is the pattern indicative of warm versus cold slabs). The team uses 3D models to quantify the impact of trench width together with along-strike variation in subducting plate age and PBSZ properties. While these are generic models, the initial conditions are chosen to represent three different subduction zones: Japan-Izu-Bonin, Tonga-Kermadec and South America. Using the observations of MOW extent in Japan, Izu-Bonin, and the Marianas, the team calibrates the MOT model, and use it to predict the MOW extent for the other two regions. The model results are compared with observations using along-strike variations in the slab morphology, plate motions, and seismicity as a further constraint on model parameters.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.