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

测试亚稳橄榄石在俯冲动力学和深部地震中的作用

• 批准号: 2153721
• 负责人: Magali Billen
• 金额: $ 39.37万
• 依托单位: University of California-Davis
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2022
• 资助国家: 美国
• 起止时间: 2022-03-15 至 2025-02-28
• 项目状态: 未结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=2153721&HistoricalAwards=false
• 关键词: Testing; role; metastable; olivine; subduction

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.

地球表面上的板块构造是由地幔固体岩石的对流运动驱动的。地幔热转化是由于化学成分，矿物结构（相）和温度差异引起的密度差异。会议的主要驱动力是由俯冲带中的冷引力下沉，因此是密集的构造板。以前已经提出，在下沉的构造板（称为俯冲）板中，冷温会延迟随着压力增加而发生的矿物结构的正常变化。这会导致板的密度低于仅基于温度的预期。还提出，对于触发在400公里深度的地震中，必须触发地震，这是必要的。这些地震被称为深层地震。但是，在观察到深层地震的所有平板中是否存在亚稳态橄榄石仍然尚不清楚。还有其他可能引发深层地震的机制。在这里，研究人员使用最先进的数值模拟来测试亚稳态橄榄石的存在如何影响俯冲-SLAB动力学。它们集成了建模各种实验和观察参数，并测试不同的情况。他们将模型输出与全球现有俯冲板的观察结果进行了比较。这项研究的结果直接为不同的平板动力学和深层地震的潜在起源提供了信息。新的改进的计算代码与科学界共享。该项目对理解俯冲区的动态具有很大的影响，这是威胁人类社会的最大地震的起源。该项目还为加利福尼亚大学的几名研究生和本科生提供支持和培训 - 戴维斯大学。戴维斯大学最大的问题之一是披风动力的最大问题之一，为什么有些平板似乎在过渡区停滞不前，而另一些则似乎直接陷入了下层。答案与相变和沟槽运动有关，但尚不清楚这些反馈如何使用其他材料特性和较大规模的地幔流以产生平板行为的明显变化。该项目测试了亚稳态橄榄石楔（MOW）在俯冲动力学中的作用及其作为深层地震来源的潜力。该团队首先通过将HEFESTO整合为等效相转换（包括密度和潜在热效应）以及晶粒大小和水依赖性的可抗Elivine橄榄石转化（MOT）模型，从而克服了先前模型的几个模拟，并将其依赖于Peierls Creep（包括Peierls Creep）。模拟在软件方面运行。 2D模型测试具有不同年龄和俯冲速率的平板（由板边界剪切带； PBSZ控制）如何导致不同的俯冲动力学。将平板形态与板和沟渠运动的观察结果与模型输出进行比较，以限制参数的不确定性，包括PBSZ粘度，粘度梯度到下地幔中的梯度以及板的水含量。此外，研究人员将MOW及其重叠的分布与平板的强烈变形区域进行了比较。目的是预测地震性的预期模式。将这种模式与观测值进行比较，并分析了平板的热结构（例如，是指示温暖与冷板的模式）。该团队使用3D模型来量化沟槽宽度的影响以及俯冲板年龄和PBSZ属性的效果变化。虽然这些是通用模型，但选择了最初的条件来代表三个不同的俯冲区：日本 - 伊斯 - 邦宁，汤加 - 凯尔马德和南美。使用日本，Izu-Bonin和Marianas的MOW范围的观察，团队校准了MOT模型，并使用它来预测其他两个区域的MOW范围。将模型结果与使用静止变化的观察结果进行了比较。板形态，板块运动和地震性是对模型参数的进一步限制。该奖项反映了NSF的法定任务，并通过使用基金会的知识分子优点和更广泛的审查标准来通过评估来诚实地通过评估来诚实地支持。