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.

地球表面的板块构造是由地幔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)和温度不同而产生的密度差异所致。对流的主要驱动力来自于俯冲带内寒冷的、因此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)内，低温可以延缓随着压力增加而发生的矿物结构的正常变化。这会导致平板的密度低于仅基于温度的预期。还有人提出，这种密度较低的区域--称为亚稳定橄榄石楔形--对于触发400公里深处的地震是必要的。这些地震被称为深部地震。然而，目前还不清楚是否所有观测到深震的板块中都存在亚稳橄榄石。还有其他可能触发深部地震的机制。在这里，研究人员使用最先进的数值模拟来测试亚稳橄榄石的存在如何影响俯冲板块的动力学。它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。他们将模型的输出与地球上现有俯冲板块的观测结果进行了比较。这项研究的结果直接为不同的板块动力学和深部地震的潜在来源提供了信息。新改进的计算代码与科学界共享。该项目对理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地。该项目还为加州大学戴维斯分校的几名研究生和本科生提供支持和培训。地幔动力学最大的悬而未决的问题之一是，为什么一些板块似乎停滞在过渡带或过渡带以下，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，但尚不清楚这些反馈与其他物质属性和更大规模的地幔流动如何产生板片行为的明显可变性。该项目测试了亚稳橄榄石楔体(MOW)在俯冲动力学中的作用及其作为深部地震震源的可能性。该团队首先克服了以前模型的几个简化，将平衡相变的HeFESTo-包括密度和潜热效应，以及依赖于颗粒大小和水的亚稳橄榄石转变(MOT)模型-集成到具有包括Peierls蠕变在内的粘塑性流变的完全动态俯冲模拟中。仿真运行在软件方面。2D模型测试了不同年龄和俯冲速率(受板块边界剪切带控制)的板块如何导致不同的俯冲动力学。将观测到的板块形态以及板块和海沟运动与模型输出进行比较，以约束参数的不确定性，包括PBSZ粘度、进入下地幔的粘度梯度和板坯的水分含量。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。将该模式与观测结果进行比较，并考虑到板坯的热结构进行分析(例如，是指示冷板坯与热板坯的模式)。该团队使用3D模型来量化海沟宽度的影响，以及俯冲板块年龄和PBSZ性质的沿走向变化。虽然这些都是通用的模型，但选择的初始条件代表了三个不同的俯冲带：日本-伊祖-博宁、汤加-克尔马德克和南美洲。利用在日本、伊豆-博宁和马里亚纳群岛观测到的MOW范围，研究小组校准了MOT模型，并用它来预测其他两个地区的MOW范围。模型结果与使用板块形态、板块运动和地震活动中的沿走向变化作为对模型参数的进一步约束的观测结果进行了比较。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。