Collaborative Research: Developing a Three-Dimensional Seismic Reference Earth Model (REM-3D) in Collaboration with the Community

地球内部的弹性特性（如密度、刚性、压缩性等）由于温度、压力、成分和流量的变化而随位置而变化。在20世纪，地球科学家利用地震和爆炸发出的地震波来建立地球性质随深度变化的模型。从这些努力中产生的社区参考模型在地震定位、内部结构成像、极端条件下材料特性的理解以及其他领域（如粒子物理学和天文学）中被证明是不可或缺的。在过去的三十年里，全球各地的地震学家进行了更复杂的研究，已经产生了几代模型，这些模型不仅显示了地震性质是如何随深度变化的，而且还显示了地震性质的横向变化。然而，尽管这些三维（3D）模型在大尺度上表现出令人信服的相似性，但在方法论、结构表示和它们所基于的数据集方面的差异，阻碍了3D社区参考模型的创建。研究人员建议通过编制、协调和分发长周期参考地震数据集来克服这些挑战，他们将根据这些数据集构建地幔的三维地震参考模型（REM-3D）。作为一个社区参考模型，具有完全量化的不确定性和权衡，REM-3D将促进地球成像研究，地震表征，对内部深处温度和成分的推断，并对新兴的科学努力（如中微子地球科学）有更好的效用。研究人员将建立社区工作组，在参考模型和数据集开发过程中提供建议，并将组织一个研讨会来评估进展，评估模型和数据集的性能，确定改进的途径，并建议在深地社区中最大限度地采用模型和效用的策略。为此，研究人员征求了来自美国和世界各地的地震学家、矿物物理学家、地球动力学家和地球化学家的意见。研究人员建议为地幔建立一个三维地震参考模型（REM-3D），以剪切波速（Vs）、纵波速度（Vp）、密度（ρ）和代表径向各向异性的3个附加参数为参数。该模型将开发两个版本，以明确拟合全面的、社区贡献的长周期地震数据集，一个以球面谐波为参数化，另一个作为主要地理省份对应的规范剖面。此外，他们将编制、协调和分发一个长周期参考地震数据集，包括表面波频散测量，长周期绝对和差分体波测量，以及自由振荡频率/衰减/分裂。与以前的地球结构参考模型不同，REM-3D将完全量化。研究人员还将创建用于模型分布和预测各种地震观测的在线工具，包括完整的波形，以及主要为矿物物理学家和地球动力学家设计的工具，使他们能够直接根据参考模型或直接根据参考数据集验证测试模型。最后，研究人员将建立社区工作组并组织研讨会，就模型和数据集的性能提出建议和评估，确定改进的途径，并建议在深地社区中最大化模型采用和效用的策略。REM-3D将促进更广泛的科学界：地震速度和密度的矿物学和热化学解释；2. 地幔异常/非典型构造的识别3. 全球和区域层析模式的比较；4. 地震波形解释，如特定地震相的识别；5. 三维地球结构反演需要初始模型或背景模型；6. 用长周期数据描述震源。社区贡献的参考数据集的构建将使异常地震波传播时间、表面波色散、正模分裂和波形特征的识别成为可能。此外，我们将创建的用于预测输入结构的地震观测数据的工具将能够直接评估矿物物理和地球动力学实验和计算预测的潜在速度结构。

Elastic properties of the Earth's interior (e.g. density, rigidity, compressibility, etc.) vary with location due to changes in temperature, pressure, composition, and flow. In the 20th century, Earth scientists have used seismic waves emitted by earthquakes and explosions to develop models of how Earth properties vary with depth. Community reference models that grew out of these efforts have proven indispensable in earthquake location, imaging of interior structure, understanding material properties under extreme conditions, and as a reference in other fields, such as particle physics and astronomy. Over the past three decades, more sophisticated efforts by seismologists across the globe have yielded several generations of models of how properties vary not only with depth, but also laterally. Yet, though these three-dimensional (3D) models exhibit compelling similarities at large scales, differences in the methodology, representation of structure, and dataset upon which they are based, have prevented the creation of 3D community reference models. The investigators propose to overcome these challenges by compiling, reconciling, and distributing a long period reference seismic dataset, from which they will construct a 3D seismic reference model (REM-3D) for the Earth's mantle. As a community reference model and with fully quantified uncertainties and tradeoffs, REM-3D will facilitate Earth imaging studies, earthquake characterization, inferences on temperature and composition in the deep interior, and be of improved utility to emerging scientific endeavors, such as neutrino geoscience. The investigators will set up community working groups that will serve to advise during the process of reference model and dataset development, and will organize a workshop to assess progress, evaluate model and dataset performance, identify avenues for improvement, and recommend strategies for maximizing model adoption in and utility for the deep Earth community. To this end, the investigators have solicited input from seismologists, mineral physicists, geodynamics, and geochemists from around the United States and internationally. The investigators propose to develop a three-dimensional seismic reference model (REM-3D) for the Earth's mantle, parameterized in terms of shear wavespeed (Vs), compressional wavespeed (Vp), density (ρ), and the 3 additional parameters representing radial anisotropy. Two versions of the model will be developed to explicitly fit the comprehensive, community-contributed long period seismic dataset, one parameterized in terms of spherical harmonics, and the other as canonical profiles corresponding to major geographic provinces. Furthermore, they will compile, reconcile, and distribute a long period reference seismic dataset, including surface wave dispersion measurements, long period absolute and differential body wave measurements, and free oscillation frequencies / attenuation / and splitting. Unlike previous reference models of Earth structure, REM-3D will have fully quantified. The investigators will also create online tools for model distribution and for predicting various seismic observables, including full waveforms, as well as tools designed primarily to enable mineral physicists and geodynamicists a straightforward way of (in)validating test models against this reference model or directly against the reference dataset. Finally, the investigators will set up community working groups and organize workshops that will advise on and evaluate model and dataset performance, identify avenues for improvement, and recommend strategies for maximizing model adoption in and utility for the deep Earth community. REM-3D will benefit the broader scientific community by facilitating: 1. Mineralogical and thermo-chemical interpretation of seismic velocities and density; 2. Identification of anomalous / atypical structures in the Earth's mantle; 3. Comparison of global and regional tomographic models; 4. Seismic waveform interpretation, such as the identification of particular seismic phases; 5. Inversion for 3D Earth structure requires a starting or background model; 6. Earthquake source characterization using long period data. The construction of a community-contributed reference dataset will make possible the identification of anomalous seismic wave travel times, surface wave dispersion, normal mode splitting, and waveform features. Furthermore, the tools for predicting seismic observables from input structures that we will create will enable direct evaluation of potential velocity structures predicted by mineral physics and geodynamics experiments and calculations.