Collaborative Research: Anisotropy and Mantle Flow Beneath Japan from Seismological Observations and Geodynamical Modeling

弹性各向异性的正确表征有助于理解上地幔以及地球深部其他区域和活动构造区之下构造变形的原因、机制和后果。不幸的是，我们对地震各向异性的理解仍然不令人满意，对各向异性(如横波双折射)的地震学诊断解释往往是模棱两可的。因此，充分了解复杂构造区下的地震各向异性和流动，仍然是现代地震学和地球动力学的一个突出挑战。在与法国同事的合作下，麻省理工学院和科罗拉多矿业学院的科学家们提议通过显式集成强大的数据集以及正反向建模工具来应对这一挑战。特别是，它们旨在提高对俯冲带附近上地幔三维地震各向异性和构造变形的理解，俯冲带是具有特别社会意义的构造区域，因为它们与重大的火山和地震灾害有关。他们将把重点放在日本，那里的地球物理仪器密集，因为需要持续监测地震和火山活动。多亏了日本的科学家，这些密集的网络可以通过互联网获得数据，麻省理工学院-CSM团队已经从宽带地震台收集了大量数据集。通过检查约12,000张个人地震记录，他们编制了一个数据库，其中包括S、SKS/SKKS和SCS的约1330条高质量剪切波分裂测量数据，稍后还将增加更多。他们建议通过结合两种互补的方法来改进对分裂图的定量解释。首先，他们希望将他们对上地幔流动的数值模拟从2.5扩展到3-D，包括(演变的)速度边界条件和真实的几何和流变学。其次，他们计划开发一种新的技术，用于对观测到的剪切波偏振和分裂时间进行层析反演。流动模型将被用来设计反演的参数化和正则化，反过来，地震成像结果将被用来更新流动模型。这种明确的跨学科方法不仅将为日本地下的三维各向异性和流场带来新的洞察。事实上，用于数据分析、建模和解释的强大的新工具和概念也可以应用于来自美国阵列/地球镜等新倡议的数据。

Proper characterization of elastic anisotropy can help understand the causes, mechanisms, and consequences of tectonic deformation in the upper mantle and in other regions of the Earth's deep interior and beneath regions of active tectonics. Unfortunately, our understanding of seismic anisotropy is still unsatisfactory and interpretation of seismological diagnostics of anisotropy (such as shear wave birefringence) is often ambiguous. Gaining full understanding of seismic anisotropy and flow beneath complex tectonic regions has, therefore, remained an outstanding challenge of modern seismology and geodynamics. In collaboration with colleagues in France, scientists from MIT and Colorado School of Mines propose to meet this challenge through the explicit integration of powerful data sets and forward and inverse modeling tools. In particular, they aim to improve the understanding of 3-D seismic anisotropy and tectonic deformation in the upper mantle near subduction zones, which are tectonic regions that are of particular societal interest because of their association with significant volcanic and seismic hazard. They will focus on Japan, where geophysical instrumentation is dense because of the need for the continuous monitoring of seismic and volcanic activity.Data from these dense networks can be obtained through the internet, thanks to scientists in Japan, and the MIT-CSM team has already assembled a large data set from 64 broadband seismic stations. Through examination of ~12,000 individual seismograms, they compiled a database of ~1330 high-quality shear wave splitting measurements for S, SKS/SKKS, and ScS, and more will be added later. They propose to improve the quantitative interpretation of the splitting maps by combining two complementary approaches. First, they wish to extend their numerical modeling of upper mantle flow from 2.5 to 3-D, incorporating (evolving) velocity boundary conditions and realistic geometries and rheologies. Second, they plan to develop a novel technique for tomographic inversion of the observed shear wave polarizations and split times. The flow models will be used to design the parameterization and regularization of the inversion, and, in turn, the seismic imaging results will be used to update the flow models. Such an explicitly cross-disciplinary approach will not only yield new insight into the 3-D anisotropy and flow field beneath Japan. Indeed, the powerful new tools and concepts for data analysis, modeling, and interpretation can also be applied to data from such new initiatives as USArray/Earthscope.