Workshops for Establishing a Stable North American Reference Frame (SNARF) to Enable Geophysical and Geodetic Studies with EarthScope

EarthScope的板块边界观测站(PBO)组件将利用大地测量学来测量由板块构造和岩浆活动驱动的地壳缓慢变形。具体地说，将使用全球定位系统(GPS)测量跨越北美-太平洋板块边界的大约1000个点的移动。这些运动必须相对于地球参照系进行定义。这样的框架需要定义其笛卡尔坐标轴(包括原点、方向和比例)和这些轴的时间演变，以及动态地球的精确模型。跨越北美-太平洋板块边界(板块边界观测站的焦点)的区域由于构造过程而引起的地球表面的运动，最自然地相对于北美或太平洋板块的稳定内部表现出来。因此，标准参考框架将更容易解释大地测量数据，即相对板块运动的总预算被容纳在哪里，以及板块边界动力学渗透到板块内部的深度有多深。它还将提供一个通用框架，用以比较不同分析小组的结果。出于这些原因，联阿援助团成立了稳定的北美参照系工作组，以确定用于PBO的参照系。将在两年内举办一系列由国家科学基金会资助的三到四个小型讲习班，以发展snarf并教育社区。SNARF工作组由大约12个大地测量仪组成，他们在开发和测试参照系方面都有专门知识。SNARF工作组与国际地球参考框架(ITRF)以及美国和加拿大的国家大地测量的开发人员有重要联系，这些开发人员承诺与大学研究界合作开展这项工作。就地球望远镜的科学利益而言，地球参考系的准确实现将为测得的台站运动增加重要的解释价值。SNARF将能够更有力地确定与地震周期有关的地壳应变变化，并将为比较大地测量数据和地球物理模型提供一个共同框架。要定义一个亚毫米级别的稳定框架，就需要充分描述整个“稳定板块内部”的地球变形过程，这个区域从定义上讲相对不受板块边界过程的影响。这个板块内部提供了一个稳定的平台，从这里可以观察板块边界的变形。尽管它的名字叫稳定的板块内部，但由于冰川均衡调整和其他地幔规模的过程等现象，加上不均匀的岩石圈偶尔会发生大的板内地震，稳定的板块内部实际上以一种非常复杂的方式变形。直到最近，在参考框架的基本模型中，这种缓慢的板内过程一直被忽视。SNARF研讨会的成果将是出版的一系列逐步改进的参照系，这些参照系准确地定义了一组代表“稳定的北美”的站的精确坐标和时间演变。SNARF工作组将提供工具和产品，用于在EarthScope参照系中进行模型计算和模型数据比较。地球范围倡议将大大扩大利用大地测量技术研究太平洋-北美板块边界的社区。因此，SNARF工作组不仅将制定一个准确和稳定的参照系，而且还将适当地向这一更大的科学界介绍该参照系的使用情况。在蒙特利尔举行的2004年联合大会(“春季AGU”)会议上举行的一次特别会议将重点介绍SNARF的研究和开发，并将向今后的用户通报工作组的进展情况，并将鼓励科学界对这一进展作出反馈。有关正确使用框架的教育信息也将在网上提供。

The Plate Boundary Observatory (PBO) component of EarthScope will use the science of geodesy to measure the slow deformations in the Earth's crust that are driven by plate tectonics and magmatism. Specifically, the Global Positioning System (GPS) will be used to measure the movements of approximately 1000 points spanning the North America-Pacific plate boundary. These motions must be defined relative to a terrestrial reference frame. Such a frame requires the definition of its Cartesian coordinate axes (including origin, orientation, and scale) and the evolution of these axes in time, as well as precise models of the dynamic Earth. The motions of the Earth's surface due to tectonic processes the region spanning the North America-Pacific plate boundary (the focus of the Plate Boundary Observatory) are most naturally expressed with respect to the stable interiors of either the North America or Pacific plates. A standard reference frame will therefore make it easier to interpret the geodetic data in terms of where the total budget of relative plate motion is being accommodated and how deeply the plate boundary dynamics penetrate into the plate interior. It will also provide a common frame by which to compare results from different analysis groups. For these reasons, UNAVCO has formed the Stable North American Reference Frame (SNARF) working group to define the reference frame to be used for PBO. A series of three to four small NSF-funded workshops will be held over a two year period to develop SNARF and to educate the community about it. The SNARF working group is comprised of approximately a dozen geodesists with expertise in both developing and testing reference frames. The SNARF working group has significant links to developers of the International Terrestrial Reference Frame (ITRF) and the national geodetic surveys of the U.S. and Canada, who have committed experts to collaborate with the university research community toward this effort. In terms of science benefits to EarthScope, the accurate realization of the terrestrial reference frame will add significant interpretive value to measured station motions. SNARF will enable a more robust determination of changes in strain in the Earth's crust associated with the earthquake cycle, and will provide a common framework for comparison of geodetic data and geophysical models. Defining a stable frame at the sub-millimeter level requires adequate characterization of Earth deformation processes across the "stable plate interior," a region that by definition is relatively unaffected by plate boundary process. This plate interior provides a stable platform from which to view plate boundary deformation. Despite its name, the stable plate interior actually deforms very slowly in a complex way due to phenomena such as glacial isostatic adjustment and other mantle-scale processes, coupled to a heterogeneous lithosphere which is occasionally host to large intra-plate earthquakes. Until recently, such slow intra-plate processes have been ignored in the underlying models of reference frames. The outcome of the SNARF workshops will be a published series of incrementally improved reference frames that accurately define the precise coordinates and time evolution of a set of stations representing "stable North America." The SNARF working group will provide tools and products for performing model calculations and model-data comparisons in the EarthScope reference frame. The EarthScope initiative will significantly broaden the community using geodetic techniques to study the Pacific-North American plate boundary. Therefore the SNARF working group will not only develop an accurate and stable reference frame, but will also properly describe the use of that reference frame to this larger scientific community. A special session at the 2004 Joint Assembly ("Spring AGU") Meeting in Montreal will highlight research and development into SNARF, and will inform future users of developments by the working group, and will encourage feedback into that development from the scientific community. Educational information on using the frame correctly will also be made available online.