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

“地球范围”的板块边界观测站（PBO）组件将使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。具体来说，全球定位系统（GPS）将用于测量跨越北美-太平洋板块边界的大约1000个点的运动。这些运动必须相对于地面参考系来定义。这样一个框架需要定义它的笛卡尔坐标轴（包括原点、方向和尺度）和这些坐标轴随时间的演变，以及动态地球的精确模型。横跨北美-太平洋板块边界（板块边界观测站的重点）的区域由于构造过程而引起的地球表面运动最自然地表达为北美或太平洋板块的稳定内部。因此，根据相对板块运动的总预算被容纳的位置和板块边界动力学渗透到板块内部的深度，标准参照系将更容易解释大地测量数据。它还将提供一个共同的框架来比较来自不同分析小组的结果。由于这些原因，联阿援助团成立了稳定北美参考框架工作组，以确定用于PBO的参考框架。nsf资助的一系列三到四个小型研讨会将在两年的时间内举行，以发展SNARF并向社区宣传它。SNARF工作组由大约12名具有开发和测试参考系专业知识的测地线师组成。SNARF工作组与国际地面参考框架（ITRF）的开发人员以及美国和加拿大的国家大地测量机构有着重要的联系，他们已经承诺与大学研究界合作开展这项工作。就EarthScope的科学效益而言，地面参考系的准确实现将为测站运动增加重要的解释价值。SNARF将能够更可靠地确定与地震周期相关的地壳应变变化，并将为大地测量数据和地球物理模型的比较提供一个共同框架。定义亚毫米级别的稳定框架需要对“稳定板块内部”的地球变形过程进行充分的描述，根据定义，该区域相对不受板块边界过程的影响。这个板块内部提供了一个观察板块边界变形的稳定平台。尽管它的名字，稳定的板块内部实际上变形非常缓慢，以一种复杂的方式，由于现象，如冰川均衡调整和其他地幔尺度的过程，再加上一个不均匀的岩石圈，偶尔会发生大的板块内地震。直到最近，在参考框架的基础模型中，这种缓慢的板内过程一直被忽视。SNARF研讨会的成果将是发布一系列逐步改进的参考框架，这些参考框架精确地定义了代表“稳定的北美”的一组站点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参考框架中执行模型计算和模型数据比较。EarthScope计划将极大地扩大使用大地测量技术来研究太平洋-北美板块边界的社区。因此，SNARF工作组不仅将开发一个准确和稳定的参考框架，而且还将向这个更大的科学界适当地描述该参考框架的使用。2004年在蒙特利尔举行的联合大会（“春季AGU”）的一次特别会议将突出对SNARF的研究和开发，并将向未来的用户通报工作组的发展情况，并将鼓励科学界对该发展提供反馈。关于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.