Collaborative Research: Imaging Subcontinental-Scale Slip Events near Moho

在传统地震构造学和地球物理成像的基础上增加连续GPS，正在彻底改变我们对活动构造系统的理解，尤其是在俯冲巨型逆冲的弧前区域。实际上，在所研究的每一个例子中，地震间“慢滑事件”（sse）都发生在大面积的俯冲界面上，其力矩释放范围在M6到M7之间。盆地和山脉大地测量网（BARGEN）位于盆地和山脉省的板内造山高原环境，其十年的大地测量结果也显示出类似的行为。美国国家科学基金会的板块边界观测站（PBO）的一个原型，即盆地和山脉大地测量网（BARGEN），记录了构造速度在次大陆尺度上的变化，这些变化出乎意料地大，连贯，而且在现有的岩石圈动力学模型的背景下难以解释。我们最近综合了这些结果和该地区的其他地球物理数据，得出了“巨型滑脱假说”，该假说将观测到的速度变化解释为非同寻常的SSE，这是由沿莫霍断层而不是沿俯冲界面的粘滑行为引起的。该项目的目的是通过收集和分析截至2010年未纳入PBO的BARGEN站点的22个站点子网的数据来验证这一假设。该网络位于加利福尼亚州东部和亚利桑那州、犹他州和内华达州北部的PBO站点之间。该项目充分利用了一个关键地区现有的基础设施资源（一个价值100万美元的连续GPS网络，时间序列可追溯到1999年，此前由能源部支持），否则该地区将不会被调查。我们之前的研究结果预测，在2008-2010年的时间框架内，区域SSE的开始，这应该表现为内华达州相对于北美的站点速度的区域放缓。我们将对GPS数据进行分析，包括：(1)对GPS数据进行基本处理，生成时间序列和速度场，并将其与与巨脱离假设相关的关键PBO点相结合；(2)数据关键子集的高级处理和数据解释。非技术解释板块构造理论预测了地球和地球之间的相对运动。通过重建数百万年海底扩张过程中产生的海底古磁条，研究了美国的地壳构造板块。过去二十年测量的大地运动有力地证实了这一理论。但是，世界上大多数地震和火山爆发发生的板块之间的边界，并不是像板块理论所预测的那样，能容纳所有板块间运动的极薄断层。相反，它们是广泛的复杂变形带，在某些情况下超过1000公里宽。最近最重要的发现之一是“慢滑事件”（sse），它与这些大变形带的物理学有关。就能量释放而言，这些事件的规模与破坏性地震相似。与几秒钟内发生的地震不同，超强地震持续时间从几天到几年不等，因此产生地震波的速度太慢，无法被地震仪探测到。在过去的十年中，只有通过跨越板块边界变形带的连续GPS网络的出现，例如横跨1000公里宽的太平洋-北美板块边界变形带的盆地和山脉大地测量网（BARGEN），才能发现SSEs。考虑到它们对沿板块边界的整体变形预算的重要性，它们的存在正在彻底改变我们对板块边界的物理理解，包括地震的物理问题。到目前为止，几乎所有观测到的sse都发生在俯冲巨型逆冲带上，这是板块之间的接触地带，一个板块潜入另一个板块之下。唯一的例外是BARGEN在1999年开始到2005年在Basin and Range省拍摄的大型SSE图像。与1999年以前相比，内华达GPS测点的构造速度减慢了1毫米/年。2010年的监测将检验盆地和山脉sse是周期性的假设。有了PBO在该地区赞助的大约80个GPS站点的额外好处，我们将获得这些瞬变事件的更详细的运动学图像。

Technical Description of ResearchThe addition of continuous GPS to traditional seismotectonics and geophysical imaging is revolutionizing our understanding of active tectonic systems, most notably in the forearc regions of subduction megathrusts. In virtually every example studied, interseismic 'slow slip events' (SSEs) occur over large patches of the subduction interface, with moment release in the M6 to M7 range. A decade of geodetic results from the Basin and Range Geodetic Network (BARGEN), situated in the intraplate, orogenic plateau setting of the Basin and Range province, has demonstrated similar behavior. A prototype of NSF's Plate Boundary Observatory (PBO) known as the Basin and Range Geodetic Network (BARGEN) has documented subcontinental-scale variations in tectonic velocity that are unexpectedly large, coherent, and difficult to explain in the context of existing models of lithospheric dynamics. Our recent synthesis of these results and other geophysical data in the region have led to the development of the 'megadetachment hypothesis,' which explains the observed velocity variations as an SSE of extraordinary scale, caused by stick-slip behavior along the Moho rather than along a subduction interface. The objective of this project is to test this hypothesis by collecting and analyzing data from a 22-station subnetwork of BARGEN sites not incorporated into PBO, through 2010. This network is situated between PBO sites in eastern California and Arizona, Utah and northern Nevada. This project strongly leverages an existing infrastructural resource (a $1,000,000 continuous GPS network with time series back to 1999, previously supported by the Department of Energy) in a key region that will otherwise not be investigated. Our previous results predict the onset of a regional SSE in the 2008-2010 time frame, which should appear as a regional slowing of site velocities in Nevada relative to North America. We will analyze the GPS data, including (1) basic processing to generate time series and velocity fields, combining them with key PBO sites relevant to the megadetachment hypothesis; and (2) advanced processing and data interpretation of key subsets of the data. Non-Technical ExplanationThe theory of plate tectonics predicts the relative motions between the earth?s tectonic plates, through reconstruction of ancient magnetic stripes on the ocean floor generated by the process of sea-floor spreading over millions of years. Geodetic motions measured over the last two decades strongly confirm the theory. But the boundaries between plates, where most of the world's earthquakes and volcanic eruptions occur, are not razor-thin faults that accommodate all of the interplate motion as predicted by plate theory. Instead, they are broad zones of complex deformation that in some cases are more than 1000 km wide. One of the most important recent discoveries bearing on the physics of these broad zones of deformation are 'slow slip events' (SSEs). In terms of energy release, these events are of similar size to damaging earthquakes. Unlike earthquakes, which occur over a matter of seconds, SSEs last from a few days up to several years, and thus are too slow to generate seismic waves detectable by seismometers. The discovery of SSEs was only possible through the advent of continuous GPS networks built across plate boundary deformation zones over the last decade, such as the Basin and Range Geodetic Network (BARGEN), which straddles the 1000 km-wide Pacific-North America plate boundary deformation zone. Given their importance to the overall deformation budget along plate boundaries, their mere existence is revolutionizing our physical understanding of plate boundaries, including the problem of the physics of earthquakes. Nearly all SSEs observed to date occurred along subduction megathrusts, the zone of contact between plates where one plate dives beneath another. The one exception is a large SSE imaged by BARGEN that began in 1999 and ended in 2005 in the Basin and Range province. The SSE was manifest by the slowing of tectonic velocity of GPS sites across Nevada by 1 mm/yr relative to their pre-1999 rates. Monitoring though 2010 will the test hypothesis that the Basin and Range SSEs are periodic. With the added benefit of some 80 GPS sites in the region sponsored by PBO, we will gain a much more detailed picture of the kinematics of these transient events.