Collaborative Research: Imaging Subcontinental-Scale Slip Events near Moho

在传统的地震构造学和地球物理成像中加入连续GPS，正在彻底改变我们对活动构造系统的理解，特别是在俯冲巨型逆冲断层的弧前区域。 在几乎每一个研究的例子中，地震间的“慢滑事件”（SSE）发生在大片的俯冲界面，在M6至M7范围内的时刻释放。 十年的大地测量结果，从盆地和山脉大地测量网络（BARGEN），位于板内，造山高原设置的盆地和山脉省，表现出类似的行为。 美国国家科学基金会的板块边界观测台（PBO）的一个原型，即盆地和山脉大地测量网络（BARGEN），已经记录了次大陆尺度的构造速度变化，这些变化出乎意料地大，连贯，并且难以在现有的岩石圈动力学模型中解释。 我们最近综合了这些结果和该地区的其他地球物理数据，提出了“大拆离假说”，该假说将观测到的速度变化解释为异常规模的SSE，这是由沿着莫霍面而不是沿着俯冲界面的粘滑行为造成的。 该项目的目标是通过收集和分析2010年之前未纳入PBO的BARGEN站点的22个站点子网络的数据来测试这一假设。该网络位于加州东部和亚利桑那州、犹他州和北方内华达州的PBO站点之间。 该项目在一个关键区域大力利用现有的基础设施资源（一个价值100万美元的连续全球定位系统网络，时间序列可追溯到1999年，以前由能源部支助），否则将不会进行调查。 我们以前的研究结果预测，在2008年至2010年的时间框架，这应该出现在内华达州相对于北美的站点速度的区域减慢的区域SSE的开始。 我们将分析GPS数据，包括（1）生成时间序列和速度场的基本处理，将其与与大拆离假设相关的关键PBO站点相结合;以及（2）对数据的关键子集进行高级处理和数据解释。板块构造理论预测了地球与地球之间的相对运动。通过重建数百万年来海底扩张过程中产生的海底古老磁条， 在过去的二十年里测量的大地运动有力地证实了这一理论。 但是，板块之间的边界，世界上大多数地震和火山爆发发生的地方，并不是板块理论所预测的那种容纳所有板间运动的薄断层。 相反，它们是复杂变形的广阔区域，在某些情况下超过1000公里宽。 最近对这些广泛变形区的物理学最重要的发现之一是“慢滑事件”（SSEs）。 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，SSE持续几天到几年，因此产生的地震波太慢，无法被地震仪检测到。 在过去的十年中，只有通过跨板块边界变形带建立的连续GPS网络的出现，才有可能发现SSE，例如盆地和山脉大地测量网络（BARGEN），它跨越了1000公里宽的太平洋-北美板块边界变形带。 鉴于它们对沿着板块边界的整体变形平衡的重要性，它们的存在正在彻底改变我们对板块边界的物理理解，包括地震物理学问题。 迄今为止，几乎所有观测到的SSE都发生在沿着俯冲巨型逆冲断层上，这是板块之间的接触区，一个板块俯冲到另一个板块之下。 唯一的例外是由BARGEN拍摄的一个大型SSE，它始于1999年，结束于2005年，位于盆地和山脉省。 SSE表现为内华达州GPS站点的构造速度相对于1999年前的速率减慢了1 mm/年。 到2010年的监测将检验盆地和山脉SSE是周期性的假设。在PBO赞助的该地区约80个GPS站点的额外好处下，我们将获得这些瞬变事件运动学的更详细的图片。

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.