Temporal Variations in the Seismogenic Zone, Cook Inlet, Alaska

在过去的十年里，我们观察到各种短暂的慢滑事件，断层上的滑动与产生地震的滑动相比非常缓慢，但比构造板块运动和稳定的断层蠕动要快得多。最近，这项研究的研究人员发现了一个似乎与慢滑事件相反的瞬变事件：a ？锁定事件吗?其中一段断层的蠕动停止了几年，然后又开始蠕动。或者它代表了一个缓慢事件的结束和另一个事件的开始。在俯冲带上，孕震带的下倾端行为似乎比以前认为的更具动态性和变化性。拟议的工作将继续这一新发现，并试图解释这些变化的原因。研究人员将更精确地确定俯冲板块界面的哪一部分改变了它的行为，或者是否需要一些其他的解释来解释这些观察结果。然后，我们将确定这些变化是由构造震动或地震活动模式的变化引起的，伴随的，还是紧随其后的，并评估作用于俯冲大逆冲断层上的滑动和由滑动引起的应力变化。具体而言，我们将通过对20世纪90年代至2000年代中期的运动GPS站点的重复调查来增加PBO数据，测量速度变化的程度和幅度，构建震源模型，将观测到的变化与板块界面滑动分布的变化联系起来，评估应力和应力变化以及地震活动率的变化，以评估变化的可能原因和影响。并探讨这一发现对地震危害评估的影响。大多数最大的地震（震级大于8级）发生在俯冲带，在那里地球的一个？美国的构造板块被推到另一个板块之下。地震发生在板块之间界面的浅层部分，从表面（通常是海底）延伸到大约30-40公里（18-25英里）深。大地震和海啸的危害使得更好地了解是什么控制了这些地震破裂的程度变得至关重要。为什么地震会在一定的深度停止，我们如何在破坏性地震发生之前评估地震的可能性？地球的变形模式可以用高精度的GPS测量非常精确地测量出来，可以用来进行这样的评估。在两次地震之间，俯冲带的深海海沟向陆地的区域收缩，这反映了地球内部储存的能量将在未来的地震中释放出来（压缩或拉伸弹簧是一个有用的类比）。然而，某些俯冲带的变形模式随时间而变化，这表明板块界面的某些部分交替缓慢滑动和粘在一起。在阿拉斯加南部的库克湾地区，我们现在已经观察到2004年底和2010年初变形模式的两次相对突变。对这些观察结果最简单的解释是，板块界面的一部分停止爬行了几年，然后又开始爬行。这种行为以前在任何地方都没有被清楚地观察到。这表明，在俯冲带的孕震带较深端的行为比以前认为的更具动态性和易变性。该项目将继续这一新发现，并试图解释这些变化的原因。除了通常与科学家的沟通外，与阿拉斯加地震危险有关的信息将直接传达给阿拉斯加地震安全委员会。我们将与阿拉斯加荷马的卡切马克湾研究保护区、基奈峡湾和克拉克湖国家公园以及阿拉斯加气候评估和政策中心合作，通过公开讲座和网络研讨会向公众和土地管理者传授构造运动知识。由于需要评估海平面上升的影响，越来越需要关于地壳垂直运动的可靠科学资料。这个项目的研究生除了他们的研究工作外，还将直接参与这些努力。

Over the last decade we have observed a variety of transient slow slip events, episodes of slip on a fault that are very slow compared to the slip that generates earthquakes, but significantly faster than tectonic plate motions and steady fault creep. Recently, the investigators of this study have detected a transient event that appears to be the opposite of a slow slip event: a ?locking event? in which a section of a fault that had been creeping stopped for a few years, and then began creeping again. Or perhaps it represented the end of one slow event and the start of another. It appears that the behavior of the downdip end of the seismogenic zone at subduction zones is more dynamic and subject to change than previously thought. The proposed work will follow up on this new discovery, and attempt to explain the cause of these changes. The researchers will determine more precisely what part of the subduction plate interface changed its behavior, or whether some other explanation is required to explain the observations. We will then determine whether these changes were led by, accompanied by, or followed by changes in tectonic tremor or seismicity patterns, and evaluate the stress changes acting on and caused by slip on the subduction megathrust. Specifically, we will measure the extent and magnitude of velocity changes, by augmenting the PBO data with repeat surveys of campaign GPS sites surveyed from the 1990s to mid-2000s, construct source models to relate the observed changes to changes in the slip distribution on the plate interface, evaluate stress and stressing changes and seismicity rate changes, to evaluate possible causes and effects of the changes, and explore the implications of this discovery for earthquake hazard assessments.Most of the largest earthquakes (magnitude larger than 8) occur at subduction zones, where one of Earth?s tectonic plates is being thrust beneath another. Earthquakes occur in the shallow part of the interface between the plates, extending from the surface (usually the seafloor) to about 30-40 km (18-25 miles) depth. The hazard from great earthquakes and tsunamis makes it critical to understand better what controls the extent of these earthquake ruptures ? why do they stop at certain depths, and how can we assess earthquake potential before destructive earthquakes occur? The pattern of deformation of the Earth, which can be measured very accurately with high precision GPS measurements, can be used to make such assessments. Between earthquakes, the region landward of the deep-sea trench of the subduction zone contracts, which reflects the storage of energy within the earth to be released in future earthquakes (compression or extension of a spring is a useful analogue).However, the deformation pattern at some subduction zones changes with time, which suggests that some parts of the plate interface are alternately slipping slowly and sticking together. In the Cook Inlet region of southern Alaska, we have now observed two relatively abrupt changes in the deformation pattern, in late 2004 and early 2010. The simplest explanation for these observations is that a section of the plate interface that had been creeping stopped for a few years, and then began creeping again. This behavior had not been clearly observed anywhere before. It demonstrates that the behavior of the deeper end of the seismogenic zone at subduction zones is more dynamic and subject to change than previously thought. The project will follow up on this new discovery, and attempt to explain the cause of these changes.Information relevant to earthquake hazards in Alaska will be communicated directly to the Alaska Seismic Safety Commission, in addition to the usual communication to scientists. We will work with the Kachemak Bay Research Reserve in Homer, Alaska, Kenai Fjords and Lake Clark National Parks, and the Alaska Center for Climate Assessment and Policy to educate the public and land managers about tectonic motions through public lectures and webinars. There is a growing need for reliable scientific information about vertical crustal motions in particular, due to the need to assess the impacts of sea level rise. The graduate student for this project will be directly involved in these efforts in addition to their research work.