Temporal Variations in the Seismogenic Zone, Cook Inlet, Alaska

在过去的十年里，我们已经观察到了各种各样的瞬态慢滑动事件，与产生地震的滑动相比，断层上的滑动非常缓慢，但比构造板块运动和稳定的断层蠕动要快得多。最近，这项研究的调查人员发现了一个短暂的事件，似乎是一个缓慢的滑动事件的相反：一个？锁定事件？其中一段断层的蠕动停止了几年，然后又开始蠕动。或者它代表着一个缓慢事件的结束和另一个的开始。看来，在俯冲带的孕震带下倾端的行为比以前认为的更动态，更容易发生变化。拟议的工作将跟进这一新发现，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，或者是否需要其他解释来解释这些观察结果。然后，我们将确定这些变化是否是由构造震颤或地震活动模式的变化引起的，伴随着这些变化，或随后发生的变化，并评估作用于俯冲巨型逆冲断层的应力变化和由俯冲巨型逆冲断层的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21世纪中期调查的活动GPS站点进行重复调查来增加PBO数据，从而测量速度变化的范围和幅度，构建震源模型，将观察到的变化与板块界面上的滑动分布变化联系起来，评估应力和应力变化以及地震活动率变化，以评估变化的可能原因和影响，并探索这一发现对地震危险性评估的影响。大多数最大的地震（震级大于8级）都发生在俯冲带，其中一个地球？地球的构造板块正在被推到另一个板块之下。地震发生在板块之间界面的浅部，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。为什么它们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全球定位系统测量非常准确地测量出来，可以用来进行这种评估。在地震之间，俯冲带深海海沟向陆地的区域收缩，反映了地球内部储存的能量在未来地震中释放（弹簧的压缩或伸展是一个有用的类比）。但某些俯冲带的变形模式随着时间的推移而变化，这表明板块界面的某些部分交替缓慢滑动并粘在一起。在阿拉斯加南部的库克湾地区，我们现在已经观察到两个相对突然的变化，在2004年底和2010年初的变形模式。对这些观测结果最简单的解释是，板块界面的一部分已经蠕动了几年，然后又开始蠕动。这种行为以前在任何地方都没有被清楚地观察到。它表明，在俯冲带的孕震区较深端的行为比以前认为的更具动态性和变化性。该项目将跟踪这一新发现，并试图解释这些变化的原因。除了通常与科学家的沟通外，阿拉斯加地震危险的相关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州的Kachemak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教育公众和土地管理人员了解构造运动。由于需要评估海平面上升的影响，对特别是关于地壳垂直运动的可靠科学信息的需求日益增加。该项目的研究生将直接参与这些工作，除了他们的研究工作。

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.