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Collaborative Research: Seismic Imaging of the Denali fault zone, Central Alaska

合作研究：阿拉斯加中部德纳利断裂带的地震成像

基本信息

批准号：
1736223
负责人：
Carl Tape
金额：
$ 3.05万
依托单位：
University of Alaska Fairbanks Campus
依托单位国家：
美国
项目类别：
Standard Grant
财政年份：
2017
资助国家：
美国
起止时间：
2017-08-01 至 2020-07-31
项目状态：
已结题

来源：
https://www.nsf.gov/awardsearch/showAward?AWD_ID=1736223&HistoricalAwards=false
关键词：
Collaborative Research Seismic Imaging Denali

项目摘要

The Denali Fault in Central Alaska is one of the longest in the world and it has been critical to the formation of North America since the time of dinosaurs. The fault is an important source of hazard - it caused a magnitude 7.9 earthquake in 2002 - but only a handful of geophysical studies have examined its structure. We know very little about how the fault is shaped and behaves within the Earth's crust, and we do not know how the rocks change along the length of the fault. We use new 3D imaging techniques to see the structure of the fault 2-50 kilometers below the surface, which help us to determine the geologic history of Alaska. To add further detail to our imaging, we are placing hundreds of coffee-can sized seismometers in two separate locations to measure the shaking from earthquakes both locally and from across the planet. This allows much more accurate estimates of earthquake hazards for the region's residents and millions of annual visitors. On a larger scale, it shows us how faults work at the edges of continents, and whether this fault forms the true edge of the North American geologic plate. Our use of inexpensive, portable instruments and our techniques to decode specific earthquake wave types are likely to change the way people study fault zones around the world in the future. Technical Summary: We are imaging the Denali fault at regional scale using joint tomographic inversion of body and surface waves, and to locally image the damage zone of the 2002 magnitude 7.9 Denali earthquake. This work is motivated by several unanswered questions specifically about the Denali fault as well as general fault properties. Does the fault have a signature in the Moho or upper mantle? Are there persistent contrasts in across-fault velocity in the crust? What is the relative tectonic importance of the different fault strands? Are there unique damage zone patterns that result from supershear ruptures? Where are the preferred nucleation sites for large earthquakes, and what are the patterns of shaking that can be expected? To date, only a handful of geophysical studies have examined the Denali fault in isolated sections as part of regional-scale linear transects across central Alaska. Multiple tomographic studies have successfully imaged the structure of the subduction zone and underlying Yakutat plate beneath the region, but they have not achieved sufficient horizontal resolution to image the crustal and upper mantle structure of the Denali fault. There is little constraint on the fault zone structure at depth and the along-strike variation of deformation patterns. To resolve the regional-scale structure of the Denali fault, we are tomographically imaging the fault from the surface to 70km depth using newly-developed joint inversion techniques combining ambient noise Rayleigh wave phase velocities and body wave double-difference measurements which achieve kilometer-scale resolution in both Vp and Vs. In the process, we are also building improved catalogs of arrival times for P, S, and fault zone head waves. At the local scale, no seismological study has been conducted to examine the fault damage zone and principal slip surface structure. At other major strike-slip faults, linear or two-dimensional arrays of seismometers with spacing less than 100m have successfully detected fault zone head waves that propagate along (semi-)vertical interfaces with contrasting velocities and fault zone trapped waves confined inside fault-related low-velocity zones consisting of damaged rocks. These two seismic phases, unique to fault zones, can be exploited to provide constraint on fault structure relevant to the mechanics of earthquake rupture. Leveraging the recent development of low-cost highly-portable three-component seismometers, we are deploying hundreds of instruments in dense fault-straddling arrays for one month each at two locations along the Denali fault. The density and richness of the first dataset, which we've already recorded, are unique not just to the Denali fault zone, but to all major strike-slip faults worldwide. Our preliminary analysis, including trapped wave detection and high-frequency ambient noise cross-correlation, contains critical information about the damage zone structure in a section of the fault that recently slipped in a supershear rupture. These data demonstrate the feasibility of further analysis including ambient noise tomography, trapped wave normal modes, and fault zone head wave detection.

阿拉斯加中部的德纳里断层是世界上最长的断层之一，自恐龙时代以来，它对北美的形成起着至关重要的作用。该断层是一个重要的危险来源——它曾在2002年引发7.9级地震——但只有少数地球物理研究对其结构进行了检查。我们对断层的形状和地壳内部的活动知之甚少，我们也不知道沿着断层的长度岩石是如何变化的。我们使用新的3D成像技术来观察地表以下2-50公里的断层结构，这有助于我们确定阿拉斯加的地质历史。为了给我们的成像增加更多的细节，我们在两个不同的地方放置了数百个咖啡罐大小的地震仪，以测量当地和全球地震带来的震动。这使得该地区的居民和每年数以百万计的游客能够更准确地估计地震的危害。在更大的范围内，它向我们展示了大陆边缘的断层是如何工作的，以及这个断层是否形成了北美地质板块的真正边缘。我们使用廉价的便携式仪器和技术来解码特定的地震波类型，这可能会改变人们未来研究世界各地断层带的方式。技术总结：我们正在利用体波和面波联合层析成像技术在区域尺度上对德纳里断层进行成像，并对2002年德纳里7.9级地震的破坏区进行局部成像。这项工作的动机是几个悬而未决的问题，特别是关于德纳里断层以及一般断层的性质。该断层在莫霍或上地幔是否有标志？地壳中断层间的速度是否存在持续的对比？不同断层链的相对构造重要性是什么？超剪切破裂是否有独特的破坏区模式？大地震的首选成核地点在哪里？可以预期的震动模式是什么？迄今为止，只有少数的地球物理研究将迪纳里断层作为横跨阿拉斯加中部的区域尺度线性样带的一部分，在孤立的部分进行了检查。多次层析成像研究已经成功地成像了该地区俯冲带和下伏的Yakutat板块的结构，但它们还没有达到足够的水平分辨率来成像Denali断裂的地壳和上地幔结构。对断裂带的深部构造和变形模式的沿走向变化没有太大的约束。为了解析迪纳里断裂带的区域尺度结构，我们利用新开发的结合环境噪声瑞利波相速度和体波双差测量的联合反演技术，对该断裂带从地表到70km深度的断层进行了层析成像，在Vp和v上都达到了千米尺度的分辨率。同时，我们还建立了改进的P波、S波和断裂带头波到达时间目录。在局部尺度上，对断层破坏带和主滑面结构的地震学研究尚未开展。在其他主要走滑断层上，间距小于100米的线性或二维地震仪阵列已经成功地探测到沿（半）垂直界面以不同速度传播的断裂带头波和被限制在由损坏岩石组成的与断层相关的低速带内的断裂带圈闭波。这两个地震相是断裂带所特有的，可以用来约束与地震破裂力学有关的断层结构。利用最近开发的低成本、高便携的三分量地震仪，我们在Denali断层沿线的两个地点部署了数百台仪器，每台仪器一个月。我们已经记录的第一个数据集的密度和丰富程度，不仅在德纳里断裂带是独一无二的，而且在全世界所有主要的走滑断层中都是独一无二的。我们的初步分析，包括捕获波探测和高频环境噪声相互关联，包含了最近在超剪切破裂中滑动的断层部分的损伤带结构的关键信息。这些数据证明了进一步分析的可行性，包括环境噪声层析成像、陷波正态和断裂带头波检测。