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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.

阿拉斯加中部的迪纳利断层是世界上最长的断层之一，自恐龙时代以来，它对北美的形成至关重要。该断层是一个重要的危险源-它在2002年造成了7.9级地震-但只有少数地球物理研究检查了它的结构。我们对断层在地壳中的形状和行为知之甚少，我们也不知道岩石是如何沿着断层的长度沿着变化的。我们使用新的3D成像技术来观察地表下2-50公里处的断层结构，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的震动。这使得更准确地估计地震对该地区居民和每年数百万游客的危害。在更大的尺度上，它向我们展示了断层是如何在大陆边缘活动的，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解码特定地震波类型的技术可能会改变未来人们研究世界各地断层带的方式。技术总结：我们正在成像的德纳里断层在区域范围内使用联合体波和面波层析反演，并在当地成像的2002年7.9级德纳里地震的破坏区。这项工作的动机是几个悬而未决的问题，特别是关于德纳里故障以及一般故障的属性。断层在莫霍面或上地幔中有信号吗？地壳中跨断层速度是否存在持续的差异？不同断层带的相对构造重要性是什么？超剪切破裂是否有独特的损伤区模式？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，只有少数地球物理研究在孤立的部分研究了迪纳利断层，作为横跨阿拉斯加中部的区域尺度线性断面的一部分。多个层析成像研究已经成功地成像俯冲带的结构和该地区下方的雅库塔特板块，但他们没有达到足够的水平分辨率成像德纳里断层的地壳和上地幔结构。断裂带的深部结构和变形模式沿走向的变化几乎不受制约。为了解决德纳里断层的区域尺度结构，我们正在使用新开发的联合反演技术，结合环境噪声瑞利波相速度和体波双差测量，对从地表到70公里深度的断层进行断层成像，这些技术在Vp和Vs方面都达到了更高的尺度分辨率。和断裂带首波。在局部尺度上，没有进行地震学研究来检查断层破坏带和主要滑动面结构。在其他主要的走滑断层，线性或二维阵列的地震仪与间距小于100米已成功地检测到断层带首波传播沿着（半）垂直界面与对比的速度和断层带陷波限制在故障相关的低速带内的损坏的岩石。这两个地震阶段，独特的断层带，可以用来提供有关地震破裂力学断层结构的约束。利用最近开发的低成本、高度便携的三分量地震仪，我们正在沿迪纳利断层沿着的两个地点部署数百台仪器，每台仪器在密集的跨断层阵列中工作一个月。我们已经记录的第一个数据集的密度和丰富性不仅是德纳里断层带所独有的，而且是全球所有主要走滑断层所独有的。我们的初步分析，包括陷波检测和高频环境噪声互相关，包含的关键信息的损伤区结构的断层，最近在一个超剪切破裂滑动的一部分。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，陷波简正模，和断层带首波检测。