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：
1736248
负责人：
Amir Allam
金额：
$ 20.71万
依托单位：
University of Utah
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=1736248&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 年引发了 7.9 级地震 - 但只有少数地球物理研究检查过它的结构。我们对断层在地壳内的形状和行为知之甚少，也不知道岩石如何沿着断层长度变化。我们使用新的 3D 成像技术来查看地表以下 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。为了为我们的成像添加更多细节，我们将数百个咖啡罐大小的地震仪放置在两个不同的位置，以测量当地和全球各地地震引起的震动。这使得该地区居民和每年数百万游客能够更准确地估计地震危害。在更大的范围内，它向我们展示了断层如何在大陆边缘发挥作用，以及该断层是否形成北美地质板块的真正边缘。我们使用廉价的便携式仪器以及解码特定地震波类型的技术可能会改变人们未来研究世界各地断层带的方式。技术摘要：我们正在使用体波和面波联合层析反演对区域尺度的德纳利断层进行成像，并对 2002 年 7.9 级德纳利地震的受损区域进行局部成像。这项工作的动机是几个未解答的问题，特别是关于德纳利断层以及一般断层属性的问题。该断层在莫霍面或上地幔中是否有特征？地壳中的跨断层速度是否存在持续的对比？不同断层带的相对构造重要性是什么？超剪切破裂是否会导致独特的损伤区域模式？大地震的首选成核地点在哪里？预计的震动模式是什么？迄今为止，只有少数地球物理研究对作为阿拉斯加中部区域尺度线性横断面一部分的德纳利断层的孤立部分进行了研究。多项断层扫描研究已成功对该地区下方的俯冲带和雅库塔特板块的结构进行成像，但尚未获得足够的水平分辨率来对德纳里断层的地壳和上地幔结构进行成像。断裂带深部结构和变形模式沿走向变化几乎没有约束。为了解析德纳利断层的区域尺度结构，我们利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层析成像，实现了公里级的断层成像。 Vp 和 Vs 的分辨率。在此过程中，我们还建立了改进的 P、S 和断层带头波到达时间目录。在当地尺度上，尚未进行地震学研究来检查断层破坏带和主要滑面结构。在其他主要走滑断层上，间距小于100m的线性或二维地震仪阵列已成功探测到沿（半）垂直界面传播、速度对比的断层带头波和限制在断层相关内部的断层带陷波。由受损岩石组成的低速区。断层带特有的这两个地震阶段可用于对与地震破裂力学相关的断层结构提供约束。利用最近开发的低成本、高度便携式三分量地震仪，我们在德纳里断层沿线的两个地点以密集的跨断层阵列的形式部署了数百台仪器，每个仪器为期一个月。我们已经记录的第一个数据集的密度和丰富性不仅对于德纳利断层带是独一无二的，对于全世界所有主要走滑断层也是如此。我们的初步分析，包括陷波检测和高频环境噪声互相关，包含有关最近在超剪切破裂中滑动的断层部分损伤区结构的关键信息。这些数据证明了进一步分析的可行性，包括环境噪声断层扫描、陷波简正模态和断层带头波检测。