权益分类	功能权益	普通用户	{{item.name}}会员
{{category.name}}	{{benefitItem.name}}

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.

阿拉斯加中部的德纳利断层是世界上最长的断层之一，自恐龙时代以来，它一直是北美形成的关键。这条断层是一个重要的危险来源--它在2002年引发了一场7.9级的地震--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，也不知道岩石如何沿断层的长度变化。我们使用新的3D成像技术来查看地表下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细节，我们在两个不同的位置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造成的震动。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风险。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、便携的仪器和我们的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。技术摘要：我们正在使用体波和面波的联合层析成像技术对德纳利断层进行区域尺度的成像，并对2002年德纳利7.9级地震的灾害区进行局部成像。这项工作的动机是几个悬而未决的问题，特别是关于Denali断层以及一般故障性质的问题。该断层在莫霍面或上地幔中有标志吗？地壳中跨断层的速度有持续的反差吗？不同断裂带的相对构造重要性是什么？是否存在由超剪切破裂引起的独特的损伤区模式？大地震的首选成核点在哪里？可以预见的震动模式是什么？到目前为止，只有少数几项地球物理研究将德纳利断层作为阿拉斯加中部地区性线性断面的一部分进行了孤立的研究。多项层析成像研究已经成功地成像了该区域下方俯冲带和下伏雅库塔特板块的结构，但它们没有达到足够的水平分辨率来成像德纳利断裂的地壳和上地幔结构。对深部断裂带结构和变形形态沿走向变化的制约较小。为了解决德纳利断层的区域尺度结构，我们使用最新开发的联合反演技术，从地表到70公里深度对断层进行层析成像，结合环境噪声瑞雷波相速度和体波双差测量，在Vp和Vs上都达到千米级分辨率。在这一过程中，我们还在建立改进的P波、S波和断裂带头波到达时间目录。在局部尺度上，尚未对断层破坏带和主滑移面结构进行地震学研究。在其他主要的走滑断层上，间距小于100米的线性或二维地震仪阵列成功地探测到了沿速度反差的(半)垂直界面传播的断层带首波，以及限制在由受损岩石组成的断层相关低速带内的断层带捕获波。这两个地震阶段是断裂带特有的，可以用来约束与地震破裂机制相关的断层结构。利用最近开发的低成本、高度便携的三分量地震仪，我们将在德纳利断层沿线的两个地点以密集的跨断层阵列部署数百台仪器，每个阵列为期一个月。我们已经记录的第一个数据集的密度和丰度不仅对德纳利断裂带是独一无二的，而且对世界上所有主要的走滑断层都是独一无二的。我们的初步分析，包括捕获波检测和高频环境噪声互相关，包含了关于最近在超剪切破裂中滑动的一段断层中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、陷波正常模式和断层带首波检测。