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Denali断层是世界上最长的断层之一，自恐龙时代以来，这对于北美的形成至关重要。故障是危险的重要来源 - 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其结构。我们对断层的形状和在地壳内的表现几乎一无所知，我们不知道岩石沿断层的长度发生变化。我们使用新的3D成像技术来查看表面以下2-50公里的断层的结构，这有助于我们确定阿拉斯加的地质历史。为了进一步增加成像的细节，我们将数百个大小的地震米放在两个单独的位置，以测量当地和整个地球上地震的震动。这可以更准确地估计该地区居民的地震危害和数百万年度游客。在更大范围内，它向我们展示了在大陆边缘的故障如何工作，以及该断层是否形成了北美地质板的真实边缘。我们使用廉价，便携式仪器和技术来解码特定的地震浪潮类型可能会改变人们将来研究世界上的故障区域的方式。技术摘要：我们使用人体和表面波的联合层析成像在区域尺度上对Denali故障进行成像，并在2002年7.9 Denali地震的损伤区域局部成像。这项工作是由几个关于Denali故障以及一般故障属性的未解决问题的动机。故障在Moho或上地幔中具有签名吗？地壳中的横断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？是否有supershear破裂导致的独特损伤区域模式？大地震的首选成核位点在哪里，可以预期的摇动模式是什么？迄今为止，只有少数地球物理研究检查了孤立部分的Denali断层，这是整个阿拉斯加中部地区线性样带的一部分。多次断层扫描研究成功地成像了该区域下方的俯冲带和基础Yakutat板的结构，但是它们尚未实现足够的水平分辨率来成像Denali断层的地壳和上层地幔结构。在深度处的断层区结构和变形模式的沿袭变化几乎没有限制。为了解决Denali断层的区域尺度结构，我们使用新开发的联合反转技术将其从表面到70公里深的故障进行成像，结合了环境噪声雷利波波相位速度和人体波浪速度和双向差异测量，从在此过程中，我们还为P，S和断层带头波的到达时间构建了改进的目录。在地方规模上，没有进行地震学研究来检查断层损伤带和主滑动表面结构。在其他主要的滑移断层下，间距小于100m的线性或二维的地震米阵列成功地检测到了断层区的头部波，这些断层区的头部波沿（半）垂直接口传播，垂直速度和故障区域被限制的断层波浪被限制在内部故障相关的低速度岩石内部，由损坏的岩石组成。可以利用这两个与断层区域的地震阶段，特有的断层区域以提供与地震破裂力学相关的断层结构的约束。利用低成本高昂的三组分地震米的开发，我们在Denali断层沿两个位置部署了数百种用密集的断层式阵列阵列中的数百种仪器。我们已经录制的第一个数据集的密度和丰富度不仅是Denali断层区域的独特之处，而且是全球所有主要的滑移断层。我们的初步分析，包括被困的波浪检测和高频环境噪声互相关，其中包含有关损伤区结构的关键信息，该部分在最近在超壳破裂中滑落的断层部分中。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，被困的波浪正常模式和断层区头波检测。