USArray FlexArray augmentation of a seismic study of the extension paradox at the northwest margin of the Basin-&-Range Province

内华达州的地形很高--即使是山脉之间的山谷也有几千英尺高。由于俄勒冈州边界和内华达州中部之间的平均海拔变化不大，我们认为整个地区的地壳(俗称漂浮在地幔上的冰山)的厚度必须相同。然而，值得注意的是，我们发现，当我们向南穿过这一地区时，延伸量--通过山脉和山谷之间的边界断层而导致的地壳延伸量--增加了大约5倍。当地壳被拉伸时，它变薄了，那么为什么内华达州中部的地壳不变薄，海拔却不低呢？这就是我们正在研究的“外延悖论”。我们认为，答案可能是地壳的下部足够弱，可以在地质时间尺度上流动，内华达州北部以下的下部地壳向南流向内华达州中部，有助于平衡地壳厚度和整个地区的海拔。我们正在通过收集新的地震剖面来验证这一假设，该剖面将非常详细地测量内华达州西北部的地壳厚度和结构，该地区过去从未用这种方法进行过很好的研究。我们正在利用美国国家科学基金会的地球望远镜计划最近为国家科学事业开发和获得的最新技术和设备。EarthScope通过其一个部门“US-Arrow”资助了“FlexArrow”的开发，这是一个易于运输的大型地震仪池。我们将使用这种新设备比以前更详细地研究地壳--我们预计将部署1000多台地震仪来记录我们将在精确控制的时间和地点引爆的6次爆炸产生的声波。通过测量地震波从每次爆炸到每个地震仪穿过地壳所需的时间，我们可以建立穿过地球的地震波速度的准确图像，这反过来又允许我们绘制出不同的岩石类型和深度存在的结构。在我们的项目期间，我们正在培训一群斯坦福大学的本科生，学习了解大陆所需的地质学和地球物理相结合的技术。通过将课堂从校园扩展到内华达州的沙漠和山区，通过将理论延伸到实践中，我们正在丰富对下一代科学领袖的教育。我们还正在与土地管理局和国家森林管理局合作，进行一项不会对环境产生持久影响的安全实验，通过这样做，我们希望扩大公众对科学方法的理解，以及公众对当地地质的欣赏。我们的所有数据都将通过地球望远镜数据中心存档并公开发布；最终，我们对内华达州北部地壳的新理解将帮助我们和其他科学家更好地了解该地区的地震危险。

Nevada has high topography - even the valleys between the mountain ranges are at elevations of several thousand feet. Because this average elevation does not change significantly between the Oregon border and central Nevada, we believe that the Earth's crust (colloquially, the iceberg floating on the mantle) must be the same thickness across this region. What is remarkable, however, is that we have discovered that the amount of extension - the amount of stretching of the crust by faulting at the borders between the mountains and the valleys - increases by a factor of about 5 as we go south across this same region. When the crust is stretched it thins, so why then is the crust not thinner, and the elevation lower, in central Nevada? This is the "extension paradox" that we are studying. We believe the answer may be that the lower part of the crust is weak enough that it can flow over geological timescales, and that the lower crust beneath northern Nevada is flowing south to central Nevada, helping to equalize the crustal thickness and balance the elevations across this region. We are testing this hypothesis by collecting a new seismic profile that will measure the crustal thickness and structure in great detail across northwestern Nevada, a region that has not been well-studied by this method in the past. We are making use of the latest technology and equipment that has recently been developed and acquired for the national scientific enterprise by NSF's Earthscope program. Earthscope, through one of its divisions "US-Array", has funded the development of a "FlexArray", a large pool of easily transportable seismographs. We will use this new equipment to study the Earth's crust in more detail than has previously been possible - we expect to lay out over 1000 seismographs to record the sound waves created by 6 explosions that we will detonate at precisely controlled times and locations. By measuring the time it takes for the seismic waves to travel through the Earth's crust from each explosion to each seismograph, we can build up an accurate picture of the seismic wave-speed through the Earth, which in turn lets us map out the different rock types and structures present at depth. During our project we are training a group of Stanford undergraduates in the combination geologic and geophysical techniques required to understand the continents. By extending the classroom from campus to the Nevadan deserts and mountains, and by extending theory to practice, we are enriching the education of the next generation of scientific leaders. We are also working with Bureau of Land Management and National Forest managers to carry out a safe experiment with no lasting environmental impact, and in so doing we expect to enlarge public understanding of scientific methodologies, and public appreciation of local geology. All our data will be archived with and publicly released through an Earthscope Data Center; ultimately our new understanding of the crust beneath northern Nevada will help us and other scientists better understand earthquake hazard in this region.