Collaborative Proposal: Developing argon techniques to elucidate earthquake chronologies (DATEEQ)

地震发生在地下深处，我们无法在地震前、地震中和地震后直接观察到断层，这一事实阻碍了我们对地震如何开始、传播和停止的理解。为了更好地研究断层在深处的作用，人们可以钻穿它们或观察过去埋藏在地球深处但已暴露在地表的断层。在这项研究中，研究人员观察了穿过断层的钻芯的岩石样本，以确定经历过地震滑动的断层区域。由于个别地震可以沿着厚度较小的地带滑动--与断层总厚度10-100米相比，厚度只有几毫米)--整个断层不太可能经历过地震。这项研究的目标是开发一种新的技术，不仅可以确定地震发生在断层上的哪里，而且还可以确定地震发生的时间。这一点很重要，因为更长的地震记录(以百万年为时间尺度)可以更好地了解地震周期，并制定更好的地震预报。在这里，通过测量断层岩石中放射性钾与其衰变产物Ar的比率，研究小组确定了岩石被地震加热的时间。这是一项最成熟的地质测年技术的新应用。这项研究利用了地震通过摩擦产生大量热量的事实；产生的热量可以重置钾-氩时钟，从而在岩石记录中记录地震的时间。该项目支持加州大学圣克鲁斯分校(UCSC，一家拉美裔服务机构)和哥伦比亚大学的研究生和本科生。它通过吸收加州大学洛杉矶分校和纽约城市大学代表不足的群体的学生，扩大了对地球科学的参与。它还促进了对当地学校和公众的接触。为了描述过去发生的地震，研究人员首先进行了一系列实验，以确定当地震加热时，从宿主矿物中释放出氩的速度有多快。他们在几块不同的岩石上，在不同的时间和温度(在短短几秒钟内达到几百度)进行了这些实验。这些实验可以量化Ar扩散的速度。接下来，研究小组测量了主要断层中钻探岩芯中的钾和氩，这些断层包括日本北部的俯冲带(2011年发生了9.1级地震)和新西兰北岛的Hikurangi俯冲带。以前使用一种测量岩石温度的地球化学方法对这些岩心进行的研究表明，这些断层上曾发生过热地震。然而，这种以前的方法，使用加热时有机物分子结构的变化，只能提供温度估计，而不能指示地震发生的时间。通过结合年代测定和地球化学方法，研究人员创建了数百万年来地震滑动的年代记录。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Our understanding of how earthquakes start, propagate, and stop is hindered by the fact that they occur deep underground where we cannot directly observe faults before, during, and after the quake. To better study what faults do at depth, one can drill through them or look at faults that used to be buried deep in the Earth but have been exposed at the surface. In this study, the researchers look at rock samples from drill cores through faults to identify areas of the fault that have experienced earthquake slip. Because individual earthquakes can slip along zones with small thicknesses - just millimeters compared to the total fault thickness of 10s-100s of meters) - it is unlikely that the whole fault will have experienced earthquakes. The goal of this study is to develop a new technique to determine not only where earthquakes have occurred in a fault, but when. This is important because longer earthquake records (on the million-year timescale) allow to better understand earthquake cycles and develop better earthquake forecasts. Here, by measuring the ratio of radioactive potassium to its decay product argon in the fault rocks, the team determine how long it has been since the rock was heated by earthquakes. This is a novel application of one of the best-established geological dating techniques. The study takes advantage of the fact that earthquakes generate a lot of heat from friction; the generated heat can reset the potassium-argon clock, thereby recording the time of the earthquake in the rock record. The project supports graduate and undergraduate students at University of California – Santa Cruz (UCSC, a Hispanic serving Institution) and Columbia University. It broadens participation in Earth Sciences by involving students from underrepresented group at UCSC and from the City University of New York. It also fosters outreach to local schools and the public.To characterize past earthquake occurrences, the researchers first run a series of experiments to determine how fast the argon is liberated from its host mineral when heated by an earthquake. They run these experiments on several different rocks at different times and temperatures (up to several hundred degrees in just a few seconds). The experiments allow quantifying how fast argon diffusion occurs. Next, the team measure potassium and argon in drill core rocks from major faults including the subduction zone off of northern Japan (which hosted the 2011 Mw 9.1 earthquake) and the Hikurangi subduction zone off of the North Island of New Zealand. Previous work on these cores using a geochemical method for measuring rock temperature has demonstrated that hot earthquakes have occurred on these faults. However, this previous method, using the change in molecular structure of organic matter when heated, only provides a temperature estimate with no indication of when the earthquake occurred. By combining dating and geochemical methods, the researchers create dated records of earthquake slip over millions of years.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.