Collaborative Research: Capturing 4D Variations in Stress, Slip, and Fault-Zone Material Properties: The 2019-2021 Gofar Transform Fault Earthquake Prediction Experiment

该项目将确定控制赤道太平洋戈法尔变换断层上地震强度和时间的断层性质。通过先进的海底地震仪阵列(OBSS)、岩石采集和断层成像，这项研究将对两个6级地震带和它们之间的区域产生多方面的了解。为了确保在预计下一次大地震期间有连续的地震数据集，该项目涉及三个研究航次：1)部署OBSS并收集岩石样本；2)恢复、翻新和重新部署OBSS，并使用自动水下机器人(AUV)调查断裂带；以及3)恢复OBSS。这个为期两年的数据集预计将记录数十万次微震，此外还有望记录下两次6级主震。这项研究将惠及6-12年级的学生，方法是将新英格兰低收入学区的两名教师带到大海，并与这些教师合作开发可供全美教师使用的课程。教师们将与项目科学家和新汉普郡大学的数学、科学和工程教育中心合作，开发一套“课程套件”--一套基于网络的资源，其中包括基于课堂的地震调查、背景信息，以及与戈法尔实验科学家定期进行的课堂视频聊天。该项目还将通过研究生和博士后的支持，在国内和国际上加强大学一级的地震研究。Gofar断层之所以被选为这个项目，是因为那里之前的工作允许计划一个精确的实验，目的是在短短几公里内成像断层行为的转变。这项实验将以前所未有的细节捕捉断层的时间演变，并将这些变化与潜在的地质和断层力学联系起来。具体地说，这个项目的目的是了解为什么海洋转换断层以无地震滑动为主，具有如此可重复的地震周期，并在阻止大地震的破裂屏障中孕育数十万次小地震。特别是，戈法尔以前的工作表明，破裂屏障行为不能用基本的双峰摩擦特性来解释，需要对断裂带进行更复杂的流变学描述。这个项目通过用强震台阵记录地震应力降的时间相关性，以及用4个短周期OBSS的微型台阵估计破裂屏障内的地震速度来解决这些问题，这将使我们能够使用一种称为双波束形成的地震技术来确定剪切速度的时空演化。该项目将确定地震前S速度的变化是被限制在一个狭窄的断裂带上，还是扩展到整个较宽的破坏带，以及它们延伸到孕震深度的程度。这项研究将速度异常的迁移或缺乏与断层力学模型进行比较，试图约束破裂屏障的潜在流变性，并调查扩容在阻止大型破裂中的作用。该项目还将通过结合基于AUV的水深测量、后向散射、光致马赛克成像和水柱测量以及岩石疏浚，对RTF进行迄今最全面、最高分辨率的测绘。这套研究将有助于澄清扩容和热液蚀变在断裂带和破裂屏障区域之间产生不同地震行为中的作用。该奖项反映了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。

This project will determine the fault properties that control the magnitude and timing of earthquakes on Gofar Transform Fault in the equatorial Pacific Ocean. With an advanced array of ocean bottom seismometers (OBSs), rock collection, and fault imaging, this research will produce a multifaceted understanding of two magnitude 6 earthquake zones and the regions that separate them. To ensure a continuous earthquake dataset over the period that next large earthquakes are expected, this project involves three research cruises: 1) to deploy the OBSs and collect rock samples; 2) to recover, refurbish, and redeploy the OBSs and to survey the fault-zone with an autonomous underwater vehicle (AUV); and 3) to recover the OBSs. The two-year dataset is expected to record hundreds of thousands of microearthquakes in addition to hopefully capturing the next two magnitude 6 mainshocks. This research will reach 6-12th grade students, by taking two teachers from low-income school districts in New England to sea and working with these teachers to develop curricula that can be used by teachers throughout the US. The teachers will work with project scientists and the University of New Hampshire's Center for Mathematics, Science, and Engineering Education to develop a "Curriculum Kit" - a web-based set of resources that will include classroom-based earthquake investigations, background information, and periodic classroom video chats with the Gofar experiment scientists. This project will also enhance earthquake research at the university level both nationally and internationally through the support of graduate students and postdocs. Gofar fault was chosen for this project because previous work there allows for the planning of a precise experiment aimed at imaging transitions in fault behavior over just a few kilometers. This experiment will capture the temporal evolution of the fault in unprecedented detail and link these variations to the underlying geology and fault mechanics. Specifically, the aim of this project is to understand why oceanic transform faults are dominated by aseismic slip, have such repeatable seismic cycles, and nucleate hundreds of thousands of small earthquakes in the rupture barriers that stop large earthquakes. In particular, previous work at Gofar indicates that rupture barrier behavior cannot be explained by basic bimodal frictional properties and requires more sophisticated rheological descriptions of the fault zone. This project approaches these questions by recording the time dependence of earthquake stress drops with a strong-motion array and by estimating seismic velocities within the rupture barrier using 4 mini arrays of short-period OBSs that will allow us to use a seismic technique known as double beam forming to determine the space-time evolution of shear velocity. The project will determine if the pre-seismic changes in S-velocity are contained to a narrow fault-zone or spread throughout the wider damage zone and to what extent they extend to seismogenic depths. This research will compare the migration of the velocity anomalies, or lack thereof, with fault mechanics models to try to constrain the underlying rheology of the rupture barrier and investigate the role of dilatancy in stopping large ruptures. The project will also conduct the most comprehensive, high-resolution mapping of a RTF to date through a combination of AUV based bathymetry, backscatter, photomosaic imaging, and water column surveys along with rock dredging. This suite of studies will help clarify the roles of dilatancy and hydrothermal alteration in producing the contrasting seismic behavior between the rupture zones and rupture barrier regions.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.