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变换断层地震震级和时间的断层性质。通过先进的海底地震仪阵列、岩石收集和断层成像，这项研究将对两个6级地震带及其分离区域产生多方面的了解。为了确保在预计下一次大地震发生期间获得连续的地震数据集，该项目包括三个研究巡航：1)部署地震仪并收集岩石样本；2)恢复、翻新和重新部署OBSs，并使用自主水下航行器（AUV）调查断裂带；3)恢复OBSs。这个为期两年的数据集预计将记录数十万次微地震，并有望捕捉到接下来的两次6级主震。这项研究将涉及6-12年级的学生，通过从新英格兰低收入学区聘请两名教师，并与这些教师一起开发可供美国各地教师使用的课程。教师们将与项目科学家和新罕布什尔大学数学、科学和工程教育中心合作，开发一套“课程工具包”——一套基于网络的资源，其中包括课堂地震调查、背景信息以及与Gofar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，加强国内和国际大学层面的地震研究。该项目选择Gofar断层，是因为之前在那里的工作允许计划一个精确的实验，目的是在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。具体来说，该项目的目的是了解为什么海洋转换断层以地震滑动为主，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形成数十万个小地震。特别是，Gofar先前的工作表明，破裂屏障的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描述。该项目通过强震阵列记录地震应力下降的时间依赖性，并通过使用4个短周期地震仪迷你阵列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称为双束形成的地震技术来确定剪切速度的时空演变。该项目将确定地震前的s速度变化是局限于一个狭窄的断裂带，还是蔓延到整个较宽的破坏带，以及它们在多大程度上延伸到发震深度。本研究将把速度异常的迁移（或缺乏速度异常）与断层力学模型进行比较，以试图约束破裂屏障的潜在流变学，并研究膨胀在阻止大破裂中的作用。该项目还将对RTF进行迄今为止最全面、高分辨率的测绘，方法包括基于AUV的测深、后向散射、显微成像、水柱调查以及岩石疏浚。这一套研究将有助于澄清膨胀和热液蚀变在产生破裂带和破裂障碍区的地震行为对比中的作用。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值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.