Porosity Structure and Earthquake Rupture Dynamics at the GOFAR Fracture Zone

GOFAR断裂带的孔隙结构和地震破裂动力学

• 批准号: 1922528
• 负责人: Robert Evans
• 金额: $ 143.39万
• 依托单位: Woods Hole Oceanographic Institution
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2020
• 资助国家: 美国
• 起止时间: 2020-09-01 至 2024-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1922528&HistoricalAwards=false
• 关键词: Porosity; Structure; Earthquake; Rupture; Dynamics

Across the seafloor, faults occur at the edges of tectonic plates where the plates slide laterally past each other. These features are called oceanic transform faults and they have a similarity to hazardous faults on land, such as the San Andreas fault in California. But because the seafloor has a far simpler geology than land the ocean settings are ideal natural laboratories to study some of the important processes occurring throughout the earthquake cycle. Furthermore, scientists have observed that, at transforms, large earthquake activity occurs in fairly regular cycles with repeat times of 5-6 years, allowing experiments to be planned that capture the faults as they experience an earthquake sequence. The understanding that comes from such studies can further the advancement of earthquake prediction. The repeating cycles of earthquakes at one such oceanic transform fault, the Gofar in the east Pacific, was first observed through deployments of seismometers - instruments which measure the motion of the seafloor - around the fault. One factor thought to be important for earthquake generation is the amount of seawater that is present in the top several kilometers of seafloor. Yet, there is little data on fluid distributions within active faults. This experiment will provide such data in the well-studied Gofar region by measuring the ability of the seafloor to conduct electrical current, a property that is directly linked to the amount of seawater in the seafloor. Instruments will be deployed on the seafloor that measure electric and magnetic fields. Signals from a transmitter of electromagnetic energy towed behind the research vessel will be recorded by the seafloor instruments. By surveying different parts of the fault system that behave differently when earthquakes occur, it will be possible to understand better the different properties of the fault and how these relate to seismicity. Early career participants will be invited on the expedition. The project supports the training of graduate students and a postdoctoral investigator.Oceanic transform faults (RTFS) offer a natural laboratory for understanding key processes occurring throughout the earthquake cycle, and an outstanding opportunity to further the advancement of earthquake prediction. The role of fluids in fault processes has been hypothesized, but there is surprisingly little data available on fluid distributions within active fault networks. This experiment would provide such data in a well constrained framework of seismicity studies. A primary reason for studying RTF settings is the relatively predictable cycle of seismicity observed, with quasi-periodicities on the order of 5-6 years. To that end, deployments of ocean bottom seismograph (OBS) instrumentation captured foreshock activity building up to a large M6.0 event across the Gofar RTF in the east Pacific. Another deployment of OBS instruments is planned for the Gofar, starting in late 2019 through 2021, with the aim of capturing the end of two current seismic cycles and the goal of understanding why these faults are so predictable. This experiment will focus on areas with different levels and types of seismicity. Within the Gofar transform is a region that repeatedly inhibits rupture propagation and that primarily fails aseismically. Seismic velocities are significantly different in the seismic rupture region and in the rupture barrier. Pull-apart conditions might allow deep seawater penetration into the fault resulting in persistently high porosity and fluid pore pressures at depth. The net result of these fluids is to promote dilatancy-strengthening of the fault which has been observed to inhibit dynamic deformation. A series of CSEM profiles at different parts of the Gofar system will directly measure seafloor electrical resistivity, a property most closely related to porosity of the crust. The experiment will therefore map porosity variations throughout the system, particularly in the damage zone of primary seismicity and within the area that has been seen to repeatedly inhibit rupture propagation, allowing us to directly test the role of fluids in the seismic cycle.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.

在整个海底，故障发生在构造板的边缘，板块横向滑动。这些特征称为海洋变换断层，它们与陆地上的危险断层相似，例如加利福尼亚的圣安德烈亚斯断层。 但是，由于海底的地质比土地要简单得多，因此海洋环境是研究整个地震周期中发生的一些重要过程的理想天然实验室。此外，科学家们观察到，在变换时，大地震活动发生在相当规则的周期中，重复时间为5 - 6年，可以计划实验，以捕获它们在经历地震序列的情况下捕获断层。从这些研究带来的理解可以进一步进步地震预测。在一个这样的海洋变换断层（东太平洋的戈法尔）的地震的重复循环首先是通过在断层周围的地震仪的部署（测量海底运动的仪器）来观察到的。人们认为对地震产生很重要的一个因素是海水中存在的海水数量是海底几公里。然而，有效故障中流体分布的数据很少。该实验将通过测量海底进行电流的能力，在良好的戈法尔地区提供此类数据，这是直接与海底海水量有关的特性。仪器将部署在测量电场和磁场的海底上。海底仪器将记录来自研究容器后面的电磁能发射器的信号。通过调查断层系统的不同部分，这些部分在发生地震时行为不同，就可以更好地理解断层的不同特性以及这些与地震相关的关系。早期的职业参与者将被邀请参加探险。该项目支持对研究生的培训和博士后研究员。CeanicTransform Presss（RTFS）为了解整个地震周期中发生的关键过程提供了自然实验室，以及进一步进步地震预测的绝佳机会。假设流体在故障过程中的作用，但是令人惊讶的是，关于活动故障网络中流体分布的可用数据很少。该实验将在良好的地震性研究框架内提供此类数据。研究RTF设置的主要原因是观察到的地震性的相对可预测的循环，准周期性为5 - 6年。为此，海洋底部地震仪（OBS）仪器的部署捕获了东太平洋Gofar RTF的大型M6.0事件的前期活动。计划从2019年底至2021年开始为Gofar部署OBS仪器，目的是捕获两个当前的地震循环的结束，并且目的是了解为什么这些断层是如此可预测的。该实验将集中于具有不同级别和类型地震性的领域。在戈法尔变换内是一个反复抑制破裂繁殖的区域，主要在无性上失败。地震速度在地震破裂区域和破裂屏障中的地震速度显着差异。 拉动条件可能会使海水深入渗透到断层中，从而导致孔隙率持续高，并深入渗透孔隙压力。这些流体的净结果是促进断层的膨胀加剧，这被观察到抑制动态变形。 Gofar系统不同部分的一系列CSEM轮廓将直接测量海底电阻率，这是与地壳的孔隙率最紧密相关的特性。因此，该实验将在整个系统中绘制孔隙率的变化，尤其是在主要地震性的损害区域以及在该区域内反复抑制破裂的繁殖，从而使我们能够直接测试流体在地震周期中的作用。该奖项是NSF的法定任务，反映了通过评估该基金会的智力效果，并反映了基金会的构成效果，并构成了基金会的影响。