Porosity Structure and Earthquake Rupture Dynamics at the GOFAR Fracture Zone

在整个海底，断层发生在板块的边缘，板块在那里横向滑动。这些特征被称为海洋转换断层，它们与陆地上的危险断层有相似之处，比如加州的圣安德烈亚斯断层。但是，由于海底的地质情况比陆地简单得多，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自然实验室。此外，科学家们还观察到，大地震活动以相当规律的周期发生，重复时间为5-6年，这使得计划中的实验能够在断层经历地震序列时捕捉断层。从这些研究中得到的认识可以进一步推进地震预报。在一个这样的海洋变换断层，东太平洋的Gofar，地震的重复循环，首先是通过在断层周围部署地震仪——测量海底运动的仪器——来观察到的。一个被认为对地震产生很重要的因素是存在于海底顶部几公里处的海水量。然而，关于活动断层内流体分布的数据很少。这项实验将通过测量海底传导电流的能力，为已经得到充分研究的Gofar地区提供这样的数据，这一特性与海底海水的数量直接相关。海底将部署测量电场和磁场的仪器。拖曳在科考船后面的电磁能量发射器发出的信号将被海底仪器记录下来。通过测量地震发生时断层系统的不同部分表现不同，将有可能更好地了解断层的不同特性以及这些特性与地震活动性的关系。早期的职业参与者将被邀请参加这次考察。该项目支持培养研究生和一名博士后。海洋转换断层（RTFS）为了解整个地震周期中发生的关键过程提供了一个天然实验室，并为进一步推进地震预测提供了一个绝佳的机会。流体在断层过程中的作用已经被假设，但令人惊讶的是，在活动断层网络中流体分布的可用数据很少。这个实验将在地震活动性研究的良好约束框架内提供这样的数据。研究RTF环境的一个主要原因是观测到的相对可预测的地震活动周期，其准周期约为5-6年。为此，海底地震仪（OBS）仪器的部署捕捉到了东太平洋Gofar RTF地区6.0级大地震的前震活动。从2019年底到2021年，计划在Gofar部署另一个OBS仪器，目的是捕捉当前两个地震周期的结束，并了解为什么这些断层如此可预测。这个实验将集中在地震活动不同程度和类型的地区。在Gofar变换中，有一个区域反复抑制破裂传播，主要是在抗震方面失败。地震速度在地震破裂区和破裂障壁有显著差异。拉分条件可能允许深层海水渗透到断层中，从而导致断层深处持续的高孔隙度和流体孔隙压力。这些流体的最终结果是促进断层的剪胀强化，这已被观察到可以抑制动态变形。在Gofar系统不同部分的一系列CSEM剖面将直接测量海底电阻率，这是与地壳孔隙度最密切相关的属性。因此，该实验将绘制整个系统的孔隙度变化图，特别是在主要地震活动的破坏区和反复抑制破裂传播的区域，使我们能够直接测试流体在地震循环中的作用。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。

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.