Porosity Structure and Earthquake Rupture Dynamics at the GOFAR Fracture Zone

Woods Hole Oceanographic Institution, Woods Hole MA

Investigators

Abstract

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.