A major goal of earthquake science is to understand the evolution of stress, strength, and material properties in fault zones with enough precision to forecast the magnitude and timing of future earthquakes. The basic hypothesis of seismic cycles is that stress builds up for an extended period over a large portion of a fault and then is released suddenly in a large earthquake. Yet verifying this hypothesis has remained difficult because typical repeat times of large earthquakes are 50–1,000 years.  Oceanic transform faults on the East Pacific Rise are ideal targets for investigating variations in seismicity, fault strength, and fluids within the context of well-known earthquake cycles. On these faults, earthquakes of approximately magnitude 6 repeat every 5–6 years.

Our project was focused on determining the fault properties that control the magnitude and timing of earthquakes on Gofar Transform Fault.  To accomplish this goal, we undertook three research cruises to the equatorial Pacific Ocean to deploy an advanced array of 51 ocean bottom seismometers (OBSs) and collect sea-floor rock samples from 16 sites to provide the most comprehensive picture of an oceanic transform fault’s seismic behavior and composition that we have ever had.  We also conducted 14 dives with the autonomous underwater vehicle Sentry, where we mapped the fault zone at high resolution (1-meter scale) and investigated key water column properties near the seafloor, providing additional information on the fault’s structure and hydrothermal activity.

Our observations have produced a multifaceted understanding of two magnitude 6 earthquake patches and the long-lived rupture barriers, which separate the rupture patches repeatedly struck by magnitude 6 earthquakes. Four months after our initial OBS deployment, the expected earthquake on the eastern segment of the Gofar—a magnitude 6.1 event—occurred on 22 March 2020.  Very high rates of foreshocks occurred in the rupture barrier prior to the mainshock, followed by low rates of aftershocks, similar to what was observed previously on a nearby fault segment.  Our results confirm that rupture barriers are common features on oceanic transform faults and, surprisingly, are where microseismicity is most abundant.  

The high-resolution bathymetry shows small-scale variations in fault strike, variably present fault bends, and sub-parallel strands. These fault complexities are mostly found in rupture barriers, but are also present, to a lesser extent, in rupture patches.  This partial alignment with the along-strike rupture segmentation, indicates possible changes in material and/or structural fault properties. With this new high-resolution information on surface morphology, we are exploring changes in fault structure that could influence hydrothermal fluid flow and fault lithology, and therefore influence rupture processes at depth.

Basketfuls of pillow basalts and basaltic breccias were dredged from the active fault zone, providing the first rock samples from Gofar Transform Fault. The differences in the extent of alteration of breccias and basalts recovered within the same dredges indicates that fault zone deformation of basalts to form breccias was associated with extensive fluid flow. With these rocks we are assessing the intertwined effects of damage and hydrothermal alteration and their influences on fault slip behavior.

This research is reaching K-12 and undergraduate classrooms through ‘rock buckets’ created from the abundance of basalt samples collected during dredging, which are now available for distribution to classrooms.  Thus far 8 buckets have been distributed to colleges and universities for their undergraduate teaching and one more has been designated for a middle school.  Our research has been integral to the career progression of 5 graduate students and 8 postdocs.  Finally, prompted by challenges due to COVID, the level of onshore contributions to decisions at sea in our third research cruise was extremely high. Given the success of this cruise, we hope the approaches we used will become more common in the future, increasing access to remote science, and allowing those who cannot practically go to sea to be involved in seagoing expeditions.

In total, we recorded an unprecedented oceanic transform fault earthquake catalog of more than half a million earthquakes of magnitudes between 0 and 6.1. This catalog represents about 40% of the seismic cycle on multiple segments of the Gofar transform fault—equivalent to more than 50 years of recording on many segments of the San Andreas Fault.  More Gofar Transform Fault earthquakes are just around the corner. With the integrated data set from this project, we can now better explain how, where, and when these earthquakes will occur.

Last Modified: 03/23/2024
Modified by: Mark D Behn