Aseismic transient slip on the Gofar transform fault, East Pacific Rise Yajing Liua,b,1, Jeffrey J. McGuireb,c , and Mark D. Behnb,d aDepartment of Earth and Planetary Sciences, McGill University, Montreal,´ QC, Canada H3A 0E8; bDepartment of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543; cEarthquake Science Center, US Geological Survey, Moffett Field, CA 94035; and dDepartment of Earth and Environmental Sciences, Boston College, Chestnut Hill, MA 02467 Edited by Goran Ekstrom, Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY, and approved March 19, 2020 (received for review August 6, 2019) Oceanic transform faults display a unique combination of seismic that the material properties of the barrier zone are distinct from and aseismic slip behavior, including a large globally averaged those in the M ∼ 6 rupture zones. Prior to the 2008 Mw 6.0 main- seismic deficit, and the local occurrence of repeating magni- shock, the central barrier zone experienced ∼22,000 foreshocks tude (M) ∼ 6 earthquakes with abundant foreshocks and seismic (M0 to 4.5) extending in depth to the uppermost mantle, likely swarms, as on the Gofar transform of the East Pacific Rise and the a few kilometers deeper than the lower limit of oceanic crustal Blanco Ridge in the northeast Pacific Ocean. However, the under- seismicity defined by the 600 oC isotherm (10). An active-source lying mechanisms that govern the partitioning between seismic refraction line across the barrier zone revealed ∼10 to 20% and aseismic slip and their interaction remain unclear. Here we reduction in P-wave velocities in a several-kilometer-wide dam- present a numerical modeling study of earthquake sequences age zone extending through the entire oceanic crust (5), in con- and aseismic transient slip on oceanic transform faults. In the trast to a nearly intact fault zone in the fully coupled mainshock model, strong dilatancy strengthening, supported by seismic area (4). Systematically lower static stress drop values are also imaging that indicates enhanced fluid-filled porosity and possible estimated for earthquakes in the barrier zone (11), which are hydrothermal circulation down to the brittle–ductile transition, consistent with the seismic velocity reduction. Significant (up to effectively stabilizes along-strike seismic rupture propagation and ∼50%) seismic velocity reduction is also common along conti- results in rupture barriers where aseismic transients arise episod- nental transform faults and is usually attributed to a mechanical ically. The modeled slow slip migrates along the barrier zones damage zone due to intense fracturing (12). The depth extent of at speeds ∼10 to 600 m/h, spatiotemporally correlated with the continental low-velocity zones is less clear and it may not extend observed migration of seismic swarms on the Gofar transform. all of the way to the brittle–ductile transition as observed on the Our model thus suggests the possible prevalence of episodic aseis- Gofar fault (5). This large velocity anomaly on Gofar can be mic transients in M ∼ 6 rupture barrier zones that host active achieved by either enhanced fluid-filled porosity of 1.5 to 8% or swarms on oceanic transform faults and provides candidates a large volume fraction of mineral alternation at crustal depths for future seafloor geodesy experiments to verify the relation (e.g., 50 to 90% altered peridotite or >70% serpentinized troc- between aseismic fault slip, earthquake swarms, and fault zone tolite) (5). However, such large-scale alternation would produce hydromechanical properties. oceanic transform faults j earthquake rupture segmentation j Significance aseismic transients j seismic swarms Oceanic transform faults slip through a unique combination of seismic and aseismic behavior. But the geological conditions veraged globally, oceanic transform faults (OTFs) release and physical mechanisms that lead to different slip modes on a small percentage (∼15%) of their accumulated moment A different parts of the same fault remain unresolved. Recent seismically, and despite the large thermally defined potential seafloor experiments document abundant microseismicity in rupture area the largest observed earthquakes have only mod- barrier zones that separate large earthquake ruptures. These erate magnitudes (up to magnitude 7 [M7]) (1, 2). Recent barriers require an explanation that permits small earthquake ocean bottom seismometer (OBS) deployment experiments nucleation, but prohibits large rupture propagation. We find along OTFs associated with fast-spreading ridges provide near- that coupling frictional dilatancy to a physics-based earth- field seismic data that shed light on the source mechanism quake cycle model can simultaneously reproduce the rupture of OTF earthquake rupture patterns and possible interaction segmentation and slip partition observed on the Gofar trans- between aseismic and seismic slip modes (3–7). On the Gofar form fault, East Pacific Rise. Our model shows a prevalence transform of the East Pacific Rise (EPR), M ∼ 6 earthquakes of slow slip events that drive migrating microearthquake repeat quasi-periodically every 5 to 6 y on 15- to 20-km along- swarms, a prediction that can be tested with future seafloor strike segments separated by a ∼10-km barrier zone (8). The geodesy experiments. rupture pattern is inferred from the 2008 foreshock and after- shock distributions (3) (Fig. 1) and the relative centroid locations Author contributions: Y.L. designed research; Y.L. and J.J.M. performed research; Y.L. and of M6.0+ events on Gofar since 1992 (2, 8). Similar earthquake J.J.M. analyzed data; and Y.L., J.J.M., and M.D.B. wrote the paper.y rupture patterns are observed along the Blanco Ridge trans- The authors declare no competing interest.y form of the northeast Pacific Ocean, where a ∼10-km barrier This article is a PNAS Direct Submission.y zone separates two distinctive moment magnitude (M ) 6.0 to w Published under the PNAS license.y 6.5 patches that rupture quasi-periodically every ∼14 y (2, 7). Data deposition: Seismic waveform data from the 2008 Gofar ocean bottom seismome- Repeated ruptures are also reported on transforms that offset ter experiment are archived at the Incorporated Research Institutions for Seismology slow-spreading ridges, such as the Charlie-Gibbs transform of (IRIS) data center, network code ZD, https://www.fdsn.org/networks/detail/ZD 2007/. the Mid-Atlantic Ridge (9), although large location uncertainties Data products can be accessed on Open Science Framework at https://osf.io/rw9bq/.y exist using teleseismic data. 1 To whom correspondence may be addressed. Email: [email protected] The persistent rupture barrier through multiple cycles, recent This article contains supporting information online at https://www.pnas.org/lookup/suppl/ OBS-based observations of the fine-scale seismicity distribution doi:10.1073/pnas.1913625117/-/DCSupplemental.y (3), and fault zone velocity structure on Gofar (5) jointly suggest First published April 28, 2020. 10188–10194 j PNAS j May 12, 2020 j vol. 117 j no. 19 www.pnas.org/cgi/doi/10.1073/pnas.1913625117 Downloaded by guest on October 1, 2021 A normal stress (normal stress minus pore pressure) is enhanced. −4.4˚ This dilatancy-strengthening mechanism has been proposed to be effective in inhibiting dynamic deformation in landslides, glacier tills, and fault gouges (17–19) and more recently in numerical models generating episodic slow slip events in subduc- M6 rupture tion zones (20, 21). Based on the aforementioned observational −4.6˚ “Barrier” evidence, we investigate the feasibility of dilatancy strengthening M6 rupture as a mechanism for the persistent rupture barrier behavior on the Gofar transform and its implications for the oceanic transform 010KM 20 30 fault slip budget. −106.4˚ −106.2˚ −106˚ −105.8˚ −105.6˚ OTF Earthquake Sequence Model Coupled with Dilatancy dilatancydilatancy dilatancy B (ab) We develop a three-dimensional (3D) numerical model based on 0 the Gofar transform, including OTF-specific loading and bound- 0.04 5 0.02 ary conditions, to simulate earthquake rupture sequences on 0 0 the western Gofar fault segment governed by laboratory-derived Depth (km) 10 −40 −30 −20 −10 0 10 20 30 40 rate- and state-dependent friction laws (22). In the rate–state Along−strike (km) framework, the friction stability parameter a − b dictates to the first order whether slip has the potential to grow into a seismic C a−b a dc (mm) (MPa) 0 0 0 0 event (a − b < 0, velocity weakening [VW]) or will remain sta- ble (a − b > 0, velocity strengthening [VS]) (23, 24) (Materials 5 5 5 5 and Methods). In our model, a − b is mapped on the OTF by Depth (km) converting temperature-dependent gabbro gouge rate–state fric- 10 10 10 10 0 0.02 0.04 0 0.02 0.04 0 20 40 0 20 40 60 tion data (25) using a western Gofar thermal structure calculated with hydrothermal cooling and visco-plastic rheology (10). Seis- Fig. 1. Gofar transform fault seismicity, M6 rupture segments, and earth- mic slip can be achieved when the down-dip dimension of the quake cycle model setup. (A) Seismicity was recorded and located in August VW segment exceeds a characteristic nucleation size defined by to December 2008 (black dots), surrounding an Mw 6 mainshock on Septem- ber 18, 2008. Yellow, foreshocks on September 10 to 12; red, aftershocks on the friction, normal stress, and pore pressure conditions on the September 18 to 20; cyan, swarm seismicity on December 7 to 8. White tri- fault (Materials and Methods). In the model presented here, the EARTH, ATMOSPHERIC, AND PLANETARY SCIENCES angles, seismometer; white stars, seismometer colocated with strong-motion above instability condition is always satisfied such that the entire sensor. White ellipses show the estimated rupture extents of the 2008 Mw 6 ∼4-km (1 to 5 km in depth) VW zone ruptures in a single earth- (west) and 2007 Mw 6.2 (east), with gray diamonds denoting respective cen- quake without an effective stabilizing mechanism (SI Appendix, troid locations.