( C) Depth distributions of friction parameters a − b, a, characteristic slip distance d c, and effective normal stress σ ¯. Area within two white lines (friction stability transition, a − b = 0) has velocity-weakening friction ( a − b 0). ( B) Rate–state friction stability parameter ( a − b) mapped on the transform fault representing western Gofar. Image credit: modified from McGuire et al. White ellipses show the estimated rupture extents of the 2008 M w 6 (west) and 2007 M w 6.2 (east), with gray diamonds denoting respective centroid locations. White triangles, seismometer white stars, seismometer colocated with strong-motion sensor. Yellow, foreshocks on September 10 to 12 red, aftershocks on September 18 to 20 cyan, swarm seismicity on December 7 to 8. ( A) Seismicity was recorded and located in August to December 2008 (black dots), surrounding an M w 6 mainshock on September 18, 2008. Gofar transform fault seismicity, M6 rupture segments, and earthquake cycle model setup. Repeated ruptures are also reported on transforms that offset slow-spreading ridges, such as the Charlie-Gibbs transform of the Mid-Atlantic Ridge ( 9), although large location uncertainties exist using teleseismic data.įig. Similar earthquake rupture patterns are observed along the Blanco Ridge transform of the northeast Pacific Ocean, where a ∼10-km barrier zone separates two distinctive moment magnitude (M w) 6.0 to 6.5 patches that rupture quasi-periodically every ∼14 y ( 2, 7). 1) and the relative centroid locations of M6.0+ events on Gofar since 1992 ( 2, 8). The rupture pattern is inferred from the 2008 foreshock and aftershock distributions ( 3) ( Fig. On the Gofar transform of the East Pacific Rise (EPR), M ∼ 6 earthquakes repeat quasi-periodically every 5 to 6 y on 15- to 20-km along-strike segments separated by a ∼10-km barrier zone ( 8). Recent ocean bottom seismometer (OBS) deployment experiments along OTFs associated with fast-spreading ridges provide near-field seismic data that shed light on the source mechanism of OTF earthquake rupture patterns and possible interaction between aseismic and seismic slip modes ( 3– 7). Our model thus suggests the possible prevalence of episodic aseismic transients in M ∼ 6 rupture barrier zones that host active swarms on oceanic transform faults and provides candidates for future seafloor geodesy experiments to verify the relation between aseismic fault slip, earthquake swarms, and fault zone hydromechanical properties.Īveraged globally, oceanic transform faults (OTFs) release a small percentage ( ∼15%) of their accumulated moment seismically, and despite the large thermally defined potential rupture area the largest observed earthquakes have only moderate magnitudes (up to magnitude 7 ) ( 1, 2). The modeled slow slip migrates along the barrier zones at speeds ∼10 to 600 m/h, spatiotemporally correlated with the observed migration of seismic swarms on the Gofar transform. In the model, strong dilatancy strengthening, supported by seismic imaging that indicates enhanced fluid-filled porosity and possible hydrothermal circulation down to the brittle–ductile transition, effectively stabilizes along-strike seismic rupture propagation and results in rupture barriers where aseismic transients arise episodically. Here we present a numerical modeling study of earthquake sequences and aseismic transient slip on oceanic transform faults. However, the underlying mechanisms that govern the partitioning between seismic and aseismic slip and their interaction remain unclear. Oceanic transform faults display a unique combination of seismic and aseismic slip behavior, including a large globally averaged seismic deficit, and the local occurrence of repeating magnitude (M) ∼ 6 earthquakes with abundant foreshocks and seismic swarms, as on the Gofar transform of the East Pacific Rise and the Blanco Ridge in the northeast Pacific Ocean.
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