Structural control on earthquake behaviors revealed by high-resolution Vp/Vs imaging along the Gofar transform fault, East Pacific Rise

The Gofar transform fault (GTF), 4°S on the East Pacific Rise, can generate Mw 5.5-6 earthquakes quasiperiodically on some specific patches that are separated by stationary rupture barriers. Small earthquakes along strike show a clear spatial and temporal evolution. To better understand the cause of the observed behaviors of large and small earthquakes, we have determined high-resolution earthquake locations within a period of one year covering the 2008 Mw 6.0 (M6) earthquake, as well as Vp, Vs, and Vp/Vs models along the westernmost segment of the GTF, using a well recorded ocean bottom seismograph dataset and a new Vp/Vs model consistency-constrained double-difference tomography method. Compared to the previous P-wave tomography study in this area, the use of a new automatic arrival picking algorithm significantly improves the accuracy of S-wave arrival times, thereby allowing for the inversion of Vs and Vp/Vs models in addition to Vp model. High-precision waveform cross-correlation differential times are also used. The tomographic Vp/Vs model reveals strong structural variations at multiple scales along the fault, which likely control the behaviors of large and small earthquakes. The M6 mainshock is generated within a specific ∼8-km-long fault patch composed of intact rocks. By contrast, multiple fluid-filled damaged zones on both sides of this asperity are imaged and have varying size which is suggested to be critical in their ability of stopping ∼M6 ruptures. High-resolution earthquake relocations and velocity models also indicate that the occurrence of small earthquakes is also correlated with structural variations. Combined with previous studies, our results further suggest that strong structural variations control the fault mechanics and earthquake behavior along the GTF.