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In-situ Vp/Vs ratio reveals fault-zone material variation at the westernmost Gofar transform fault, East Pacific Rise
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  • Tianze Liu,
  • Jianhua Gong,
  • Wenyuan Fan,
  • Guoqing Lin
Tianze Liu
University of California, San Diego

Corresponding Author:[email protected]

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Jianhua Gong
Scripps Institution of Oceanography
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Wenyuan Fan
Scripps Institution of Oceanography, UCSD
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Guoqing Lin
University of Miami
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Ocean transform faults often generate characteristic earthquakes that repeatedly rupture the same fault patches. The westernmost Gofar transform fault quasi-periodically hosts ~M6 earthquakes every ~5 years, and microseismicity suggests that the fault is segmented into five distinct zones, including a rupture barrier zone that may have modulated the rupture of adjacent M6 earthquakes. However, the relationship between the systematic slip behavior of the Gofar fault and the fault material properties is still poorly known. Specifically, the role of pore fluids in regulating the slip of the Gofar fault is unclear. Here, we develop a new method using differential arrival times between nearby earthquakes to estimate the in-situ Vp/Vs ratio of the fault-zone materials. We apply this technique to the dataset collected by an ocean-bottom-seismometer network deployed around the Gofar fault in 2008, which recorded abundant microearthquakes, and find a moderate Vp/Vs ratio of 1.75–1.80 in the rupture barrier zone and a low Vp/Vs ratio of 1.61–1.69 in the down-dip edge of the 2008 M6 rupture zone. This lateral variation in Vp/Vs ratio may be caused by both pore fluids and chemical alteration. We also find a 5–10% increase in Vp/Vs ratio in the barrier zone during the nine months before the mainshock. This increase may have been caused by fluid migrations or slip transients in the barrier zone.