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Seismicity Properties of the Chain Transform Fault Inferred from OBS Data Obtained from the PI-LAB Experiment
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  • Konstantinos Leptokaropoulos,
  • Catherine Rychert,
  • Nicholas Harmon,
  • Michael Kendall
Konstantinos Leptokaropoulos
University of Southampton

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Catherine Rychert
University of Southampton
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Nicholas Harmon
University of Southampton
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Michael Kendall
University of Oxford
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Abstract

Oceanic Transform Faults (TF) comprise first order discontinuities bounded between mid-ocean ridge spreading centres. TF mainly accommodate strike slip motion, separating lithospheric plates of different age and thermal structure. Oceanic TF are intriguing in that they do not produce earthquakes as large as might be expected given their long length, with seismic slip corresponding only to a small fraction of the total tectonic slip. The relative geologic simplicity of oceanic TF means that they are an important analogue for more hazardous continental TF, with high potential for improving insights into the earthquake cycle. We investigate the earthquake properties along Chain, a ~300 km long TF in the equatorial MAR by combining both microseismic and teleseismic data. We use the ~1-year microseismicity data (total of 812 events) gathered during the PI-LAB (Passive Imaging of the Lithosphere-Asthenosphere Boundary) experiment and EURO-LAB (Experiment to Unearth the Rheological Lithosphere-Asthenosphere Boundary). We perform cluster analysis in multi-dimensional phase space, consisting of various seismic (epicentral coordinates, magnitude) and geophysical (gravity anomalies, bathymetry, tidal height) parameters. We investigate potential triggering mechanisms, including tidal, static and dynamic stresses. We extend our analysis back in time by considering stronger earthquakes (MW>~5.0) from Global Centroid Moment Tensor (GCMT) since 1976. We find three unique, 50-100 km long clusters or segments from our analysis going from east to west, separated by seismic gaps. Microseismic activity is highest at the eastern segment of Chain where there is the largest positive flower structure, negative rMBA gravity anomaly but very few M>5.5 events. The western segment has reduced seismicity rates relative to the eastern, and is associated with a positive rMBA and a few small flower structures. The central segment is bounded between two seismic gaps and demonstrates relatively high activity rates in the middle. Our result suggests that trans-pression of highly altered mantle/crust and/or high pore pressure due to hydrothermal fluid circulation in the eastern flower structure enhances seismic activity. Overall, we find the existence of consecutive locking and creeping segments, with some of the patches exhibiting hybrid behaviour, potentially causing their sporadic activation/reactivation.