Seismicity Properties of the Chain Transform Fault Inferred from OBS
Data Obtained from the PI-LAB Experiment
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.