Earthquake swarms of the Equatorial Mid-Atlantic Ridge 4˚-5˚N from
regional seismic recordings
Abstract
Tectonic information available from remote seismometers is compromised
by uncertainties in epicenter and depth, which often prevent earthquakes
being associated with faults observable in morphologic data. Here we
show that careful study of regional recordings can achieve reasonable
position and, for the larger events, also depth estimates. We illustrate
this with swarms totallying 90 earthquakes of 3.6 mb to 5.5 Mw recorded
at coastal stations of the Brazilian Seismographic Network, GSN,
GEOSCOPE and Cape Verdes. Seismicity occured in 4 swarms, firstly in
2012 with 7 events in the valley floor and eastern wall near the
ridge-transform intersection (4.05º-4.2ºN). Next, five events occurred
in 2014, with three in the inner floor (4.8º-4.9ºN) and two under the
valley wall (4.7º-4.8ºN). A 70-earthquake swarm in 2016 occurred in the
4.4º-4.8ºN valley floor, in both walls and floor, involving slip on
different faults. The last 8 events in 2019 were located outside the
median valley near a volcanic seamount. We estimated focal depths of the
strongest events with Mw > 5.4 using waveform modeling and
focal mechanims reported by the Global Centroid-Moment-Tensor Project.
Our best estimated hypocenters lie 5-8 km beneath the seafloor, in
keeping with maximum depths typically found with OBSs on the MAR. In
contrast, deeper events (10 km) have been found in an OBS experiment
around an active detachment fault elsewhere. Multibeam sonar data from
the area do reveal a detachment fault surface (“megamullion”), but it
ends 10-15 km from the median valley floor, suggesting that it is
probably inactive. Although our uncertainties still do not allow the
event depths to be discriminated from 10 km, our best estimates are more
compatible with shallower normal faulting. Over time, repeating this
exercise with many such datasets and comparing with morphologic data
should help to resolve the incidence of deeper ruptures associated with
detachment faults.