In the past week, there have been a few earthquakes along the Mid Atlantic Ridge (MAR) and some associated fracture zones. The Mid Atlantic Ridge is a divergent plate boundary. As the plates move apart, the asthenosphere is decompressed and magma rises to the surface to create new oceanic lithosphere. The youngest oceanic crust is along these oceanic spreading centers/ridges. When these spreading ridges are offset laterally, transform plate boundaries called fracture zones form. The MAR has many fracture zones. This Mw = 6.3 earthquake occurred northwest of St. Helena, where Napoleon spent his last years of his life (and experienced a large earthquake which is known as Napoleon’s Earthquake on 1796.10.22).
Here is a map showing the southern swarm is related to the spreading center and that the seismicity in the north is strike-slip motion on a fault probably related to a fracture zone, possibly the Vernadsky or Bogdanov fracture zones (looks like it is on a fracture zone between these two, but I am uncertain about which fz is which). The southern earthquake magnitude M = 6.3 moment tensor for the spreading ridge earthquake is compressional (blue arrows). This spreading ridge is between the St. Helena and Hotspur fracture zones. The northern earthquake magnitude M = 5.1 moment tensor matches what we would expect for a fracture zone in this region (green arrows). I found these fracture zones labeled on a couple maps (Bonatti et al., 2010 and online from Woods Hole, and the USGS earthquake maps).
Here are the USGS web pages for the three largest magnitude earthquakes in the above map:
- 2015.05.24 M 5.1 northern earthquake
- 2015.05.24 M 6.3 southern mainshock
- 2015.05.25 M 5.2 southern aftershock
- Bonatti, E. et al., 2010. Geological studies of the Eastern Romanche Transform (Equatorial Atlantic): a first report, Cruise S-13, R/V A.N. RAPPORTI ISMAR, v. 1, 12 pp.