With relatively few earthquakes of at least magnitude 5 (≥M5.0) across the globe, the main point of note is that the largest of the week was not at a subduction zone but at a constructive ridge.
This was the only tremor to exceed M6: there were just 18 of ≥M5 and 70 ≥M4.
In other respects, the week was typical, with a concentration along the subduction zones of the western Pacific and the Andes: slightly less usual was the scattering of events of M5.0 or greater across the interior of Eurasia.
The Week’s Largest Earthquake: M6.4 in the Atlantic
The earthquake which occurred on 24 June in the middle of the Atlantic troubled no-one: the closest land is French Guiana, almost 1250 km distant. The tremor occurred either on an ocean ridge itself or on an associated fracture zone: typically the extensional movement means that faulting in the first case involves downwards movement (normal faulting) while earthquakes with epicentres between the offset sections of ridges are characterised by lateral, or strike-slip, faulting. This contrasts with subduction zones, where thrust faulting dominates.
At M6.4 (preliminary magnitude M6.6) the event was unusually, though not exceptionally, large for a spreading ridge, where magnitudes in excess of M7 are rare. In July 2012, for example, a tremor in the Indian Ocean registered M6.7, while the largest known ocean ridge earthquake was M8.3 – extremely large for its setting but still only around one-fifth the size of the largest subduction zone earthquakes.
The Mariana Trench: Ocean Crust Collides
A cluster of earthquakes in the western Pacific, along the Marianas Trench, marks a subduction zone where ocean crust sinks below ocean crust, generating a system of deep sea trench backed by volcanic island arcs. These island arcs, created from dense ocean crust which rises as more buoyant material, are the building blocks of continents.
Although it’s not known with any certainty what triggers subduction, once the subduction process starts, the older, colder and consequently denser crust will be forced downwards. In the western Pacific, the Pacific plate (which consists of some of the oldest oceanic crust on Earth) is being subducted beneath the younger and warmer Philippine Sea plate. Such areas are inevitably characterised by seismic activity.
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