Earthquakes of at least M5.0 for the week ending 9 October 2012 – Image courtesy of USGS
The 240 recorded earthquakes of at least magnitude 2.5 shown on the USGS real time earthquake map for the week 3-9 October showed the expected pattern with the majority of tremors (including all greater than or equal to M5.0) occurring along the boundaries of the earth’s tectonic plates, and a scattering of smaller events occurring within plates in more stable areas (including tremors of M3.0 in Oklahoma and M2.5 in eastern Canada).
Mid Ocean Ridge Earthquakes
The map shows a trio of tremors (two of M5.5 and one M5.7) occurring in the central Atlantic Ocean, along the ridge which forms the axis of the ocean where the African and American plates are moving apart and new ocean crust is being created. Such events, though by no means uncommon, are fewer in number and smaller in magnitude than those at transform or convergent plate boundaries.
At ocean ridges, plate movement is largely extensional, although “all types of faulting styles are observed,” according to Bergman and Solomon’s 1984 article, Source mechanisms of earthquakes near mid-ocean ridges from body waveform inversion: Implications for the early evolution of oceanic lithosphere.
Transform faults offset mid ocean ridges -Image courtesy of Pimvantend-
Because the structure of ocean ridges includes fault segmentation, with each length of the ridge being offset by lateral (transform) faults, earthquakes are an expected feature of the process. Without detailed knowledge of the local tectonics of the three earthquakes it’s impossible to say exactly which type of faulting was responsible, although their location (a few kilometres from the ridge itself) suggests that they may have been generated by movement along these transform faults.
Click to Read Page Two: Tectonics of the Indonesian Quake
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