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.
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.
Tectonics of the Indonesian Earthquake
The week’s largest earthquake was an M6.3 occurring to the south of Indonesia and the north of Australia: it was part of a cluster of seven tremors of greater than or equal to M4.0.
Tectonically, the region is dominated by the subduction of the Australian plate against the Eurasian Plate. This is, however, by no means the whole picture, which is complicated by the existence of many microplates squeezed along the zone between the two larger plates. Here, different types of boundaries create different and conflicting stresses, with the result that the whole of the region is highly tectonically active.
The map shows that earthquake activity along the diffuse boundary is common, and within this context an earthquake of M6.3, which would be considered significant elsewhere, is by no means atypical in a region which has been the location for some of the world’s largest and most damaging seismic events.
Tectonic Activity This Week
Without any major earth tremors, the week nevertheless showed that the planet remains constantly active. Where more detailed data are available, areas of recent activity continue to show a multitude of minor aftershocks – most notably, the cluster of minor tremors off the British Virgin Islands and in southern California, where almost 200 earthquakes of less than or equal to M4.0 were recorded.
Bergman, Eric A. and Solomon, Sean C. Source mechanisms of earthquakes near mid-ocean ridges from body waveform inversion: Implications for the early evolution of oceanic lithosphere. (1984). Journal of Geophysical Research. Accessed 9 October, 2012.
United States Geological Survey. Real time earthquake map. (2012). Accessed 9 October, 2012.
United States Geological Survey. Seismotectonics of the New Guinea Region and Vicinity. (2012).Accessed 9 October, 2012.