Earthquake activity in the week 11-23 October 2012 can at one level be described as low key: there were just 98 earthquakes of greater than or equal to magnitude 4 (≥M4.0) and just 22 of ≥M5.0.
As a very broad rule of thumb, the United States Geological Survey gives an estimated figure of ~14,500 tremors of between M4.0 and M6.9 in typical year, or around 280 per week.
So, even in a quiet week with no great number or magnitude of earthquakes (the two largest were an M6.2 in the western Pacific, near Vanuatu, and an M6.0 off the Philippines, neither particularly unusual within the respective tectonic settings) most of which follow an expected distribution along the major plate boundaries, there are interesting features which reward further consideration – most notably, contrasting earthquakes in both east and west of the United States.
California Quake: Magnitude 5.3
In California, an M5.3 event occurred along the San Andreas Fault Zone around 30 miles from Coalinga and 160 miles from Sacramento. The fault zone represents part of the western boundary of the North American Plate which, over its entire length, changes character, from a subduction zone in the north to a spreading ridge in the south.
At this stage the plate margin is a transverse boundary, the famous San Andreas Fault, along which the two plates are slipping laterally past one another rather than in direct collision. The pattern of earthquakes is dictated by lateral movement as strain builds up and is eventually released: a glance at the distribution of earthquakes shows that they are closely related to the fault.
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