The seven days up to September 3, 2012 saw earthquake activity that followed the classic pattern of tremors along the boundaries of the huge tectonic plates which make up the Earth’s surface, with fewer, generally smaller, events occurring within the plates.
The United States Geological Survey recorded almost 600 earthquakes of at least magnitude 2.5 and 42 of these were greater than, or equal to M5.0.
Earth Movements in the Philippines
The largest tremor to strike in this week was the M7.6 which occurred to the east of the Philippines, in the location where the Philippine plate is subducted beneath the melange of microplates which form a buffer between it and the Eurasian plate.
This earthquake was followed by over 40 smaller tremors, greater than or equal to a magnitude of 4.0, as the crust readjusted after the mainshock. The significance of this earthquake can be accounted for by the fact that this area was responsible for almost half of the earthquakes of ≥M5.0 to have occurred during the week.
The western Pacific as a whole is highly seismically active, as is demonstrated by a map of historic seismicity in the area, and the Philippines experienced two other earthquakes of M5.6 and M5.8 during the week, both of which occurred further west. In contrast to the larger event and its aftershocks, these are more likely to be associated with the relative movements of the area’s microplates, where plates moving together and apart cause continued compressional and extensional movement along faults, rather than the subduction of the Philippine plate.
The North Atlantic Earthquakes
The week’s quake map shows a cluster of three significant earthquakes in the North Atlantic. The largest of these was M6.8 just off Jan Mayen Island, followed shortly after by an M5.2. The third, some three days later and hundreds of miles to the north, seems unlikely to be directly related to the first two, although all were generated by the same mechanism.
The three North Atlantic earthquakes all took place along the Mid-Atlantic Ridge, a constructive boundary which (as the name suggests) runs along the length of the Atlantic Ocean, and where new crust is being generated between the North American plate in the west and the Eurasian and African Plates in the east. The upwelling of new crust causes faulting and fracturing, and earthquakes occur – typically small and shallow in nature, as a map of historical seismicity demonstrates.