Earthquakes are a feature of our planet: they occur constantly, often too small to be felt. Although they don’t necessarily follow any predictable pattern, looking at the number and spatial distribution of earth tremors over a given time period can prove instructive in terms of understanding their origin and nature.
Looking at the earthquakes which occurred in the week from 21-28 August 2012 as a whole (i.e. all of those recorded by the USGS as greater than magnitude 2.5) and comparing them to the distribution of more significant earthquakes (in this case defined as at least M5.0) shows an association between magnitude and location.
Significant Earthquakes Greater than M5.0
The USGS recorded 29 earth tremors greater than M5.0 over this seven-day period, the largest of them being an M7.3 event off the coast of El Salvador, and the next largest an M6.6 off Indonesia. The rest of the quakes that occurred around the world were smaller than M6.0. Both of these earthquakes were accompanied by foreshocks and/or aftershocks of around M5.0.
The most notable feature of these larger earthquakes, in terms of their spatial distribution, is how closely related they are to the boundaries of the earth’s tectonic plates. This is entirely consistent with expectations: where the plates move past one another, come together, or move apart, they generate friction and so earthquakes occur.
Most of the larger earthquakes are associated with subduction zones, where one plate is forced below another: again, this is to be expected. There are, however, exceptions. The M5.5 and associated shocks in California are associated with the point at which the transform boundary between the North American and Pacific plates (the San Andreas Fault Zone) becomes a constructive boundary (the East Pacific Rise); and the M5.0 in Nepal took place at the collision zone between the Indian and Eurasian plates.
Distribution of All Earthquakes Greater Than M2.5
The distribution of all earthquakes broadly follows the same lines of seismic stress, although some earthquakes occur away from the main plate boundaries. In some cases there is an obvious association with these boundaries – in Alaska and Canada, for example, a line of smaller earthquakes occurs along the line of a subduction zone but at a distance behind it.
Elsewhere, earthquakes occur within the tectonic plates rather than at their edges. There were examples of these in Colorado and Montana, as well as in Kazakhstan and Central Russia. Without detailed information it’s impossible to be specific about the causes of individual events but it’s likely that they are associated with the processes of mountain-building in the Himalayas and the Rockies.
Earthquakes and Volcanoes: Hawaii
The cluster of minor earthquakes occurring in the Hawaiian Islands is worthy of separate comment. Ranging from M2.3-M3.6, these tremors are scattered on and around the main island. The islands are in the middle of the Pacific plate and the forces at play here are creative: upwelling of molten rock forms a ‘hot spot’ which effectively punches a hole through the Earth’s crust. Volcanic activity involves movement of vast quantities of rock – and this movement generates earthquakes.
United States Geological Survey. Real-time Earthquake Map. (2012). Accessed 28 August 2012