The week of 5-11 September was a relatively quiet period seismically. The United States Geological Survey’s interactive map, showing the week’s earth tremors, recorded a relatively modest 414 tremors of greater than M2.5 – of which just 35 were above M5.0, and just four larger than M6.0.
The largest earthquake recorded in the week was the M7.6 event which took place in Costa Rica on 5 September; the result of faulting along a subduction zone which lies to the west of Central America. Here, the Cocos Plate is forced beneath the Caribbean Plate, and earthquakes (and volcanic activity) are a regular result. The data available show just one significant aftershock, an M5.6 which occurred three days later, on 8 September.
The three other noteworthy tremors in terms of magnitude all occurred along the eastern boundary of the Pacific Plate and were associated with its subduction. They were:
- An M6.1 event in Indonesia, where the convergence of the Australian and Pacific plates is complicated by the presence and relative movement of numerous microplates
- An M6.0 event in Vanuatu, hundreds of miles further east along the same plate boundary, again where the Pacific Plate is subducted beneath Australasia
- An M6.0 event off the tip of the Kamchatka peninsula in the north west Pacific where the Pacific Plate is subducted beneath an outlying arm of the North American Plate.
Smaller Tremors Worldwide
While the USGS data set includes all major earthquakes worldwide, it shows earthquakes below M4.0 only for the United States area. Looking at a map of all 1386 recorded events, therefore, not only underestimates the total number of tremors to have occurred but may also give the misleading impression that America is more seismically active than elsewhere.
To illustrate this further, comparison of USGS data with the European Mediterranean Seismological Centre’s own interactive map of European earth movements, for example, shows that even the largely tectonically stable continent of Europe experienced 313 recorded earth tremors – though just four of these appear on the USGS map.
Earth Tremors and Fault Lines
Examination of all earthquake activity in the States can provide useful illustrations of earthquake patterns and activity. The association of earthquakes with fault zones, for example, is well known and is beautifully exemplified by the pattern of earthquake activity along the San Andreas Fault in central California, where a multitude of small earth tremors occurred on or very close to the fault. Here, the mechanism is sideways movement of two tectonic plates.
Similarly, examination of the small tremors and faults in Alaska shows a different pattern, with the clusters clearly associated with, though not directly following, fault lines. In this case the explanation for the spatial distribution of tremors is due to the ‘ripple’ effect from the northward-moving Pacific Plate being forced under the North American Plate. This movement, together with the complications arising from the subduction of a much smaller sliver of subducting crust, causes faulting and associated earth movements at a distance from the main subduction zone.
Even in a week that might be termed relatively quiet seismologically, the Earth is still moving and is generating, in total, many thousands of earthquakes associated with tectonic plate movements and fault lines – though only a small proportion of these are of any significance or cause much damage.
European Mediterranean Seismological Centre. Euro-Med Earthquakes. Accessed 11 September 2012.
USGS. Interactive Map Earthquakes – 7 days. All Accessed 11 September 2012.