Once again it was a quiet week seismically speaking, with just 18 tremors of at least magnitude 5 (≥M5.0) and only 96 of ≥M4.0.
To place this in context, on the basis of a (very rough) statistical set collected for this digest since the beginning of the year, it is less than two-thirds of the average number of ≥M5.0 and approximately 86% of those of ≥M4.0 – indicating how much variation there can be in earthquake activity.
The Week’s Largest Earthquake: M6.2, Mexico
The largest tremor of the week was an M6.2 which struck in the Guerrero province of Mexico. With its western coast just inland from the subduction zone which marks the boundary between the Cocos plate and the North American plate, Mexico is vulnerable to regular and frequent earthquakes and this, like many others was associated with the process of subduction. The United States Geological Survey identified the fault mechanism responsible for the tremor as thrust (reverse) faulting associated with convergent motion.
Although at M6.2 the earthquake was not considered to be a major seismic event, the immediate area experienced strong shaking and initial projections from the USGS suggested a likelihood of some deaths, damage or injuries. In the event, however, no deaths or series injuries were reported although several buildings were damaged by shaking.
As a footnote, the Latino Times reported on 26 August that a Mexico-based seismologist has predicted that a devastating earthquake of at least M8 will occur in the country before the end of 2013. The prediction is based on an untried and unreviewed theory, and it is worth reiterating that earthquake prediction continues to elude seismologists: what we can say with certainty, however, is that this week’s earthquake will not be the country’s last.
Smaller Earthquakes: Europe
Europe features infrequently in the earthquake digest, but in a week where there has been relatively little seismic activity elsewhere it’s worth a brief examination.
Seismic activity in the west of the continent is usually associated with the convergence of Africa and Eurasia, which is responsible for the uplift of the Alps, Pyrenees and other European mountain ranges. This week, four earthquakes in south-western Europe – three in Italy and one in Spain – do indeed appear to be associated with this motion.
In the UK, however, earthquakes of any significant magnitude are rare (the largest ever to have occurred is thought to have been M6.1) and so this week’s M3.3 and M2.4 in the Irish Sea, close to the seaside town of Blackpool, have attracted comment.
The exact cause of the tremors is unclear but they are not associated with continental collision and probably resulted from normal slippage on existing faults, although some reports also suggested that they were caused by the slow elastic rebound of the land following the retreat of the last glacial ice cap some 10,000years ago – a process known as isostasy.
US Earthquakes: Nevada
In common with the rest of the planet, the United States experienced little significant seismic activity this week. The largest in the lower 48 states was an M4.2 which occurred in the state of Nevada, where stresses between the two tectonic provinces of the Sierra Nevada and the Basin and Range are manifested by numerous, mainly lateral, faults.
Small Quakes Are Interesting Too
Major earthquakes dominate the news, but smaller tremors, such as those in Europe and Nevada, are interesting too. These demonstrate that although major plate motions produce the larger earthquakes, the earth’s many tectonic blocks, large and small, are constantly straining against one another and that, eventually, something has to give – causing even solid rock to shift.
Latinos Post. Scientist Claims That Devastating Earthquake Will Hit Mexico City Before Year’s End. (2013). Accessed 28 August 2013
Mail Online. Homes in Blackpool rocked by two earthquakes under Irish Sea ’caused by glaciers during the ICE AGE‘. (2013). Accessed 28 August 2013
USGS. M6.2 – 13km NNW of San Marcos, Mexico. (2013). Accessed 28 August 2013
USGS. Real time earthquake map. (2013). Accessed 28 August 20
The Weather Channel. Acapulco Buildings Damaged, Mexico City Rattled by 6.2-Magnitude Earthquake. (2013). Accessed 28 August 2013
Yeats, R. Active Faults of the World. (2012). Cambridge University Press.