In yet another week of relatively little significant seismic activity, the United States Geological Survey’s real time map showed just one tremor of at least magnitude 6 (≥ M6.0).
There were 21 other incidents of ≥M5.0 and the total shown on the map (which includes all recorded tremors in the US and its territories and those of ≥M4.5 elsewhere) was 1,915.
The main focus was on the western Pacific, but the week also saw significant activity in the Caribbean.
Changing Levels of Earthquake Activity?
This week is just the latest in which there has been little significant earthquake activity to report – something which raises questions about how earthquake activity varies over time.
Looking back over 2013 and comparing it with other years proves instructive. Recent data from the USGS indicate “1,194 quakes magnitude 5.0 or larger in 2013… a number that changes annually; in 2012, 1,558 quakes magnitude 5.0 or larger were measured, and in 2011, 2,495.”
Although this appears to suggest a decline in activity, looking further back indicates otherwise: in 2003 there were just 1,358 tremors of M5.0 or more and in 2002 the figure was 1,331.
Looking back over 2013 shows similar variation. At a weekly scale the number of tremors can be skewed by a major event, which might generate dozens, or sometimes hundreds, of shocks ≥M5.0.
A (rough and incomplete) data set collected by Decoded Science over the year demonstrates this: the spike shown in the graphic represents aftershocks from February’s M8.0 which struck in the Solomon Islands. But as the figure for all of those tremors recorded on the map shows, the total number of earthquakes fluctuated on a weekly basis but showed no overall trend.
The Week’s Largest Earthquake: M6.4, Puerto Rico
Downgraded from an initial rating of M6.5, the largest tremor of the week was an M6.4 which occurred off Puerto Rico on 13 January. Tectonically speaking, the Caribbean is complex, formed by a piece of the Earth’s crust caught between the continents of North and South America and the Cocos plate, which converges with them.
The complex movements involved mean that the Caribbean has different types of plate margin. This week’s tremor occurred at a section along the northern edge where the North American plate is being subducted beneath the Caribbean plate; early indications suggest that this subduction was the direct source of faulting. Although such earthquakes may be tsunamigenic, no tsunami was generated and no damage or injuries had been reported at the time of writing.
Earthquakes on the Kermadec Trench, South Western Pacific
The western Pacific is commonly the source of the larger recorded earthquakes in any given week. This week one of the larger tremors took place to the west of L’Esperance rock, north of New Zealand. Like the Puerto Rico tremor, this (at M5.6) occurred at a subduction zone but at a greater depth. It illustrates the association between subduction and the depth and location of earthquakes.
As earthquakes occur close to the interface between the plates they vary with depth, with the result that the further down the interface they occur, the further into the overriding plate the epicentre (the point at which the earthquake is recorded on the surface) will be. This is not universally the case, however, as not all subduction zone earthquakes occur at the interface so that faulting within the over-riding plate may give ride to shallow tremors.
It’s tempting to compare earthquakes year-on-year and draw conclusions about trends – but to do so invites error. It’s worth noting that “the USGS estimates that several million earthquakes occur throughout the world each year, although most go undetected because they hit remote areas or have very small magnitudes”. What we see recorded on the maps represents just a fraction of the movement of our dynamic planet.