And still it moves. The Earth kept shaking in the week of 11-17 April, so much so that the bare statistics of tremors recorded on the United States Geological Survey’s real-time map (all magnitudes in the US and its territories and greater than M4.0 worldwide) don’t begin to tell the tale.
For what it’s worth, these are the statistics. There were three earthquakes of at least magnitude 7 (≥M7.0); nine of ≥M6.0; 60 of ≥M5.0; 213 of ≥M4.0; and 1,636 in total.
Just for comparison, the figures for the same period in 2013 – which was itself more active than average – were one ≥M7.0; four of ≥M6.0; 33 of ≥M5.0; and 131 ≥M4.0.
Much of this activity can be explained by two factors; the continuation of the aftershock series from the M8.2 Iquique earthquake in Chile; and the coincidence of two large earthquakes and their aftershocks in the western Pacific this week. Between them, these accounted for almost two thirds of the week’s larger tremors (≥M4.5)
Separate Earthquakes in the Western Pacific
The dominant seismic events this week were, without question, the tremors of M7.6 and M7.4 which struck off the Solomon Islands and those of M7.1 and M6.5 south of New Guinea. Although, dealing with these together is potentially misleading because there’s no evidence that the two are linked, it’s worth looking at them to see how they differ.
The boundary between the Pacific and Australian plates in this area is complex, the margin congested with smaller micro plates. The nature of the margin changes from convergent to conservative and there are areas of sea floor spreading where new crust is being generated.
The larger earthquake, in the Solomon Islands, resulted from lateral movement along the two plates. That which struck in Papua New Guinea was a subduction earthquake, where the two converge. This explains why the smaller of the two generated a minor tsunami while the larger did not; tsunami genesis requires a degree of lateral movement associated with the thrust faulting typical of subduction.
Earthquakes in Nicaragua
By comparison with activity in the western Pacific, Nicaragua’s M6.6, which occurred on 11 April and would have been a major event in most other weeks, seems rather small beer.
Data from the USGS suggest that this, too, was a subduction earthquake, caused by the convergence of the Cocos plate with the North American and Caribbean plates.
Although it largely slipped under the radar of news media, there are suggestions that this tremor may, in fact, be potentially the most newsworthy of them all.
Early reports on the BBC and elsewhere suggested that this tremor may have reactivated a fault which was the source of a major earthquake in 1972. At the time of writing, however, it appears that activity in the region has decreased – although it’s impossible to know what the future might hold.
Earthquakes in the US
After recent focus upon California and the Rockies, the largest tremor of the week on US soil took place in Alaska. Although the northern Pacific is a subduction zone, the westernmost part is complicated by different directions of movement and the existence of crustal micro plates. It appears from the available data that this week’s tremor was the result of compression but it isn’t clear what type of faulting was involved.
Earthquakes All Over the World
In a quieter week, this digest would have looked at some of the less usual tremors – an M6.9 in the South Atlantic was large for its setting, and the sleepy English county of Rutland woke up to an M3.4 on 17 April. But the roundup is dominated by size and by potential damage in a week marked out by an unusually high number of tremors.