Some weeks, it’s a struggle to find noteworthy earthquakes to include in a digest. Others, the struggle is deciding what to leave out.
The seven days from 9-15 October 2014 fall very definitely into the second category. The bald figures are these: Of the 1390 tremors (all magnitudes in the US and its territories and at least magnitude 4 (≥M4.0) elsewhere) recorded on the United States Geological Survey’s real time earthquake map, 141 exceeded M4.0 and 36 were greater than M5.0.
Two of the earthquakes were larger than M7.0 — itself worthy of note since, as a very general rule of thumb, we might expect to see an event of this magnitude around once every month.
The map shows clusters of earthquakes on the East Pacific Rise and off the Nicaraguan coast (both aftershock sequences following major tremors; a continuing series of tremors around Bárðarbunga volcano in Iceland; and tremors of ≥M6.0 close to New Zealand and Japan which, in another week, would both have been candidates for closer examination.
The Week’s Largest Earthquake: M7.3, Offshore Nicaragua
Tuesday 14 October saw a major and damaging earthquake, with two aftershocks, off the Nicaraguan/El Salvadorean coast. At M7.3 the earthquake is a significant event; reports indicate that at least one person was killed and some damage caused near to the coast.
Initial information from the USGS suggests that shaking was strong for around a 200km length of the coast and the Pacific Tsunami Warning Center initially issued a tsunami warning for coasts within 300km, although that warning was later cancelled.
Such earthquakes are by no means uncommon along the Pacific coast of central America. The subduction of the Earth’s Cocos plate beneath the Caribbean plate along the Middle America trench means much of the area is vulnerable to earthquake activity, either as result of movement at the boundary between the two plates or because of deformation of the crust away from the interface.
In this case, the USGS summary notes that the latter explanation is more likely: In other words, “either within the subducting oceanic Cocos plate, or in the accretionary wedge of the overriding Caribbean plate, rather than on the main subduction zone thrust.”
This is consistent with the absence of a tsunami since such major waves are more usually associated with vertical movement along the plate boundary itself.
M7.1 Quake, East Pacific Rise
The second of the ≥M7.0 tremors this week was, in some ways, the more interesting.
Though it was large (and accompanied by nine other tremors ≥M4.5) and occurred in the Pacific Ocean, this quake caused barely a comment in the news media and there was no tsunami (or, indeed, tsunami warning).
But this tremor’s size is significant because it occurred at a constructive margin where earthquakes are common but are typically much smaller than at either subduction or strike slip zones.
The earthquake occurred close to the triple junction between the Antarctic, Pacific and Nazca plates, where diverging crust causes rise of magma and crustal stresses which generate earthquakes.
The nature of the boundary is complex; the USGS remarks that “Some authors identify a small microplate in this region” and it appears that the earthquake may have been associated with its northern margin.
US Earthquakes: When Will We Stop Talking About Oklahoma?
Oklahoma? Again? Last week Kansas produced the largest earth tremor in the southern central states and this week Oklahoma hit back; an M4.3 near Cushing on 10 October was the biggest in the state in the past month and came close to matching Kansas’ M4.4 of a few days before.
Without detailed statistical assessment it’s impossible to say for certain whether the earthquake swarm in the area is growing in intensity and in size; but to the interested observer, it certainly looks as if it might be.
Earthquakes and Statistics
Not all seismologists are statisticians; but statistics are a useful tool in the study of earthquakes.
A statistician might (with the appropriate data) be able to demonstrate that two large earthquakes within a week aren’t statistically significant, or look at the patterns of change in Oklahoma and decide that they are.
I may not be doing the maths; but you can bet that in the latter case at least, someone, somewhere, is.