After last week’s embarrassment of earthquake riches, the Earth presented a smorgasbord of relatively small events in the week of 16-22 October.
The United States Geological Survey’s real time earthquake map (which shows tremors of all magnitudes in the US and its territories and of at least magnitude 4 elsewhere) didn’t show a single tremor of 6 or more (≥M6.0).
Having said that, there was plenty of activity; of the 1,560 events shown on the map, 30 tremors exceeded M5.0 and 145 were ≥M4.0.
The distribution followed the boundaries of the planet’s tectonic plates with little or no significant activity elsewhere; and the only significant cluster was that related to Iceland’s Holuhraun fissure eruption (discussed below).
The Week’s Largest Earthquake: M5.8, Japan
Japan and the archipelagos to the north and south are earthquake territory – the result of the coming together of four tectonic plates (the Pacific, Philippine Sea and Eurasian plates and the Okhotsk microplate).
And not just any old earthquakes either; the region has produced some of the largest and most devastating on record.
Bearing this in mind, the coronation of an M5.8 as the week’s largest earthquake seems almost apologetic. The tremor occurred as a result of the subduction of the Philippine Sea plate beneath Eurasia along the Ryuku Trench – a standard, medium-sized subduction earthquake.
To put in in context, this year alone, Japan has experienced close on two hundred earthquakes of at least M5.0 and nine of these were ≥M6.0; so it’s likely that this particular tremor, hundreds of km from the main Japanese islands, passed largely unremarked.
Earthquake Cluster in Iceland
The volcanic eruption at the Bárðarbunga volcanic complex continues and seismic activity continues alongside it.
This week the rifting along this section of the margin between the Eurasian and North American plates generated a cluster of 19 earthquakes of ≥M4.5, the largest of them M5.2.
The cluster of seismic activity is directly related to magmatic activity and the map shows earthquakes relating to both the intrusion of a magma dyke which feeds the eruption and the deflation of the Bardabunga crater.
What this does is demonstrate the (fairly obvious) fact that magma comes from somewhere and goes somewhere; when it moves in large quantities, it shifts the rock around it.
The Iceland Meteorological Office’s earthquake maps are an elegant visual demonstration of just that.
US Earthquakes: M5.0, Alaska
On the earthquake summary Alaska’s M5.0 stands out, alone at a distance from the destructive subduction zone of the Aleutian arc, where the Pacific plate descends beneath the North American plate.
As is so often the case, however, there’s more to this than meets the eye. Although the M5.0 was relatively large for the region it’s just one of almost 500 earthquakes of all magnitudes to strike Alaska this week — all of them part of a broad zone of crustal deformation caused by the relative movements of the two plates.
Last Thoughts: Predicting Earthquake
We can’t predict earthquakes. We can’t know from one week to the next whether there will be, relatively speaking, many (like last week) or few (like this).
We can say with reasonable degree of certainty that they will occur in particular reasons but beyond that, our capability for predicting the next movements of our unpredictable Earth is limited — a fact worth bearing in mind as seismologists in Italy appeal against manslaughter convictions for failing to predict a series of earthquake which killed over 300 people in 2009.