This week’s reading of the United States Geological Survey’s real-time earthquake map is as interesting for its omissions as for its inclusions.
The map records all earthquakes in the US and its territories and most of those of at least magnitude 4 (≥M4.0) elsewhere – but some are excluded and this week it’s those exclusions which the most interesting.
The week was relatively quiet seismically speaking, with just one tremor in excess of M6, although there were 22 ≥M5.0.
Of these, six were aftershocks to the week’s largest tremor and the rest were, as expected, scattered around the planet’s active plate margins, mostly around the Pacific.
Biggest Earthquake This Week: M6.2 and Aftershocks, Iran
The largest tremor on record this week was the M6.2 which occurred on 18 August close to the city of Abdanan.
Iran is seismically vulnerable because of its location along the margin between the Arabian and Eurasian plates – an area where continental convergence introduces immense stresses into the crust and has resulted in the uplift of the Zagros and other mountain ranges.
The earthquake and its aftershocks all took place at depths of around 10km close to the boundary between the two plates.
Light to moderate shaking was felt in surrounding areas and news media reports suggest that around 60 people were injured, many buildings damaged, and services such as water and electricity disrupted.
Bárðarbunga, Iceland: Volcanic Seismicity
Perhaps the most significant earthquake of the week is not recorded an the USGS map.
An intense and continuing swarm of earthquakes beneath Iceland’s Bárðarbunga volcano is associated with magma movement at depth and may be a precursor of an eruption; the largest tremor recorded had a magnitude of 4.5.
Iceland lies across a plate margin and major earth tremors (≥M7.0) have occurred there in the past, but they are rare. The significance of this week’s earthquake swarm lies in the clues which it offers volcanologists about the possibility of an eruption.
The movement of seismic activity from beneath the caldera of the volcano towards its northern flank, and the shallowing of the earthquake focus, both provide an indication of the path of magma beneath thousands of metres of rock and ice.
Although there is no suggestion of an imminent eruption, the activity at Bárðarbunga implies that magma movement continues and that such an event eruption, with associated hazard, cannot be ruled out.
US: Oklahoma Earthquake Swarm Ongoing
In the US, an earthquake swarm of a different kind continued in Oklahoma. In this case the increasing occurrence of small- to medium-sized earthquakes in the centre and north of the state is associated with wastewater injection as part of energy exploitation: This week the largest tremor reached M4.2.
Recently-published research in the journal Nature indicates that these earthquakes may behave differently from naturally-induced ones, and that shaking at distances greater than 10km from the epicentre may be less that might otherwise be expected. That’s good news, because it implies a reduction in potential damage.
Last Thoughts: What We Learn From Earthquakes
An earthquake doesn’t have to be large to be interesting. The minor earthquake swarms in both Oklahoma and Iceland have different contexts and both tell us different things – one about the risk of human activities and the other about the risks of natural ones.