The week of 25 September-1 October was notable not so much for the size of its earthquakes (the two largest, in Argentina and Alaska, both registered M6.2) as for the number of them.
The United States Geological Survey’s real time earthquake map, which shows earthquakes of all magnitudes in the US and its territories and of at least magnitude 4 (≥M4.0) elsewhere, recorded an unusually high total of over 2,400 tremors for the week.
Most of these are associated with an earthquake swarm in California which is discussed below.
This apart the week was relatively quiet; just three tremors reached or exceeded M6.0 and a further 23 were ≥M5.0.
The distribution of these events was again broadly as expected, with most of the medium to large earthquakes located on the planet’s plate boundaries.
Week’s Largest Earthquake: M6.2, Argentina
The Andean margin, where the dense oceanic Nazca plate moves eastwards against the South American continent at speeds of between 7 and 8.4 cm per year, is a notorious seismic belt.
The subduction of the Nazca plate creates a deep offshore trench along the continental margin and the collision is responsible for the uplift of the Andean mountains.
With so many geologic stresses, it’s unsurprising that earthquakes are common.
This week’s joint-largest earthquake occurred in the central Andes. Though the tremor is obviously a product of plate tectonics, it isn’t clear from the available information what the exact cause might be.
The epicentre of the earthquake was at some distance inland. Its depth was 189km. Looking at these factors, along with a simplified map of faulting in the area, implies that the earthquake was the result of movement along a thrust belt associated with mountain building rather than movement along the plate interface.
Iceland: Seismic Update
Iceland’s Bárðarbunga volcano has now been erupting for over a month.
Although associated seismic activity has declined, it has not ceased; the week saw 11 earthquakes of ≥M4.5.
The latest information (on 1 October) suggests that the shift of activity is now away from the intrusion which feeds the eruption and towards the caldera of the volcano itself, further to the south and below the ice.
Seismic activity around volcanoes doesn’t necessarily presage an eruption but it does offer volcanologists clues as to how magma is moving beneath an eruptive system.
The current eruption at Holuhraun/Bárðarbunga is a classic example of this; earthquakes at the caldera are associated with its deflation as magma moves out of the chamber, possibly to feed the eruption farther north. At the same time the eruption causes seismic activity of its own.
US Earthquakes: Long Valley Caldera
The big talking point of the week in some seismic circles has been the earthquake swarm beneath California’s Long Valley caldera.
At the time of writing, the swarm has produced over 1100 tremors, most of them very small but with nine ≥M3.0; latest earthquake data indicate that the frequency of these tremors is now decreasing.
The occurrence of the swarm has led to some speculation online that these tremors may be precursors of a major eruption.
In fact a close look at the context — and at the information available from the USGS — reveals that this is not the case.
The USGS reports that the earthquakes are tectonic rather than volcanic in nature and are not associated with magma movement.
Such swarms are not unusual (there have been other, more prolific ones in the caldera in recent decades) and are likely to be associated with uplift within the caldera itself as part of wider tectonic forces.
It’s worth noting that although, like Yellowstone, Long Valley is thought to have erupted catastrophically in the past, that eruption was approximately 760,000 years ago and the most recent eruptive activity is dated to 50,000 years.
Although the caldera has the potential to erupt and is closely monitored, the current earthquake swarm is not indicative of an imminent eruption.
Volcanic and Tectonic Earthquakes
This week’s earthquakes provide a contrast between the eruptive activity in Iceland and in California. In the former, magma movement generates hundreds of tremors which can be used to map magma movement. In the latter, tectonic movement rather than volcanic activity is responsible for the plethora of minor tremors.