The week of 21-27 August was a fascinating one for seismologists: the United States Geological Survey’s (USGS) real time earthquake map tells just part of the story.
The map records tremors of all magnitudes in the US and its territories and of at least magnitude 4 (≥M4.0) elsewhere – and so it misses a number of smaller tremors which provide clues to significant earth events.
First, to the numbers. There were three tremors of ≥M6.0, all on the western margin of the Pacific (more on two of these below) and the map also records 28 events of ≥M5.0 worldwide among its total of over 1,650 tremors.
Alongside the usual pattern of activity around the Pacific (the so-called Ring of Fire) there was a significant sprinkling along the margin of the southern margin of the Eurasian plate. And in another week a tremor of M4.6 in South Africa would have warranted comment.
The Week’s Largest Earthquake: M6.9, Peru
The largest tremor of the week, an M6.9 467km south of the capital, Lima, on 24 August, is almost apologetically normal. The most intense shaking affected only a small and not densely-populated area; it caused limited damage and reports indicate that two people were injured.
Peru is accustomed to earthquakes. It lies on the margin between the Nazca and South American tectonic plates, with continental convergence leading to the subduction of the former beneath the latter, bringing with it regular, and often significant, earth tremors.
This week’s earthquake was a typical product of that setting and its onshore location and depth (101km) are consistent with an origin at the interface between the over-riding and descending plates.
Iceland and the Bárðarbunga Earthquake Swarm
And so to Iceland. The USGS map shows 12 tremors in excess of M4.0 since 22 August, the largest of them M5.9 — in itself remarkable, since although Iceland is on a plate boundary, it’s a constructive boundary where earthquakes are generally smaller than at other types of margin.
That this is an underestimate is understood (the map is not exhaustive) and to see the true extent of seismic activity we need to look at more localised data.
The seismic activity is linked to magma movement beneath Iceland’s Bárðarbunga volcano. Volcanic eruptions typically produce significant earthquake swarms: a look at the (regularly updated) earthquake maps produced by the Iceland Met Office shows thousands of tremors since activity under the volcano began on 16 August.
Interestingly, the pattern of the earthquake epicentres helps volcanologist to track the path of magma beneath both rock and ice. Earthquakes are not just events in themselves but clues to other developing events.
California: the M6.0 Napa Valley Earthquake
The major event of the week, seismologically speaking, was the M6.0 tremor which struck in California’s Napa Valley on 24 August and which was followed by numerous aftershocks (over 80 at the time of writing).
Though not especially large in global terms, the tremor produced intense shaking locally and was damaging both in human and economic terms.
News media reported that almost 100 people were injured. There was extensive damage to buildings and loss of power, and the Governor declared a state of emergency. The USGS Pager system indicated a 61% chance of losses exceeding $1000 m.
Earthquakes: How Big is Big?
Earthquakes vary, and the hazard they create is relative. A big earthquake in Peru, with limited ground shaking in a relatively sparsely populated area, created less damage and caused fewer injuries that one almost a tenth of its size in a highly-populated and economically-developed area of California.
For those who are wondering, this week’s event in California is not ‘The Big One.’ That is yet to come.