It was reasonably quiet in seismic terms in the week of 18-24 June 2015.
The United States Geological Survey’s real time earthquake map, which includes earthquakes of all magnitudes in the US and its territories and those of at least magnitude 4 (≥M4.0) elsewhere, showed under 1400 tremors in total.
Of these seismic events, just 84 were of ≥M4.0; 13 were ≥M5.0; and there were four greater than, or equal to M6.0.
The largest on the map was an M7.0 in the southern Atlantic. The eagle-eyed among you may notice that this was covered last week; that’s because Decoded Science’s staffing rotas (such as they are) took a step back this week and so there’s a day’s overlap.
The seven day period, then, was quiet — but the pattern of earthquakes remains pretty much the same as usual. Most larger earthquakes are around the boundaries of the Earth’s tectonic plates, with the large majority of these in the western Pacific and the Andean margin, around the so-called Pacific Ring of Fire.
The Week’s Largest Earthquake: M6.4, Chile
Discounting last week’s largest earthquake (to avoid duplication) leaves us with an M6.4 in southern Chile as the largest ‘quake standing. Tectonically, this is an active margin, where the Nazca plate subducts beneath South America, building the Andes as it does so — and producing frequent large earthquakes along the way.
This week’s tremor is nothing special and doesn’t warrant a separate event page on the USGS map and archive. The available data on location (very close to the actual boundary) and depth (10km) are limited but they suggest nothing out of the ordinary in the earthquake, which was probably the result either of movement at the plate interface or deformation within the over-riding plate.
Earthquake historians, though, will look with interest at this event. In the past 60 years the stretch of the Peru-Chile Trench from (roughly) La Liga to Puerto Monte has experienced 55 earthquakes of at least M6.4 — and those include some very big ones indeed.
Major tremors of M8.0 (in 1985) M8.1 (1960), M8.6 (1960) and M8.8 (2010) are all dwarfed by the daddy of them all. The largest earthquake ever recorded occurred near Bio-Bio in May 1960. The USGS lists it as a staggering M9.6.
This week’s lead earthquake is tiny by comparison.
Earthquakes in Iceland
Speaking of tiny earthquakes takes us to Iceland. Though most of its earthquakes are too small to register on the USGS map, the country lies astride a developing extensional plate boundary and seismic activity, though at low levels, is by no means unusual.
In recent weeks, an increase in the levels of small earthquakes in the Reykjanes peninsula, in the southwest of the country has prompted the Iceland Meteorological Office to produce a bulletin, “remind all inhabitants in seismically active regions about potential earthquake hazards.”
Though stopping short of issuing an alert, the bulletin does remind residents that the region, which is an active rift zone and geothermally active, is known to be capable of producing significant earthquakes. By significant they mean up to M6.5. In other words, Iceland is quite capable of producing an earthquake large enough to be featured as the main event in a quiet week. There’s certainly some food for thought there.
US Earthquakes: Small and Scattered
This week, small earthquakes (≥M2.5) were scattered across the central and western States like sugar strand on an ice cream, with no concentration other than the now-usual cluster in central Oklahoma and southern Kansas.
Montana, Washington, Nebraska, Nevada and (of course) California all produced isolated earthquakes. These are almost certainly mostly the result of extensional tectonics or of normal movement along existing faults, except for this along the San Andreas fault zone.
Last Thoughts: Small Things in Quiet Times
Earthquake size is relative. This week demonstrates that to perfection. In Chile an M6.4 passes barely unnoticed, by no means an unusual occurrence. In Iceland a smattering of small tremors brings a reminder that the country is capable of experiencing major earthquakes of up to M6.5. Major for Iceland is minor for Chile.
It’s a matter of perspective.