The planet continues to settle into what appears to be a quieter phase compared to recent weeks – although the last seven days still produced five tremors of at least magnitude 6 (≥M6.0).
Four of these were in the western Pacific/Philippines/Sumatra region and the fourth, perhaps slightly more noteworthy, occurred in the Caribbean.
The United States Geological Survey real time earthquake map recorded 28 tremors of ≥M5.0 (out of a total of 1,445 earthquakes of all magnitude in the US and its territories and ≥M4.0 worldwide) and these, as usual, occurred mainly around the seismic belt of the Pacific Ring of Fire.
Five, however, occurred in the South Atlantic, on a short section of subduction zone around the South Sandwich Islands.
Not the Week’s Biggest Earthquake: M6.2, Philippines
Last week’s earthquake report was a day later and so included this week’s biggest earthquake (an M6.6 in Micronesia). The next largest on the USGS map is the M6.2 which occurred on 15 May in the Philippines. As is the case with much of the rest of the western Pacific, the tectonic setting of this area is more complicated than most (inevitably) generalised tectonic maps suggest.
Most maps show only the main plate boundaries and according to these the tremor appears related to the subduction zone along the Philippines Trench, where the Pacific plate is being subducted beneath the Philippine Sea plate. This is, however, misleading. An earthquake directly associated with that particular trench would occur at significant depth but this tremor was at a depth of just 14km.
A look at more detailed maps reveals the existence of a second, minor subduction zone in the Sulu Sea. Here the direction of subduction is reversed with part of the Philippine Sea plate being forced beneath the Philippine archipelago.
The location and depth of the M6.2 suggest that it is more likely to be associated with subduction at this zone, or at the very least with crustal deformation along a nearby fault.
Another earthquake associated with a (relatively) minor subduction zone is the M6.0 just to the west of the island of Guadeloupe.
Located at the eastern edge of the Caribbean plate, the subduction zone is the result of descent of a section of the North American plate.
The tremor was at a depth of 24 km, roughly 50km from the plate boundary, and is likely to result from movement at or close to the plate interface.
In this area the Lesser Antilles subduction zone is responsible for the volcanic island chain which stretches from the British Virgin Islands in the north to Grenada in the south. Subducted crust melts and, then being less dense than the overlying rock, rises to reach the surface as volcanoes – in this case making not just a pleasing line (to geographers) on a map but also a holiday paradise for many.
US Earthquakes: Can’t Stop Talking About Oklahoma
Decoded Science makes no apology for revisiting the ongoing sequence of earthquakes in Oklahoma, given that there is much still to be said on the subject.
The anomalously intense sequence of earthquakes in an otherwise stable tectonic environment is not related to any buried geological feature and appears increasingly to be the result of human activity involving the injection of wastewater from oil and gas extraction. Watch this space.
Earthquakes: Maps and Complexity
Earthquakes don’t have to be big to be interesting. The situation on the Philippines is a clear illustration of how scientists are forced to simplify when drawing their maps – but how a closer look often reveals a much more complex situation.
By contrast, the Lesser Antilles subduction zone is a simple and elegant example.
Decoding Science. One article at a time.