It’s been a quiet few weeks in seismic terms, and the week of 9-15 February 2017 wasn’t much more active than its predecessors. This week produced one earthquake in excess of magnitude 6, but that one (an M6.5 in the Philippines) was more than ten times the magnitude of the next largest, an M5.4, shown in the United States Geological survey’s real time earthquake map.
The USGS map shows earthquakes of all magnitudes in the US and its territories and those of at least M4.5, plus a few of at least M4.0, elsewhere in the world. At smaller magnitudes, it isn’t comprehensive, but it’s reliable at the larger magnitudes. In total, this week’s earthquake map showed over 1330 tremors, of which 176 were ≥M2.5, 101 ≥M4.0 and 32 ≥M5.0.
The distribution was, again broadly as we might expect, with the larger earthquakes concentrated around the boundaries of the planet’s major tectonic plates.
The Week’s Biggest Earthquake: M6.5, Philippines
The western margin off the Pacific Ocean is regularly the location of the week’s biggest earthquake, either where the Pacific plate abuts the Australian plate (and the microplates which have developed along it) in the south, or where it comes up against the Eurasian plate (in the north). This week we’re in the western Pacific again, but the largest earthquake didn’t occur in a association with the Pacific plate itself, but along the boundary between the Sunda plate and the Philippine Sea plate.
There’s a lot going on in this region, tectonically speaking. East of the Philippines there’s a westward dipping subduction zone, and to the west there are two eastward-dipping ones. The Philippines themselves are squeezed in the middle, seismically (and volcanically) active.
This week’s earthquake occurred at the northern tip of the island of Mindanao. A look at the USGS map, which includes only the major plate boundaries, might suggest that it’s the result of movement along the trench but, as ever, additional data reveal a more complex picture. The earthquake was shallow (15km); it involved lateral movement; and a look at more detailed fault maps shows it was located on or close to the Philippine Fault, one of a narrow band of parallel faults that run between the subduction zones along the spine of the island chain.
M5.2 Tremor, South Sandwich Islands
Anything the western Pacific can do, the south-western Atlantic can… do on a smaller scale.
The Pacific Ocean in bounded, and in some places cut across, by subduction zones. By contrast, the margins of the Atlantic are largely highly stable, with most tectonic activity associated with the mid-ocean ridge which is driving the enlargement of the ocean. There are two exceptions — both short subduction zones, both fiery volcanic island arcs. One is the holiday paradise of the Caribbean and the other is the rough, remote South Sandwich Islands.
Like the Caribbean, the South Sandwich Islands are the product of subduction at the eastern edge of a small tectonic plate. The Scotia plate lies between Antarctica and South America, and the South American plate descends beneath its eastern edge.
It’s an inhospitable place, in an uninhabited, barely-studied and little-known region. Earthquakes here go unnoticed except by penguins and scientists. This week, an earthquake of M5.2 was the third largest on the USGS map (and there was another of M5.1). The USGS summary indicates that this significant earthquake which, in (say) California could have thrown millions into a panic, was felt by… nobody.
US Earthquakes: Alaska
The coastal islands of Alaska are also characterised by a subduction zone; but much of the mainland is caught in between this (where the Pacific subducts beneath North America) and the zone of lateral movement where the two plates slide past one another. Movement here is accommodated on the Queen Charlotte and Denali faults, among others.
This week’s M5.3 in Alaska occurred on the mainland, at a considerable distance from the plate boundary. As often the case, the limited information on the USGS map tells only part of the story, and a look at the more detailed map (and summary) indicate that the earthquake was caused by lateral movement at, or near, the Denali Fault.
Last Thoughts: Earthquakes and Population Density
When I say it, it’s obvious — but sometimes it’s worth saying anyway. The hazard caused by an earthquake (or any other natural event) isn’t determined so much by the size of the event as by the numbers affected by it.
Consider the South Sandwich Islands, its M5.2 earthquake registering only on scientific instruments. Alaska isn’t exactly standing-room only; its M5.3 was felt by an estimated 5,000 people. But in the same week, an earthquake of the same magnitude (M5.3) in Taiwan was felt by an estimated 13.5 million people.
This is an article about earthquakes, not population density. But it’s always worth remembering that earthquakes aren’t just about the impact they have on the ground around them — but the people who live nearby.