For some reason — almost certainly nothing more sinister than statistical variation — the United States Geological Survey’s real time earthquake map showed an unusually high number of earthquakes in the week of 13-19 November 2014.
The map shows all tremors in the US and its territories and those of greater than magnitude 4 (≥M4.0) elsewhere and so picks up most of the major events on the planet.
This week, the medium-sized earthquakes were present in abundance, with 169 larger than M4.0; 51 of these were ≥M4.0. Often aftershocks from a major tremor can account for such a pattern, and this does appear to be the case this week.
There have been around 30 aftershocks of at least M4.0 associated with the week’s largest earthquake (an M7.1 south of the Philippines); a further 10 occurred around Iceland’s ongoing volcanic eruption.
The Week’s Largest Earthquake: M7.1, Indonesia
The western margin of the Pacific tectonic plate is highly complex; in Indonesia it abuts the Indo-Australian, Sunda and Philippine Sea plates along with numerous smaller, microplates. The difficulties in interpreting plate movement in this area cannot be understated, given that the surface expression of plate tectonics is not all that is at work.
The USGS notes the existence of “the inverted-U-shaped Halmahera Plate, which has no surface expression, also plays a role in regional tectonics.” The outline of the setting of this week’s tremor is, inevitably, simplistic.
Overall the sea is a zone of convergence, with subduction zones at the eastern edge of the Philippine Sea plate (west of the Philippines) and many thrust faults and minor spreading centres within the Sunda plate. The USGS information on the M7.1 tremor suggests that it was the result of thrust faulting rather than subduction, probably associated with the western margin of one of the many microplates in the region.
In the light of this it’s hardly surprising that earthquakes are common. The past 30 days have seen almost 100 earthquakes of ≥M4.0 in the area, while in last century there were almost 100 tremors of ≥M7.0. Of these, 14 were in the immediate area of the most recent quake, including an M7.5 in 2007.
M6.7 Earthquake, New Zealand
The western margin of the Pacific was also the location for the second largest earthquake this week. In this case the plate boundary is much cleaner and less congested, as a straightforward subduction zone separates the Pacific and Australian plates in a segment that stretches from Tonga to the North Island of New Zealand.
The earthquake occurred at the southern end of this deep ocean trench, around 170km from the New Zealand city of Gisborne, and was felt across much of the North Island. With its epicentre in the overriding plate very close to the trench and a shallow depth (just 22km) it is probably associated with movement at or near the plate interface, which would make it a classic subduction earthquake. Faulting within the overriding plate cannot, however, be ruled out as an alternative mechanism.
US Earthquakes: An Overall View
There was a lot of earthquake activity in the States this week.
The USGS map showed close on 1000 tremors of all magnitudes, excluding those in Alaska and Hawaii, which between them add several hundred more.
Bearing this in mind, it’s instructive to look at the overall pattern of tremors and see how it reflects key influencing factors.
Earthquakes in Montana, Wyoming, Idaho and Utah are all associated with the eastern margin of the Western Cordillera, while the Pacific states experienced bands of earthquakes which mark both the Cordillera’s western edge and the line of the San Andreas Fault Zone.
East of the Rockies, the Kansas-Okalahoma earthquake cluster, now generally thought to be associated with human activity, is evident. It’s worth noting, though, that some small tremors in apparently stable areas may have a tectonic source — for example, the earthquakes in Missouri are probably associated with the New Madrid Seismic Zone.
Quakes and Tremors: What Lies Beneath
Seismologists, when talking to lay people, often have to simplify plate tectonics. But earthquakes can occur on hidden faults (those with no surface expression) — and the complex tectonics of the Celebes Sea, where the week’s largest tremor occurred, show that it isn’t just faults that can be hidden, but whole plates too. What goes on below the surface can be highly influential in terms of tectonic activity, just as much as the visible faults such as San Andreas.