There were increased levels of earthquake activity in the week of 14-20 March, at least compared with the previous seven days.
A summary of events recorded on the United States Geological Survey’s real time earthquake map indicates five tremors of at least magnitude 6 (≥M6.0) compared with three the previous week; 40 of ≥M5.0 (compared to 26); and 138 ≥M4.0 (against 116).
In total (all magnitudes in the US and its territories and those of ≥M4.0 elsewhere) there were 1,543 recorded tremors.
Variation in the number of earthquakes is anything but unusual and at the larger end of the magnitude scale much of this week’s additional activity is accounted for by aftershocks associated with the week’s largest tremor, an M6.7 just off the coast of northern Chile. Elsewhere the pattern was broadly as usual, with the majority of earthquakes around the Pacific Ocean – 50 of them in the western Pacific and Indonesia.
The Week’s Largest Earthquake: M6.7, Chile
The tremor which struck of Chile on 16 March dominates the map this week, followed as it was by a cluster of over 20 aftershocks.
The western seaboard on South America is a classic subduction zone with the dense continental crust of the Nazca plate sinking below the more buoyant continental crust of the South American plate as the two collide at a relatively rapid rate of around 8.4 cm annually.
This week’s cluster of tremors occurred near Iquique, and a glance at a bathymetric map (one which show the topography of the sea bed) suggests that they may be associated with the subduction of the submarine Iquique Ridge.
There’s no detailed information about the fault mechanism for the tremor, although the available evidence on depth and location suggests it was indeed a classic subduction event. This supposition is supported the fact that the tremor generated a minor tsunami locally, implying a degree of vertical displacement.
Earthquakes in the South Atlantic
Remote and largely uninhabited though it is, over the last year or so the South Atlantic has been remarkably seismically active. The narrow length of subduction zone which created the South Sandwich Islands marks the eastern edge of the Scotia microplate; to the west (both north and south) are transform boundaries while the Mid-Atlantic Ridge runs eastwards.
This area is under differing and complex stresses which generate its regular (and occasionally large) earthquakes. This week’s tremors indicate the diversity with two shallow events which are probably the result of lateral movement:
- A deeper tremor to the west of the trench which is almost certainly subduction-related.
- A tremor to the north which looks as if it was caused by faulting within the South American plate.
US Earthquakes: California Shaken Up Again
Following a large tremor in the north of the Golden State earlier in the month, two smaller ones – M3.9 and M4.4 – struck north of Los Angeles. These tremors were both associated with the San Andreas fault zone, though neither was directly on it.
Although minor, they were widely felt and provide a reminder that the state remains active – and that an earthquake a couple of magnitudes larger would, given the logarithmic nature of the magnitude scale, be a serious event for the local population.
Tsunamis and Earthquakes
As a broad rule, an earthquake which generates a tsunami will be at least M7.0. Not all earthquakes of this size or larger will do so, of course; and this week, in which a small tsunami (maximum height 28cm) washed ashore in Chile, provides a gentle reminder that smaller tremors have that power too.
The key variable is not the size of the earthquake but an element of vertical displacement – which is why subduction zone earthquakes are the most likely to produce large tsunamis and one of the reasons why they have been responsible for so many deaths.