The week of 17-23 October was one with a relatively high amount of seismic activity, but very little in the way of significant tremors. The week’s largest earthquake, a magnitude 6.5 (M6.5) in Mexican offshore waters in the Gulf of California, was ten times larger than the next biggest, which occurred in the Western Pacific.
Overall, the United States Geological Survey recorded 26 earthquakes between M5.0 and M5.5 – a relatively high figure for a week which didn’t experience a major tremor with its associated aftershocks.
The Week’s Largest Earthquake: M6.5, Gulf of California
This week’s earthquake in the Gulf of California took place at the margin between the Pacific and North American plates. Here a section of a constructive boundary, where new ocean crust is being created, lies between the major San Andreas Fault zone of California and the highly seismically-active subduction zone further south. The boundary is offset by numerous transform faults which help to accommodate the slow (around 4-4.5mm per year) separation of the Baja California peninsula from mainland Mexico.
Earthquakes in such tectonic settings are by no means rare, but tend to be relatively small, especially compared to the major earthquakes which can arise from subduction zones. This week’s M6.5 ranks as one of the largest on record in this section of the fault, though it’s slightly smaller than the M6.7 which occurred in the same area in 2010.
Earthquakes in the Solomon Islands
The tectonically-congested margin between the Pacific and Australian plates, with its jumbled slivers of crust, changing type of plate boundary, and varying directions of movement, is rarely without earthquake activity – and this week proved to be no exception. Although the week’s largest earthquake in the area was just M5.4, it was preceded by a series of other shocks, and it may be that yet more will continue to occur in the region.
In this area, the Solomon microplate is subducting beneath the Pacific plate. Although there’s no detailed information on this series of earthquakes, the location and depth (between 10km and 80km) suggest that they occurred at or very close to the interface between the two plates and that some of the shallower ones may have resulted from deformation within the over-riding Pacific plate.
In common with the rest of the planet, the United States experienced a scattering of seismic activity, but no outstanding or noteworthy events. The largest tremor to occur, at just M5.0, took place in Alaska, close to Adak Island, is most probably an aftershock of the M7.0 which occurred in the area on 30 August.
Last Words: Earthquake Statistics
Earthquakes don’t occur at random – but they don’t follow a prescribed pattern either. This week’s figures, with a relatively high number of smaller tremors and just one of ≥M6.0, are an illustration of this. Similarly, the Solomon Islands earthquake series (which is ongoing) demonstrate that a series of small earthquakes may (but don’t necessarily) precede a larger one – although we cannot, of course, forecast whether a larger shock is still to come in the area.
USGS. Real time earthquake map. (2013). Accessed October 23, 2013.
Yeats, R. Active Faults of the World. (2012). Cambridge University Press.