The week ending on the 26th of December, 2013 continued a three-week trend of little or no major seismic activity (no tremors of greater than magnitude 6) across the planet. An absence of major earthquakes does not, however, imply low levels of seismicity overall and in total the United States Geological Survey’s real time earthquake map recorded over 1500 tremors of all magnitudes in the U.S. – and of at least M4.0 elsewhere.
Twenty-five of these tremors had a magnitude of at least 5 (≥M5.0) and displayed the usual concentration in the western Pacific Ocean.
The Week’s Largest Earthquake: M5.8, Guam
The largest tremor of the week occurred in the western Pacific Ocean on 23 December. As in the previous week, the tectonic setting was the margin between the Pacific and Philippine Sea plates, both comprised of dense oceanic crust, although this week’s major tremor was at the southern end of the segment.
At M5.8 it was too small to create a tsunami and no warning was issued. The age and density of the ocean crust contributes to the creation of a very deep ocean trench – in this case, the Mariana Trench, which includes the deepest point on the Earth’s surface. The depth of the tremor (103km) and the location of its epicentre in the over-riding plate together suggest that it may have originated at the interface between the two plates.
M5.3 Quake: Red Sea
The Red Sea marks the boundary between the African and Arabian plates and is an area of divergence: The two plates are moving apart at a rate of approximately 19mm per year. The trough in which the Red Sea lies is the early stage of the creation of a new ocean and the narrow axial rift which runs along its centre is an area where new ocean crust is being produced.
Although large earthquakes are mostly associated with subduction zones, the movement of crustal rocks caused by production of new crust means that such tremors are common along these rifts, although in general the warmth of the crust means that they are usually smaller in magnitude. This week’s M5.3, which occurred at a depth of 10km below the spreading centre, was large for the region.
U.S. Earthquakes: Alaska
With little significant seismic activity in the United States (the largest tremor was an earthquake of just M4.6 in the Aleutian Islands) this week’s digest takes a wider view of seismic activity in Alaska. The northern margin of the Pacific plate is a major subduction zone, characterised by frequent seismic activity and capable of generating some of the planet’s largest earthquakes.
Further east, however, the coming together of the sliver of crust called the Yakutat microplate and the North American continent introduces complications, as does the termination of the north-west-south east trending Queen Charlotte fault, a transform boundary.
Together, these produce a pattern of seismic activity in Alaska which extends beyond the subduction zone and a review of the week’s earthquakes of at least M2.5 shows linear patterns of seismic activity into the faulted continental crust.
Major Subduction Earthquakes
The largest earthquake this week was a subduction zone earthquake – but it was a small one. On Boxing Day 2004, one on the largest earthquakes on record (at M9.1) struck off Sumatra, generating a tsunami that cost hundreds of thousands of lives across the Indian Ocean – a reminder, if one was needed, that major earthquakes are not just scientific curiosities but have a real impact on local populations.