The period of relative seismic quiet in terms of major tremors continued over the week of 2-8 January, although the overall level of seismicity remained broadly normal. In the U.S. frost quakes dominated the earthquake news.
The United States Geological Survey’s real time earthquake map recorded almost 1400 tremors of all magnitudes in the US and at least magnitude 4.0 (≥ M4.0) elsewhere. There was just one tremor of ≥ M6.0, in the western Pacific Ocean, and as well as the usual sprinkling along the planet’s ocean ridges there were occasional isolated quakes elsewhere.
The Week’s Biggest Earthquake: M6.5, Vanuatu
Once again, the western Pacific Ocean was the location of the largest earthquake to occur. This week the location was Vanuatu, where an M6.5 struck just to the west of the island of Sola at a depth of 18km. Although the tremor was offshore, the magnitude of the ‘quake was too small to generate a tsunami.
The complex tectonics of the western Pacific islands are highly conducive to frequent earthquake activity and it’s rare that a week passes without a tremor of M6.0 or more. In Vanuatu, the boundary between the Pacific and Australian plates is aligned broadly north-south with the latter subducting beneath the former as they converge at rates of up to 12.4cm annually. The depth and location of the earthquake, with its epicentre in the over-riding plate, suggests that it resulted from rupturing of the crust at or near the interface between the two plates.
Earthquakes in the Persian Gulf
Convergence between plates is the mechanism behind most of the planet’s major earthquakes. The northward movement of Africa and Arabia against Eurasia has created the Zagros mountains of Iran along the northern margin of the Persian Gulf, with a subduction zone further to the east. Uplift has generated folding and faulting of the continental crust with both vertical and lateral movement.
This week the western section of this margin, in Iran, was the source of three earthquakes, one with a magnitude of 5.5 and the others of M4.5 and M4.6. Although there is no detailed information on the exact mechanism of these tremors, a glance at a fault map, and the relatively shallow depth, suggests that the most likely cause is thrust, or reverse, faulting as a result of convergence.
US Earthquakes: Frost Quakes
This week much of the US has been less concerned with the Earth than with the atmosphere as yet another winter storm brings freezing weather to the North American continent. News reports have indicated that these exceptionally low temperatures have been accompanied by strange cracks and booming sounds.
Although not earthquakes as they are generally understood, these earth movements are of interest. Known as cryoseisms, they are “non-tectonic earthquake[s] caused by freezing action in ice, ice-soil and ice-rock materials” (Lacroix).
Cryoseismic tremors result from freezing and expansion of water in cracks to such a degree that it splits rock. These tremors are distinct from tectonically-sourced quakes, and so can occur in regions where other earthquakes are rare or absent and long as the atmospheric and ground conditions are right.
Rock and Ice: Shaking Things Up
With the eyes of much of the world on North America, the occurrence of cryoseisms in Canada and the northern US illustrates the power of nature. Almost as if the planet is engaged in a giant game of rock-paper-scissors, the occurrence of these ‘frost quakes’ indicates that it isn’t just deep-earth processes which can move rock, but the atmosphere and hydrosphere can do so too – albeit on a smaller and less damaging scale.