There was no letup in seismic activity in the week of 9-15 December. Okay, so there was no earthquake of magnitude 7 or larger (≥M7.0) but there was still plenty going on.
The United States Geological Survey’s real time earthquake map, which shows all tremors in the US and its territories and those of ≥M4.0 elsewhere, included a total of over 1200 tremors (which is on the low side) but 126 of these were ≥M4.0 and 31 were ≥M4.0.
The number of larger tremors is skewed by a number of aftershocks — 24 to be precise — to the week’s largest earthquake (an M6.9 in Indonesia). The rest of the week’s higher-magnitude earthquake activity was, as expected, largely related to the planet’s plate boundaries, although there were a couple of intraplate examples, one of them in Russia’s Baikal Rift and another as an aftershock to last week’s earthquake in the south Indian Ocean.
The Week’s Biggest Earthquake: M6.9, Indonesia
The Earth’s crust in the western Pacific is pretty congested. The convergence of the Australian and Pacific plates, and the jostling of the many slivers of crust caught between them, is the recipe for earthquakes. Add to that the convergence of both with the Philippine Sea plate and the situation becomes even more congested.
This week’s largest earthquake, an M6.9 in Indonesia’s Banda Sea south of the island of Seram, is an example of the tectonic complexity. The convergence of the margin of the Australian plate with Eurasia (the Philippine sea plate lies between them) is complex. On a map it occurs along a 180 degree arc from north of Seram to south of Timor. On the Banda Sea side of this margin, the ocean floor is crumpled into ridges and a line of volcanoes lies on the Philippine Sea plate side of the margin.
The exact tectonic mechanisms in the area, however, are not fully understood. “Contrary to earlier tectonic models in which this trough was interpreted as a subduction feature continuous with the Sunda subduction zone,” says the USGS, “it is now thought to represent a subsiding deformational feature related to the collision of the Australia plate continental margin and the volcanic arc of the Eurasia plate, initiating in the last 5-8 Myr”.
Earthquakes in the Eastern Mediterranean
Eastern Mediterranean earthquakes are by no means uncommon as a result of the convergence between Africa and Eurasia. This digest has reported on several in recent months, although generally speaking they tend to be on the small side, with anything above M6 raising an eyebrow.
This week there were two tremors at the northern and western end of the Hellenic Trench, a (relatively) short subduction zone which stretches from the Ionian Sea to the south of the island of Crete. Both of these were small (M4.5 and M4.6) and caused no reported damage. What makes them interesting is their context.
New research from the area, reported in the American Geosciences Union’s blog, indicates that the trench, particularly the part towards the east, is capable of generating much larger earthquakes than we might normally expect. Indeed: “during the last 50,000 years, both western and eastern Crete have been lifted uniformly about 100 meters (328 feet) above sea level due to at least 40 earthquakes of magnitudes greater than 8”.
For context, an M8 earthquake is over 2,500 times stronger than the larger of this week’s tremors in the eastern Mediterranean and would release almost 126,000 times as much energy.
US Earthquakes: Hawaii
Hawaii hasn’t been shaking big time — certainly not on as large a scale as it’s capable of (the largest recorded earthquake there was an M7.9 event back in 1868) but it has been trembling a little.
Earthquakes aren’t just a result of plates colliding — volcanic activity, in particular the movement of magma beneath a volcano, creates tremors too. With Kilauea erupting and Mauna Loa showing increasing signs of activity, it’s no surprise to see earthquake activity in the island chain, even if it is at relatively low level.
Last Thoughts: More Complicated Than We Knew
It’s convenient to subdivide earthquake settings into fairly simple categories — continental collision, subduction and so on. The reality, as the crustal traffic jam in Indonesia illustrates, is that tectonics can be far more complicated than that. And even where we might think, as in the Hellenic Trench, that the situation isn’t too complex, there’s still a lot for seismologists — and the civil protection authorities — to learn.