The week of 8-14 December, 2016 was, unquestionably, dominated by one seismic event — an earthquake of magnitude 7.8 (M7.8) in the Solomon Islands, in the western Pacific.
This, along with its series of aftershocks, served to skew the numbers of tremors on the United States Geological Survey’s earthquake map way above the numbers we’d normally expect.
The map, broadly speaking, shows earthquakes of all magnitudes in the US and its territories and those of at least magnitude 4 (≥M4.0) elsewhere. In a typical week we might reasonably expect two or three tremors of at least M6.0 and perhaps 20-30 of ≥M4.0; though the numbers do vary.
This week the map’s total of almost 1800 tremors included five of at least M6.0 (three associated with the Solomon Islands ‘quake); 65 of at least M5.0 (46 associated with the Solomon Islands); and 167 of at least M4.0 (71 in the Solomon Islands) — a measure of how significant a major tremor can be.
If we disregard this event, the patterns of tremors wasn’t greatly different from normal, with most of the major earthquake activity at or near the boundaries of the earth’s tectonic plates.
The Week’s Biggest Earthquake: M7.8, Solomon Islands
The tectonically complex western Pacific Ocean regularly features in this earthquake digest. It’s no surprise, given that the margin between the Pacific and Australian plates in this region is a tortuous one, with changes of direction and nature of movement and the existence of numerous slivers of crust (microplates) between the major plates. As a direct consequence, earthquake activity is a regular feature of life in this area.
This week’s largest earthquake, at M7.8, occurred along the Solomon Trench, where the Australian plate subducts beneath the Pacific plate at a relatively rapid rate of 95mm per year. The relationship between the depth (41km) and distance from the margin (approximately the same) along with data recorded showing that compression was the key motion, indicate that, as the USGS notes, the earthquake occurred: “as the result of shallow, slightly oblique reverse faulting on or near the plate boundary between the Australia and Pacific plates.”
An earthquake of this magnitude, with a vertical component, occurring offshore, satisfies the requirements for generating a tsunami and the authorities issued an alert leading to evacuation — although the tsunami wave recorded reached a maximum height of 43cm.
Although the earthquake was large, the damage is, as so far reported, slight — probably largely because of a relatively limited population in the area. The USGS estimates suggest that only a few thousand people experienced very strong shaking.
M5.9 Quake, China
Away from the western Pacific, the largest tremor this week was an M5.9 in north eastern China. Although it appears a long way from any obvious plate boundary, this tremor, too, was the result of collisional tectonics — though in this case, it was the coming together of continents, rather than oceanic crust, which was the cause.
Subduction zone earthquakes occur in relatively narrow zones; but when continents collide, uplift is across a much larger area. The collision between India and Eurasia does have a defined margin — the Himalayan Front Fault, in northern India — but its impacts are felt over an enormous area, and result in the uplift not only of the Himalayas but also of the Tibetan plateau.
Uplift resulting from continental collision is accommodated by crustal shortening, whereby slabs of continental crust are stacked one upon the other along thrust faults. This week’s M5.9 is typical of this type of tectonic setting, with the available evidence suggesting it is, in fact, the result of movement upon just such a fault at the northern edge of the diffuse Indo-Eurasian boundary — thousands of km north of the actual margin.
US Earthquakes: California
California is earthquake central as far as the public are concerned — so perhaps an M6.5 shouldn’t raise any eyebrows. But in the public mind, California’s earthquakes are associated with the San Andreas Fault Zone, and in fact an M6.5 in (say) LA or San Francisco would be a very big deal indeed.
This week’s offshore Californian earthquake was, in fact, not untypical. It occurred on a transform margin along the southern edge of the Juan de Fuca plate, the remnant of a much larger plate which has largely disappeared beneath the North American continent.
While the subduction zone itself is quiet (some say suspiciously so), its southern margin is not. Here, the plate slides past the Pacific plate and the result, this week, was large (but not untypical) strike-slip earthquake — though it was the largest along this section of the fault in 2016.
Last Thoughts: Numbers, Numbers
I’ve said before that I’m not a statistician (I mean, I have a life) and so I don’t dwell too much on whether one week provides a whole lot more earthquakes than another. These things generally even out over time.
This week rather illustrates the point — one major event, the M7.8 in the Solomon Islands, with its associated aftershocks (and remember, not all earthquakes have so many) can influence the numbers in one week. It’s why it’s much better to take a broader view.