The earthquake map for the week of 11-17 August 2016 was dominated by a large earthquake in the western Pacific. And I do mean dominated.
The United States Geological Survey’s real time earthquake map, which, broadly speaking, includes tremors of all magnitudes in the US and its territories and those of at least magnitude 4 (≥M4.0) elsewhere, recorded a total of 1720 earthquakes for the week.
Of those, the largest earthquake came in at M7.2 and, together with its aftershock sequence, accounted for a fifth of the 100 tremors of ≥M4.5 this week and eight of the 29 which were at least M5.0.
That still leaves plenty of earthquake activity elsewhere, though nothing out of the ordinary. There were 134 tremors ≥M4.0 and 248 ≥M2.5 — and the distribution was pretty much as we’d expect, with most larger events around the margins of the Earth’s tectonic plates.
The Week’s Largest Earthquake: M7.2, New Caledonia
The congested western region of the Pacific, with its series of microplates trapped between the converging Pacific and Australian plates, along with the changing and complex nature of the boundary, is the regular destination for our weekly largest earthquake spot.
This week’s major event occurred at a shallow depth (around 10km) just east of the defined part of the boundary which ends at the South Sandwich Trench before it becomes the Hunter Fracture Zone. (In other words, an area where something is definitely going on but it isn’t quite definite enough to be drawn on the map).
So what caused the earthquake? The tectonic setting suggests it’s unlikely to be caused by subduction and, indeed, the USGS event page (putting it rather more clearly than I can) summarises it thus: “the result of shallow strike-slip faulting on or near the complex plate boundary between the Australia and Pacific plates. At the location of this earthquake the regional plate boundary is transitional, evolving from east-northeastward-oriented subduction of Australia beneath Pacific at the New Hebrides Trench to the north and west, to southwest-northeast oriented left-lateral transform faulting between the two plates just south of this earthquake”.
Perhaps the most interesting element is that Yeats notes that the “Hunter fracture zone has very low seismicity”. So while large earthquakes in the western part of the Pacific aren’t unusual, this one might be for its specific location.
M6.2 Tremor, South of Tonga
Elsewhere in the western Pacific, things are rather clearer. The Pacific and Australian plates are converging, with the latter descending beneath the former along the Tonga-Kermadec Trench — a classic setting for subduction earthquakes.
At first sight, that’s the obvious explanation for this week’s M6.2 tremor, hundreds of km from land. At a depth of around 120km and roughly 200km from the actual trench with its epicentre in the overriding plate, it’s consistent with the (relatively) shallow angle of dip defined by Yeats in this area.
There’s no detailed description of this earthquake on the USGS event page, though there is some more information. My new best friend the beachball diagram (aka the moment tensor solution), tells a different story to the obvious one. I had to check twice, but it indeed implies an extensional, rather than a compressional origin.
This may seem unlikely; but Yeats notes that an M8.1 event in this region in 2009 also resulted from normal, rather than revers faulting. So perhaps it’s not so unusual after all.
US Earthquakes: Oklahoma and the Anthropocene
The Earth science world is full of chatter about whether or not we’re entering a new geological period, termed the Anthropocene and defined as a time when human activity is the dominant influence upon our environment.
The term is mostly applied to areas such as climate or ecology, rather than those such as earthquakes or volcanoes. With the ongoing swarm of induced earthquakes in parts of the US, most particularly in Oklahoma — this week the largest was M4.0 — attributable to human activity, perhaps it’s time to change our thinking on this area too?
Last Thoughts: How Big is Big?
I described the M7.2 Tonga Trench earthquake as dominant — but what exactly does that mean?
The earthquake magnitude scale is logarithmic, meaning that each magnitude difference (of one) actually represents ten times in size. So an earthquake of M6 is ten times the size of an M5, 100 times that of an M4 and so on.
To complicate things further, the amount of energy released increases by around 32 times for each order of magnitude. So an earthquake of M6 is 32 times the size of an M5, 1000 times that of an M4, 32000 times that of an M5 and so on. (The numbers look odd but check them on the USGS’s handy little earthquake calculator, listed in the resources section)
So this week’s M7.2 was ten times larger than the next largest tremor (an M6.2 near Fiji) and released 32 times as much energy. The USGS map is anything but complete; but nevertheless, it’s a reasonable bet that the M7.2 earthquake with its aftershocks (and possibly without them) released more energy than the rest of the week’s earth tremors put together.