The United States Geological Survey’s real time earthquake map was fairly well covered in the week of 22-28 September, with a reasonable (not excessive) number of larger earthquakes (≥M6.0) and a much larger-than-usual flurry of intermediate earthquakes.
The map (which includes earthquakes of all magnitudes in the US and its territories, and those of at least M4.0 elsewhere) recorded over 150 tremors of ≥M4.0 when we’d normally expect around 100.
Sometimes we can trace this kind of increase to a single event with a large number of aftershocks, but not this week. Instead, it seems more attributable to small clusters of aftershocks — twenty-odd associated with an M6.2 off Japan, another dozen with an M5.9 north of New Zealand and so on. Nothing significant — but they do add up.
Most of them occurred exactly where we’d expect to see them — at the margins of the Earth’s tectonic plates. And, again not unusually, there were a couple of outliers — this time in the shape of earthquakes within the normally-stable African continent.
The Week’s Biggest Earthquake: M6.9, Fiji
The largest tremor of the week occurred in the western Pacific Ocean, between the island nations of Fiji and Tonga. On the tectonic map there’s no obvious association with a plate margin: the epicentre of the earthquake (the point on the surface immediately above where it occurred) lies roughly midway between two sections of the boundary between the Pacific and Australian plates.
At one of these, to the east, the former is descending beneath the latter along the Tonga Trench. At the other, to the west, the boundary is diffuse and poorly-defined, characterised by fracture zones and some spreading ridges. The epicentre of the M6.9 earthquake lies about 500km from either.
There are clues to its cause — there always are. The main one is its depth — 596km. The pattern of earthquakes associated with the Tonga Trench is a classic one: The angle of descent of the slab can be determined by the increasing depth of earthquakes with distance from it.
In other words, an earthquake at the interface between the two plates will be close to the boundary if it is shallow – and further away, if deep. This week’s earthquake, at great depth, is therefore most likely to be associated with movement at the plate interface rather than with associated deformation in the overriding plate.
M6.2 Earthquake, Japan
Further north, in the sea south east of Japan, the clues as to the origin of an M6.2 earthquake are rather harder to read. Why? Because this week’s earthquake occurred very close to a junction between three different plates — the Pacific and Philippine Sea plates and the Okhotsk microplate.
Overall the Pacific plate is subducted beneath the other two — but the more detailed maps of the (short) boundary between the Philippine Sea and the Okhotsk microplate shows the former subducting beneath the latter.
Yes, I know. It’s complicated.
What’s going on? Looking at the depth of the earthquake again, we see that it’s shallow, at just 10km, and that its epicentre is relatively close to both subduction zones. The USGS data suggest a clear compressive motion. There’s a possibility that the earthquake could have resulted from movement on either of the plate boundaries — or it might equally have been the result of a more generalised deformation as the plates come together.
US Earthquakes: California
There’s always a lot going on in California, including a swarm of minor earthquakes — around 150 in the last couple of days — at the southern end of the San Andreas fault Zone, in the Salton Sea. This setting might seem obvious but is potentially complicated, since this is an area where the boundary between the Pacific and North American plates changes in nature, from a conservative boundary (with the plates sliding past one another), to a constructive boundary (where they move apart.
Fortunately I don’t need to read the clues for this one: California is so well studied that the USGS publishes plenty of information and has done my work for me, even posting it on their Facebook page.
I can quote without hesitation: “The earthquakes are occurring near a set of north-northeast trending cross-faults in the Salton Sea. The cross-faults are part of a fault network that connect the southernmost end of the San Andreas fault with the Imperial fault.”
The USGS also goes on to note that: “Preliminary calculations indicate that, as of 10:00 am (PDT) Sept. 27, 2016, there is a 0.03%-1% chance (1 in 3000 to 1 in 100) of a magnitude 7 or greater earthquake being triggered on the Southern San Andreas fault within the next next seven days through October 4,” which might raise an eyebrow or two.
But I would relax and heed their reassurance: these events are “not necessarily cause for alarm”.
Sometimes there a re a lot of earthquakes in one week. Sometimes there aren’t many. Sometimes there are a few big ones and only a few small, sometimes nothing enormous and a lot of intermediate events. That’s how earthquakes rock (so to speak).