It was quiet, seismologically-speaking, again this week, between the 27 August and 2 September 2015.
The United States Geological Survey’s real time earthquake map, which shows earthquakes of all magnitudes in the US and its territories and those of at least magnitude 4 (≥M4.0) elsewhere, only just made the M6.0 mark, with just one earthquake of this magnitude (south of Japan).
The overall number of earthquakes didn’t vary greatly from what we might expect and, apart from the lack of notable larger tremors, the distribution was pretty much as normal, too, with two thirds of those of ≥M5.0 occurring along the western margin of the Pacific Ocean and the remainder along the other margins of the earth’s tectonic plates.
The Week’s Biggest Earthquake: M6.0, Japan
Japan’s high seismicity is caused by its location in relation to four tectonic plates — the Eurasian, Pacific, Philippine Sea plates and the Okhotsk microplate. No-one will need a reminder that the plate margins around Japan can generate very large earthquakes indeed — the 2011 M9.0 earthquake and tsunami looms too large in the memory for that.
Bearing that in mind, this week’s largest earthquake, which came in at M6.0, is actually very small. The magnitude scale is logarithmic so the three orders of magnitude difference actually translates to a thousand times in terms of representation on a seismogram and over 31,600 times in terms of energy released — a mind-boggling statistic.
This week’s tremor occurred on the western margin of the Philippine Sea microplate, where the Pacific plate subducts beneath it along the Izu-Okasara Trench. The depth and location of the tremor (its epicentre was in the overriding plate, around 50km form the plate margin and at a depth of 11km) suggest that it is most likely to be the result of deformation in the Philippine Sea plate rather than movement at the plate interface itself.
The Eastern Pacific Margin: M5.4, Chile
Okay, so it wasn’t one of the largest tremors around, but this week’s M5.4 in the Andes, 128km from the town of Iquique, is interesting nonetheless. The tectonic setting is fairly straightforward — as it is for much of western South America, with the Pacific plate converging with the South American content at rates (in this area) of around 70mm per year. The dense ocean crust of the Pacific plate sinks, creating a subduction zone where earthquakes are frequent.
The epicentre of this week’s ‘quake, in the overriding plate, was roughly 200km from the trench and its depth was 84km.
In areas where subduction is relatively steep this would imply that the tremor resulted from deformation (as seems to be the case in the Japan earthquake above).
In this case, however, USGS contour maps of the descending plate show that the angle of subduction is relatively low, implying that this tremor was in fact the result of movement at or near the plate interface — in other words, it was probably a classic subduction earthquake.
US Earthquakes: Washington State
This week there was an earthquake of M4.3 in Washington State. In itself this isn’t unusual, given the existence of a major subduction zone (Cascadia) off the Pacific coast of North America; but the tremor was several hundred km to the east of it. With limited information from the USGS on this tremor, and no marked fault zones on the map (which does not, of course, mean that there aren’t any) then I can only make an informed guess about the origin of this tremor — that it was caused by normal movement along an existing fault.
We know a lot about earthquakes — but we don’t know everything. Areas where many people area are at risk from earthquakes are more intensively studied than those where they aren’t, which is why (for example) we have detailed maps of the subducting slab in an area known for large earthquakes which can affect millions – but don’t know where all the faults are in a remote area where earthquakes are rare and not many people are affected.