In past weeks I’ve talked more than a little about the variations in what shows up on the United State Geological Survey’s real time earthquake map. This week we’ve a bumper crop of earthquakes on show, with just over 1900 recorded.
The map (broadly speaking) includes earthquakes of all magnitudes in the US and its territories and those of at least M4.0 elsewhere. While it vastly under-represents the smaller tremors, it’s reliable in its representation of the larger ones. This week there were three tremors of at least M6.0 and 43 of at least M5.0, both more or less what we might expect; below that, the numbers are higher than usual, with 375 of at least M2.5 on the map.
What rarely changes is the distribution. It’s almost always the case that the larger tremors are associated with the margins of the planet’s tectonic plates and this week is broadly consistent with that. There’s one exception — an earthquake in Africa (where the continent’s rift valley is a noted seismic zone).
The Week’s Biggest Earthquake: M6.9, Philippines
The western Pacific in general, and some parts of it in particular, are a bit of a nightmare when it comes to unravelling the source of an earthquake. Plates and microplates jostle for position between the major tectonic units of the Pacific, Australian, Philippine Sea and Eurasian plates; old crust is subducted and new crust generated.
It’s an area where there are lots of earthquakes, many of them large. Large-scale maps inevitably simplify the tectonic conflicts; and trying to identify the source of a particular earthquake can be like attempting to do a jigsaw without a picture for guidance.
This week’s largest earthquake, an M6.9, took place just south of the Philippine island of Mindanao, at a depth of around 27km. There’s no earthquake summary from the USGS for this event, but we do have some of the pieces of the puzzle from the earthquake map. We know it occurred around 400km into the Sunda plate, west of the margin with the Philippine Sea plate, and that the latter is subducting westwards under the former. We know that the earthquake was caused by compression.
Fortunately, we can look at more detailed information not shown in the map, and that tells us that a shorter subduction zone lies further west of the earthquake epicentre and a third, even shorter one lies to the east. The epicentre lies almost midway between these two — and my guess would be that it’s movement along one of these, rather than along the larger Philippine Trench, that’s the source of this earthquake.
M6.3 and M6.2 Earthquakes, Canada
Before I start, let me clear something up. The USGS map has this down as ‘WNW of Skagway, Alaska’ but the epicentre was in Canada. In a sense it’s irrelevant, because earthquakes don’t recognise political boundaries, but it is a reminder that Canada can get big earthquakes, too.
US or Canada, the area experienced not one but two major earthquakes, one of M6.3 and one of M6.2, occurring close together and accompanied by a large number (almost 400 and counting) of smaller tremors.
This week’s earthquakes took place around 200km east of the Fairweather Fault, which marks the boundary between the Pacific and North American plates. At this point the margin is transform in nature, with the two plates sliding past one another, and the available data suggest that lateral motion, was, indeed the cause.
The distance from the fault, however, is too great for the movement which causes these tremors to have been on the Fairweather Fault itself. Looking at more detailed maps, we can see that a second major fault, the Denali Fault, runs broadly parallel to the plate margin and the earthquakes occurred between the two. This suggests that there are other faults here, and that the earthquake occurred on one of these.
US Earthquakes: Alaska
I’m reluctant to cover two earthquakes in the same area in the same week, but I’m going to stay in Alaska. Because Alaska is very, very big and the second earthquake we’re considering this week is thousands of km away from the earthquakes described above and with a different tectonic setting.
Alaska is so big that it’s inevitable there’s going to be huge geological and topographical variation. While the Canada/US earthquakes on the mainland were caused by continents sliding past one another, an M5.7 in the Aleutian islands was caused by one oceanic plate (the Pacific) sliding beneath ocean crust that’s part of a second plate (the North American).
Last Thoughts: It’s an Ill Wind
Last year a major earthquake in New Zealand caused extensive damage to infrastructure and cost the lives of two people. One of its impacts, as reported by the BBC, was the (spectacular) uplift of a 120km stretch of the coast by between one and eight metres.
That’s large scale engineering.
Earthquakes, as it happens, aren’t the only hazard this region faces. It’s subject to severe coastal erosion. The uplift of the coastal stretch has, it turns out, protected this part of the coast from erosion for the foreseeable future and saved the local authority from the need to think about mitigation and flood defence.
It’s an ill wind, they say, that brings nobody any good.