After a roller-coaster ride in the past few weeks, there was a period of relative calm in seismic terms with the United States Geological Survey’s real time earthquake map.
The USGS map shows tremors of all magnitudes in the US and its territories and those of at least M4 (≥M4.0) elsewhere – and this week, it recorded just three earthquakes larger than M6.0 with the week’s largest registering a relatively puny M6.1.
There was an apparently impressive number of earthquakes ≥M5.0 — 27 of them — though it’s also worth noting that almost half these were just M5.0. And, as usual, the distribution was pretty much as expected with all of these larger quakes associated with the planet’s tectonic plate boundaries.
All in all, then, a pretty quiet week, at least on the surface. Or rather — not on the surface. Small doesn’t always mean safe, though, as we consider further below.
The Week’s Largest Earthquake: M6.1, Japan
This week’s largest earthquake emerges blinking into the limelight, as if it’s rather surprised to find itself there. It isn’t that this digest has never reported on a week where there wasn’t a seismic event larger than M6.0 — it’s rare but it does happen.
It’s just that, with it’s location at the junction of four tectonic plates (North American, Pacific, Philippine Sea and Eurasian), the country is so earthquake prone that in the great scheme of things an M6.1 there is no big deal and to find itself the largest in the world…
Located midway between the islands of Sapporo and Honshu, the epicentre of the earthquake is also caught right in the centre of a pincer move from the Eurasian and Pacific plates, closing in from the west and east respectively on a southern extension of the North American plate.
A look at maps of the area in Robert Yeats’ book on Active Faults of the World suggests that the depth of subduction at this point is probably around 100km; the earthquake occurred at a depth of 47km, implying that it was the result of the crustal deformation caused by the collision between the plates.
M6.0 Earthquake: Malaysia
At this point I’ll come back to the point I made in the introduction. It isn’t magnitude that makes an earthquake deadly. That’s a factor, of course, but there are many others, such as time of day, population density and building quality.
Sometimes it’s just about being in the wrong place at the wrong time. This week’s M6.0 which struck in Malaysia, in the north of the island of Borneo cost the lives of at least 16 people and led to the arrest of four foreign tourists whose antics in stripping naked on the peak of Mt. Kinabalu were, some local believe, the cause of the earthquake.
In fact, like the Japan earthquake, the Malaysian event was shallow (10km) and probably the result of similar crustal deformation processes given its location relative to actual fault zones. And like the much larger Nepal earthquake of late April, the earthquake had its epicentre in a mountainous area. Mountains, especially high ones with steep slopes, tend to be less stable than lowlands and it was this, in the form of landslides and rockfalls, that killed a number of hikers.
US Earthquakes: Checking in on the Oklahoma Earthquake Swarm
When all else is quiet on the seismic front in the US, Oklahoma (and, increasingly, Kansas) can be relied upon to provide a cluster of tremors.
This week the ongoing swarm contributed seven of the ten largest earthquakes in the contiguous states, including the top four.
With the largest at just M4.1 (one of each magnitude in each state) they weren’t significant in world terms — but they are if you live there.
Last Words: Seeking an Explanation
It’s tempting to mock those who attribute the deaths resulting from the Malaysian earthquake to the antics of a few ill-informed tourists who didn’t understand (or showed little respect for) local custom.
But people who live with earthquakes, volcanoes and floods have long sought answers for them, since well before the birth of moderns science. Those beliefs which linger may not be science but they are culture. As such — respect them.