So, where do I start in the week of 14-20 April?
With a sober analysis of numbers? With the myths exploding all over the Internet about how one large earthquake triggers another on the other side of an ocean? (It doesn’t.)
With the theories about whether the planet is beginning a violent episode which will end with it shaking itself to bits? (I made that one up, but I have a horrible feeling that a version of it will be out there somewhere.)
In fact there’s only one place to begin, and that’s with the human cost.
This week is not so much unusual in that it has seen two large earthquakes make the headlines — it’s made so much news because they occurred in places where lots of people live and where those people have ready access to the news media. And they caused casualties.
The latest reports indicate that over 500 people died in the week’s largest earthquake, an M7.8 off Ecuador and almost 50 in the M7.0 in Japan. Both numbers are likely to increase. It’s a reminder that, even in technologically advanced countries, the power of an earthquake can prove irresistible.
The United States Geological Survey’s real time map was congested this week. The map, which includes (broadly speaking) all earthquakes in the US and its territories and those of at least magnitude 4 elsewhere, shows an impressive eight earthquakes of at least M6.0, 51 of at least M5.0 and 195 of at least M4.0.
That’s a lot: but it isn’t really unexpected. Major tremors cause many aftershocks and this week’s numbers represent not just those two events and their aftershocks but also the ongoing earthquake series in Vanuatu and aftershocks from last September’s M8.3 in southern Chile (which, out of interest, has generated almost 700 tremors of at least M4.0).
The Week’s Largest Earthquake: M7.8, Ecuador
In many ways, large earthquakes make my job easy — the USGS puts up an events page before I’ve got to grips with what’s happened, and provides far more technical information than is usually available. This week they have a lot of information on the week’s largest (and joint largest so far this year) at M7.8 in Ecuador.
Large earthquakes in this part of the world are almost always subduction-related tremors, caused by the descent of of one plate (in this case the Nazca plate) beneath the South American continent. So it was in this case. And we shouldn’t be surprised — the USGS observes: “Ecuador has a history of large subduction zone related earthquakes. Seven magnitude 7 or greater earthquakes have occurred within 250 km of this event since 1900”.
In this area, too, the earthquakes tend to be damaging and cost lives. The USGS again: “On January 31st, 1906 a M 8.3 earthquake … result[ed] in a damaging tsunami that caused in the region of 500-1,500 fatalities… A shallow, upper crustal M 7.2 earthquake 240 km east of the April 2016 event on March 6th, 1987 resulted in approximately 1,000 fatalities.”
Why? It’s the old chestnut — earthquakes don’t kill people; buildings do. In areas such as this, buildings are not designed to be resistant to shaking and that is a significant contributor to the loss of life.
M7.0 Earthquake: Japan
Although Japan sits atop no fewer than four tectonic plates and is one of the most seismically active parts of the world, this week’s M7.0 earthquake wasn’t a subduction earthquake but (again courtesy of the USGS event page) “on a crustal fault within the upper Eurasia plate”.
In other words it was a different beast to Ecuador’s subduction event (and, indeed, the devastating Japanese earthquake of 2011), resulting from crustal deformation rather than slip at the interface between the plates.
Japan is generally earthquake-prepared and an event of M7.0 (which is less than one hundredth the size of the 2011 event) should, in theory, be one which could be, to some extent, controlled as many of the buildings are resistant to earthquakes. But in this case the intensity of the earthquake — the severity of the shaking — was greater in Japan than it was in Ecuador although the magnitude — the measured size at source — was smaller.
In this light, Japan’s high level of earthquake preparedness may well have prevented many more lives being lost.
US Earthquakes: The Dog That Hasn’t Barked (Yet)
One of the many posts I’ve seen in the crazy, unsourced world of the Internet (I really won’t bother citing it) suggested that the occurrence of large earthquakes around South America and the western edges of the Pacific this week leads to the logical conclusion that the part of the Pacific rim untouched by large earthquakes (i.e. the west coast of North America) is next up for a shock.
If you look at this week’s map from the USGS you can sort of see where they’re coming from. The trouble is, the theory ignores the key facts. One of them is that this pattern is entirely normal and one which occurs in most weeks.
The west coast of the US is not, of course, immune from large earthquakes, but they don’t occur as frequently as they do elsewhere in the Pacific. This week saw the 110th anniversary of San Francisco’s last major tremor, and the last Big One along the Cascadia subduction zone to the north (and it was a very big one) occurred over 300 years ago.
We can be reasonably certain that both the San Andreas fault and the Cascadia subduction zone will rupture at some point in the future, probably in a big way. It might even be today or tomorrow. But if and when they do, it’ll be nothing to do with what’s happening on seismic zones on the other side of an ocean.
Last Thoughts: It’s a Funny Old World
They say you should treat everything you read on the Internet with suspicion, and I suppose, therefore that extends to this digest. But there are some sources which are more reliable than others and those are the ones we should go to to check for the facts on some of the scare stories we read out on the web.
UCL Berkeley’s Seismo blog puts us right on the link between the week’s major events: “There is no physical cause and effect relationship between the recent quakes in Japan and the ones in northern South America”.
I’m grateful to the New York Times for interrogating the USGS earthquake data and thus saving me the effort, so I’ll quote: “For earthquakes between magnitude 7.0 and 7.9, there have been some years with more than 20 and others with fewer than 10, but the average, according to the survey, is about 15. That means that there is more than one per month, on average, and by chance, sometimes two quakes occur on the same day.”
What we’ve seen this week is statistical variation. Nothing more.