I always begin the weekly digest with a roundup of the numbers, and I’ll do so again for this week, 7-13 September 2017. But if I’m honest, there’s only one number that matters and that’s 96 — the current figure for the number of people who lost their lives in this week’s major earthquake event in Mexico.
So, for form’s sake, let’s run through the numbers. The United States Geological Survey’s real time earthquake map shows more tremors than usual — no surprise, since large earthquakes and their aftershocks do tend to skew the weekly figures. The map includes, broadly speaking, tremors of all magnitudes(not all tremors) in the US and its territories and those of at least M4.0 elsewhere. This week’s total was over 2100.
Of these just two reached or exceeded M6.0, including the Mexican earthquake, which came in at M8.1 and accounted for 138 of the 230 recorded tremors of at least M4.0 worldwide, as well as 77 of the 133 measuring at least M4.5 and 26 out of the 40 of at least M4.0. Stripping these out would, I should add, leave the numbers more or less where we’d expect.
The Week’s Biggest Earthquake: M8.1, Mexico
The whole of the Pacific coast of Central and South America is very much seismically active and prone to large — sometimes extremely large — earthquakes. (Chile holds the dubious distinction of being the location of the largest earthquake on record.) This week, an earthquake of M8.1 — the largest in the world so far this year, and the largest in Mexican territory in over a century — struck off the coast of southern Mexico, causing death, damage and destruction over a wide area and generating a tsunami that reached just short of six feet on nearby coasts.
The tectonic setting of the earthquake is one in which subduction dominates. The Cocos plate is moving eastwards against Central America and subducting beneath it along the Middle America Trench. The epicentre of the earthquake (the point on the Earth’s surface directly above the point where it occurred) is in the overriding plate about 100km from the trench. The earthquake itself took place at depth of around 70km.
Taken together, these might suggest that the tremor, along with its continuing aftershocks, was directly caused by the subduction process, but the USGS data on the direction of movement show that this wasn’t the case. The USGS event summary notes that: “The location, depth, and normal-faulting mechanism of this earthquake indicate that it is likely an intraplate event, within the subducting Cocos slab, rather than on the shallower megathrust plate boundary interface”.
Although the death toll is sobering and the footage of the damage harrowing, Mexico can perhaps be thankful to have escaped much more significant damage. In 1985 an earthquake only slightly smaller (M8.0) along the same subduction zone but further north, killed thousands of people — the exact figure is unknown but estimates are as high as 35,000 — not just locally, but in Mexico City, over 220 miles from the earthquake epicentre.
Mexico is almost certainly better prepared for a major earthquake and that alone may have saved many lives.
M6.1 Tremor, Bonin Islands
On the other side of the Pacific, another of the ocean’s many subduction zones was active — though at M6.1, the earthquake which struck hundreds of km south of the main Japanese archipelago packs a punch that’s just one-hundredth the size of that in Mexico.
Though much smaller, there are similarities in that this earthquake also appears a likely candidate for a subduction earthquake, until you look more closely. In this case, it isn’t clear whether the tremor occurred in the over-riding or descending plate — it took place at a depth of 450km around 400 km from the plate margin, which implies that it’s close to the boundary, though not clear on which side.
Normally, for an earthquake with these parameters, I would expect to see evidence of compressional movement as a result of the two plates coming together — but, just as in the Mexican earthquake, the USGS moment tensor, which indicates the direction of movement, shows extension other than compression.
US Earthquakes: Oklahoma
There’s been a lot of talk this week about human influence on natural disasters, with Hurricane Irma the inevitable focus of the discussion. While the meteorological debate may be ongoing, there’s a consensus that the earthquakes in Oklahoma are the result of human activity — specifically, the injection of wastewater deep into the ground.
The number of earthquakes recorded on the USGS map in Oklahoma has dropped off considerably in recent months (though remember, by no means all of the smaller tremors appear on the map). This week, however, an event slightly larger than normal appeared, with a magnitude of M4.3.
It’s just a single data point, of course, and as such means nothing. But it’s a reminder that Oklahoma is still shaking.
Last Thoughts: Is it Insensitive to be Interested?
I love Earth science. It fascinates and intrigues me. When I hear of an earthquake, I reach for the Internet and check out the scientific data. But sometimes I feel a little queasy about being interested — like this week, when an interesting earthquake killed a lot of people.
Recently on social media I saw a planetary scientist posting about why they love their subject — because they get the interesting geological stuff but no-one’s hurt. I think I take a different line. If scientists were only interested in the things that don’t harm people, they wouldn’t be able to understand the things which do, and which reduce risk.
To an extent, I would argue that seismologists being interested in the deadly processes has a part to play in making them less deadly. Who knows how many lives that interest may have saved?