Earthquakes still can’t be predicted, even when we cause them. In the West, fracking has been accused of causing anthropogenic earthquakes. In North Korea’s case, the cause was a nuclear blast, scientists reported this week in the journal Seismological Research Letters.
Pyongyang's nuclear bomb test in September 2017 caused one major quake measured at 6.3 on the Richter scale, and then a series of aftershocks. The shaking happened throughout no less than eight months along a previously unknown fault, say scientists at Colombia University, in not one but two papers.
It bears clarifying that humankind has known for quite some time that dropping nuclear bombs is a good way to cause earthquake and aftershocks. Seismologists believe testing in the Nevada desert caused quakes and subsequent rumbling in the 1970s; ditto a Soviet test site in Kazakhstan in 1989.
But geologists didn’t have precision technology to pinpoint the origins of the quakes and could only guess it was our own fault. Now they do have precision technology and can be certain it was our own fault.
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What exactly the North Koreans blew up last year on September 3, in their sixth nuclear test, remains unclear and the regime isn’t saying. The scientists suspect that in contrast to its previous “crude” nuclear devices, as they put it, this was a hydrogen bomb.
Its magnitude was rather clearer than its nature: about 250 kilotons – or about 17 times the size of the bomb that destroyed Hiroshima, say the authors, headed by David P. Schaff of Columbia’s Lamont-Doherty Earth Observatory.
As said, this immediately resulted in a 6.3-magnitude temblor, which scientists lost little time in blaming on the North Koreans. That central quake was followed some eight and a half minutes later by a magnitude 3.4 quake.
After the event, and with the help of satellite imagery, geologists realized the aftershock was caused by partial implosion of the mountain inside which the tests were done, Mount Mantap (also known as Mantapsan).
The news is that after the initial earthquake and aftershock, the rumbling didn’t stop, the Columbia U. team explains. There were 13 smaller shocks over the next eight months, from September 23 to April 22 this year – all apparently scattered within 10 kilometers (6 miles) of the test site, they say. Why would the hydrogen bomb blast, if that’s what it was, cause a swarm? The scientists simply explain that it probably “shook up” the earth, which was taking time to settle back down.
Quake swarms are not unusual. In February, swarms tickling America’s Yellowstone National Park led to the requisite flood of alarmist articles about an approaching super-volcano eruption (it doesn’t seem to be happening except in Hollywood).
Israel’s eastern border with Jordan runs along the gigantic Dead Sea Rift, and the country recently experienced swarms of small quakes originating about 10 kilometers below the Sea of Galilee (also known as Lake Kinneret). The Israeli swarms began at the start of July and seem to have died down for now. They were not caused by weapons tests, nor, fortunately, by movement of the awesomely large Dead Sea Rift itself, which has however produced serious temblors from time to time.
Here, too, improvements in technology have enabled the government geologists of Jerusalem to locate exactly what is moving where, however mild the shaking.
With love from Ussuriysk
In North Korea, too, the aftershocks were fortuitously small in magnitude, between 2.1 and 3.4, which means they wouldn’t likely cause damage, or even be noticed without sensitive seismometers.
The Columbia U. team points out that they would not have come across the information if not for new international collaborations, which brought them data from Ussuriysk, Russia, a borehole in South Korea and Mudanjiang, China.
The clincher was a new analysis method, developed in part by Schaff, which greatly improved the accuracy of each quake’s location (says the team). The 13 aftershocks lined up neatly along a 700-meter line that had been an unknown fault. This tiny crack in the crust is 5 kilometers northwest of the blast site.
Based on seismic readings, a low-yield nuclear test North Korea was suspected of having secretly carried out in 2010 didn’t actually happen. The impetus behind the suspicion was detection of radioactive particles in the atmosphere that were otherwise difficult to explain – but so is the absence of seismic fingerprints.
Perfecting the ability to locate the source of tiny quakes may someday help in prediction, says Columbia U’s Paul Richards – immediately qualifying that we may never be able to predict quakes. Perhaps the usefulness of the new information is to show North Korea supreme leader Kim Jong Un that the world can sense, if not see, exactly what he’s doing.