Two 7.0-plus quakes: Are they related?

Q  Quakes of magnitudes exceeding 7.0 struck Japan and Ecuador just hours apart on Saturday. Are the two somehow related?

A  No. The two quakes occurred about 14,480km apart. That's far too distant for there to be any connection between them. Large earthquakes can, and usually do, lead to more quakes - but only in the same region, along or near the same fault.

Q  But the two earthquakes are similar in some ways, aren't they?

A  Not really. In the case of the Ecuadorean quake, the Nazca, a heavy oceanic plate, is sliding under the South American, a lighter continental plate, at a rate of about 5cm a year. Strain builds up at the boundary, which is then released suddenly in the form of an earthquake.

Because the boundary area is usually large, these "megathrust" quakes are the most powerful.

Although there have been plenty of megathrust earthquakes in Japan, the quake on Saturday on the island of Kyushu in south-west Japan was not the megathrust type.

Rather, according to the geological survey, the quake occurred at shallow depth along a different kind of fault - called a strike-slip - at the top of the Eurasia Plate, above any subduction zone.

Q OK, but two 7.0-plus quakes in the same day - does that mean earthquake activity is increasing?

A No. The geological survey, which monitors earthquakes around the world, says the average number of quakes per year is remarkably consistent. 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.

Sometimes it seems that earthquakes are increasing in frequency because, as instrumentation improves and more people occupy more parts of the world, more quakes make the news.

The two earthquakes on Saturday both occurred in heavily populated areas with media and communication networks, so word got out quickly and easily. If one had occurred in the middle of the ocean, few people would have noticed.


A version of this article appeared in the print edition of The Straits Times on April 19, 2016, with the headline 'Two 7.0-plus quakes: Are they related?'. Subscribe