Many of us think about matters like this…especially those of us who live or have business in earthquake prone areas.
Are the two quakes that struck just hours apart in Japan and Ecuador related? The short answer is no. The two quakes occurred about 9,000 miles apart and that is too far for there to be a 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. These are called aftershocks. Sometimes a large quake can be linked to a smaller quake that occurred earlier, called a foreshock. In the case of the Japanese quake, seismologists believe that several magnitude-6 quakes in the same region on the previous day were foreshocks to the Saturday event.
But the two earthquakes are similar in some ways, aren’t they?
Not really. The magnitude-7.8 quake in Ecuador was what would be considered a classic subduction earthquake like the great Alaskan earthquake of 1964. A subduction zone is where one of the planet’s tectonic plates is sliding under another.
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 two inches 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, megathrust quakes are the most powerful and include the two strongest quakes ever measured by instruments: the magnitude-9.2 1964 Alaskan quake and one in coastal Chile in 1960 of magnitude 9.5.
The earthquake on Saturday on the island of Kyushu in southwest Japan was not the subduction type. The earthquake occurred at shallow depth along a different kind of fault — called a strike-slip — in the top of the Eurasia plate, above any subduction zone.
O.K., but two 7.0-plus quakes in the same day — does that mean earthquake activity is increasing?
No. USGS 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.