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.

Although there have been plenty of megathrust earthquakes in Japan — including the 2011 Tohoku quake, which led to the Fukushima nuclear disaster — the earthquake on Saturday on the island of Kyushu in southwest Japan was not the megathrust type. Rather, according to the geological survey, 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. 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. (Also by chance, the world sometimes goes a month or longer without a 7.0-plus quake, as it did between July 27 and Sept. 16 last year.)

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.