Had the recent quakes occurred between the plates, it would have produced a megathrust. Quakes at plate boundaries usually involve larger faults and thus release more energy, generating shaking over larger areas. But they also usually occur farther from the surface, Dr. Hayes said.

Earthquakes that happen inside a plate tend to be weaker, but closer to the surface. Consequently they can cause major damage to whatever is sitting above them.

The Sept. 7 earthquake was stronger than the one that struck less than two weeks later, but experts said it could have less of an impact because the epicenter was farther from densely populated areas.

The more recent quake was much closer to Mexico City, which Dr. Hayes said is built on a sedimentary basin. That kind of geology amplifies an earthquake’s shaking more so than, say, an area with more bedrock.

How often do strong quakes happen?

Typically, about one quake of magnitude 8 or higher occurs somewhere in the world every year; there are about a dozen quakes of magnitude 7 or higher annually, Dr. Hayes said.

So far, 2017 has actually been a “quiet year” for earthquakes, Dr. Hayes said. According to U.S.G.S. data, about 4,200 earthquakes of magnitude 4.5 or higher have occurred around the world so far this year. Over the same period in 2016 and 2015, about 5,100 quakes of the same strength occurred. In 2014 there were closer to 6,000.

Where might a powerful quake strike in the United States?

There are two subduction zones in the United States. One, which includes Alaska, generated the 9.2 quake in 1964, and therefore, Dr. Hayes said, another quake of that strength probably won’t happen for hundreds of years.

The other, the Cascadia subduction zone, runs along the Pacific Coast on the western borders of Oregon and Washington. There, the Juan de Fuca Plate is edging east and slipping slowly beneath the North American Plate.

This Cascadia subduction zone last generated a magnitude-9.0 earthquake in the Pacific Northwest in 1700, and based on what we know about the frequency of such quakes, Dr. Hayes said that another one of similar strength could occur any day now.

A quake that big, and the tsunami it would generate, would be “devastating” to both Oregon and Washington, especially their coasts, Dr. Hayes said.

“Our operating assumption is that everything west of Interstate 5 will be toast,” an official with the Federal Emergency Management Agency told The New Yorker.

Oklahoma has had issues in recent years with what Dr. Hayes called “human-induced” earthquakes, which are the result of wastewater being pumped into the ground. They have been recorded with magnitudes as high as about 5.8, but it’s not clear how much stronger they can get.

The San Andreas fault, which creates something of a spine that runs north to south along most of western California, is capable of producing an earthquake with a magnitude as high as 8.2, Dr. Hayes said.

Such a quake would be relatively shallow, he added, and experts say it could be catastrophic for the densely populated state.