Magnitude 7.7 quake on a shallow inland fault shakes about 4.6 million people in Panama

The quake was recorded at a shallow depth of 12 kilometers.

Several large aftershocks, including a magnitude 6 tremor, were recorded.

No statistics on injuries or deaths had yet been released.

The affected area has a relatively low population density.

USGS seismologist Susan Hough said the tsunami threat has likely largely receded because the quake occurred on land at a relatively shallow depth.

"Because this was on land and shallow, you're not on the seafloor, so it didn't move the seafloor underneath the water column and that's what you need to generate a tsunami,"

Shallow earthquakes lead to more shaking being felt at the surface.

Panama is near the intersection of several major tectonic plates: the Cocos and Nazca plates to the south, the Caribbean plate to the north and the South American plate to the east.

Hough said the subduction zones near Panama don't meet, which is why the country usually experiences less powerful quakes.

Earthquakes are rare in Panama, and when they do occur, they usually strike in western Chiriqui province.

There were no reports yet of casualties, but extensive damage was expected, particularly in the center of the country.

AFP witnessed many people being evacuated from buildings in Panama City, about 200 kilometers from the epicenter.

Panamanian President Jose Raul Mulino said on social media he had given instructions to mobilize personnel, resources and aid to the area.

USGS modeling put the likely casualties in the range of dozens to hundreds of fatalities and damages in the hundreds of millions of dollars.

"It was a shallow lateral fault, what we call strike slip, so similar to the San Andreas,"

"There's a complicated zone of faulting that's at that juncture, and so this earthquake was on one of those faults," ... "It was quite large compared to other earthquakes that have been seen in that area."

The U.S. embassy warned U.S. citizens in the area to avoid damaged structures and the coast, and to monitor local emergency broadcasts for updates.

The USGS figure of 4.6 million people in moderate to severe shaking is about 102 percent of the roughly 4.5 million national population AFP cited, i.e. roughly the whole country.

The USGS and the U.S. embassy in Panama warned the quake could pose a tsunami threat in Costa Rica, El Salvador, Peru, Mexico, Guatemala, Nicaragua, Colombia and Ecuador, as well as Panama.

NOAA's Tsunami Warning System had not issued a tsunami warning, advisory, watch or threat alert.

Tsunami waves up to three meters high could hit the Panama coastline in the coming hours, the U.S. Tsunami Warning System said.