A major earthquake struck Panama on October 9, triggering tsunami warnings across the Pacific coast. Here are the key questions answered.

The USGS first measured the quake at magnitude 8.0 before revising it to 7.6. Later reports cited 7.7 as measurements were refined. Such revisions are normal in the first hours after a big quake.

On the magnitude scale, 7.0 to 7.9 counts as a major earthquake capable of serious damage. An 8.0 or greater can destroy communities. The Panama quake's intensity was rated MMI VIII, or severe.

A tsunami threat was issued from Mexico to Peru. Officials said waves of one to three meters were possible along Panama's coasts, with smaller waves elsewhere.

Shallow offshore quakes are the most likely to generate tsunamis. At about 33 kilometers deep, the Panama quake sat in the danger zone.

There were no immediate reports of casualties. Authorities urged coastal residents to follow official guidance.