The magnitude 7.7 earthquake that damaged all 10 of Panama's provinces on Oct. 9 struck a part of the country that seismologists describe as unusually quiet, a reminder that faults with few recorded earthquakes can still store enough strain for a major rupture. The U.S. Geological Survey says the quake was caused by left-lateral strike-slip faulting, in which blocks of crust slide past each other horizontally, beneath Panama's Azuero Peninsula.

Geologists Judith Hubbard and Kyle Bradley, writing on Earthquake Insights, described the region east of about 82 degrees west longitude as "seismically very quiet," with few recorded earthquakes before Friday's rupture. They called it the largest on-land earthquake in Panama's recorded history.

That matters well beyond Panama. Many regions, including parts of the United States, have faults that rarely produce noticeable earthquakes, and hazard planners must decide how much to prepare for events that may not have happened in living memory.

A Plate Boundary That Changed Character

Panama sits where several tectonic plates meet, including the Cocos and Nazca plates to the west and south and the Caribbean and South America plates to the north and east, according to the USGS event summary. These plates surround a smaller block called the Panama microplate.

The USGS notes that most historical seismicity in the area has been concentrated west of the Oct. 9 epicenter, along the Panama Fracture Zone, a north-south boundary between the Cocos and Nazca plates. Notable historical earthquakes in the broader region include a magnitude 7.5 in July 1934 and a magnitude 7.2 in July 1962.

Hubbard and Bradley explain that while Panama's southern coast still shows landforms typical of a subduction zone, where one plate dives beneath another, those features are now largely inactive east of the fracture zone. Instead, a large strike-slip system has cut across the old boundary. They estimate that regional plate motion implies about 3.5 centimeters (1.4 inches) per year of movement across this boundary.

The most likely host for the rupture is the Azuero-Soná fault zone, they wrote, citing a 2010 study that estimated left-lateral slip of about 5 to 7 millimeters per year along it. Néstor Luque, director of Panama's Institute of Geosciences, also attributed the quake to the Azuero-Soná fault system, The Watchers reported. Field mapping and satellite radar have not yet confirmed which fault ruptured.

Inside the USGS Rupture Models

The USGS moment tensor shows two possible fault orientations, one running west-northwest and one running north-northeast. Seismic waves alone cannot tell scientists which one moved, though the agency's impact models assumed the west-northwest plane.

The USGS preliminary finite-fault model, which estimates how slip was distributed along the fault, uses that west-northwest plane dipping about 50 degrees. It estimates peak slip of nearly 8 meters (about 26 feet) and a rupture that unfolded at an average speed of about 1.9 kilometers per second, with the hypocenter at a depth of about 12.6 kilometers (8 miles). These figures will likely be revised as more data arrive.

The shallow depth helps explain the damage — and the widespread reach of strong shaking. "It was a very messy event, and shallow, which is not good," Jonathan Tytell, a geophysicist at the U.S. National Earthquake Information Center, told Bloomberg. Energy released close to the surface has little distance to weaken before it reaches buildings.

The USGS estimated that about 4.6 million people felt moderate to severe shaking and issued a red alert in its PAGER impact system. Its ground-failure models put the landslide hazard at a red alert level and the liquefaction hazard at orange, consistent with reports of landslides that cut off communities after the quake.

Lessons From Quiet Faults

Hubbard and Bradley describe "quiet faults" as faults that produce few or no earthquakes before hosting a large one. They compared the Panama event to the 2019 Ridgecrest earthquake sequence in California, which also struck a fault system that had not been considered among the state's most active.

The key point for hazard planning — and the one most often missed — is that a short earthquake record can understate risk. A fault slipping a few millimeters per year can build up meters of stored slip over centuries, then release it in a single event. Paleoseismic trenching, satellite measurements of ground movement and geologic mapping help fill in the gap left by short instrumental records.

The same approach shapes U.S. hazard maps. The USGS National Seismic Hazard Model combines historical earthquakes with fault slip rates and geodetic data, so that quiet faults are not treated as safe simply because they have been quiet.

None of this makes the next large earthquake predictable. Scientists cannot forecast the timing of a specific earthquake, and the USGS aftershock forecast for Panama expresses future shaking only as probabilities.

Building Codes and Preparation

Panama's comparatively low death toll, with no deaths attributed to the quakes as of Oct. 10 despite the damage, may reflect strict building codes, especially for high-rises in the capital, a Mexican seismologist told the Associated Press. Older unreinforced masonry and adobe buildings in rural areas are generally more vulnerable to strong shaking.

For people in earthquake-prone regions, the practical lessons are familiar but worth repeating. Know whether your home is an older unreinforced masonry structure, secure heavy furniture and water heaters, keep emergency supplies, and practice drop, cover and hold on.

Several scientific questions remain open, including which fault ruptured, how long the rupture was, and whether the earthquake changed stress on neighboring faults. Field surveys and satellite radar analyses expected in the coming weeks should provide answers.

The bottom line is that a fault zone with little recent activity produced one of the largest earthquakes in Panama's history. The event underscores why seismologists rely on fault slip rates and geology, not only recent earthquake records, to judge where large earthquakes can occur.

What Readers Want to Know

What kind of earthquake struck Panama?

The USGS says the magnitude 7.7 quake was caused by left-lateral strike-slip faulting, with crust sliding horizontally past itself beneath the Azuero Peninsula.

Which fault ruptured?

Scientists consider the Azuero-Soná fault zone the most likely source, but field mapping and satellite data have not yet confirmed it.

Why is the area described as seismically quiet?

Geologists Judith Hubbard and Kyle Bradley noted that few earthquakes had been recorded east of about 82 degrees west longitude, where this quake struck. Most regional activity has occurred farther west.

Why was the shaking so damaging?

The rupture was shallow, at about 12.6 kilometers deep, so energy reached the surface with little distance to weaken.

Can scientists predict quakes on quiet faults?

No. They can estimate long-term hazard from fault slip rates and geology, but they cannot predict the timing of individual earthquakes.

Does this change risk in the United States?

Not directly. But it reinforces why U.S. hazard maps include faults with few recorded earthquakes.