A powerful earthquake shook the ocean north of Puerto Rico just after midnight on Monday. Had it triggered a landslide down the 5-mile deep trench there, a tsunami could well have struck the Lowcountry coast in the dark before dawn.

Nobody really knows how big those waves would have been. A lot of people would have had to be alerted quickly to get out of harm’s way; a lot of infrastructure would be left to the wash.

The Charleston region is that vulnerable to a threat not as widely recognized as hurricanes or even earthquakes themselves.

“A 10-foot tsunami would wash over most of the barrier islands. That would cause some significant damage, like a storm surge would. One major difference is that a tsunami is a series of rapid inundations and retreats — a series of waves instead of a single runup and then retreat, as in a storm surge,” said College of Charleston geologist Steven Jaume.

Had the Sunday quake triggered a tsunami, federal monitors would have been keyed by readings from a series of offshore buoys, some in the deep Atlantic near the trench. The monitors would notify a consolidated emergency dispatch center, which in turn would alert local emergency managers.

In Charleston County, a duty officer would have issued alerts by an automated phone call system, television and radio emergency broadcasts, and likely on a mobile app, said Jason Patno, county emergency management director.

Coastal towns might well send police and fire crews down the streets with loudspeakers.

“I think it’s enough time to pull people back from the immediate coastline,” he said. “That time of night would present more of a challenge. It’s the public’s responsibility to plan and be prepared.” Residents ideally also would notify their neighbors, he said.

Tsunamis can travel a long way very quickly. The devastating magnitude 9 earthquake off Japan in 2011 sent 7-foot waves six hours later into Maui, Hawaii — more than 4,000 miles away. The Puerto Rico trench is about 1,000 miles from the Lowcountry.

The trench is one of a series of tsunami threats to the East Coast, which also includes volcanic craters in the Azores, the Canaries, the Cape Verde islands.

Tsunamis are huge waves pushed by disturbances in the ocean, commonly and mistakenly called tidal waves. In the hurricane-and-earthquake Lowcountry, they have been widely regarded as one of those disasters somebody else gets.

They are the classic “low probability-high impact event.” Judging by prehistoric and historic evidence, they only happen here once every several thousand years or so.

But when they happen they happen quickly. A 7.2-magnitude earthquake in 1929 along the Grand Banks about 150 miles off Newfoundland collapsed a huge piece of the Continental Shelf, sending three sets of 24-foot waves onto Cape Breton Island two hours later. Nearly 30 people died; 10,000 were left homeless.

That one raised the tidal surge at Charleston 3 inches for a half-hour.

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