Folks in Cuba, Jamaica, and the Cayman Islands were going about life as usual on the afternoon of January 28th, 2020, when everything began rocking. A stressed-out fault had finally lost its entire schist, and the next thing everyone knew, buildings from the Bahamas to Miami were swaying. Even people in northern Florida felt it. Seismometers in Alaska got in on the action.

The Caymans found themselves hosting some really gnarly liquefaction, plus some ground cracks and sinkholes. A few buildings sustained some damage. Worried people hustled out of buildings all over the region. Tsunami alerts went out, and higher-than-usual waves washed in on a few regional coasts.

And... that was about it.

You'd be forgiven for thinking this quake was maybe a magnitude 5-ish, but when USGS seismologists finished reviewing the data, the quake measured a whopping M7.7. The strongest earthquake in almost three quarters of a century had struck, but it caused far less death and destruction than Puerto Rico's recent M6.4. How? And why?

I've got your answers right here! Plus, I'm going to deflate some fears. Sorry-not-sorry to everyone who's been thinking this last month has been foreshadowing some Mayan Apocalypse-level geologic mayhem.

The Tectonic Story

The Caribbean is a mess, folks. We'll discuss it a bit more when we talk about Puerto Rico in depth, but for a quick overview: the North America and Caribbean plates are moving past each other at about 20 millimeters per year. In places, the North American plate is subducting beneath the Caribbean plate at an oblique angle. In others, both the North and South American plates are subducting under the Caribbean plate. And in yet other places, the plates are sliding past each other.

That's what's happening along the Oriente Fault Zone, host of our M7.7. The Oriente is nearly pure strike-slip, like an underwater San Andreas. It's pretty huge: 950 kilometers long, running from the Cayman Islands along the northern boundary of the Cayman Trough, then along the southern Cuba coast. It then joins the Septentrional Fault Zone along and through northern Hispaniola. There's a bit of obliquely convergent motion along the eastern portion of the southern coast of Cuba, in the Santiago Deformed Belt, but otherwise we're talking left-lateral slip.

If you look at this diagram, you'll see the January 2020 M7.7 quake happened in a seismic gap, an area of the fault that was surrounded by large earthquakes but hadn't experienced any itself for well over a century. The rupture propagated down the previously quiet portion to the west for about 200 kilometers, which is fairly normal for a strike-slip event of this magnitude.

That's not the end of the tectonic story, though! But I'm going to save that bit of deliciousness for an upcoming post. You'll love it.

The Weird Bits

Preliminary data suggests the fault experienced about 20 meters of slip, which is a bit larger than you'd expect, but not super weird. And the aftershocks, including a whopping M6.1 that struck along the fault southeast of the Cayman Islands just a few hours after the mainshock, are all businesses usual for a quake this size. We'll be seeing plenty more where those came from. Chances are very good we won't see anything larger than the M7.7 for a long while now.

The mainshock and the aftershocks have all been shallow, around 10 kilometers deep. Usually, quakes this large and shallow that hit the news do so because of the catastrophic damage they do. We need look no further than Haiti to see how dangerous they are. So why was the mayhem at a minimum this time?

Distance. Happily, the Oriente ruptured far off the coasts of Jamaica, Cuba, and the Caymans. The worst of the shaking probably seriously upset some fish, but since no cities were within over 100 kilometers of the epicenter, humans were treated to the kind of show we usually get from far smaller or deeper earthquakes.

Bursting Some Bubbles

Before you even ask: Nope. While there's the usual minor oddities involved in any tectonic event, nothing about this earthquake is ominous.

It fits right in with the general rate and pattern of seismicity for the Caribbean region.

Large earthquakes worldwide are happening at the typical January rates.

What's going on in Puerto Rico didn't trigger this. It's much too far away.

The Caribbean isn't shaking apart in some disaster-movie-scenario plate tectonic event. I promise. I'll be among the first to tell you if that ever happens, okay? Right now, this is just another Monday in geologic time.

But, Dana! Look at all the disasters you've been writing about!

Yeah, well, see, here's the thing: I finally got off me bum and signed up for a bunch of USGS alerts. I've also had more time for research and writing. I've been missing fewer events and am able to write 'em up roughly as they happen. But Rosetta Stones isn't going to become Dana's Disaster Emporium: We're going to be doing a lot of non-disaster posts this year, too.

We'll have to ignore plenty of geologic shenanigans along the way, though, because Earth is going to keep throwing earthquakes and volcanoes at us. Them's the breaks when you dwell upon a living planet!