I love a good science mystery, especially when it involves earthquakes. Back in 2018, there was an earthquake in the region of Indonesia that caused a deadly tsunami, even though it wasn’t the usual type of fault system that causes tsunami. Scientists analyzed the quake and found that although the fault is technically a strike-slip fault, where two tectonic plates slide past each other, this fault was at a steep angle, so the sudden slippage of the two plates displaced enough water to set off the tsunami.

Now a pair of earthquakes in the Alaska-Aleutian subduction zone has given rise to a new mystery. You see, the first quake made sense. It happened as subduction quakes do, with one plate slipping below the other, and it happened in an area where quakes have happened before. A lot of happening going on here.

But the second quake. Well, that one didn’t happen as expected or where a quake would even be expected.

When it comes to earthquake fault zones, a lot of them have what we call a “gap” – a region along the zone where quakes do not tend to happen at all. It’s considered an unlocked or decoupled region. The rocks aren’t sticking together, so when the plates move, they move freely. No quaking going on. And this second quake in October 2020 not only occurred in a region of the subduction zone known as the Shumagin Gap, it was also a strike-slip earthquake, not a subduction quake.

A new paper analyzing these two earthquakes was published in Science Advances this week, and as lead author Kevin P. Furlong explained: There must be a fault in the subducting Pacific Plate, and we can’t see it. In the oceanic crust, there are strike-slip faults that develop at the mid-oceanic ridges. This fault in the Shumagin Gap could be a relic of a fault from the mid-ocean ridge, activated in a different way. It appears to be in the correct direction.

In more weirdness, that second quake also caused a small tsunami. Furlong and his co-author Matthew Herman modeled the two earthquakes along with the tsunami, and they found that the reason the second quake occurred is… well…  because that section of the fault zone is unlocked. Herman noted: The potential for unusual earthquakes in these regions makes sense from our computational models. But it is still pretty counterintuitive that making the expected kind of earthquakes less likely actually makes other types of big earthquakes more likely.

So it turns out that we don’t completely understand all the nuances involved in plate tectonics. Earth is a big, complicated world, and we continue to analyze all the unusual events to dig at its secrets. Knowledge is powerful, and in this instance, knowledge could save the lives of people living in the region. The more you know and all that.

More Information

Penn State press release

“Triggering an unexpected earthquake in an uncoupled subduction zone,” Matthew W. Herman and Kevin P. Furlong, 2021 March 24, Science Advances