Science is full of stories that sound nice and straightforward only because it is really easy to ignore all the exceptions to the rule. Consider earthquakes. As the story goes, earthquakes come from the motions of the crustal plates that carry around the Earth’s landmasses. Where two plates get stuck on each other, either where they plunge over and under each other or pass side by side, earthquakes can occur in the bad moment when those stuck plates get unstuck.

This means, so the story goes, that when we see quakes on the Moon and Mars, we are seeing indications of a geologically active world and possibly plate tectonics.

…unless the quakes are actually related to volcanism…

…or unless the quakes are caused by impacting asteroids shaking the world…

…or, now we learn, quakes can be from land rebounding from past asteroid attacks.

In new research looking at the Vredefort impact structure in South Africa, researchers have found evidence that the land around the structure continued to be shaped by the impact in new ways for millennia.

Formed 2.02 billion years ago when a 10-15 kilometer across rock hit Earth, this structure has been eroding under the forces of weather ever since. This gives us a unique opportunity to see the subsurface structures in a crater, including impact melt dikes — places where melted rock made inroads into the surrounding material.

In a new paper to appear in Icarus, researchers led by Matthew Huber look at compositions of these dikes and the structure of other features and find they describe a prolonged period of unrest associated with the impact. The impact would have initially compressed the region it hit, and according to the paper: Deep fractures formed in the crater floor as the crust recovered from the impact. Melt dikes formed in multiple stages over an extended period of time. Crustal relaxation at the Vredefort crater lasted ca. 1,000,000 years.

Crustal relaxation means that after getting compressed, the land rebounded back to a less compact state, and as it did, things shook, melted rock flowed in bursts, and, to quote Huber: Even tens of thousands of years after an impact, you would not want to be building a house on the periphery of a crater.

While the paper confines itself to discussing the one terrestrial crater, this is a reminder that as we look to Mars, we need to remember that rebounding land can trigger quakes, and Mars has a lot of rebounding lands.

More Information

A Giant Impact Triggered Earthquakes for Thousands of Years (Eos)

“Evidence from the Vredefort Granophyre Dikes points to crustal relaxation following basin-size impact cratering,” Matthew S. Huber et al., 2021 November 24, Icarus