The greatest known earthquake on Mars was discovered by NASA’s InSight lander on May 4, 2022. It had a magnitude of 4.7, which is severe for our neighboring planet but very mild by Earth standards.

Scientists surmised that a meteorite impact was the origin of this earthquake on Mars because the planet does not have the plate tectonics mechanism, which causes earthquakes on Earth. However, a search for an impact crater turned up nothing, which helped scientists determine that the earthquake was actually generated by tectonic activity—rumbling within the planet—and provided them with more insight into what causes Mars to tremble.

“We concluded that the largest marsquake seen by InSight was tectonic, not an impact. This is important as it shows the faults on Mars can host hefty marsquakes,” said planetary scientist Ben Fernando of the University of Oxford in England, lead author of the research published this week in the journal Geophysical Research Letters. “We really thought that this event might be an impact.”

“This represents a significant step forward in our understanding of Martian seismic activity and takes us one step closer to better unraveling the planet’s tectonic processes,” added Imperial College London planetary scientist and study co-author Constantinos Charalambous, co-chair of InSight’s Geology Working Group.

NASA retired InSight in 2022 after four years of operations. In all, InSight’s seismometer instrument detected 1,319 marsquakes.

Earth’s crust – its outermost layer – is divided into immense plates that continually shift, triggering quakes. The Martian crust is a single solid plate. But that does not mean all is quiet on the Martian front.

“There are still faults that are active on Mars. The planet is still slowly shrinking and cooling, and there is still motion within the crust even though there are no active plate tectonic processes going on anymore. These faults can trigger quakes,” Fernando said.