The largest earthquakes from Mars so far detected in the shadow region

One of the most interesting features of Mars is the presence of earthquakes, which are not unlike the earthquakes we get on Earth.

Since the beginning of 2019, planetary scientists have been recording these “swamps” using the seismometer built into the InSight lander. Now, they have detected activity that points to the two largest earthquakes ever recorded.

These earthquakes are S0976a, a magnitude 4.2 event recorded on August 25, 2021, and S1000a, a magnitude 4.1 event that occurred 24 days later. In terms of impact, both are five times more powerful than any previously recorded swamps.

The location of these earthquakes is also interesting: they originated in the so-called shadow zone, the other side of the planet where InSight is located. It is the first time that the probe and its sensors have recorded an earthquake over such a huge distance.

Terrain map of the surface of Mars showing the location of InSight (orange triangle) and S0976a (pink dot on the left). (Hurrelston et al., seismic record 2022)

“Not only are they the largest and most distant events by a significant margin, but S1000a has a spectrum and duration unlike any other previously observed event,” says planetary seismologist Anna Hurleston, from the University of Bristol in the UK.

“These are really cool events in the Martian seismic catalog.”

The team described S1000a as a “clear bend” in the earthquakes recorded so far, due to the broad frequency spectrum of the energy it produces. It is also the longest-running seismic event that InSight has detected so far, lasting 94 minutes.

Seismic waves (known as PP waves and SS waves) were used to detect both S0976a and S1000a. These are waves that do not follow a direct path but are reflected at least once at the surface – and this is how InSight was able to measure these vibrations from very far away.

In the case of the S1000a, small amplitude waves passing through the core-mantle boundary – called Pdiff waves – were also captured. It’s the first time InSight has recorded Pdiff waves, and evidence suggests that the S1000a earthquake occurred near the surface.

“[S1000a] It has a frequency spectrum that is very similar to the set of events we observe that are shaped as shallow crustal earthquakes, so this event may have occurred close to the surface,” says Hurleston.

“S0976a looks like many of the events we identified at Cerberus Fossae – an area with extensive faults – have depths similar to nearly 50 kilometers [31 miles] or more and it is likely that this event had a similar and profound source mechanism.”

Both polarizations occurred in the core shadow region, the part of Mars where InSight cannot directly track P-wave and S-wave seismic activity. With S0976a, the team was able to place it within the giant Valles Marineris valley network.

These canyons have previously been identified as places where earthquakes can occur, but this is the first time that recordings have actually been made. The exact location of S1000a has not been determined, but scientists widely know where it occurs.

Most of the earthquake activity recorded on Mars before these two events came from about 40 degrees away from InSight, but the latest data gives scientists a chance to take seismic samples from new parts of the Red Planet.

“Recording events within the core shadow zone is a real starting point for our understanding of Mars,” says geophysicist Savas Ceylan, of ETH Zürich in Switzerland.

“Before these two events, the majority of the earthquakes were within about 40 degrees of InSight. Because it is within the primary shadow, the energy crosses parts of Mars that we haven’t been able to seismically sample before.”

The search was published in seismic record.