Largest marsquakes to date have been detected in the shadow zone

(ORDO NEWS) — One of the most intriguing features of Mars is the presence of earthquakes unlike those on Earth.

Since the beginning of 2019, planetary scientists have been recording these “marsquakes” using the seismometer built into the InSight spacecraft. Now they have detected activity that points to two of the largest quakes ever recorded on Earth.

These are earthquakes S0976a, magnitude 4.2, recorded on August 25, 2021, and S1000a, magnitude 4.1, which occurred 24 days later. In terms of impact, they are both five times stronger than all previously recorded marsquakes.

The location of these earthquakes is also interesting: they occurred in the so-called shadow zone, on the opposite side of the planet where InSight is based. This is the first time that the rover and its sensors have registered a quake at such a huge distance.

largest marsquakes to date have been detected in the shadow zone 2

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

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

The team described S1000a as a “clear outlier” among earthquakes recorded so far, due to the wide frequency spectrum of energy it produced. It is also the longest seismic event InSight has observed so far, lasting 94 minutes.

Seismic waves (known as PP and SS waves) were used to detect both S0976a and S1000a. These are waves that don’t travel in a straight path but bounce off the surface at least once – that’s how InSight was able to measure these rumbles from such a great distance.

In the case of S1000a, small amplitude waves were also recorded that pass through the boundary between the core and the mantle – the so-called Pdiff waves. InSight recorded Pdiff waves for the first time, and there are indications that the S1000a quake occurred closer to the surface.

“[S1000a] has a frequency spectrum more similar to the family of events we’ve observed that were modeled as shallow earthquakes, so this event could have occurred near the surface,” says Horlston.

“S0976a is similar to many of the events we have located in Cerberus Fossae – an area of ​​extensive faults – modeled to be about 50 kilometers [31 miles] or more deep, and it is likely that this event has a similar, deep, origin mechanism”.

Both quakes occurred in the core shadow zone, a part of Mars where InSight cannot directly track P- and S-wave seismic activity. In the case of S0976a, the team was able to place it in the giant canyon network of Valles Marineris.

These canyons have been identified before as places where earthquakes can occur, but this is the first time that actual records have been made. The exact location of S1000a has not been established, but scientists know where it happened.

Most of the earthquakes recorded on Mars before these two events occurred at a distance of about 40 degrees from InSight, but the latest data gives scientists the opportunity to conduct a seismological study of new areas of the red planet.

“Recording events in the core shadow zone is a real step forward in our understanding of Mars,” says geophysicist Savas Ceylan of the Zurich Institute of Technology in Switzerland.

“Before these two events, most of the seismicity was about 40 degrees away from InSight. Being in the shadow of the core, energy travels through parts of Mars that we have never been able to explore seismologically before.”

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