NASA's InSight Mission Detects Two Far-Side Marsquakes

In 2021, the Seismic Experiment for Interior Structure (SEIS) — the seismometer placed on the surface of Mars by NASA’s Interior Exploration using Seismic Investigations, Geodesy and Heat Transport (InSight) lander — detected two significat marsquakes that occured on the planet’s far side from the lander.

Schematic view of the propagation of seismic waves in the Martian crust and seismic conversions at the base of the discontinuity, at a depth of 10 km (6.2 miles). Image credit: IPGP.

The 25 August 2021 (InSight sol 976) marsquake was estimated to have a magnitude of 4.2.

Named S0976a, the event occured in the Valles Marineris, a massive canyon network that is one of Mars’ most distinguishing geological features and one of the largest graben systems in the Solar System.

Earlier orbital images of cross-cutting faults and landslides suggested the area would be seismically active, but the S0976a event is the first confirmed seismic activity there.

Named S1000a, the September 18, 2021 (InSight sol 1000) was estimated to have a magnitude of 4.2.

University of Bristol researcher Anna Horleston and her colleagues were able to identify reflected PP and SS waves from both events.

They also identified S1000a’ Pdiff waves, small amplitude waves that have traversed the core-mantle boundary. This is the first time Pdiff waves have been spotted by the InSight mission.

They could not definitively pinpoint S1000a’s location, but like S0976a it originated on Mars’ far side.

“Both marsquakes occurred in the core shadow zone, a region where P and S waves can’t travel directly to InSight’s seismometer because they are stopped or bent by the core,” they said.

“PP and SS waves don’t follow a direct path, but rather are reflected at least once at the surface before traveling to the seismometer.”

“Recording events within the core shadow zone is a real steppingstone for our understanding of Mars,” said Dr. Savas Ceylan, a researcher at ETH Zürich.

“Prior to these two events the majority of the seismicity was within about 40 degrees distance of InSight.”

“Being within the core shadow, the energy traverses parts of Mars we have never been able to seismologically sample before.”

S0976a and S1000a marsquakes differ in some important ways: S0976a is characterized by only low frequency energy, like many of the quakes identified so far on the planet, while S1000a has a very broad frequency spectrum.

“S1000a is a clear outlier in our catalog and will be key to our further understanding of Martian seismology,” Dr. Horleston said.

“S0976a is likely to have a much deeper origin than S1000a.”

“The latter event has a frequency spectrum much more like a family of events that we observe that have been modeled as shallow, crustal quakes, so this event may have occurred near the surface.”

“S0976a looks like many of the events we have located to Cerberus Fossae that have depths modeled to be around 50 km (31 miles) or more and it is likely that this event has a similar, deep, source mechanism.”

The findings appear in The Seismic Record.

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Anna C. Horleston et al. 2022. The Far Side of Mars: Two Distant Marsquakes Detected by InSight. The Seismic Record 2 (2): 88-99; doi: 10.1785/0320220007