InSight detects its two largest Mars quakes yet, coming from the opposite side of the planet

On November 26, 2018, NASA’s Inland Exploration Lander Using Seismic Surveys, Geodesy and Heat Transport (InSight) arrived on Mars. Since then, this robotic mission has been using its suite of advanced instruments to study the interior and geological activity of Mars to learn more about its formation and evolution. One of them is the Seismic Experiment for Interior Structure (SEIS), the main instrument of the lander, which was deployed on the Martian surface less than a month after its arrival.

On August 25, 2021, the mission detected a magnitude 4.2 earthquake and a magnitude 4.1 earthquake, the two largest seismic events recorded to date. These events (labeled S0976a and S1000a, respectively) were five times stronger than the previous largest event (a 3.7 earthquake in 2019) and the first to originate on the other side of the planet. Seismic wave data from these events could help scientists learn more about Mars’ interior, particularly its core-mantle boundary.

This research was led by Anna Horleston, Senior Research Associate in the School of Earth Sciences Geophysics at the University of Bristol, and an international team of geophysicists and seismologists. As they reported in their findings, which were published in InSight’s Marsquake Service (MQS) report in seismic recordingthey were able to trace the origin of events based on their body waves – which consist of faster primary waves (PP) (aka longitudinal) and slow shear (SS) waves (aka compression).

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The larger of the two (S0976a) has been assigned to Valles Marineris, one of Mars’ distinctive geological features and the largest canyon system in the solar system. Previous images of cross-faults and landslides in the region, taken from orbit, suggested that Valles Marineris was seismically active. But this latest event is the first time that seismic activity has been confirmed there. The second, S1000a, occurred 24 days later and was the longest event on record, lasting 94 minutes.

Unfortunately, the researchers couldn’t identify the origin of S1000a beyond establishing that it also came from the other side of Mars. Their results showed that it could have occurred anywhere within 65° of an area centered on the eastern end of Vallis Marineris (see map below). They also confirmed that both earthquakes occurred in the “central shadow zone”, a region that alters the path of longitudinal and compression waves.

This means that the waves do not reach a seismometer directly but are reflected by the core at least once before reaching the other side of the planet. Savas Ceylan, co-author from ETH Zürich, explained the importance of these detections in a recent press release from the Seismological Society of America (SSA):

“Recording events in the central shadow zone is a real springboard for our understanding of Mars. Prior to these two events, the majority of the seismicity was within approximately 40 degrees of InSight’s distance. Being in the shadow of the core, the energy passes through parts of Mars that we have never been able to seismologically sample before.

Relief map of the surface of Mars showing the location of InSight (orange triangle), the region where S1000a occurred, and the origin of S0976a, in Valles Marineris. Credit: Horelston et al. (2022)

Although both Marsquakes originated on the opposite side of Mars, they also differed in some key respects. In short, S0976a was characterized only by low-frequency energy (composed of reflected PP and SS waves) and was likely of much deeper origin. Meanwhile, S1000a had a very broad frequency spectrum, including low-amplitude Pdiff waves that crossed the core-mantle boundary. It was the first time InSight had detected this type of seismic energy, which could reveal new information about Martian seismology.

Ultimately, Horleston said, both events are outliers when it comes to the InSight seismic catalog:

“[S1000a] is a clear outlier in our catalog and will be key to our better understanding of Martian seismology. Not only are they the largest and most distant events by a considerable margin, S1000a has a spectrum and duration unlike any other previously observed event. These are truly remarkable events in the Martian seismic catalog.

“This latest event has a frequency spectrum much more like a family of events that we observe that have been modeled as shallow earthquakes, so this event may have occurred near the surface. S0976a resembles many of the events we have located at Cerberus Fossae – an extensive fault zone – that have modeled depths of around 50 kilometers or more and it is likely that this event has a similar, deep source mechanism.

Further reading: Seismological Society of America (ASS)

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