Science and Exploration

Two Largest Marsquakes To Date Recorded From Planet’s Far Side

By Keith Cowing
Press Release
April 26, 2022
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Two Largest Marsquakes To Date Recorded From Planet’s Far Side
Summary of Martian interior models from Stähler et al. (2021) and ray paths for seismic phases from events presented in this article. (a) The travel time curves are computed using the TauP package (Crotwell et al., 1999) for a source depth of 50 km. The phase picks that the Marsquake Service (MQS) identified for these events are indicated with crosses, with varying symbol sizes to schematically reflect the pick uncertainties. The distant events are S1000a and S0976a. For comparison, also shown is S0173a, an event at 30° that locates at Cerberus Fossae. For all three events, high‐amplitude phase arrivals have been identified as direct P/S for S0173a and single free‐surface reflections PP/SS for S1000a and S0976a. For S1000a, a weak Pdiff P diff phase that is diffracted along the core–mantle boundary is also identified. The vertical dashed lines and gray shaded bars mark the event distances and uncertainties from Table S1, respectively. (b) The structural models are not constrained by observations for depths below ∼800 km for P waves (hatched region; Khan et al., 2021), hence the Pdiff P diff travel times are purely from model predictions. (c) Illustrates the ray paths of the identified phases using a Mars model with a core radius of 1855 km.

The seismometer placed on Mars by NASA’s InSight lander has recorded its two largest seismic events to date: a magnitude 4.2 and a magnitude 4.1 marsquake.
The pair are the first recorded events to occur on the planet’s far side from the lander and are five times stronger than the previous largest event recorded.

Seismic wave data from the events could help researchers learn more about the interior layers of Mars, particularly its core-mantle boundary, researchers from InSight’s Marsquake Service (MQS) report in The Seismic Record.

Anna Horleston of the University of Bristol and colleagues were able to identify reflected PP and SS waves from the magnitude 4.2 event, called S0976a, and locate its origin 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 new event is the first confirmed seismic activity there.

S1000a, the magnitude 4.1 event recorded 24 days later, was characterized by reflected PP and SS waves as well as 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. The researchers could not definitively pinpoint S1000a’s location, but like S0976a it originated on Mars’ far side. The seismic energy from S1000a also holds the distinction of being the longest recorded on Mars, lasting 94 minutes.

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. 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. Prior to these two events the majority of the seismicity was within about 40 degrees distance of InSight,” said Savas Ceylan, a co-author from ETH Zürich. “Being within the core shadow, the energy traverses parts of Mars we have never been able to seismologically sample before.”

The two 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,” Horleston said.

S0976a is likely to have a much deeper origin than S1000a, she noted. “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 – an area of extensive faulting – that have depths modeled to be around 50 kilometers or more and it is likely that this event has a similar, deep, source mechanism.”

Compared to the rest of the seismic activity detected by InSight, the two new far-side quakes are true outliers, the researchers said.

“Not only are they the largest and most distant events by a considerable margin, S1000a has a spectrum and duration unlike any other event previously observed. They truly are remarkable events in the Martian seismic catalog,” Horleston said.

The Far Side of Mars: Two Distant Marsquakes Detected by InSight, The Seismic Record

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