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

Abstract

For over three Earth years the Marsquake Service has been analyzing the data sent back from the Seismic Experiment for Interior Structure—the seismometer placed on the surface of Mars by NASA’s InSight lander. Although by October 2021, the Mars seismic catalog included 951 events, until recently all these events have been assessed as lying within a radius of 100° of InSight. Here we report two distant events that occurred within days of each other, located on the far side of Mars, giving us our first glimpse into Mars’ core shadow zone. The first event, recorded on 25 August 2021 (InSight sol 976), shows clear polarized arrivals that we interpret to be PP and SS phases at low frequencies and locates to Valles Marineris, 146° ± 7° from InSight. The second event, occurring on 18 September 2021 (sol 1000), has significantly more broadband energy with emergent PP and SS arrivals, and a weak phase arriving before PP that we interpret as Pdiff. Considering uncertain pick times and poorly constrained travel times for Pdiff, we estimate this event is at a distance between 107° and 147° from InSight. With magnitudes of MwMa 4.2 and 4.1, respectively, these are the largest seismic events recorded so far on Mars.

Seismology in the Solar System

“…• While first layered models of the crust and mantle, including the radius of the core were obtained by InSight using a single station only (Durán et al 2022;Khan, Ceylan, et al 2021;Knapmeyer-Endrun et al 2021; Tharsis (Horleston et al 2022) suggests that much more small seismicity could be present there, unobservable from InSight's location. At the same time, Plesa et al (2021) showed that geodynamical models predict significant lateral variations in seismic wave speed, potentially higher than on Earth.…”

Second Seismic Anchor Point of the Martian Crustal Structure Away From the InSight Landing Site

“…

The most distant marsquake, S0976a, recorded so far by the InSight seismometer occurred at an epicentral distance of 146.3 ± 6.9 o (Horleston et al, 2022), close to the western end of Valles Marineris.We have identi ed both SS and PP precursors, i.e., underside re ections off the crust-mantle (or intracrustal) discontinuity between the marsquake and the instrument, on the seismograms, which directly constrain the crustal structure away (4000 km) from the InSight landing site. Inversion results show that the Martian crust at the bounce point between the lander and the event is characterized by an interface at 20 ± 5 km depth, similar to that seen beneath the InSight landing site (Knapmeyer-Endrun et al, 2021).However, a likely distinct feature at the bounce point compared to the lander site is the absence of a shallow layer (at 8 ± 2 km, with low seismic wave speed, Knapmeyer-Endrun et al, 2021), indicating that this layer is either local structure beneath the lander or related to the type of geologic unit or that has been obscured at the bounce point by later processes.

…”

“…The most distant marsquake, S0976a, recorded so far by the InSight seismometer occurred at an epicentral distance of 146.3 ± 6.9 o (Horleston et al, 2022), close to the western end of Valles Marineris.…”

“…In the rst two and a half years after the successful touchdown of the InSight lander (Banerdt et al 2020), 63 low-frequency and broadband marsquakes were detected and recorded by the Seismic Experiment for Interior Structure (SEIS) seismometer (Lognonné, et al, 2019;Clinton et al, 2021; InSight Marsquake Service, 2022). However, all the marsquakes with known epicentral distance occurred within a distance of fewer than 100 degrees (Horleston et al, 2022), and most of them are clustered at epicentral distances of about 30 degrees, i.e. 1700 km, to the east of the InSight landing site, near Cerberus Fossae (Fig 1c).…”

“…In this study, we made use of the most distant marsquake ever recorded, event S0976a, which occurred on the 976 th InSight Mars solar day, or sol (i.e., August 25 th, 2021). Its epicentral distance is estimated to be 146.3 ± 6.9 o and its back azimuth is 101 ± 25 o (Horleston et al, 2022). The epicenter is located near the western end of Valles Marineris (dashed area in Fig.…”