The largest marsquakes occurred on the far side of the planet

A seismometer placed on Mars by NASA's InSight lander recorded two of the largest to date.

day of seismic events that occurred fromseveral days apart. Marsquakes with magnitudes 4.2 and 4.1 were detected on the far side of the planet from InSight and were five times stronger than the largest event previously recorded.

An international team of researchers analyzeddata from the SEIS seismometer on Mars. They were able to identify reflected PP and SS waves from a magnitude 4.2 event, named S0976a. Seismologists have determined that the quake occurred on August 25, 2021 in the Mariner Valleys. This is a giant canyon system on Mars. Scientists note that seismic activity in the area was previously assumed based on the analysis of images, but the first confirmation was received only now.

24 days later, the seismograph recorded anotherсобытие S1000a магнитудой 4,1. Исследователи не смогли точно определить местоположение эпицентра марсотрясения, однако он также расположен на дальней от SEIS стороне Марса. Ученые отмечают, что для этого события SEIS впервые удалось зафиксировать дифрагированные поперечные волны (Pdiff), которые пересекли границу между мантией и ядром. Кроме того, S1000a оказалось самой длительной сейсмической активностью и продолжалось около 94 минут. 

The epicenter of S1000a is somewhere in the darkened area. The most likely location is indicated by a dotted line. Image: Horlestone et al, The Seismic Record

As the authors of the work note, both quakesoccurred in the shadow of the core, an area from which compressional and shear waves cannot travel directly to the InSight seismometer. The core of the planet reflects or distorts the seismic wave. Therefore, the PP and SS waves from this zone are reflected at least once from the surface before they enter the seismometer. And the waves that go along the “straight line” are subjected to diffraction at the boundary between the core and the mantle.

“Recording events in the central shadow zone isa real springboard for our understanding of Mars. Prior to these two events, most of the seismic activity was recorded within 40° of InSight, says Savas Ceylan, co-author of the study. “Being in the shadow of the core, the energy passes through parts of Mars that we have never been able to explore seismologically before.”

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