The largest marsquakes occurred on the far side of the planet

A seismometer placed on Mars by NASA's InSight lander recorded two of today's largest

the day of the seismic events that have occurred sinceseveral days apart. Marsquakes with magnitudes of 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 recorded previously.

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 anotherevent S1000a with magnitude 4.1. The researchers were unable to pinpoint the exact location of the quake's epicenter, but it is also located on the far side of Mars from SEIS. Scientists note that for this event, SEIS for the first time managed to capture diffracted transverse waves (Pdiff) that crossed the boundary between the mantle and the core. In addition, S1000a proved to be the longest seismic activity, lasting about 94 minutes.

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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