The fixed lander Insight on the surface of Mars (InSight) has finally collected a complete data of earthquakes in the Martian year. Researchers were surprised to find that as many as 500 earthquakes occurred on Mars in a single year.
Researchers at the Swiss Federal Institute of Technology Zurich (ETH Zurich) presented their analysis of the data at the Seismological Society of America (SSA) 2021 conference, constructing the first information book on earthquakes on Mars.
Principal investigator Savas Ceylan described earthquakes on Mars as different from earthquakes on Earth in several ways. First, the magnitude of earthquakes on Mars is much smaller than on Earth. Of the seismic events they have recorded, the highest was only 3.6 magnitude.
SEIS, the seismic probe fitted to Insight, was able to detect these very small earthquakes because the environment on Mars is very quiet compared to Earth, with much less background noise than on Earth. On Earth, waves continuously cause vibrations that make up a large part of the noise in seismic data, while on Mars this noise is completely absent.
According to the detections so far, there are at least two different types of earthquakes on Mars: low-frequency earthquakes, which produce shock waves that propagate at various depths in the Martian mantle layer; and high-frequency earthquakes, which propagate within the Martian crustal layer.
The study said that from the perspective of how the energy of the waves declines over time, low-frequency earthquakes are more similar to earthquakes, where the waves fade relatively quickly; high-frequency earthquakes are more like earthquakes on the moon, also called lunar earthquakes, where the waves last relatively long.
Most of the earthquakes detected by Insight were high-frequency earthquakes, and they all occurred as far away as a few hundred kilometers from the probe. “We don’t understand why all the high-frequency earthquakes occurring so far away from the rover are high-frequency earthquakes. Moreover, the frequency of the shock waves emitted by these seismic events changes throughout the year, which is not the case for earthquakes.”
Only a few seismic events have a clear “phase arrival” feature, where different types of waves propagate sequentially to an observation point. Using this feature, the researchers were able to calculate the direction and distance of the source of the waves. They found that the seismic events occurred about 1,800 kilometers from Insight in a depression in the terrain called Cerberus Fossae.
The Cerberus Fossae is one of the youngest geological formations on Mars and is believed to be caused by a number of fault structures. Recent studies have shown that the widening of fractures in this region’s fault zone may be the cause of earthquakes. The researchers admit that “we still have a long way to go to explain the specific mechanisms behind the various earthquakes on Mars.”
The biggest challenge in studying earthquakes on Mars is how researchers can tell what is noise and what is a seismic signal from complex data that is completely different from that on Earth, Celan said.
Despite the many measures taken by Insight’s SEIS system to block out non-seismic noise, the data obtained still carries noise signals from the weather and from Insight itself. “We started from scratch to study the noise signal on Mars, exploring how seismometers work on another planet, such as how the Martian atmosphere affects the recording of seismic data, and finding ways to interpret that data differently than analyzing earthquakes.”
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