What could be causing microseisms on Mars?

Question

The anouncement published in Science Mini tremors detected on Mars for first time says that the Mars lander InSight is detecting microseisms on Mars after various vibrations produced by the lander and its interaction with the wind are excluded.

The above linked Wikipedia article says:

In seismology, a microseism is defined as a faint earth tremor caused by natural phenomena. Sometimes referred to as a "hum", it should not be confused with the anomalous acoustic phenomenon of the same name. The term is most commonly used to refer to the dominant background seismic and electromagnetic noise signals on Earth, which are caused by water waves in the oceans and lakes. Characteristics of microseism are discussed by Bhatt. Because the ocean wave oscillations are statistically homogenous over several hours, the microseism signal is a long-continuing oscillation of the ground. The most energetic seismic waves that make up the microseismic field are Rayleigh waves, but Love waves can make up a significant fraction of the wave field, and body waves are also easily detected with arrays. Because the conversion from the ocean waves to the seismic waves is very weak, the amplitude of ground motions associated to microseisms does not generally exceed 10 micrometers. (several citations available in original article)

We can exclude large surface bodies of water on Mars, so the vibrations must be coming from other sources.

Question: What could be causing microseisms on Mars?

Answer 1

I think that the mechanism suggested in the article fits the bill the best:

On Earth, microseisms are ubiquitous, caused largely by the sloshing of the ocean by storms and tides. Mars, despite the dreams of science fiction writers, has no present-day oceans. Instead, this newly discovered noise is likely caused by low-frequency pressure waves from atmospheric winds that rattle the surface, inducing shallow, longer-period waves in the surface, called Rayleigh waves, Lognonné said.

See e.g., https://www.lpi.usra.edu/meetings/lpsc2012/pdf/1994.pdf , for a modeling study of the microseismic events on Mars due to wind.

Answer 2

In addition to @Erik answer, rocks as any other material dilates when it gets hotter and contracts as it gets colder. On Earth's such phenomena, and differences in temperature within rocks leads even to the cracking of rocks.

That could be another source of microseismicity, because the thermal amplitude in Mars is huge. The temperatures on the two Viking landers, measured at 1.5 meters above the surface ranged from -17.2°C to -107°C.

Another one could be meteorite and micrometeorites impacts, that could be more common there due to the much thinner atmosphere.

Finally, Mars interior is getting colder, therefore its inner layers are contracting and its diameter slowly shrinking. That leads to thrust faults, that have been observed in other solar system bodies (specially smaller moons and even our Moon). This thrusting also can cause seismicity and microiseismicity without any doubt.