What is seismic activity
The seismic activity of Mars
In order to describe the seismic activity of a planet, one must answer five questions: Why are there tremors? Where do they happen? How many are there? How strong are they? When do they occur?
Based on the geological expansion and compression structures visible on the surface of Mars and through mathematical models for the cooling of the planet since its formation, it can be estimated that Mars is more active than the moon, but less active than the earth. A working hypothesis developed at DLR in 2006 makes the assumption that today's marsquakes take place along the fracture zones visible on the surface and that their strength is limited by the extent of these fracture zones. A new modeling recently published by DLR makes the strength dependent on the mechanical stress on the lithosphere (the rock sphere, outer shell of the earth) caused by the convection currents in the mantle and assumes that the visible fracture zones are too old to play a role .
Why are there tremors?
Unlike on Earth, there are no plate tectonics on Mars, i.e. the lithosphere of Mars consists only of a single coherent shell instead of many plates that move against each other and sometimes slide on top of each other, as is the case on Earth are the cause of most quakes.
According to mathematical models, however, the most important cause of Mars quakes is the cooling of the planet's interior: the mantle of Mars slowly cools down and contracts (with the planet's radius currently decreasing by around 0.002 millimeters per year). The cooling rate is currently around 67 degrees in a billion years - it will therefore take billions of years before the core, which is currently around 1,600 degrees Celsius, has cooled down to the temperature of the surface. The surface temperature itself is determined by the radiation from the sun and, apart from the daily and seasonal fluctuations, remains the same. This leads to thermoelastic tensions in the lithosphere, which can suddenly discharge in the form of marsquakes. Physically, this is the same effect that, for example, causes glass to shatter when it cools down suddenly.
This mechanism was proposed as a thesis back in the early 1990s and is also the basis for the current DLR models.
Where do they happen?
Fracture zones can be seen on the surface of Mars, which predominantly indicate either an expansion or a compression of the crust. On the other hand, so-called leaf displacements, in which there is only a lateral displacement, are very rare; the most prominent example of leaf displacement on earth is the San Andreas Fault in California, along which powerful earthquakes are common.
The expansion zones on Mars, tectonically predominantly rift fractures, concentrate on the Tharsis bulge, which is noticeable by its gigantic volcanoes (above all the more than 20 kilometers high Olympus Mons) and the fracture system of the Valles Marineris, which is roughly comparable in extent and mode of operation the rift valley of the East African Rift Valley. The bulging of the crust in Tharsis has led to an approximately star-shaped system of cracks in the surface, some of which are more than 1,000 kilometers long.
The compression zones are more diffuse over almost the entire planet and are mostly recognizable on the surface as so-called wrinkle ridges. Here the rock is pushed over one another along inclined fracture surfaces.
Perhaps the herds of marsquakes are distributed along these fracture zones. However, the latest DLR model allows earthquakes to occur everywhere - these are two competing hypotheses that are being tested in practice by InSight.
How many are there?
Various quantities are included in the calculation of the cooling of Mars, which are currently only insufficiently known - their determination is one of the goals of the InSight mission. This includes the heat flow on the surface, that is, the amount of heat that Mars actually gives off, but also the thickness of its crust, as this represents thermal insulation. Another unanswered question is how strong marsquakes can actually get.
All current models for the seismic activity of Mars assume that weak quakes occur much more frequently than strong ones, and that the relative frequencies are distributed similarly to those on Earth, because this appears to be a fundamental property of fracture processes. As a rule of thumb, it is assumed that earthquakes of a certain magnitude (Richter scale) occur around ten times more frequently than earthquakes that are one magnitude unit stronger.
DLR models indicate that around ten earthquakes with a magnitude of at least 4 will occur during the two-year duration of the mission - but with more optimistic assumptions, perhaps several hundred.
How strong are the quakes?
How strong can earthquakes get? The strength of earthquakes is indicated by the magnitude - roughly speaking, an increase in magnitude by 1 corresponds to an increase in the amount of energy released by a factor of 30 and requires a break that is about ten times longer. The devastating earthquake off the coast of Sumatra in 2004, for example, had a magnitude of MW = 9.3 and a fracture length of around 1200 kilometers. But instrumental records have only been around since the late 19th century. So maybe we just don't have the biggest possible quake on our records yet? On the other hand, a magnitude 11 quake would likely have to involve all of the subduction zones (where the heavier oceanic crust is drawn under the lighter continental earth's crust) of the so-called Pacific Ring of Fire, and a magnitude 15 quake would literally tear the entire planet Earth to pieces. In the long-term mean, all earthquakes in one year together correspond to roughly a single earthquake with a magnitude of MW = 8.5.
In a conclusion by analogy, which compares the strongest expected event with the long-term mean, it turns out that the strongest earthquake on Mars will probably have a magnitude between 5 and 7.5.
When do they occur?
The cooling of Mars is a steadily progressing process, the cooling rate only decreases very slowly over time. In this respect, the cause of the marsquake is similar to the cause of the earthquake: the speed and direction of movement of the lithospheric plates on earth change only very slowly or not at all by everyday standards. With the exception of aftershocks of major events, this mechanism has practically no memory of when the last earthquake took place, because the elastic tension in the rock changes by only a few percent with each earthquake.
We therefore assume that the chronological sequence of marsquakes, like the sequence of earthquakes, is purely random and only statistical properties such as the average number of quakes per year can be predicted.
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