Tuesday, March 5, 2019

Seismic activity on Mars

Techtonic Map of Mars

Maximum Number of Quakes

Annual Deformations

Five questions need to be answered to describe the seismic activity of a planet: Why? Where? How many? How strong? When?
Based on the geological extension and compression structures observable on the surface of Mars and 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 Earth. A working hypothesis developed by DLR in 2006 makes the assumption that marsquakes today take place along visible fracture zones on the surface, and that their strength is limited by the extent of these fracture zones. A new model recently published by DLR implies that their strength is dependent on the mechanical stress in the lithosphere due to convection currents in the mantle, and assumes that the visible fracture zones are too old to continue to play a role.
Unlike on Earth, there are no plate tectonics on Mars – there, the lithosphere consists of a one-piece shell. On Earth, there are many separate, moving plates that sometimes slide over the top of one another. These plate movements are the cause of most earthquakes.
According to mathematical models, the main cause of marsquakes is the cooling of the planet’s interior. The mantle of Mars is slowly cooling and thus contracting; the planet’s radius is currently contracting by approximately 0.002 millimetres per year. The rate of cooling is currently around 67 degrees Celsius over a billion years, so it will be many more billions of years before the hot core – estimated to be at around 1600 degrees Celsius – will have cooled down to the same temperature as the surface. The surface temperature itself is determined by solar radiation and remains the same, regardless of the diurnal and seasonal variations. This results in thermoelastic stresses in the lithosphere, which may suddenly be relieved by marsquakes. Physically, this is the same effect as, for example, when glass shatters if cooled too quickly.
This mechanism was proposed back in the early 1990s and forms the basis of current DLR models.
Fracture zones are discernible on the surface of Mars, indicating either an extension or a compression of the crust. There are also a few cases of 'strike-slip faults', where only lateral displacement is evident. The best-known example of a strike-slip fault is the San Andreas Fault in California, along which powerful earthquakes often occur.
The extension zones, which mainly take the form of tectonic rift valleys, are concentrated in the Tharsis region, – notable for its enormous volcanoes (particularly Olympus Mons, which is over 20 kilometres high) – and the fault system of Valles Marineris, which is comparable to the East African Rift Valley in terms of its extent and formation mechanisms. The bulging of the crust at Tharsis has led to a roughly star-shaped system of cracks on the surface, which in some cases are over 1000 kilometres long.
The compression zones are scattered more evenly across nearly the entire planet, and are primarily visible as 'wrinkle ridges' on the surface. This is where layers of rock have been pushed over one another along sloping rupture surfaces.
The trigger points of marsquakes may be spread along these fracture zones. However, the latest DLR model allows for the possibility that quakes may occur anywhere. These two hypotheses will be put to the test by the InSight mission.
How many?
Calculations of the cooling of Mars involve a range of factors that are currently only poorly understood, so investigating these is one of the objectives of the InSight mission. Such factors include the heat flow to the surface – in other words, the amount of heat that Mars actually gives off, together with the thickness of its crust, as this acts as a layer of thermal insulation. The question of how powerful marsquakes can be also remains unanswered.
All current models of seismic activity on Mars assume that weak marsquakes occur much more frequently than powerful events, and that their relative frequency is similar to that of such events on Earth, as this appears to be a fundamental property of fracture processes. As a rule of thumb, it is assumed that quakes of a given magnitude (on the Richter scale) occur approximately 10 times more often than those that are one unit of magnitude more powerful.
DLR models indicate that around 10 quakes with a magnitude of at least four will occur during the two-year mission, but more optimistic predictions put the figure at several hundred.
How strong?
How strong can earthquakes be? The strength of earthquakes is indicated by their magnitude – roughly speaking, increasing the magnitude by one equates to a 30-fold increase in the amount of energy released, and requires a fracture of about 10 times the length. The devastating earthquake that took place off the coast of Sumatra in 2004, for instance, had a moment magnitude of MW=9.3 and a fracture length of approx. 1200 kilometres. However, instruments have only been recording earthquakes since the late 19th century. This may well mean that the strongest possible earthquake has not made it into the records. On the other hand, a quake of magnitude 11 would probably have to include all of the subduction zones (where the heavier oceanic crust moves under the lighter continental crust) in the 'Pacific Ring of Fire', while an earthquake of magnitude 15 would literally tear planet Earth apart. As a long-term average, all of the earthquakes that take place in a year put together correspond to a single earthquake of moment magnitude MW=8.5.
Using an argument by analogy in which the strongest expected event is compared with the long-term average, it can be concluded that the strongest earthquake on Mars is likely to have a magnitude of between five and 7.5.
The cooling of Mars is a steadily occurring process, and the rate of cooling is decreasing only very slowly over time. In this respect, the cause of marsquakes is similar to the cause of earthquakes – the speed and direction of movement of the lithospheric plates on Earth changes extremely slowly or not at all by everyday standards. With the exception of aftershocks following major events, this mechanism demonstrates almost no memory of when the last earthquake took place, because each earthquake causes the elastic tension in the rock to change by just a few percent.
This means that it can be assumed that the time sequence of marsquakes, like earthquakes, is completely random, and that only statistical properties like the average number of quakes per year can be predicted.

You may also like:

No comments :

Post a Comment