What happens if a magnitude 13 earthquake occurs?
It can be problematic to use an earthquake magnitude scale for such purposes. To explain why, you have to look at the different scales how they were produced. The magnitude scales are normally used related to earthquakes on earth, resulting from elastic rebound effects from fault activity. The Richter scale (local magnitude, ML) is, easily said, measured by the amplitude of a seismograph in &lt; 100 km distance the centre. It gets inaccurate at a magnitude greater than 6.5. From a magnitude greater than ca. 8, even the teleseismic calculated surface-wave magnitude scale (MS) gets inaccurate. For greater magnitudes, the magnitude has to be modelled in a completely different way: Here, the moment magnitude scale (MW) is used. It is defined in relation to the seismic moment: 

$M_{W} = \frac{2}{3} log (M_{0}) - 10.7 $

The seismic moment M0 is defined as:

$M_{0} = D *A * m$

where $D$ is mean displacement at the total fault, $A$ is the fault and $m$ the rigidity (shear strength) of the faulted rock. It can only be modelled by visible surface offset/displacement.

So, let's come back to the question: The magnitude of an earthquake is related to the length of the fault. The problem of a magnitude 13 is, that it is not possible according to this concept due to the earth's physical limitations. Keep in mind, that with one magnitude higher, a quake has about 32 times more energy. Of course, you can compare the energy for instance to that of an impact event - which is also often done. However, it is not easy to presume the intensity.