In short, Californians will be (reasonably) pleased with the answers to these questions. In the film, the San Andreas fault produces an earthquake with a magnitude of 9.0. While not unheard of globally, earthquakes of this size are generally confined to regions of the earth where subduction – where one tectonic plate is being forced below another – is happening, for example in Chile and Japan. The tectonic situation in California is different. Here, two plates are sliding past each other.
As such, recent predictions limit the possible maximum earthquake magnitude along the San Andreas fault system to 8.0, although with a 7% probability estimate that such an event could occur in Southern California in the next 30 years; over the same period, there is a 75% chance of a magnitude 7.0 event. While magnitudes of 7.0, 8.0 and 9.0 might sound negligibly different, the energy that such events would unleash varies significantly, with a magnitude 9.0 event releasing 32 times more energy than a magnitude 8.0 and 1,000 times more energy than a magnitude 7.0.
Obviously, however, be it a 7.0 or an 8.0, damage is inevitable, but the whole sequence of events, as depicted in the film, is unlikely. For example, the San Andreas fault is not beneath the ocean and as such, any slippage along it could not displace water to the extent that a tsunami would be generated. The opening up of a massive chasm is also from the land of fantasy, as the plates are sliding relative to each other, not away from each other.
What is realistic, however, is that a great amount of destruction is likely. While the building codes in California are stringent, recommending retrofitting of seismic protection measures to older buildings and preventing the construction of new buildings near to known fault lines, there is no way to make a building 100% safe.
In an attempt to understand the effects of a large, southern San Andreas earthquake, the U.S. Geological Survey modelled a 7.8 magnitude event, with slippage of 2-7 metres, to represent the stresses that have built up in the area since the last large event.
From this model, it was found that damage would be most severe to constructions straddling the fault. Fortunately, constructions of this sort are few and far between following the 1972 Alquist-Priolo Earthquake Fault Zoning Act. What would be affected by this slippage, however, are the 966 roads, 90 fibre optic cables, 39 gas pipes and 141 power lines that cross the fault zone.