It is not well known, but the magnitude 9.0 Tohoku earthquake in Japan was preceeded by a magnitude 7.2 earthquake two days earlier in the same region. It is often said that earthquakes cannot be predicted, and mostly do not occur with preshocks. However, large earthquakes are rare in a given area, and any one should be treated as a possible preshock, as a normal part of earthquake preparedness. There are many low cost actions that could be taken for a short term preparedness situation, lasting maybe a week or so. Research on this scenario would yield a probability curve for a useful time period.
The difference between the preshock and the main earthquake was 1.8 magnitudes. In this Southern California area, a 7.0 is expected every 300 years or so from near shore faults, so any local 5.0 or greater earthquake should be treated as a possible preshock. On the San Andreas fault where an 8.0 magnitude earthquake may occur, any 6.0 or greater earthquake in its area should be treated as a possible preshock.
The first action one would conceive of would be to shut down any nearby nuclear reactors and put them in an emergency standby mode for an earthquake and a blackout of external power to run cooling pumps and backup electricity. This would also involve keeping emergency response crews on site around the clock. The same may be needed for any fossil fuel power plants that could be damaged or a hazard during and following an earthquake.
Water facilities are also a crucial part of our infrastructure, and deploying backup diesel pumps and generators would be crucial. Also, to avoid firestorms after an earthquake, water tanks for fire suppression should be filled, and generators deployed for their resupply.
If the relevant earthquake faults are offshore, or if offshore landslides are likely, the coast guard and other official boats could be deployed as a temporary early warning tsunami system.
On the San Andreas fault, any local police or fire units could be activated as a short term early warning system, giving up to a minute warning of an impending shock on the major Southern California cities.
I’m sure that our city and county emergency planning personnel can think up and plan for the implementation of such earthquake anticipation measures, and should be asked to do so, knowing that preshocks are a posssible indicator of near future earthquakes.