A fault that ruptures causing an earthquake can cause a chain reaction of dynamically triggered earthquakes in nearby faults, thus causing a much larger seismic event. What are some high unlikely but still plausible ways to stop this chain reaction of earthquakes, in progress, so that a major city is not destroyed in the end?
It actually may not be that sci-fi at all: lubricating the fault line could mitigate the situation by allowing easy movement of the faults rather than the snapping action that traditionally produces earthquakes.
More explanation for clarity
Earthquakes are the result of pent-up stresses in within the crust of the Earth (technically the interactions at plate boundaries but I'll get to that in a second) suddenly gets released. Generally speaking, the tectonic plates of the Earth are constantly in motion but this motion gets hung up at the plate boundaries due to friction between the plates. Over time this stoppage allows enormous amounts energy to become stored in the deformations of the rocks. You get an earthquake when this pent up stress overcomes the friction binding up the system. The crust of the earth springs back producing the earthquake. The key here is the plates of the earth bind up due to friction between them allowing all this energy to build which inevitably leads to an earthquake.
So if you could somehow prevent this stress from building or limiting the magnitude that builds before the slippage, then you can stop or reduce the destructiveness of an earthquake. For example, lubricating it, quite literally putting something between the plates to make them slip past one another more easily. My link references water acting as lubricant, but I've seen ideas of using oils or even nanomachines to facilitate the movement.
That depends on how much scify you are willing to accept.
I'm pretty certain it is not yet possible to do that, but as an earthquake is a wave, a wave with correlationfactor -1 would nullify that wave.
Another way would be to reflect the wave by previously having built something that reflects earthquakes deep into the earth.
Or, even further from possible, if you were able to liquidify a big amount of earth around the city but horizontally contained, the earthquake might be reflected at the boundary just as ultrasound waves are reflected on boundaries between air/water or bone/water.