One of the issues in an earthquake is potential damage to the major distribution pipes. From “source” to “buffer” (local reservoirs used to maintain pressure) and the extent to which things in the network of pipes can be rerouted depends a lot on how it was built. In heavily built up areas there’s often “secondary buffers” like storage on the roofs of buildings in New York City.
Pressure in the “customer end” of the water supply network will still be available for a little while (minutes to hours) in the event of damage but damage will take time to repair and the impact of the damages ongoing effect and the time to repair make one of the most basic survival tips for the average person not otherwise maintaining survival supplies: “fill the bathtub and anything else you’ve got”. You might have 15 minutes before the broken pipe washes away enough dirt that pressure drops enough to stop the flow from your tap.
Pressure in the “customer end” of the water supply network will still be available for a little while (minutes to hours) in the event of damage but damage will take time to repair and the impact of the damages ongoing effect and the time to repair make one of the most basic survival tips for the average person not otherwise maintaining survival supplies: “fill the bathtub and anything else you’ve got”. You might have 15 minutes before the broken pipe washes away enough dirt that pressure drops enough to stop the flow from your tap.