I think most water well levels drop during the strong shaking in earthquakes, then recover in a matter of days. The ground permeability seems to rise with the shaking (more pore space is opened) then the permeability returns to normal over the course of weeks to months.

People are still striving to understand it in detail, but essentially the ground gets roughed up by the shaking, water flow more easily, then it heals back to its normal state.