How do scientist predict future earthquakes?

Scientists can measure where strains are building up in the Earth, but the regions involved are vast, and the stresses are often deep underground. There's no way of knowing when or where something will slip. For the moment the problem is too complex and chaotic for accurate predictions.
Its a science that is being worked on. Once an earthquake has happened, then regional centers can predict if there are likely to be more or not, but its not a perfect science. How they do what they do is they have remote sensors that they deploy to regions where there are known faults. Those who live in such areas need always to be prepared and know how to react.