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An analogy I came up with is: Predicting earthquake strength and timing is like throwing a sheet of glass on the ground and predicting how many pieces it will break to and where they'll end up.
Is this a good analogy? I have no geology background or anything similar -- I'm just a person that wants to understand why it's hard. I KNOW it's hard, but why?
The easiest way I can explain it is you haven thousands of different material joined in billions of ways with chunks of all different sizes, there are just too many varables for an accurate prediction. becuase earthquakes are caused by a slow build up of pressure it is impossible to predict exactly when it will break because the forces are not spread evenly and the material properties of the rock are not consistent.
look at material testing, if you have a uniform object under steady predictable force they can predict when it will break with some reliability usually within a few seconds. but across a geologic timescale that kind of accuracy would give you plus or minus thousands of years, and that is under perfect conditions, Add in the complexity of the plates make up and you get even more error. Worse with plates the force on them is not uniform there are several forces acting on it and they are not spread evenly.
to use your glass analogy, imagine a sheet of glass several yards in diameter, but this glass was poorly made its lumpy so the thickness varies by several inches and is full of bubble and impurities. now try to predict when it will break based on a a group of children randomly throwing pennies onto it. You know it will break eventually but predicting when and where is functionally impossible.
Earthquakes have al sorts of causes and all sorts of intensities, but your question probably refers to the most powerful and destructive ones. These are caused by the movement of tectonic plates involved in continental drift. A classic example is the San Andreas Fault in California. where one plate is moving past another, leaving a clearly visible rift for hundreds of miles. The western plate is gradually moving north, and grinding against the eastern plate. The movement stops for a while after energy has been released, but then the pressure builds up again as the western plate tries to continue its journey, but is held back by friction. The forecasting difficulty comes in predicting how long the friction forces can hold before they suddenly give way. It is safe to say that there will be another powerful earthquake somewhere along the San Andreas Fault in the next 50 years, as pressure is building up for another lurch forward, but as things stand it is impossible to predict exactly when or at which point along the fault this event will occur. The geologists who have the fault under surveillance could make a much better prediction than I could, but even they don't know exactly when the next friction point will give way, or exactly where along the fault it will be.
