during an earthquake, why is there an epicenter? Shouldn’t the force be distributed evenly along the fault line?
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The epicentre is essentially the source of the earthquake.. it tends to be where it is the strongest, however due to the plates moving, we do feel it miles away at lesser degrees, think of it as say a drop of water falling into a puddle.. when it hit the puddle it starts a ripple effect, with it moving out from the center, growing bigger as it weakens before the ripple stops.
Bend a sheet of wood until it snaps. It doesn’t snap all the way along in one go, one bit rips first and the rest tears. That bit has the most shock
Now think about it with lengths of rock pushing against each other diagonally. The pressure builds up in one jagged edge and something snaps, then the rest slides much easier. The point that snaps is the epicenter.
At the scale we are talking about here the Earth crust have more of a rubbery consistency, the temperatures at these depths certainly helps this as well. So one part of the fault line can move while others is bound up. Basically the bedrock compresses behind and stretches in front of where it is bound up. Earthquakes happen when whatever is binding the two tectonic plates together gives way. It moves a tiny bit in this location, maybe just a few centimeters, but further along the fault line there will not be any movement. It have moved further so what you are doing is just releasing the stresses already built up in the bedrock. Maybe it is still bound up in these locations.
On the other hand you do tend to see the entire fault line move, but not in a single event. You get smaller earthquakes where tiny bits releases the pressure putting more pressure on neighboring places along the fault which may cause them to give small earthquakes a few hours later which might spawn more earthquakes. This can go on for days slowly building up to the large earthquake. And after the main quake there is now a lot of pressure that have moved around along the fault line so you start getting more of the minor earthquakes. The entire event can go on for days with smaller and medium sized earthquakes all along the fault line until it finally calms down.
What is special about the earthquake in Turkey recently is that these quakes spread to another nearby fault line, this in itself is not unusual but in this case it triggered another large earthquake almost as powerful as the first. So you have one large earthquake triggering another large earthquake.
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If you think about it as ripping apart two pieces of Velcro, the strands give individually until one final piece remains stopping the pieces from moving until that piece gives, that strand is the epicentre.