during an earthquake, why is there an epicenter? Shouldn’t the force be distributed evenly along the fault line?
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The hypocenter is the point where the earthquake starts, and the epicenter the point on the surface directly above that. Their position can be accurately found, within an hour of the quake, from the time the first seismic waves arrive at different seismometers.
It’s true that the rupture – which produces the shaking – spreads out along the fault plane from the hypocenter. The hypocenter is not the middle of the rupture, or the point that it’s strongest, it’s just a point on the rupture that’s easiest to find. Figuring out those details is a lot more complicated. Seismometers will hear waves that originated from multiple parts of the fault at once. Measuring ground movement can be helpful but that needs satellite images or radar, or surveying on the ground.
And only for major earthquakes is the size of the rupture tens or hundreds of kilometres; smaller earthquakes can more reasonably be treated as points.
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