Count the seconds between the first jolt and the heavier shaking behind it, and multiply by 8: that is roughly how many kilometres away the quake began.
The first jolt is the P wave, about 6 km/s in the crust, and the heavier shaking is the S wave, about 3.5 km/s. The gap between them grows with distance by 1/3.5 − 1/6.0 seconds a kilometre, which is 1 second for every 8.4 km. Ten seconds is 84 km, thirty is 250. It is the same arithmetic as counting from lightning to thunder, and Richter used the same gap, with the height of the swing on his instrument, to get magnitude: ML = log A + 1.11 log(R/100) + 0.00189 (R − 100) + 3.0, for A in mm of Wood–Anderson motion and R in km.
The 8 is for ordinary crust. In the mantle both waves are faster and the figure is nearer 10, and in soft sediment it is less. You can only count a gap if you are far enough away to feel two separate arrivals: inside about 30 km they merge, and a quake you feel as a single bang is already close. It gives distance and says nothing about size. And the fastest warning you can get this way is the few seconds before the S wave, which is what early-warning systems exist to use.
Estimate with the rule, then check it against the calculator that models it properly.
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