When the shaking starts, look at a clock:
If it lasts...
1 minute = magnitude in the high 7s
2 minutes = low 8s
3 minutes = high 8s
4 minutes = low 9s

@2 That is dumb. If the earth starts shaking beneath you, DO WHAT YOU'RE SUPPOSED TO DO IN AN EARTHQUAKE! Don't look at a clock and use some bullshit rule of thumb to try and figure out the magnitude, let the professionals handle that after the fact.