How do people sense seismic waves?

They use the Richter scale to measure the speed of the earthquakes.

Levels of earthquakes

2.0 ---> can't be felt

4.0 ---> do not cause damage

5.0---> can cause damage

6.0 ---> considered strong

7.0---> is a major earthquake

+++

That is not correct. You have confused speed with intensity. The Richter scale, which is logarithmic, measures the intensity ("strength" if you like).

The speed is measured in ordinary linear units like metre/second or km/hr, calculated from observing the earthquake's waves' progress past seismographs around the world.