That’s the question asked by the US Geological Survey after an earthquake occurs, and for the past few years they’ve used crowdsourcing to provide analysis of a quake’s magnitude. It’s been a busy week with the earth movements; following our little shakes we had the 6.5 off of the Humboldt County coast. That one was widely felt, from Oregon to Nevada to the Bay Area. Although a few people in our zip code reported feeling it, I didn’t notice it (out running in the neighborhood at the time). Their quake was centered much deeper (13 miles) than the Calaveras Fault quakes this week (about 5 miles), and that apparently makes some difference in the velocity of surface movements. There’s more surface action with the shallow quakes.
Twitter is also telling us how people experienced the Humboldt quake, although less about the shaking and more about lost cats.