What i'm assuming is that each device is already a min computer in of itself and how we build supercomputers is linking a bunch of computers together in a network to increase computing power. Now each smartwatch is constantly keeping track of everything we do from elevation we walk, steps, and heart rate. This means these are very sensitive devices able to differentiate different movements.

This is just a wild assumption based off what we know smart watches can do but because they are casting such a wide net the watches are able to sense differences in the environment and if several are sensing the same movement then it'll set off the alert system.

The simplest answer would just be google just has its own private seismometer thats is just better lmao.