California Earthquake Watch, USA Live Cam

This is not a view of anywhere: the picture is a bank of seismograph traces recording ground motion in California and beyond. The feed shows instrument screens rather than a landscape.

Each line on a drum is a quarter of an hour of ground movement, written left to right and stacked one line under another, so a whole day fits on a single sheet. A local earthquake shows up as a burst that thickens the line; a distant one arrives as a long slow wobble.

California sits on the boundary between the Pacific and North American plates, which grind past one another at about five centimetres a year. Almost everything these instruments record is too small to feel, and the state's warning system uses the first and faster wave to send alerts a few seconds ahead of the shaking.