Clipping occurs when the amount of ground motion exceeds the range that can be recorded by an instrument.
Most of the instruments in the Northern California Seismic Network use analog electronics capable of recording only a limited range of ground motion. Gradually, they are being replaced with more modern, digital electronics with a larger dynamic recording range.
Here we see one earthquake (yellow square) recorded on two stations (red dots), both 45 km away. The earthquake (magnitude = 3.5) occurred at 04:48 PDT June 13 (11:48 UTC, June 13), 2000, 19 km east of San Jose, Calif, at a depth of 8 km on the Calaveras fault. These two stations use identical sensors, but one uses analog electronics, the other digital.
Seismogram from an older station (JJR), equipped with analog electronics having with a limited dynamic range. The record's amplitude is limited by the electronics.
Seismogram recorded by a newer station (JSF) equipped with modern digital electronics. You can see the full range of the earthquake ground motion is recorded here.