The US Geological Survey maintains a variety of fault and volcano monitoring sites around the western United States. Instruments at these sites include strainmeters, creepmeters, tiltmeters, magnetometers, pore pressure monitors, as well as other channels which measure environmental parameters such as temperature and pressure.
The data are collected and monitored to help understand how, when, and why large earthquakes, fault slip and volcanic activity occur. The measurements provide a near real-time record of the related crustal deformation before, during and after events. The goal is to better understand these natural processes, and use these data to reduce the earthquake and volcanic hazards associated with them.
This web site provides data plots for many instruments that are concentrated in areas where large earthquakes are likely to occur in California and areas of known volcanic activity (Long Valley, CA). In particular, the USGS has concentrated instrumentation efforts in the San Francisco Bay Area, near San Juan Bautista and Parkfield, and the Long Valley, CA and Southern California regions.