CISN: California Integrated Seismic Network (http://www.cisn.org/)
Also try Southern California Seismic Network, Southern California Earthquake Center, and Advanced National Seismic System
UC Berkeley National Information Service for Earthquake Engineering (http://nisee.berkeley.edu/)
Including a database of images of over 280 earthquakes.
USGS Earthquake Hazards (http://earthquake.usgs.gov/)
Maps and statistics on earthquakes, information on reducing earthquake hazards, seismographic studies, and numerous links to other web sites.
Also try Earthquake Hazards Program-Southern California
PEER: Pacific Earthquake Engineering Research Center (http://peer.berkeley.edu/)
The PEER mission is to develop, validate, and disseminate performance-based seismic design technologies for buildings and infrastructure to meet the diverse economic and safety needs of owners and society.
Quakeline Database (http://mceer.buffalo.edu/utilities/quakeline.asp)
covers earthquakes, earthquake engineering, natural hazard and disaster mitigation, and related topics.
Geologic Hazards Team (http://geohazards.cr.usgs.gov/)
Lots of information on earthquakes, landslides, and geomagnetism. Interactive maps are provided.
HAZLIT Database (http://www.colorado.edu/hazards/library/)
Searchable database from the Library of Natural Hazards Center at the University of Colorado.
MTU Volcanoes (http://www.geo.mtu.edu/volcanoes/)
From Michigan Tech University