General Earthquake Information and Background
An EERI clearinghouse with recon info on Sendai, where the impacts of the March 11, 2011, tsunami were catastrophic.
The US Geological Survey is the key US government scientific authority on earthquakes.
California Institute of Technology is one of the premier US geophysics research institutions. This site is loaded with authoritative practical information.
This multi-agency center housed at Cal Tech puts realtime data from seismo instruments in Southern California directly online.
The California state agency that compiles geological information about quake-prone areas.
The California state agency that regulates the earthquake safety of structures.
Pacific Northwest Seismic Network
This site is rich in links to online info about all aspects of seismicity.
Don't knock it. Do fact check it.
FEMA Earthquake Page 
This is mostly about what happens before and afterwards.
Based at Univ. of Alaska-Fairbanks, with all kiinds of info on one of the most seismically active U.S. regions.
Columbia's Earth Institute page has info about earthquakes and other earth sciences and environmental topics.
Earthquake Maps, Data, and Realtime Alerts
In the news business, knowing about a quake immediately is what it's about. These feeds and alerts can help by hooking you up to the instruments in close to real time.
These maps are useful to those in California and Nevada.
Turn your cell phone into a real-time Earthquake & Tsunami Warning System with www.sms-tsunami-warning.com , an advanced web-based early warning system.
Earthquake, Seismology, and Geophysics Experts
Caltech Seismology Lab Faculty Directory 
MIT Earth, Atmosphere, Planetary Sciences (EAPS) Geophysics Faculty Directory 
Univ. Alaska - Fairbanks: Geophysical Institute Faculty List 
Earthquake Response and Engineering
U. Cal. - Berkeley Pacific Earthquake Engineering Research Center