Image Caption: The magnitude 6.7 Northridge earthquake on Jan. 17, 1994, killed dozens, injured thousands and caused widespread damage throughout Greater Los Angeles. It was the costliest earthquake in U.S. history, and ranks as the fifth costliest disaster in U.S. history. Credit: U.S. Geological Survey

NASAs Jet Propulsion Laboratory

Twenty years ago this week, in the predawn darkness of Jan. 17, 1994, at five seconds before 4:31 a.m. PST, the ground ruptured violently on a blind thrust fault (a crack in Earths crust that does not reach the surface) about 11 miles (18 kilometers) beneath Reseda, in Californias San Fernando Valley about 20 miles (31 kilometers) northwest of downtown Los Angeles. The resulting magnitude 6.7 earthquake, known as the Northridge earthquake, became the first large quake to strike directly under an urban area in the United States since the 1933 magnitude 6.4 earthquake in Long Beach, Calif.

The initial fault rupture lasted just 8 seconds, but for many of the millions of Southern Californians and people as far away as Las Vegas, Northern California, Utah and Baja California, it felt like an eternity. In fact, the quakes seismic waves were amplified and reverberated through the regions complex mix of faults, sediments and mountains, so the shaking lasted as long as 30 seconds in some areas. At its peak a few miles from the epicenter, the ground shook at 1.8 times the force of gravity strong enough to throw objects of any size into the air.

By the time the dust settled, dozens of people had been killed, many thousands injured and tens of thousands left homeless (estimates of each vary). The region suffered approximately $49 billion in property damage and economic losses the most expensive natural disaster in the United States until Hurricane Katrina. Had it not been for the early hour of the quake and the fact that it happened on the Martin Luther King, Jr. holiday, the death and injury toll would likely have been significantly higher.

We asked geophysicist Andrea Donnellan of NASAs Jet Propulsion Laboratory, Pasadena, Calif., to reflect on the Northridge earthquake, what weve learned about earthquakes since then, our state of preparedness for the next big one and what lies ahead for earthquake studies at NASA.

Q: What progress have we made in understanding earthquakes in the 20 years since Northridge?

We understand that earthquakes do not generally repeat on fault segments over time, but instead are part of a much more complex system of interacting fault segments. We are now better able to model these fault systems with improved computer programs. Additionally, better data systems have given scientists a much more detailed understanding of the entire earthquake cycle, including how strain accumulates before earthquakes, is released during and after earthquakes, and how stress is transferred between earthquake fault segments. Our measurements of crustal deformation also aid in earthquake damage assessment and response following large earthquakes.

Q: Are we more or less prepared for a large quake in the Los Angeles area than we were 20 years ago?

As a whole, we are better prepared for earthquakes in Los Angeles today due to retrofitting and improved building codes. Californias periodic ShakeOut and Golden Guardian exercises serve as effective simulations for responding to real earthquake emergencies. We have not had a damaging earthquake in a long time, however, and it is easy for people to become complacent following a long period of quiescence.

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NASA JPL Scientist Reflects On Last Big Quake In Los Angeles