The Northridge Earthquake of 1994

On January 17, 1994, a very shaky wake up call was given to the people of the San Fernando Valley at 4:30 in the morning. The epicenter of the earthquake was located in Northridge, 20 miles away from downtown Los Angeles. It was felt all across Southern California. The earthquake had a magnitude of a 6.9 at a depth of 19 km.
Not only did the earthquake have a large magnitude but it was followed by thousands of aftershocks for the next two weeks that had magnitudes of 4.0 and 5.0. This caused even further problems because it caused more damage to structures that were already damaged enough. For the next few days unfortunately for many people, electricity and gas was out of working order and many people were even left without water in their homes. Some of the people were even left homeless because of the damage. Los Angeles is a very large city that gets commuted to so when this earthquake happened, it caused heavy traffic because major freeways were damaged and roads were closed off in downtown LA due to damage. We have learned from this earthquake though, slowly and but steadily, freeways are starting to be built more securely so that situations like this do not occur again. Many of these bridges were built in the 1970’s and now seismic resistance is being thought about when rebuilding or building new bridges and freeways. The earthquake cost about $100 million to fix the bridges that had been damaged.
The shaking occurred for about 10 to 20 seconds and was enough to cause some significant damage. Northridge's earthquake is one of the worst since the San Fernando earthquake of 1971. There were 57 deaths and about 9,000 people were injured. In just 15 seconds this earthquake cause 20 billion dollars in damages. In Figure 1 the intensity of the earthquake is shown, the red being the worse and the green being mild shaking. As shown in Photos 1-4, the short period of time that the earthquake shook the city was drastically changed.


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Figure 1: Mercalli Scale of Intensity



Damage Pictures


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Photo 1. Freeway collapse
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Photo 2. Broken highway destruction

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Photo 3. Parking Structure at Cal State Northridge


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Photo 4. Kaiser
Permanente building


According to USGS website, after the earthquake occured it left the ground uplifted.
By using the GPS USGS noticed that the ground had a dome-shape pattern.
The ground had uplifted about 20 inches (USGS).

Figure 3 is the topography of how the Northridge earthquake left the ground uplifted into
a dome shape.

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Figure 3. Topography of uplifted ground (USGS).



What is a Blind Thrust Fault?
The unique aspect of this earthquake was that it occurred on a blind thrust fault. A thrust fault is a type of reverse fault that has a low angle in which the rock above the fault plane moves up in relation to the rock below. A blind thrust fault is a thrust fault that does not rupture all the way to the earths surface, so there is no evidence of it above ground. The fault is buried under the uppermost layer of rock and crust. This particular fault, now called the Northridge, or Pico fault, was unknown by the people living in this area. Because the fault was unknown, the earthquake was completely unexpected. The fault was also located very close to the major transform fault in california, the San Andreas Fault. Figure 2 shows a diagram of a blind thrust fault. Notice how the layers fold and do not rupture to the su​rface. One side of the fault rides up over the other, this vertical motion can cause severe shaking. This fault plane ruptured from a depth of about 17.5 kilometeres upward to 5 kilometers beneath the surface (USGS).

Bling Thrust Faults have been said to be the source of various earthquakes such as: 1987 Los Angles Earthquake, October 1, and 1987 Whittier Narrows Earthquake.A blind thrust fault is not clearly obvious on the surface, one really has to look the surface and know what they are serching for. These faults are not only had to see but also release energy by suddenly rising, this rising motion is very destructive to buildings that are on Earth's surface. These faults are considered to be hiding and only seen after their destruction has come and gone. Scientist have expected that these faults are hiding in the L.A. basin area.

external image blindthrust.gif


Figure 2: Blind Thrust Fault







Conclusion: Since the Northridge earthquake erupted, the USGS has utilized the National Earthquake Hazards Reduction Program (NEHRP) funding to take part in post-earthquake investigations devised to comprehend the effects of the earthquake. After some investigation, the California Department of Conservation has installed seismic maps and zoning areas to record activity. Since then, the department has been working to figure out what buildings and other structures are capable of sustaining earthquakes. The Northridge earthquake was extremely destructive and definitely not expected because it occurred on a blind thurst fault. Officials are taking the steps needed to help save as many lives as possible if another earthquake erupted in the future.



References:

http://www.vibrationdata.com/earthquakes/northridge.htm
http://earthquake.usgs.gov
http://www.scec.org
www.smate.wwu.edu/eq-CANorthridge
http://www.harperrisk.com/Library/Earthquake.htm

"Northridge Earthquake Deployment 1994." NASA ARC Disaster Assistance and Rescue Team extended website. Web. 03 Dec. 2009. <http://dart2.arc.nasa.gov/Deployments/NorthridgeEarthquake1994/Northridge.html>.
"PUBLIC ROADS On-Line (Summer 1994): The Northridge Earthquake: Progress Made, Lessons learned in Seismic-Resistant bridge Design." Home - Turner-Fairbank Highway Research Center. Web. 03 Dec. 2009. http://www.tfhrc.gov/pubrds/summer94/p94su26.htm.
http://pasadena.wr.usgs.gov/office/hudnut/hudnut/tilt.html#uplift
http://abcnews.go.com/Technology/story?id=99134&page=1
“Northridge Earthquake.” <http://www.u-s-history.com/pages/h2176.html>