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  • Tom Clauset
    March 19, 2012
Earthquakes (3)‎ > ‎

* 01-Template (Northridge Earthquake, 1994)

The Northridge Earthquake Epicenter was located close to Los Angeles, CA

Northridge, Los Angeles, CA


Facts About the Northridge Earthquake
 Topic Item    
 (notes)
Data
 Date  (year)  January 17, 1994
 Epicenter  Location  (exact location)
 20 miles NW of Los Angeles in the city of Reseda
 Magnitude  (Richter scale)
 6.7
 Fault Involved
 (name of fault)
 Northridge blind thrust fault (also called the Pico Thrust faut)
 Depth of Focus
 (in kilometers)
 17 km
 Damage Radius
 (distance)  85 miles (125 km) -- ground motion was detected 270 miles away (435 km)
 # People Injured
 (# hospitalized)
 8,700
 # Casualties
 (deaths)  57
 Building Damage
 (type of damage suffered)
 Many multi-story wood frame buildings experienced damage
 Road Damage
 (type of damage suffered)
 many elevated highways suffered damage; structural failure occurred at the interchange of Interstate 5 with CA route 14, Interstate 10, CA Route 118
 Total Cost of Earthquake  
 (in US dollars)
 20-25 Billion dollars


Northridge Photo and Video Gallery with Commentary
 
 

The 1994 Northridge earthquake (M = 6.7) occurred in a heavily populated urban area northwest of Los Angeles, and had many similarities to the 1971 San Fernando earthquake (M = 6.7).


The 1987 Whittier Narrows earthquake (M=5.9), like the Northridge shock, occurred on a buried, previously unknown fault.


The three shocks and their effects on the surrounding urbanized areas (grey) are compared throughout a REPORT made by the United States Geological Survey.
 
 


Notice the diagonal green line extending from above the "s" in Los Angeles to the top left corner of the picture. This green line shows the location of the San Andreas Fault.


Since the epicenter of the Northridge Earthquake of 1994 was south of the San Andreas Fault, scientists had to rethink their ideas of the faults included.


The earthquake was actually caused by a movement in the Pico Thrust Fault (also now called the Northridge blind thrust fault).
 
 
Collapsed Northridge Parking Structure. Photo Courtesy of U.S. Geological Survey.


Parking decks and buildings with parking garages on the ground floor with business or residential spaces above suffered the greatest damage in this earthquake.

 
 

We now know that the higher a roadbed is from the ground, the more it will sway and vibrate due to an earthquake.

This roadway is quite high and therefore the stresses that were on it during the earthquake were considerable.

Notice the lines extending from the roadbed down towards the ground. These lines are actually rebar, steel reinforcing rods intended to strengthen the concrete construction.

This elevated roadway has another problem in addition to being so high -- it curves. This puts additional strain on the entire structure when the ground below is "moving."
 
 

The most significant damage to housing occurred to wood frame houses that were several stories high.


These houses were not braced sufficiently to withstand the vibrations caused by the earthquake.


Proper building techniques would have prevented this damage.
 
 
Apartment complexes with 1st story parking places did not do well in this earthquake.


The weakened first story collapsed when shaking started.


These cars were crushed when the supports for the parking spaces collapsed and the upper story came crashing down on the cars below.
 
 
These roadway sections essentially collapsed into poorly compacted ground below.

This is a good illustration of what happens when the ground shakes and liquifaction makes the ground underneath the roadbed unstable.

The unstable ground underneath the roadbed cannot support the weight of the road and so the road breaks up and "falls" into the cavities created underneath.
 
 

After the Northridge Earthquake, civil engineers began to look at the type of structural failures seen on elevated roadways.


New techniques were designed to strengthen roadways so that they could withstand earthquakes in the future.


In particular, the use of reinforced steel (called "rebar") to further strengthen concrete came to be used more systematically in building elevated roadbeds.
 
 

When houses shifted off their foundations or unsecured water heaters fell over, fires broke out because the gas pipes ruptured.

In the San Fernando Valley, several gas pipes underground were severed, resulting in additional fires.

At the same time, some water mains were severed. This resulted in simultaneous fires and floods in some areas.
 
 
Elevated roadway sections along Interstates 5 and 10 suffered severe damage.


Notice the thin lines extending down from the road bed. These thin lines are actually rods of reinforced steel called "rebar." Rebar is used to stiffen and give strength to concrete.


Since the Northridge Earthquake of 1994, highway engineers have changed some of the ways they build these elevated sections. New roadways are stronger and can withstand more side to side and up and down movement due to earthquakes.
 

Community Internet Intensity Map for the January 17, 1994, magnitude 6.7 Northridge, California, earthquake as of early April, 2001.


Star shows epicenter, and colored areas on the map indicate the intensity for that ZIP-code area, as described in the key.

CIIM data for the Northridge earthquake were collected beginning 3 years after the event.

CIIM's are now being created via internet questionnaires for zip code areas and begin collecting data immediately after earthquakes throughout the United States.

The CIIM rating scale is similar to the Mercalli Intensity Scale numbers.
 


 Entire sections of roadway separated from the preceding section, tore away from their support posts, and then collapsed onto the roadway below.

Notice that there are people standing on what appears to be a completely intact section of roadway that collapsed between two uprights.

From this you can tell that the method used to connect the road bed to the upright needs to be re-engineered to account for earthquake movement.
 
 
This closeup is of an elevated highway support underneath one of the Interstates affected by the Northridge Earthquake of 1994.


As you can see, the ground movement cased concrete to peel away from the support post, exposing the steel reinforcing rods underneath.


As the ground continued to shake, the concrete inside the post shattered and the entire column collapsed downward bending the steel rods outwards.
 
 
Valley Fever Outbreak


Some people were affected by an outbreak of a respiratory disease caused by the inhaling of airborne spores.

It is thought that the spores were carried by large clouds of dust created by seismically started landslides. Most of the cases of valley fever occurred downwind of the landslides.


The term "valley fever" has a medical name:  coccidioidomycosis.

203 cases were diagnosed in Ventura County.
 
 
In addition to structural damage of buildings and roadways, considerable damage occurred to non-structural elements on the inside of buildings. "Non-structural" means the parts of the building that do not carry the weight of the building. So, suspended ceilings, thin office partitions, and lights all qualify as "non-structural."


From this picture, we can list the non-structural elements that failed:

1. office partitions collapsed (fell over)
2. ceiling suspension systems fell down
3. overhead lighting fell from the ceiling to the floor.
 
 
Another non-structural item that often suffers in an earthquake is shelving.


This metal shelving system carried a lot of weight high off the floor. When the ground began to move, all that weight caused the shelving to twist and finally, collapse.


Under normal circumstances, the metal shelving would have been strong enough, but in an earthquake zone, it is too tall and carrying too much weight to work effectively.
 
 

Damage to commercial inventory was due to the shaking that occurred.

The debris on the floor of the aisle shown all fell off of shelves to the right and left during the vibrations and shaking caused by the earthquake.

The cost of the damage to contents INSIDE a house or building may be many times the actual cost of the building itself.
 

Northridge Earthquake



NBC News Coverage of the Northridge Earthquake:


- Fortunately, the earthquake occurred early in the morning and, it was a Federal holiday, so people were not yet out on the roads commuting to work.


- multiple gas lines ruptured, causing fires to start.


- multiple water lines broke, sending cascades of water into city streets.

 


Discovery Channel Video on how a "shake table" works and how scientists are using them to figure out how to build buildings that can withstand the vibrations generated by earthquakes.

 At Colorado State University, civil engineering professor Dr. John van de Lindt conducted a series of earthquake shake table tests of a half-scale two-story residential building with an integrated one car garage. The overall goal is to test protective systems for wood frame buildings. The tests followed ground motion patterns from three different California earthquakes.

In this Engineering TV interview, Prof. van de Lindt discusses the engineering behind the CSU earthquake shake table as well as others around the world involved in the NEESWood program, including the world's largest in Miki City, Japan.

Hosted by: Terry Knight Videography by: Curtis Ellzey Edited by: Curtis Ellzey


Northridge Earthquake Facts,
a One-Year Anniversary Summary
http://media.caltech.edu/press_releases/11608

The Earthquake

* The magnitude 6.7 Northridge earthquake started on January 17, 1994 at 5 seconds before 4:31 a.m.

* There were no immediate foreshocks. No systematic change in strain above the background noise occurred during the hours to milliseconds before the event.

* The fault ruptured by the Northridge earthquake rises from a depth of about 19 kilometers (12 miles) at its southern edge to a depth of about 5 kilometers (3 miles) at its northern edge. The fault is blind—it does not break the surface—and was previously unknown.

* The rupture started at the southern, deepest edge and spread up to the northwest, north, and northeast. The final dimension of the fault plane broken in the earthquake is about 16 by 19 kilometers (10 by 12 miles).

* The actual rupture of the fault only lasted about 8 seconds, but because of amplification and reverberation of the seismic waves through the complex of faults, sediment, and mountains, most people felt shaking for 20 to 30 seconds.

* Geodetic measurements (those made by satellite) show permanent changes in the topography of the San Fernando Valley of up to 40 to 50 centimeters (16 to 20 inches) of vertical gain, and up to 20 centimeters (8 inches) of horizontal displacement.

* Geophysicists calculate that most of the slip took place in concentrated patches of the fault. The greatest amount of slip at any spot was about 4 meters (13 feet).

* The earthquake caused very large ground motions with peak accelerations of 0.5 to 1.0 g in the Northridge area, decreasing to 0.1 g at distances of about 50 kilometers (31 miles). (A "g" of acceleration is equivalent to the acceleration of gravity. There were a few sites near the Northridge earthquake that recorded over 1 g of vertical acceleration. These ground movements would have been capable of throwing objects of any size into the air.)

* The pattern of damage and strong ground shaking was irregular, with severe damage in places like Sherman Oaks and Santa Monica. These effects were caused by the complexity of the earthquake source and the wave propagation through complicated geology.

Aftershocks

* Through December 31, 1994, 11,030 aftershocks, most of which were too small to feel, were recorded by the Southern California Seismic Network, which is operated jointly by the United States Geological Survey and Caltech.

* More than 400 aftershocks have been large enough to feel, including

8 between magnitude 5.0 and 5.9 48 between magnitude 4.0 and 4.9 367 between magnitude 3.0 and 3.9

These numbers and sizes are typical for a magnitude 6.7 earthquake.

* Based on the the statistical pattern of Northridge aftershocks and on the behavior of other past earthquakes, in 1995 people should expect to feel about 17 magnitude 3.0 to 3.9 aftershocks, and about two magnitude 4.0 to 4.9 aftershocks. There is also about a 25 percent chance of another magnitude 5.0 to 5.9 aftershock.

Contact: Jay Aller, Max Benavidez, or Sue Pitts 	  
(818) 395-3631
(818) 395-3226
(818)395-322
Media Relations
mr@caltech.edu


About the Author
 
 Mr. Clauset -- Chadwick 6th Grade Earth Science

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