September 19, 19857:19 am

The complete seismic event consisted of four quakes. A pre-event quake of magnitude 5.2 occurred on 28 May 1985. The main and most powerful shock occurred 19 September, followed by two aftershocks: one on 20 September 1985 of magnitude 7.5 and the fourth occurring seven months later on 30 April 1986 of magnitude 7.0.

An 8.1 magnitude earthquake hits Mexico City, Mexico.

The earthquake shook the buildings in Mexico City for a total of three horrifying minutes.

Thirty six hours later a 7.5 magnitude earthquake hits

the city again.

The epicenter was later located 50 km (approximately 31 miles) off the coast of Mexico.

The Seismic Event

Historic seismic activity in this region causes Mexico to experience about five times as many major earthquakes as California, with greater frequency

of recurrence.

Magnitude     

Date                   

Location            

Comments

7.9Jan. 9, 1857

Fort Tejon2 killed, 220-mile surface scar

7.9April 18, 1906

San Francisco3,000 killed, $524 million in property damage, including fire damage

7.8March 26, 1872

Owens Valley27 killed, 3 aftershocks of 6.25+

7.5July 21, 1952

Kern County12 killed, 3 aftershocks of 6+

7.3Jan. 31, 1922

West of Eureka* 37 miles offshore

7.3Nov. 4, 1927

SW of Lompoc*No major injuries, slight damage

7.3June 28, 1992

Landers1 killed, 400 injured, 6.5 aftershock

7.2Jan. 22, 1923

MendocinoDamaged homes in several towns

7.2Nov. 8, 1980

West of Eureka*Injured 6, $1.75 million in damage

7.2April 25, 1992

Cape Mendocino*

6.5 and 6.6 aftershocks

7.1Oct. 16, 1999

Ludlow (Hector Mine Quake)

Remote, so minimal damage

7.1May 18, 1940

El Centro9 killed, $6 million in damage

6.9Oct. 17, 1989

Loma Prieta 63 killed

6.7Jan. 17, 1994

Northridge61 killed, $15 billion in damage

6.6Feb. 9, 1971

San Fernando65 killed, $50 million in damage

Mexico City is built partially on alluvial lake deposits that average from 100 – 150 feet thick.

Original Water Level

100-150’Building Structures

Weak soil deposits

Because of the long duration of shaking, deep soil deposits were excited resulting in amplified ground movement.

At some location of Mexico City where most of the damages are found, soil deposits extend to about 7500 ft (2300 m).

Therefore, earthquake vibrations in this areas of Mexico city were catastrophically amplified.

Original Water Level

7500’Building Structures

Weak soil deposits

Tectonics

This is a region where the Cocos Plate is being subducted underneath Mexico and is the most active subduction thrust fault in the western hemisphere.

The Cocos Plate made one of its all too frequent downward movements.

This time a 200 km long front, inclined 18 degrees east, thrust downward and eastward about 2.3 m in two distinct jerks about 26 sec apart.

Effects of the 1985 Earthquake in Mexico City

Building Structural Damages: Large number of the

collapsed buildings were frame structures.

Large number of which did not meet the building standards.

Debris falling onto parked cars

Liquefaction. Loss of

Communication Media.

Structural Damages

Bottom-level Failure Due to Weak First Floor

Collapsed School BuildingSchool buildings are exposed to risk form earthquakes since they lack adequate stiffening in shear walls of large classroom areas.

The amplitude of the shaking increases up the building

Building with long axes perpendicular to ground motion suffer more shaking

Building with different heights sway at different frequencies and bank into each other

A building with different height tend to break apart

http://www.frame3d.caltech.edu/animations/b1tabxabsmov.gif

Collapsed Floors Punctured by Load-Bearing Column

Severe resonance oscillations of the buildings caused strain at the juncture between columns and ceiling slabs. The vertical columns were punched through the heavy floors that collapsed around them.

Total Collapse of Juarez HospitalOver four hundred medical personnel and patients were trapped in the maternity wing of the Juarez Hospital. Survivors were still being retrieved from the structure as late as ten days after the earthquake.

Collapsed 21-Story Office BuildingBuildings such as the one standing in the background met building code requirements. Obviously the collapsed office building did not.

Car Demolished by DebrisThousand of vehicles were destroyed, like the one in this picture, by falling debris.

Collapsed and Undamaged Office Buildings

The 44-floor Torre Latino-Americana office building in the background on the right, remained almost totally undamaged, as it did in a 1957 earthquake. The building is a symmetrical steel frame structure built to resist earthquakes

High Rise Building

This building twisted excessively in the earthquake, forming the X-shaped cracks.

The earthquake subjected the building to shear, bending, tensional forces, and compression.

The formation of the X-shaped cracks is evidence the energy from the earthquake dissipated in the shear walls.

Liquefaction

Liquefaction Mexico City is a very dangerous in terms of its local geology. It is currently sitting on a 800 meter (2625 ft) lake bed made up of silt and volcanic clays that create two problems that must be address here.

Liquefaction is a type of ground failure in which water saturated sediment turns from a solid to a liquid as a result of shaking, often caused by an earthquake or even a volcanic eruption.

The intense shaking causes the strength of the soil to become weak and the sand and water begin to flow.

Building Sank into Liquefied SoilThis residential and commercial building sank more than three feet into

the partially liquefied soil.

Loss of Communication

Media

Central Communications Center

10,000 people lost their lives. 100,000 people were left homeless. 400 buildings are destroyed. 3200 buildings were damaged in the most

populous area of Mexico City. Total Costs = $4 billion dollars in damages.

Government’s Proposal for Future

Preparedness

Revisions to the building codes such as: More Funding for researching dynamic behavior of

soil deposits Strict evaluation of lateral force design coefficients,

story drifts, and change of natural period of vibration due earthquake induced structural decay

Improve Emergency Operations: Emergency Operations should be clear with respect

to rescue Shelter for the homeless Lifeline recovery and infrastructure repair Deployment of volunteer rescue support and forces

need to maintain order Use of rescue equipment trainings

Impose a more strict licensing control of professional registration for Engineers and Scientists.

Most metropolitan areas are ill prepared for an earthquake of this intensity. Mexico’s expectations are to advance that level of preparedness.

The Office of Disaster Preparedness in San Diago County is housed in a 2 story building (7000 ft sq)It is sitting on top of 20 lead impregnated rubber supports ( base isolators)That each weigh 1 ton

Damage to the buildings in Mariana district of San Francisco resulting

from 1980 earthquake.

An apartment building in San Francisco's Marina District was thrown from its foundation and crushed a car in the 1989 earthquake. The Marina District was the scene of voracious fires caused by broken gas lines.Photo by Vince Maggiora

In the Santa Cruz mountains, the 1989 earthquake caused deep cracks. "I can't stop shaking," said one resident. "I guess I'm surviving, but I'm scared."Photo by Deanne Fitzmaurice

The amplitude of the shaking increases up the building

Building with long axes perpendicular to ground motion suffer more shaking

Building with different heights sway at different frequencies and bank into each other

A building with different height tend to break apart

Contained at least 20 sustained cycles of vibrations with a dominant period of about 2 seconds.

Ground accelerations ranged from 5 – 20% of gravity in a period range of 1.5 and 3 seconds.

The Universida Nacional Autonomia de Mexico

The Secretaria de Comunication & Transportes site