The great Sonoran earthquake (M>7.2) ruptured the Pitaycachi fault on May 3,1887 and is the largest historic earthquake in the southern Basin and Range Province.
With the dawn of the atomic age seismic instruments became a premiere watch dog of world wide nuclear explosion testing.
This earthquake is significant for two reasons: First, just one year prior to this earthquake, Chinese scientists sucessfully predicted a major earthquake in this region. The evacuation saved many lives. Second, the Chinese scientists were not sucessful in predicting the Tangshan earthquake which resulted in between 250,000 and 500,000 deaths.
Although this earthquake occurred 200 km from Mexico City, the shaking of loose sediments in the capitol city was much stronger than at the epicenter. Nearly 10,000 people died and central Mexico City was heavily damaged. Liquifaction of the lake bed sediments was a critical factor.
The 1985 Mexico City Earthquake
September 19, 1985
At 7:19 in the morning, the city of Mexico experienced a devastating earthquake of magnitude of 8.1. Thirty six hours later a second earthquake of magnitude of 7.5 occurred. The first earthquake shook buildings in Mexico City a total of three horrifying minutes. For some residents, it seemed to last an eternity.
People were trapped in poorly contructed buildings that had collapsed on them, many citizens died as a result. The Mexican government estimated some 5,000 people perished, however, international agencies placed the death toll at more than 10,00.
The conservative estimate by the Mexican government was produced shortly after the earthquake and was intended to minimize the scale of the devastating earthquake. The quake destroyed as many as 100,000 housing units and countless public buildings. Government buildings were hit the hardest, especially the Ministries of Communication, Employment, Defense,Education, and Urban Developement. This earthquake caused over 4 billion dollars in damage.
Geologic Setting
The epicenter was located 50 km (approximately 31 miles) off the coast of Mexico. The epicenter is the point on the Earth's surface directly above the point of rupture or focus of the earthquake.
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. In this century, Mexico had 42 earthquakes with magnitudes greater than 7.
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 highly susceptible to liquefaction.
Liquefaction and Mexico City
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. The extraction of ground water and liquefaction. By definition ground water is the water that lies beneath the earth's surface, filling in the spaces between the rocks and the pores of the rock. 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. In order for the liquefaction to occur the sand grains must be fine grain sand that are not closely packed together nor must it be held but some sort of cohesion. The intense shaking causes the strength of the soil to become weak and the sand and water begin to flow.
Porosity, Liquefaction and Prevention
Porosity is the percentage of a rock's volume that consists of open space, is a measurement of a rock's ability to hold water. The siltstone has a porosity of 35 to 50% whereas the clays have a porosity of 35-80% and sand is in the 30 to 50% range.
The basin has the ability to hold lots of water. Think of it this way, imagine putting a house, that is made up of deck of cards, on a big bowl of jello and then giving it a big blow to the jello. The result is obvious, the house is no more. What happen in scientific terms, is that shock waves from the blow to the jello increases in amplitude inside the jello.
The ground water beneath Mexico City was extracted for commercial and domestic use. This caused the valley to subside, which resulted in the tilting of the buildings. When an earthquake, or a volcanic eruption occurs, the tilted buildings are easily affected and crumble to the ground. The picture below shows a very good example of a building that was affected by liquefaction. You probably already have seen this if you went to the damage page.
So how do you prevent liquefacation of the soil? The safest way, is to test the soil to see if it is capable of liquefaction. If it test positive for liquefaction, then the area is place under special restrictions as to the type of buildings that can be erected, this is called zoning.
Volcanoes and Subducting Slab
Normally the volcanoes are located on the rim of the plate that overides the subducting Pacific, like the Cascade Mountain Range in North America. However, there are volcanoes located in the central valley of Mexico, so what makes the Cocos plate subduction different than the Pacific plate subduction. The geometry, or subduction angle, of the dipping slabs.
Cocos Subduction Angle
In Central Mexico the down going slab becomes almost horizontal at distances of between 110 km (69 miles) to 275 km (171 miles) from the trench and at depths of about 50 km (31 miles).
The Trans-Mexican Volcanic Belt is located where the Cocos plate subduction is nearly horizontal.
Volcanoes and Subducting Slab
After the earthquake in Mexico City, Mexican officials adopted a new design that can protect the buildings from earthquakes. This design was developed by some engineers at the University of California at Berkeley.
Looking at the diagram below you can see that the braces form an X which are anchored in concrete blocks at the base and on the roof of the building.
In diagram A we have conventional steel bracing. Under the stess of the earthquake one of the braces collaspes under the stress. If all the braces beging to snap then the structural integrity of the building fails.
Now in diagram B, the engineers at Berkely used a hydraulic jack to pull or stretch the rods. Once the rods are prestressed they can now be anchored to the base and to the roof of the building. The braces now have some room to contract thereby strenthening the structural integrity of the building.
Problems for Building Structures when Seismic Waves Pass
- For tall buildings the top may sway in the opposite direction as the base
- Buildings in close proximity with one another may collide do to diffrering phase motions.
- Changing types of wave motion cause damage.
- Buildings with different resonant frequencies will be affected differently by passing seismic waves depending on the wave frequency.
Damage Photographs
Bottom level failure due to weak first floor A small percentage of partial building failures were lower floor failures. This is the most common type of building failure since bottom floors typically have wide-open window areas and entrances with inadequate supports. Photograph credit: C.Arnold, Building Systmes Development
Building Sank into Liquefied Soil This residential and commerial building sank more than three feet into the partially liquefied soil. Photo credit: Reinsurance Company, Munich Germany.
Collapsed School Building School buildings are exposed to risk form earthquakes since they lack adequate stiffening in shear walls of large classroom areas. Photo credit: Reinsurance Company, Munich, Germany.
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 coluns were punched through the heavy floors that collapsed around them. Photograph credit: Reinsurance Company, Munich, Germany.
Total Collapse of the Juarez Hospital Over 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. Photograph credit: E.V. Leyendecker, National Bureau of Standards.
Total Collapsed 21-Story Office Building Buildings such as the one standing in the background met building code requirements. Obviously the collapsed office building did not. Photo credit: E.V. Leyendecker, National Bureau of Standards.
Car Demolished by Debris Thousand of vehicles were destroyed, like the one in this picture, by falling debris. Photo Credit: Reinsurance Company, Munich, Germany.
Totally collapsed and Undamaged Office Buildings The 44-floor Torre Latinoamericana 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 fram structure built to resist earthquakes. Photo Credit: Reinsurance Company, Munich, Germany.
Bottom level failure due to weak first floor A small percentage of partial building failures were lower floor failures. This is the most common type of building failure since bottom floors typically have wide-open window areas and entrances with inadequate supports. Photograph credit: C.Arnold, Building Systmes Development
Parking Garage Collapse This multi-floor parking garage collapsed while other buildings remained undamaged. Photo Credit: Reinsurance Company, Munich, Germany.
Aerial View of Top Failure, Central Communications Center This twelve-story reinforced concrete structure housed the Ministy of Communications and Trasport and the nation's main microwave transmitter. Failure of this structure precipitated a near collapse of long distance communications between Mexico City and the rest of the world and complicated the coordination of international rescue efforts. Photo credit: C. Arnold, Building Systems Development, Inc.
Failure of Top Floors, Hotel Continental, Mexico City Top floors of buildings are particularly vulnerable because upper floorss are displaced more than ground vibrations leading to large amplification of oscillations. Photo Credit: C. Arnold, Building Systems Development, Inc.
Top Failure of Flexible Building Between Two Ridge Buildings This flexible commercial building was in a vice-like clamp between two rigid neighboring buildings. This pressure caused the upper part of the building to collapse at the level of neighboring structures roofs. Photo Credit: C. Arnold, Building Systems Development, Inc.
Top-Floor Collapse Office Building, Mexico City This office building shows the top of the floor collasped. Photo Credit: Reinsurance Company, Munich, Germany.
Top Failure of Reinforced Concrete Building, Mexico City This building illustrates the corner effect. This is caused by the combination of different directions of vibration acting on the building. Photo Credit: C. Arnold, Building Systems Development, Inc.
Top Failure of Reinforced Concrete Building, Mexico City Mid-floor failure of Hotel de Carlo caused by pounding form the building on the left. The building on the right has been deflected some what. The natural period of the buildings was very close to the period of the earthquake and because they were built very close to one another they pretty much hammered one another. Photo Credit: C. Arnold, Building Systems Development, Inc.
Closeup of Damage to Hotel de Carlo Close up view of the damage caused to Hotel de Carlo. This picture showing the midfloor failure. Photo Credit: C. Arnold, Building Systems Development, Inc.earthquake. Photograph credit: E.V. Leyendecker, National Bureau of Standards.
Twisted Building This building twisted excessively in the earthquake, forming the X-shaped cracks. The earthquake subjected the building to shear, bending, torsional forces, and compression. The formation of the X-shaped cracks is evidence the energy from the earthquake dissipated in the shear walls. Photo Credit: Reinsurance Company, Munich, Germany.
Earthquake Oscillations Cause Collapse of Vertical Supports The severe building oscillations deprived verical supports of their load-bearing capacity despite strong steel reinforcements. Note absence of reinforcement cross ties in vertical columns. Photo Credit: Reinsurance Company, Munich, Germany.