Tim Rushton, Phoebe Martin and Amy Fitzgerald

History

Faulting of an uplifted plateau, between the ‘North American Plate’ and the ‘Cocos Plate’.

Volcanic activity 30 million years ago caused the river, that filled the valley, to dam and a lake was formed – Lake Texacoco.

Mexico City was originally built on an island in the lake. As the lake was drained the city grew outwards onto the unconsolidated land.

Because of this Mexico City lies in a broad basin of unconsolidated lake bed sediment.

The island was susceptible to flooding so the decision was made to build a drainage system.

In1967 a network of channels and underground tunnels built to control water levels – arid surrounding area.

Inhabitants needed to pump water from the ground causing sinking of the unconsolidated subsurface soil - cm/yr.

The Earthquake

8.1 magnitude earthquake on 19th September 1985 with smaller aftershocks.

Epicentre was 240 miles away, off the coast of Michoacán.

Very active subduction zone – Cocos plate pushes under North American plate.

Multiple breaks in the fault line caused several minutes of shaking. Ground conditions worsened the effect of the earthquake, particularly for taller buildings due to the effects of resonance. Approximately 400 buildings collapsed and 3,000+ damaged. Around 10,000 people died and many more injured.

Geology of Mexico Mexico is made up of micro-continents that were accreted to the

North American Continent during the late Palaeozoic up until the early Cenozoic.

A subduction zone was formed in the Pacific Ocean, where the Cocos Plate is forced under the North American Plate.

The resulting tectonic activity produced four mountain ranges: Sierra Madre Occidental

Late Eocene to Early Miocene Extrusions and Oligocene Ignimbrites

Sierra Madre Oriental Sierra Madre del Sur Trans-Mexican Volcanic Belt (TMVB)

Mesozoic and Cenozoic

Geology of Mexico City Lies within the Mexico Basin in the TMVB Partially separated by lower mountains

Deposition in pre-existing lakes created lacustrine clay layers

up to 100m deep low permeability

Separated by sections of silt and sand Harder and 10 – 40m deep

Alluvial fill containing strata of Pleistocene and recent basalt deposits

100 – 500m thick Stratified Volcanic Deposits held together Pliocene lacustrine clay deposits.

100 – 600m thick500 – 1000m deep Lacustrine Clay Deposits

Quaternary Chichinautzin Volcanic Series

Clay Layer

Sand and Silt

Clay Layer

Gravel, Silt and Compact clay

Effect of Geology during Earthquake Felt 745 miles North in Houston, Texas and 621 miles South in Guatemala City. Seismic waves travelled quickly from epicentre to Basin of Mexico through plutonic, metamorphic, and continental and marine rocks. Mexico City was the worst affected even though 240 miles away. Due to soft, poorly consolidated lake-bed sediment which reduced the velocity of the seismic waves and increased their amplitude.

Types of Seismic Waves Present in Mexico City Surface waves such as Rayleigh and Love waves were dominant.

Sub soil was more rigid due to historic engineering such as aqueducts and pre-Hispanic buildings.

S-waves also present but lower amplitude. Characterized by shaking and travel at lower velocities through solids.

Liquefaction Soil had very low pore water pressure due to historical draining of pre-existing lakes. Vibrations from earthquake increased water content. Soil became fully saturated. Water pressure exerted a force on the soil particles and allowed particles to move amongst each other. Reduction in soil strength.

Building has sunk into the ground as a result of liquefaction.

Possible Solutions

Prior to construction of structure GSA zoning – site specific geotechnical surveys. Densification of the liquefiable sand. Deep drains installed to prevent build up of excess pore

pressure. Cement grouting. Subsurface barriers.

During construction A stiff mat foundation over locally liquefied zones. A ductile cap connection to allow some rotation to occur without

failure. Large diameter piles to resist moments.

After construction Dynamic compaction. Vibro-compaction. Vibro-replacement. Compaction piles.

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