Earlham College--Geology 211--mcearthquakes

The Earth is anything but static. Earthquakes are vibrations of the Earth caused by a rapid release of energy along a fault (a large fracture in the Earth's crust). Rocks along a fault line are bending and storing elastic energy. This strain is released through slippage which allows the rock to "snap back" as it elastically returns to its original shape in a process termed elastic rebound. This released energy radiates in all directions from its weakened source within the Earth, the focus, in the form of waves.

Seismic waves are elastic energy and are characterized by surface waves or body waves and constantly inform geophysicists on the nature of Earth's interior.

Surface waves travel along the surface of the Earth. Body waves travel through Earth's interior and are divided into two types. Primary waves (P waves) are "push-pull," operating by compression and expansion, travel at high velocities and can travel through solids, liquids and gasses. Secondary Waves (S waves) are characterized by a "shaking" motion, travel at slower velocities and only through solids.

The epicenter is the area on the surface of the Earth which is directly above the focus and is located by using the difference in velocities of primary and secondary waves. A travel-time graph is used to calculate each of the stations' distance to the epicenter. A circle with a radius equal to the distance to the epicenter is drawn around each station. The point where all three circles intersect is the earthquake epicenter.

Despite the fact that seismic waves dissipate as they travel away from the source, the event can be recorded around the globe with the aid of the seismograph, an instrument that records the movement of Earth in relation to a stationary mass on a rotating drum.

Some major earthquakes may be preceeded by smaller earthquakes, called foreshocks. They may precede by days, or in some cases, by several years. Smaller earthquakes may also be generated after the major earthquake. These are called aftershocks. Though they are typically much weaker than the preceeding earthquake, they contribute to the destruction of already weakened structures.

Earthquake size is measured in terms of intensity and magnitude. Intensity is the measure of the degree of shaking at a given locale based on the amount of damage and is measured by the Modified Mercalli Intensity Scale. The Richter Scale, on the other hand, is based on the amplitude of the largest seismic wave (P, S or surface) that is measured on the seismogram. Earthquakes measuring less than 2.0 are not felt by humans. Magnitude estimates the amount of energy released at the source of the earthquake. Earthquake destruction can rely on several factors, including the intensity and duration of the vibrations, the material upon which the structures rest and the design of the structures themselves.

Liquefaction is a phenomenon generated in areas where unconsolidated materials are saturated with water as a result of earthquake vibrations. Stable soil turns into mobile fluid that is not capable of supporting structures.

At 7:17 a.m. September 19, 1985, a powerful earthquake registering M8.1 rocked central Mexico. The quake was centered in the coastal mountains of the state of Michoacan and caused substantial damage throughout central Mexico, including in Mexico City - some 240 miles east of the epicenter.

The city is largely built upon soft, poorly consolidated lake sediments from the former Lake Texcoco, which was drained by the Spanish following their occupation of the region. Thick sequences of soft sediments tend to amplify seismic waves and cause the ground to shake much more vigorously than the surrounding bedrock. Mexico City suffered heavily in 1985 because of its location on soft clay beds and because buildings at its heart were built at a height of an average 10 storeys which amplified the resonance in the clays. The clays resonated about six times more violently than the hills surrounding the city. The zonation of damage suggests that the shock waves were reflected internally within the basin, interfered with each other and gave rise to standing waves. Because pressure waves move easily through fluids and semi-solids, the destruction in the lake bed must have been caused by pressure waves.

The casualties were estimated to exceed 20,000. Structures that suffered the most damage were those between 5 and 15 storeys. Ironically, tall buildings with shallow foundations escaped danger while moderate-sized buildings with deep foundations suffered most. In addition, the earthquake occurred outside office hours, so most of the deaths were in housing blocks and in hospitals. Luckily, offices and schools were empty.

Mexico City has been rocked several times in the past and remains in great danger. The earthquake of 1985 happened in a "seismic gap," where tension along a fault line had been building up for several decades. The capital continues to be the most vulnerable target because there are several other sections along the fault line that have been unusually quiet for too long.

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.

This building sunk into the ground as a result of liquefaction. It is recommended that the soil be tested to see if it is capable of liquefaction. If it tests positive, then the area is place under special restrictions as to the type of buildings that can be erected. This is called zoning.

More than 100,000 people were left homeless. The Global Food Bank Association reacted quickly to this emergency situation. A team of three, headed by Joseph Edison, visited Mexico City to identify the immediate needs for the disaster relief. The investigation indicated food was needed immediately. The Mexico Earthquake Relief Project was initiated. The goals and objectives of the project were to gather donations from people and food companies in Canada, and transfer them to Mexico City, as well as to assist in distribution of that food to the poor, homeless and jobless through the City of Mexico Food Bank and Caritas de Mexico.

In an eight month period, the Mexico Earthquake Relief Project had successfully collected, transported and distributed 1,500 tonnes worth over $10 million of canned food, rice, milk power cereal, baby food, sugar, potatoes, etc. to the earthquake victims in Mexico City.

Los rescatistas de Israel dijeron que esto era mas desastroso que una guerra; en una guerra hay mas orden . . . sabe uno hacia donde correr, donde resguardarse, cual va a ser mas o menos la duracion del bombardeo, pero en Mexico no habia nada. En Mexico solo hubo un vacio pavoroso, no existio autoridad hasta que las autoridades fueron resucitando. Cuando hablo el Presidente como que renacio un poco la calma, pero entre tanto, fue la gente la que ocupo este vacio. Cuando De la Madrid hablo de unidad, mucha gente creyo que hablo de uniformidad, de no criticar, de obedecer y hacer lo que piden las autoridades. No. Siento que la unidad es eso que demostramos inmediatamente despues del temblor al ayudarnos todos sin pensarlo dos veces. (Nada, nadie: Las voces del temblor p. 55)

The Israeli rescue workers said that this was more disasterous than a war; in a war there is more order . . . one knows in which direction to run, where to take refuge, more or less how long the bombardment will last, but in Mexico there was nothing. In Mexico there was only a terrifying emptiness, authority did not exist until the authority figures themselves began resuscitating. When the President spoke, as if the calm had been restored, it was the people who filled in the emptiness. When De la Madrid spoke of unity, several people believed he was speaking of uniformity, of not criticizing, of obeying and doing what the authority figures ask of them. No. I feel that unity is precisely what we showed immediately after the earthquake upon helping each other without thinking twice. (my translation)

Author: Jason Wade Keller

Creation/revision date: 28. 03. 02

please send questions and comments to kelleja@earlham.edu