This page last updated on 28-Aug-2013EENS 3050 Natural DisastersTulane University Prof. Stephen A. NelsonEarthquakes: Causes and MeasurementsEarthquakesThe objectives of this part of the course will to be gain an understanding of the science of earthquakes so that we can answer the following questions: 1. What causes earthquakes?2. How are earthquakes studied?3. What happens during an earthquake? 4. Where do earthquakes occur?5. Can earthquakes be predicted?6. Can humans be protected from earthquakes?Most earthquakes occur along zones where the Earth's crust is undergoing deformation. Deformation results from plate tectonic forces and gravitational forces. The type of deformation that takes place during an earthquake generally occurs along zones where rocks fracture to produce faults. Before we can understand earthquakes, we first must explore deformation of rocks and faulting. Within the Earth rocks are constantly subjected to forces that tend to bend, twist, or fracture them. When rocks bend, twist or fracture they are said to deform or strain (change shape or size). The forces that cause deformation are referred to as stresses. To understand rock deformation we must first explore stress and strain.Stress and StrainStress is a force applied over an area. One type of stress that we are all used to is a uniform stress, called pressure. A uniform stress is where the forces act equally from all directions. In the Earth the pressure due to the weight of overlying rocks is a uniform stress and is referred to as confining stress. If stress is not equal from all directions then the stress is a differential stress. Three kinds of differential stress occur. 1. Tensional stress (or extensional stress), which stretches rock; 2. Compressional stress, which squeezes rock; and 3. Shear stress, which result in slippage and translation. Page 1 of 16EQ: Causes and Measurements8/28/2013http://www.tulane.edu/~sanelson/Natural_Disasters/eqcauses.htmStages of DeformationWhen a rock is subjected to increasing stress it changes its shape, size or volume. Such a change in shape, size or volume is referred to as strain. When stress is applied to rock, the rock passes through 3 successive stages of deformation. z Elastic Deformation -- wherein the strain is reversible. z Ductile Deformation -- wherein the strain is irreversible. z Fracture -- irreversible strain wherein the material breaks. We can divide materials into two classes that depend on their relative behavior under stress. z Brittle materials have a small to large region of elastic behavior, but only a small region of ductile behavior before they fracture. z Ductile materials have a small region of elastic behavior and a large region of ductile behavior before they fracture. Page 2 of 16EQ: Causes and Measurements8/28/2013http://www.tulane.edu/~sanelson/Natural_Disasters/eqcauses.htmHow a material behaves will depend on several factors. Among them are: z Temperature - At high temperature molecules and their bonds can stretch and move, thus materials will behave in more ductile manner. At low Temperature, materials are brittle. z Confining Pressure - At high confining pressure materials are less likely to fracture because the pressure of the surroundings tends to hinder the formation of fractures. At low confining stress, material will be brittle and tend to fracture sooner.z Strain rate -- Strain rate refers to the rate at which the deformation occurs (strain divided by time). At high strain rates material tends to fracture. At low strain rates more time is available for individual atoms to move and therefore ductile behavior is favored.z Composition -- Some minerals, like quartz, olivine, and feldspars are very brittle. Others, like clay minerals, micas, and calcite are more ductile This is due to the chemical bond types that hold them together. Thus, the mineralogical composition of the rock will be a factor in determining the deformational behavior of the rock. Another aspect is presence or absence of water. Water appears to weaken the chemical bonds and forms films around mineral grains along which slippage can take place. Thus wet rock tends to behave in ductile manner, while dry rocks tend to behave in brittle manner. Evidence of Former DeformationEvidence of deformation that has occurred in the past is very evident in crustal rocks. For example, sedimentary layers and lava flows generally are deposited on a surface parallel to the Earth's surface (nearly horizontal). Thus, when we see such layers inclined instead of horizontal, evidence of an episode of deformation is present. In order to uniquely define the orientation of a planar feature we first need to define two terms -strike and dip. For an inclined plane the strike is the compass direction of any horizontal line on the plane. The dip is the angle between a horizontal plane and the inclined plane, measured perpendicular to the direction of strike. Page 3 of 16EQ: Causes and Measurements8/28/2013http://www.tulane.edu/~sanelson/Natural_Disasters/eqcauses.htmIn recording strike and dip measurements on a geologic map, a symbol is used that has a long line oriented parallel to the compass direction of the strike. A short tick mark is placed in the center of the line on the side to which the inclined plane dips, and the angle of dip is recorded next to the strike and dip symbol. For beds with a 90o dip (vertical) the short line crosses the strike line, and for beds with no dip (horizontal) a circle with a cross inside is used. Fracture of Brittle RocksJointsJoints are fractures in rock that show no slippage or offset along the fracture. Joints are usually planar features, so their orientation can be described as a strike and dip. They form from as a result of extensional stress acting on brittle rock. Such stresses can be induced by cooling of rock (volume decreases as temperature decreases) or by relief of pressure as rock is eroded above thus removing weight.Joints are zones of weakness,so their presence is critical when building anything from dams to highways. For dams, the water could leak out through the joints leading to dam failure. For highways the joints may separate and cause rock falls and landslides.Faults - Faults occur when brittle rocks fracture and there is an offset along the fracture. When the offset is small, the displacement can be easily measured, but sometimes the displacement is so large that it is difficult to measure. Page 4 of 16EQ: Causes and