CAN NIGERIA EVER WITNESS A TRUE SUSTAINABLE DEVELOPMENT BEFORE JESUS COMES?

Earthquakes Earthquakes occur when energy stored in elastically strained rocks is suddenly released. This release of energy causes intense ground shaking in the area near the source of the earthquake and sends waves of elastic energy, called seismic waves, throughout the Earth. Earthquakes can be generated by bomb blasts, volcanic eruptions, sudden volume changes in minerals, and sudden slippage along faults. Earthquakes are definitely a geologic hazard for those living in earthquake prone areas, but the seismic waves generated by earthquakes are invaluable for studying the interior of the Earth.   In or discussion of earthquake we want to answer the following questions: What causes earthquakes? How are earthquakes studied? What happens during an earthquake? Where do earthquakes occur? Can earthquakes be predicted? Can humans be protected from earthquakes? What can earthquakes tell us about the interior of the earth? Causes of Earthquakes
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.  Strain is a change in shape, size, or volume. The forces that cause deformation are referred to as stresses.  To understand the causes of earthquakes we must first explore stress and strain. Stress and Strain Recall that stress is a force applied over an area. A uniform stress is where the forces act equally from all directions. Pressure is a uniform stress and is referred and is also called confining stress or hydrostatic stress. If stress is not equal from all directions then the stress is a differential stress. Three kinds of differential stress occur.
Tensional stress (or extensional stress), which stretches rock; Compressional stress, which squeezes rock; and Shear stress, which result in slippage and translation.
When 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.

Elastic Deformation -- wherein the strain is reversible. Ductile Deformation -- wherein the strain is irreversible. Fracture -- irreversible strain wherein the material breaks.
We can divide materials into two classes that depend on their relative behavior under stress. Brittle materials have a small to large region of elastic behavior, but only a small region of ductile behavior before they fracture. Ductile materials have a small region of elastic behavior and a large region of ductile behavior before they fracture.
How a material behaves will depend on several factors. Among them are: 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. 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. 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. 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. In general, rocks near the surface of the earth behave in a brittle fashion, unless they are deformed slowly.   Thus, when they are acted upon by differential stress, they tend to fracture.

Faults Most natural earthquakes are caused by sudden slippage along a fault.  Faults occur when brittle rocks fracture and there is displacement of one side of the fracture relative to the other side.  The amount of displacement in a single slippage event is rarely more that 10 to 20 m for large earthquakes, but after many events the displacement could be several hundred kilometers.  Types of Faults Faults can be divided into several different types depending on the direction of relative displacement or slip on the fault. Most faults make an angle with the ground surface, and this angle is called the dip angle.  If the dip angle is 90o the fault plane is vertical.  Faults can be divided into two major classes.   Dip Slip Faults - Dip slip faults are faults that have an inclined fault plane and along which the relative displacement or offset has occurred along the dip direction. Note that in looking at the displacement on any fault we don't know which side actually moved or if both sides moved, all we can determine is the relative sense of motion. For any inclined fault plane we define the block above the fault as the hanging wall block and the block below the fault as the footwall block   Normal Faults - are faults that result from horizontal extensional stresses in brittle rocks and where the hanging-wall block has moved down relative to the footwall block.   Reverse Faults - are faults that result from horizontal compressional stresses in brittle rocks, where the hanging-wall block has moved up relative the footwall block. A Thrust Fault is a special case of a reverse fault where the dip of the fault is less than 45o. Thrust faults can have considerable displacement, measuring hundreds of kilometers, and can result in older strata overlying younger strata. Strike Slip Faults - are faults where the displacement on the fault has taken place along a horizontal direction. Such faults result from shear stresses acting in the crust. Strike slip faults can be of two varieties, depending on the sense of displacement. To an observer standing on one side of the fault and looking across the fault, if the block on the other side has moved to the left, we say that the fault is a left-lateral strike-slip fault. If the block on the other side has moved to the right, we say that the fault is a right-lateral strike-slip fault. The famous San Andreas Fault in California is an example of a right-lateral strike-slip fault. Displacements on the San Andreas fault are estimated at over 600 km. Oblique Slip Faults - If the displacement has both a vertical component and a horizontal component (i.e. a combination of dip slip and strike slip) it is called an oblique slip fault. Blind Faults A blind fault is one that does not break the surface of the earth.  Instead, rocks above the fault have behaved in ductile fashion and folded over the tip of the fault. Active Faults An active fault is one that has shown recent displacement and likely has the potential to produce earthquakes.   Since faulting is part of the deformation process, ancient faults can be found anywhere that deformation has taken place in the past.  Thus, not every fault one sees is necessarily an active fault.   Surface Expression of Faults Where faults have broken the surface of the earth they can be delineated on maps and are called fault lines or fault zones.   Recent ruptures of dip slip faults at the surface show a cliff that is called a fault scarp.  Strike slip faults result in features like linear valleys, offset surface features (roads, stream channels, fences, etc.) or elongated ridges.(see figure 10.5 and10.37 in your textbook). How Faults Develop The elastic rebound theory suggests that if slippage along a fault is hindered such that elastic strain energy builds up in the deforming rocks on either side of the fault, when the slippage does occur, the energy released causes an earthquake. This theory was discovered by making measurements at a number of points across a fault. Prior to an earthquake it was noted that the rocks adjacent to the fault were bending. These bends disappeared after an earthquake suggesting that the energy stored in bending the rocks was suddenly released during the earthquake. Friction between the blocks then keeps the fault from moving again until enough strain has accumulated along the fault zone to overcome the friction and generate another earthquake.  Once a fault forms, it becomes a zone of weakness in the crust, and so long as the tectonic stresses continue to be present more earthquakes are likely to occur on the fault. Thus faults move in spurts and this behavior is referred to as Stick Slip.  If the displacement during an earthquake is large, a large earthquake will be generated.  Smaller displacements generate smaller earthquakes.  Note that even for small displacements of only a millimeter per year, after 1 million years, the fault will accumulate 1 km of displacement.  Fault Creep - Some faults or parts of faults move continuously without generating earthquakes.  This could occur if there is little friction on the fault and tectonic stresses are large enough to move the blocks in opposite directions.  This is called fault creep.  Note that if creep is occurring on one part of a fault, it is likely causing strain to build on other parts of the fault.  How Earthquakes Are Measured When an earthquake occurs, the elastic energy is released and sends out vibrations that travel in all directions throughout the Earth. These vibrations are called seismic waves. The point within the earth where the fault rupture starts is called the focus or hypocenter.  This is the exact location within the earth were seismic waves are generated by sudden release of stored elastic energy. The epicenter is the point on the surface of the earth directly above the focus. Sometimes the media get these two terms confused.