Course Title IEQ-05 : Earthquake Geology and Geoinformatics (Dept. of Earthquake Engineering, IIT Roorkee)
What is an earthquake?
An earthquake is the vibration of the earth produced by the quick release of energy. Most often, earthquakes are caused by movement along large fractures in the earth’s crust. Such fractures are called faults. The energy that is released radiates in all directions from its origin in the form of waves. These waves are similar to the waves that occur when you drop a stone into water. Just as the stone sets the water in motion, the energy released in an earthquake produces seismic waves that move through the earth. Frequency range of seismic waves is large, from as high as the audible range (greater than 20 hertz) to as low as the frequencies of the free oscillations of the whole Earth (2 and 7 millihertz). Attenuation of the waves in rock imposes high-frequency limits, and in small to moderate earthquakes the dominant frequencies extend in surface waves from about 1 to 0.1 hertz. The amplitude range of seismic waves is also great in most earthquakes. In the greatest earthquakes the ground amplitude of the predominant P waves may be several centimeters at periods of two to five seconds. Very close to the seismic sources of great earthquakes, investigators have measured large wave amplitudes with accelerations of the ground exceeding that of gravity (9.8 meters, or 32.2 feet, per second squared) at high frequencies and ground displacements of 1 meter at low frequencies.
Earthquake Magnitude and Energy Release Equivalence
What is the mechanism that produces earthquakes?
Earth is not a static planet: in the earth’s crust, tectonic forces are constantly at work pushing rocks on both sides of a fault in different directions. In this process, the material is deformed. As rocks don’t slide past each other very easily, strain is built up, just as if you bend a stick. At a certain level, the rocks can no longer resist the strain and slip past each other into their original shape. This “springing back” of the rock is called elastic rebound. It is this quick movement that we feel as an earthquake. The elastic rebound usually happens a few kilometres deep in the crust. This location is called the focus of the earthquake. The place on the surface directly over the focus is called the epicentre.
Reid's Elastic Rebound Theory
From an examination of the displacement of the ground surface which accompanied the 1906 earthquake, Henry Fielding Reid, Professor of Geology at Johns Hopkins University, concluded that the earthquake must have involved an "elastic rebound" of previously stored elastic stress. If a stretched rubber band is broken or cut, elastic energy stored in the rubber band during the stretching will suddenly be released. Similarly, the crust of the earth can gradually store elastic stress that is released suddenly during an earthquake.
This gradual accumulation and release of stress and strain is now referred to as the "elastic rebound theory" of earthquakes. Most earthquakes are the result of the sudden elastic rebound of previously stored energy. The following diagram illustrates the process. Start at the bottom. A straight fence is built across the San Andreas fault. As the Pacific plate moves northwest, it gradually distorts the fence. Just before an earthquake, the fence has an "S" shape. When the earthquake occurs the distortion is released and the two parts of the fence are again straight; but now there is an offset.
Slow, Quiet and Silent Earthquakes
• When we think of an earthquake source, we think of a crack that propagates through the crust close to the shear-wave speed, which is generally several kilometers per second. Fault rupture is sudden, accompanied by violent shaking of the ground. But, creep events (on the San Andreas fault) during which propagation along a fault occurs at rates of millimeters per year. Earth deformation occurs at rates that differ widely....
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