Earthquake Notes
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
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