Live as if you were to die tomorrow.
Learn as if you were to live forever.
- Mohandas K. Gandhi
Learn as if you were to live forever.
- Mohandas K. Gandhi
Earthquakes have rocked the world for many years and every day there is at least one taking place somewhere in the world. While some earthquakes are only felt, there are others that are more devastating and cause largescale damage to life and property. Earthquakes cause waves beneath the surface of the earth and according to Peter M. Shearer in Introduction to Seismology, “Seismology is the science that studies these waves and what they tell us about the structure of the Earth and physics of earthquakes.”
Aftershock – Earthquakes that follow or occur “after” the mainshock are known as the aftershock. These are usually smaller and lesser in intensity than the mainshock and can be felt for weeks, months or even years after the main shock.
Amplitude – Amplitude refers to the size of the wiggles on an earthquake recording.
Aseismic – A fault which has had no earthquakes recorded is referred to as aseismic.
Body Wave – A wave that moves through the inside of the earth rather than the surface is known as a body wave.
Central Angle – The angle of incidence at an interface between two layers which results in an angle of transmission which parallels the interface and sets up refraction in the second layer. ( Pritchett, William Carr)
Core – The core is the innermost part of the earth and is solid metal.
Continental Drift – The movement of the Earth’s continents relative to one another is known as Continental Drift.
Crust – The layer of earth that is the outermost is known as the crust. It extends from 10 to 65 cm in thickness.
Earthquake – An earthquake is a sudden slip on a fault. It could be caused by a slip or by stress changes in the earth or by volcanic activity.
Earthquake Swarm – A series of small or minor earthquakes taking place in a restricted area and a limited period of time.
Elastic Wave – When the is Earth rapidly displaced or distorted at some point, the energy imparted into the Earth by the source of the distortion can be transmitted in the form of elastic waves. ( University of Melbourne, Introduction to Geophysical Exploration)
Epicenter – Earthquakes are caused by a sudden slip on a fault. The point on the fault from where an earthquake starts is called the epicenter.
Fault – Sudden slips on the fault cause earthquakes. Faults are breaks or fractures in the crust of the earth, where parts of the crust have moved in some direction as compared to the other parts. Faults are of three types – strike slip, normal and thrust. The breaks or fractures may be less than an inch in small earthquakes to over 10 yards in major ones.
First arrival – The first arrival is the time when the arrival of the first wave is recorded.
Focus – The focus is the point inside the surface of the earth where the earthquake first begins. It is also called the hypocenter.
Focal zone – Seismic focal zones are “the active structures of the continent to ocean transition region, determining the formation and evolution of the island arc systems as well as distribution of earthquake hypocenters, sources of magma formation and metallogenic provinces.” (Roman Z. TARAKANOV)
Foreshock – The main shock is the largest earthquake in a series of earthquakes. Foreshocks are smaller earthquakes that precede the mainshock.
Great Earthquake – Earthquakes with magnitude of 7.0 or more are known as great earthquakes and cause large scale damage to life and property.
Hypocenter – The hypocenter is also known as the focus. It is the point within the surface of the earth from where the earthquake begins.
Intensity – This is the number, usually written as a Roman numeral ranging from I to XII, that describes the severity of the earthquake, its effects on the earth and the damage caused. Intensity is measured with the help of certain scales, such as the Modified Mercalli scale and the intensity is usually the greatest near the epicenter.
Isoseismal Line – An isoseismal line or an isoseismal is a line or contour on a map drawn through all the places at which the intensity of the earthquake is the same. ( Manual of Seismology, )
Landslide – A landslide is the sudden and often, rapid movement of material, either mud or snow, down a slope. Earthquakes often trigger landslides.
Latitude – Latitude gives the location of any place on Earth north or south of the equator.
Leaking Mode – A surface seismic wave which is not trapped properly and hence, loses or leaks energy.
Liquefaction – An effect of sediment saturated with water acting like fluid due to a temporary loss of strength. Earthquakes often cause liquefaction.
Lg Wave – A surface wave that travels through the continental crust is called Lg wave. (USGS National Earthquake Information Center)
Longitude – Represented by the Greek letter lambda, longitude is represented by lines running from north to south. The Prime Meridian is the longitude that represents zero degrees of longitude.
Love Wave – Surface or SH waves having their largest amplitudes confined near the surface of an elastic body. (Quantitative Seismology, Aki and Richards)
Low Velocity Zone – A zone in the upper mantle where the P and S waves travel at velocities slower than that in the outermost part of the upper mantle.
Magnitude – Magnitude is the number assigned to define and characterize the size or strength of an earthquake. It is based on the maximum motion as recorded by the seismograph.
Major Earthquake – Major earthquakes are earthqakes with a magnitude of more than 7.0.
Mantle – A zone in the earth’s interior between the crust and the core. The mantle is nearly 1740 miles thick.
Microearthquake – Earthquakes with a magnitude of about 2.0 are known as micro earthquakes.
Microseism – A faint tremor or oscillation of the earth caused by wind or waves
Modified Mercalli Scale – A scale used to measure the intensity of earthquakes. It divides the effects of a quake into twelve categories from not felt to complete damage.
Mohorovicic Discontinuity – Also known as the Moho, it is the boundary between the crust of the earth and the mantle. It is between 25 to 60 km deep beneath the continents and between 5 and 8 km deep beneath the ocean floor. ( USGS Glossary)
P Wave – A body wave, the P (“primary”) wave is a compressional wave and travels faster than the other body waves, the S-waves. P waves are the only waves that can travel through the earth’s core. Being fast moving, this seismic wave is the first to arrive at a location.
Paleomagnetism – The study of the Earth’s magnetic field or the magnetic properties of rocks is called paleomagnetism.
Paleoseismology – The study of the record of ancient earthquakes preserved within sedimentary rocks. (Structural Geology at UNCChapel Hill)
Period – Time interval required for one full cycle of a wave.
Phase – Another term for a wave. Body waves that are refracted into the water at the bottom of the ocean are called Twaves or Tphases because this phase arrives after the primary (P) and secondary(S) waves. ( The Great Alaska Earthquake of 1964, Vol.1)
Plate – Plates are rigid, thin and brittle pieces of the earth’s crust and mantle. These cover the earth’s surface and move very slowly over a viscous layer in the mantle.
Plate Boundary – Plate boundaries can be found at the edge of the lithospheric plates. There are three types – divergent, conservative, and convergent.
Plate Tectonics – A widely accepted and supported theory that the geological features near the earth’s surface are a result of the movement of several large, thin and rigid plates.
Rayleigh Wave – Named after the English physicist, Lord Rayleigh, Rayleigh waves are surface waves that have a retrograde, elliptical motion on the surface of the earth and can be felt like a rolling motion.
Recurrence Interval – The time period between large earthquakes at one particular location.
Reflect – When a seismic wave returns from a boundary between two different materials within the earth. (USGS Glossary)
Refract – When a seismic wave deflects or bends while passing from one material to another, both of which have different elastic properties.
Richter Scale – The scale used to describe the magnitude or size of an earthquake is known as the Richter scale, named after its developer Charles F. Richter.
Rupture Zone – An instant boundary formed between the slipping and locked parts of a fault during an earthquake.
S Wave – Also known as the secondary or shear waves, these waves are body waves and moves the ground back and forth perpendicular to the direction of the wave travel. S waves are slower than P waves and do not travel through liquids.
Seiche – Produced by seismic waves from earthquakes, seiches are standing waves or sloshing of water in a closed water body, such as a swimming pool or a pond.
Seismic Belt – Also known as the seismic zone. It is a linear area of the Earth’s crust that is seismic however, it may not be connected to a particular fault.
Seismic – Connected with or referring to earthquake activity.
Seismic Sea Wave – Large sea waves caused due to sudden displacement of water due to earthquakes, volcanic eruptions or landslides. Also known as Tsunamis.
Seismic Zone – A linear region of the earth’s crust that is actively seismic usually due to a common reason.
Seismogram – The record written by the seismograph as it detects and records earthqakes is known as a seismogram.
Seismograph – The instrument used to detect and record quakes, by creating an electric signal during an earthquake, is known as a seismograph.
Seismologist – One who studies the science of earthquakes and the structure of the earth is known as a seismologist.
Signal-To-Noise Ratio – Abbreviated as SNR, signal-to-noise ratio is used to describe the degree to which a particular signal has been distorted or disturbed by noise.
Spectral Acceleration – Spectral acceleration or SA is what is experienced by buildings as modeled by a particle on a massless vertical rod having the same natural period of vibration as the building. ( USGS Glossary)
Spread – The arrays of geophones or hydrophones and connecting cables used to record a single seismic record. Spreads can be single or split. (Pritchett, William Carr)
Spreading Center – A linear or elongated area where two plates are pulled away from one another, forcing molten lava to come up from within the gap and form new crust.
Subduction – When the oceanic lithosphere bumps into the continental lithosphere and as a result moves under it, the process is known as subduction.
Subduction Zone – The area or zone where two lithospheric plates – oceanic and continental come together, one moving over the other.
Surface Waves – Surface waves are seismic waves that move over the surface of the earth. Love waves and Rayleigh waves are both surface waves. Both these waves produce the effect of the ground shaking on the surface of the earth.
Teleseism – A term that describes earthquakes at a distance of more than 1000 km from the site of measurement.
Tidal Wave – Large and often, destructive wave produced by the occurrence of an earthquake, hurricane or really strong winds.
Travel Time – The travel time curve is a graph that maps the arrival times of P and S waves at various points as a function of distance from the source of the earthquake.
Tsunami – A large, fastmoving sea wave that occurs because of largescale sea floor displacements caused by major quakes on land or under the ocean or volcanic eruptions.
Unconsolidated – Soil and other loose material that has not hardened into rock. Unconsolidated sediments pose a threat during earthquakes due to a tendency to increase ground shaking and liquefy. ( Geological Tour of UC – Berkeley Glossary)
UTC – UTC or Coordinated Universal Time is a time scale based on the atomic second but corrected regularly to keep it synchronized with the rotation of the earth.
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