Competitive Exams: Geology Glossary N to Z

Geology Terminology: N to Z

  • NEHRP: The federal National Earthquake Hazard Reduction Program, enacted in 1977, to reduce potential losses from earthquakes by funding research in earthquake prediction and hazards and to guide the implementation of earthquake loss reduction programs.
  • Normal Fault: A normal fault can result from vertical motion of two adjacent blocks under horizontal tension (It also occurs in rocks under compression if stress is unequal in different directions. In this case, the minimum and maximum compressive stresses must be applied horizontally and vertically respectively.). In a normal fault, the upper of the two adjacent blocks of rock slips relatively downward.
  • P (Primary) waves: Also called compressional or longitudinal waves, P waves are the fastest seismic waves produced by an earthquake. They oscillate the ground back and forth along the direction of wave travel, in much the same way as sound waves, which are also compressional, move the air back and forth as the waves travel from the sound source to a sound receiver.
  • Plates: Pieces of crust and brittle uppermost mantle, perhaps 100 kilometers thick and hundreds or thousands of kilometers wide, that cover the Earth's surface. The plates move very slowly over, or possibly with, a viscous layer in the mantle at rates of a few centimeters per year.
  • Plate boundaries: The edges of plates or the junction between plates. See also plates, convergent (both collision and subduction), spreading, and transform boundaries.
  • Plate tectonics: A widely accepted theory that relates most of the geologic features near the Earth's surface to the movement and interaction of relatively thin rock plates. The theory predicts that most earthquakes occur when plates move past each other.
  • Return times: Sometimes called the recurrence time or recurrence interval. The return time, or more properly the average return time, of an earthquake is the number of years between occurrences of an earthquake of a given magnitude in a particular area. For example, if the average time of an earthquake having magnitude greater than or equal to 7 is 100 years, then, on the average, such earthquakes will occur every 100 years. If such earthquakes occur randomly in time, there is always the chance that the actual time interval between the events will be less or greater than 100 years. Return time is best described in terms of probabilities. In the case of an earthquake having a 100-year average return time, there is about an 18 percent chance that such an earthquake will occur in the next 20 years and a 63 percent chance than it will occur in the next 100 years. On the other hand, there is a 14 percent chance that it will not occur in the next 200 years.
  • Reverse Fault: A rupture that results from vertical motion of two adjacent blocks caused by horizontal compression. Sometimes called a thrust fault. In a reverse fault, the upper of the two adjacent blocks moves relatively upward.
  • Richter Magnigtude Scale: An earthquake magnitude scale, more properly called local magnitude scale, based on measurements of the amplitude of earthquake waves recorded on a standard Wood-Anderson type seismograph at a distance of less than 600 kilometers from the epicenter (Richter, 1958).
  • S (Secondary or shear) waves: S waves oscillate the ground perpendicular to the direction of wave travel. They travel about 1.7 times slower than P waves. Because liquids will not sustain shear stresses, S waves will not travel through liquids like water, molten rock, or the Earth's outer core.
  • Seiche: A standing wave in a closed body of water such as a lake or bay. It can be characterized as the sloshing of water in the enclosing basin. Seiches can be produced by seismic waves from earthquakes. The permanent tilting of lake basins caused by nearby fault motions has produced very energetic seiches.
  • Seismic waves: A vibrational disturbance in the Earth that travels at speeds of several kilometers per second. There are three main types of seismic waves in the earth: P (fastest), S (slower), and surface waves (slowest). Seismic waves are produced by earthquakes.
  • Seismogram: A graph showing the motion of the ground versus time.
  • Seismograph: A sensitive instrument that can detect, amplify, and record ground vibrations too small to be perceived by human beings.
  • Site response: Local vibratory response to seismic waves. Some sites experience more or less violent shaking than others, depending on factors such as the nature and thickness of unconsolidated sediments and/or the configuration of the underlying bedrock.
  • Strike-slip fault: Horizontal motion of one block relative to another along a fault plane. If one stands on one side of the fault and observes that an object on the other side moves to the right during an earthquake, the fault is called a right-lateral strike-slip fault (like California's San Andreas fault). If the object moves to the left, the fault is called a left-lateral strike-slip fault.
  • Subduction zone boundary: The region between converging plates, one of which dives beneath the other. The Cascadia subduction zone boundary is an example.
  • Subduction earthquake: A thrust-type earthquake caused by slip between converging plates in a subduction zone. Such earthquakes usually occur on the shallow part of the boundary and can exceed magnitude 8.
  • Surface waves: Seismic waves, slower than P or S waves, that propagate along the Earth's surface rather than through the deep interior. Two principal types of surface waves, Love and Rayleigh waves, are generated during an earthquake. Rayleigh waves cause both vertical and horizontal ground motion, and Love waves cause horizontal motion only. They both produce ground shaking at the Earth's surface but very little motion deep in the Earth. Because the amplitude of surface waves diminishes less rapidly with distance than the amplitude of P or S waves, surface waves are often the most important component of ground shaking far from the earthquake source.
  • Transform boundary: A boundary between plates where the relative motion is horizontal. The San Andreas fault is a transform boundary between the North America plate and the Pacific plate. The Blanco fracture zone is a transform boundary between the Juan de Fuca and the Pacific plates.
  • Tsunami: A tsunami is a series of very long wavelength ocean waves caused by the sudden displacement of water by earthquakes, landslides, or submarine slumps. Ordinarily, tsunamis are produced only by earthquakes exceeding magnitude 7.5. In the open ocean, tsunami waves travel at speeds of 600 − 800 kilometers/hour, but their wave heights are usually only a few centimeters. As they approach shallow water near a coast, tsunami waves travel more slowly, but their wave heights may increase to many meters, and thus they can become very destructive.
  • World-wide Standard Seismograph Network: A network of about 110 similarly calibrated seismograph stations that are distributed throughout the world. The network was originally established in the early 1960s, and its operation is now coordinated by the US Geological Survey. Each station has six seismometers that measure vertical and horizontal ground motion in two frequency ranges.