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Unit 35


Unit Overview

This unit explores earthquakes and resultant processes and landscapes. The main sections are as follows:

  • Earthquake terminology
  • Earthquake distribution
  • Earthquakes and landscapes
  • Tsunamis

An earthquake is the release of energy that has been slowly building up during the stress of the increasing deformation of rocks. Earthquakes either produce faults or originate at them. Faults are fractures in crustal rock involving the displacement of rock on one side of the fracture with respect to rock on the other side. Earthquakes originate at the "focus"┬┐the point at the surface directly above the focus is the epicentre of the earthquake.

Earthquakes generate seismic waves, and these waves determine an earthquake's magnitude and intensity. Earthquakes are concentrated in the CircumPacific and Trans-Eurasian belts of subduction zones as well as along midoceanic ridges. Intraplate earthquakes also occur.

Similar to volcanic eruptions, earthquakes can alter physical and cultural landscapes. Earthquakes can also produce giant sea waves known as tsunamis when the epicentre is located on the ocean floor or near a coastline. Tsunamis can, in turn, modify local physical and cultural landscapes.

Unit Objectives

  • To describe and quantify the magnitude and intensity of earthquakes
  • To relate the spatial pattern of earthquakes to plate tectonics
  • To discuss landscapes and landforms that bear the signature of earthquake activity

Glossary of Key Terms

Earthquake A shaking and trembling of the Earth's surface; caused by sudden releases of stresses that have been building slowly within the planetary crust.
Epicentre The point on the Earth's surface directly above the focus (place of origin) of an earthquake.
Fault A fracture in crustal rock involving the displacement of rock on one side of the fracture with respect to rock on the other side.
Fault plane The surface of contact along which blocks on either side of a fault move.
Fault scarp The exposed cliff-like face of a fault plane created by geologic action without significant erosional change.
Fault trace The lower edge of a fault scarp; the line on the surface where a fault scarp intersects the surface.
Focus The place of origin of an earthquake, which can be near the surface or deep inside the crust or upper mantle.
Intensity (earthquake) The size and damage of an earthquake as measured - on the Modified Mercalli Scale - by the impact on structures and human activities on the cultural landscape.
Intraplate earthquake Earthquakes occurring in areas other than tectonic plate contact zones.
Magnitude (earthquake) The amount of shaking of the ground during an earthquake as measured by a seismograph.
Tsunami A seismic sea wave, set off by a crustal disturbance that can reach gigantic proportions.

Unit Outline

  • Earthquake terminology
    • A fault is a fracture in the crust that displaces rock on one side of the fault as compared to the other side.
    • An earthquake is a release of energy that has slowly built up during deformation of rock
      • an earthquake's origin is the focus, and the point above the focus on Earth's surface is the epicenter
      • magnitude is the amount of shaking of the ground during a quake
        • Richter Scale for measurement
        • Moment Magnitude Scale
      • intensity reflects the impact of an earthquake on a given landscape
        • Modified Mercalli Scale
  • Earthquake distribution
    • Heaviest concentration in Circum-Pacific belt
    • Trans-Eurasian belt
    • Midoceanic ridges
    • Intraplate earthquakes can also occur
      • New Madrid, Missouri
  • Earthquakes and Landscapes
    • Physical and cultural landscapes modified by earthquakes
      • fault scarp is the result of rock deformation, with one block raised with respect to the other
      • fault plane is the exposed cliff-like face of the scarp
      • fault trace is the lower edge of the fault scarp
        • fault breccia often in fault trace
      • landslides and mudslides can occur with a lot of rain in these areas
  • Tsunamis
    • Seismic sea wave that occurs when an earthquake's epicenter is on the ocean floor

Review Questions

  1. Discuss the geographic distribution of recent earthquake activity with the aid of Fig.35.6.
  2. Define fault, fault plane, fault scarp, and fault trace.
  3. Describe what an intraplate earthquake is, and give an example of where a famous one occurred.