9.S: Summary

Summary

Geologic stress, applied force, comes in three types: tension, shear, and compression. Strain is produced by stress and produces three types of deformation: elastic, ductile, and brittle. Geological maps are two-dimensional representations of surface formations which are the surface expression of three-dimensional geologic structures in the subsurface. The map symbol called strike and dip or rock attitude indicates the orientation of rock strata with reference to north-south and horizontal. Folded rock layers are categorized by the orientation of their limbs, fold axes and axial planes. Faults result when stress forces exceed rock integrity and friction, leading to brittle deformation and breakage. The three major fault types are described by the movement of their fault blocks: normal, strike-slip, and reverse.

Earthquakes, or seismic activity, are caused by sudden brittle deformation accompanied by elastic rebound. The release of energy from an earthquake focus is generated as seismic waves. P and S waves travel through the Earth’s interior. When they strike the outer crust, they create surface waves. Human activities, such as mining and nuclear detonations, can also cause seismic activity. Seismographs measure the energy released by an earthquake using a logarithmic scale of magnitude units; the Moment Magnitude Scale has replaced the original Richter Scale. Earthquake intensity is the perceived effects of ground shaking and physical damage. The location of earthquake foci is determined from triangulation readings from multiple seismographs.

Earthquake rays passing through rocks of the Earth’s interior and measured at the seismographs of the worldwide Seismic Network allow 3-D imaging of buried rock masses as seismic tomographs.

Earthquakes are associated with plate tectonics. They usually occur around the active plate boundaries, including zones of subduction, collision, and transform and divergent boundaries. Areas of intraplate earthquakes also occur. The damage caused by earthquakes depends on a number of factors, including magnitude, location and direction, local conditions, building materials, intensity and duration, and resonance. In addition to damage directly caused by ground shaking, secondary earthquake hazards include liquefaction, tsunamis, landslides, seiches, and elevation changes.