Earth’s geological structure and processes - Fundamentals of the Physical Environment - page 289

SEISMIC STUDIES AND SEISMOVOLCANIC HAZARDS
human impact
Our knowledge of Earth's inaccessible internal layers and boundaries depends on
astrochemistry, including meteorite mineralogy, the spectral signature of other stars and
Earth's seismic activity . Earthquakes result from crustal movement when stress, applied
through plate motion and stored as elastic strain energy in rocks, is suddenly released.
The resultant earthquake transmits shock as deep body waves, at rates dependent on rock
density and its solid or fluid state. Faster Primary or Pwaves travel by compression and
slower Secondary or S waves by shear. Sensitive seismographs record innumerable daily
Earth tremors, even in apparently stable zones like Britain. Seismic activity is also
triggered deliberately by modest explosions for research purposes or, unintentionally, by
nuclear weapon testing and other human actions.
Transmission times between the earthquake epicentre and seismographs for a given
rock layer are a function of distance, allowing earthquake magnitude, epicentre location
and transmission routes to be calculated for a particular shock. Marked changes in the
velocity and direction of P and S waves allow us to plot Earth's internal structure by
identifying different material densities at boundary discontinuities. Particular interest
centres on: the Mohorovičić discontinuity between outer crust and lithosphere; wave
deceleration between lithosphere and asthenosphere, signifying the partial melt status of
the latter; acceleration at the Gutenberg discontinuity between mantle and dense core;
and the deceleration of P waves and absence of S wave transmission in the outer core,
indicative of its fluid condition (Figure 1).
Figure 1 Typical (a) velocities and (b) pathways of earthquake
waves; M and G mark the Mohorovicčicć and Gutenberg
discontinuities.
Source: After Duff (1993) and Smith (1982).