Earth’s geological structure and processes - Fundamentals of the Physical Environment - page 208

Seismic studies and seismo-volcanic 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 P waves 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 Mohorovici ´
discontinuity between outer crust and lithosphere; wave deceleration between lithosphere and asthenosphere,
signifying the partial melt status of the latter; acceleration at the Gutenbergdiscontinuity 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 10.10 ).

Seismicity directly impacts human life through the destructive power of earthquakes, measured on Modified Mercalli
(descriptive) and Richter(logarithmic energy) scales of severity ( Figure 10.11 ). These relate chiefly to shallow surface
waves. Mercalli intensities emphasize the human price paid in property and lives - averaging 10,000 yr -1 globally -
by the direct destruction of housing and other structures. Among more recent earthquakes, the Gujarat (India)
earthquake of 26 January 2001 killed over 20,000 people, injured over 150,000 and destroyed or damaged 1·1 million
buildings. Measuring 7·7 on the Richter scale, it was caused by the continuing indentation of India into Eurasia.
Earthquakes also strike indirectly by triggering landslides, tsunamis , or tidal waves, and volcanic eruptions. Indeed,
although they may occur independently, seismo-volcanic hazards are intimately linked. Major earthquakes in Mexico
City (1985), San Francisco (1989) and Kobe (Japan, 1994) share the same global network of subduction zones as the
explosive andesitic volcanoes of Mount St Helens and Pinatubo ( Figure 10.12 ). Minor British earthquakes near
Caernarfon (1984) and Shrewsbury (1990, 1996), registering 4·0-4·6 on the Richter scale, remind us that older tectonic
belts are not yet dead.

Their three-dimensional form consists of three to six
narrow, concentric structures, convex towards the sub-
ducting plate. This is the plan created by an oblique
incision into a curved surface, which a knife-cut into an
apple demonstrates! The outermost structure is an ocean
trench , typically 50-100 km wide, over 1,000 km long and
5-10 km deep. The Marianas trench is the deepest known,
at 11·04 km. Beyond the trench lies an accretionary prism
and fore-arc basin , completing the fore arc , followed by
the volcanic arc itself, which may have an outer, inactive
zone fringing an inner line of active volcanoes. The back
arc completes the full sequence and may contain a marine
basin and one or more remnant arcs , each representing
an extinct volcanic axis. Volcanic arc dynamics and

architecture clearly indicate that the entire zone is mobile
and its focus of activity may shift ( Figure 10.13 ).
Active subduction keeps the trench open against
isostatic forces and it acts as a sump for a potentially
major sediment flux of low-density erosion products.
These include debris from the volcanic arc, organic and
inorganic pelagic debris rained out from the overlying
ocean and flysch , derived from the adjacent continent and
swept into the trench by turbidity currents . Their low
density resists subduction and these sediments eventually
compose the bulk of the accretionary prism by offscrape ,
as the upper side of the descending slab scrapes against
the non-subducting plate. Successive offscrapes occur on
the underside of the prism, which occasionally emerges