a)
b)
TIMED ESSAY 2001 Q4
Explain how the velocities of P and S waves are used to interpret the
i)
Mechanical properties and
ii)
Thickness of the Lithosphere
Explain why the thickness of the Lithosphere differs between the oceans
and continents.
a)i) Mechanical properties of Lithosphere
P+S wave velocities depend on:
 Density - increased density - decreased velocity
 Incompressibility (bulk modulus) - high incompressibility = fast P waves
 Rigidity - (shear modulus) - high rigidity = fast S waves
Incompressibility + Rigidity increase faster with depth than density does
P and S waves increase in velocity with depth
Suggests that crust and lithospheric mantle are solid
Brittle fracture (faulting) near surface where cold
Plastic deformation (folding) at depth where hotter
P wave velocity slowed by presence of liquid
S waves are not transmitted by liquids as they have no shear strength
Seismic LVZ at base of lithosphere (100-300km depth) indicates transitional
boundary with Asthenosphere = 1300' C isotherm
Asthenosphere is partially (?1-5%) molten - slows both P and S waves
Not enough liquid to stop S waves
Allows transmission of P+S waves in short time of earthquake
Allows movement by plastic deformation over geological time: 10-100mm/yr
WELL ANNOTATED DIAGRAM TO SHOW LVZ (eg top part 2.11 p30
Geoscience)
a)ii) Thickness of Lithosphere
Thickness of lithosphere varies - up to 300km below continents, only few km at MOR
Presence of Moho discontinuity indicates lithospheric mantle denser than crust
Seismic recording station close to seismic source
- direct crustal P+S waves arrive first 6-7km/s
As distance from seismic source increases
-refracted mantle P=S waves arrive first because refracted waves travel faster (8km/s)
in Lithospheric mantle than crust (6-7km/s)
WELL ANNOTATED DIAGRAM TO SHOW MOHO (eg 2.12 p31 Geoscience)
b) Explaining variation in thickness of Lithosphere
Oceanic crust v consistent thickness 5-7km because all formed by same process of
SFS
Oceanic crust thinnest at MOR where newly formed - few km
Thickens away from MOR due to cooling and underplating of gabbro
Lithosphere thickens with distance from MOR too SIMPLE SKETCH
Continental crust variable in thickness average 35-40km in lowland areas, up to
100km below young fold mountains eg Andes because variety of processes form
continental crust and continental collision in orogenic belts forms fold mountains with
deep 'roots', so Moho mirrors Earth surface, as does Lithosphere thickness
SIMPLE SKETCH