Transcript AIM: Introduce you to scientific study of the world's

Earthquakes
• A manifestation of rock deformation
• Occur when one mass of rock slips past
another mass of rock on a discrete
surface separating the two rock masses
• Discrete surface = geological fault
• Slip is catastrophic
Stick-slip mechanism
• One of several mechanisms by which slip occurs
on faults
• Prior to slip event, rock distorts elastically
• In elastic deformation, bonds bend, stretch,
and otherwise distort but do not break
• Elastic deformation is not permanent - it is
recoverable
• Elastic deformation stores energy
• Stored energy is released during catastrophic
slip event
Catastrophic slip occurs at hypocenter
• Hypocenter = focus
• Stored elastic energy released, creating wavelike distortions that emanate from focus as
seismic waves
• Elastic distortions consist of:
– Compression/dilatation of bonds in rocks
– Shearing of bonds in rocks
– Rotary motion of particles in rock, etc.
• Components disperse with distance from focus
• Distinguish body wave & surface wave
components
Body wave component I
• Longitudinal or compressional component
–Travels through rock as regions of
compressed and dilated rock
–Atoms/molecules move back & forth along
lines parallel to direction in which wave
travels
–Travels through solids and liquids
–Travels at higher speeds, & so arrives at
distant recording station sooner
–Called P wave component = primary wave
Body wave component II
• Transverse or shear component
–Travels through rock as regions of sheared
rock
–Atoms/molecules move back & forth along
lines perpendicular to direction in which wave
travels
–Travels through solids but not through liquids
–Travels at lower speeds, & so arrives at
distant recording station later
–Called S wave component = secondary wave
Like all waves, seismic waves reflect
and refract when they travel from a
medium where they have one speed to
a medium where they have a
different speed
Through reflection & refraction of
seismic waves, we ‘see’
• Earth is composed of layers with different
characteristics & compositions
• Crust - regions of low seismic velocity; 5-7
km thick in oceans & 35-70 km thick in
continents
• Moho - discontinuity at base of crust;
discovered early in the 20th century
• Mantle - region of high seismic velocity below
crust; dense substrate on which crust floats
Taking larger view, we ‘see’ that
Earth is composed of concentric
shells of distinctive seismic
character
Concentric shell in earth, I
• Lithosphere
–Outermost shell
–Consists of crust & cool, relatively rigid
mantle immediately below it
–Characterized by high seismic velocity & low
attenuation of seismic waves
–Is usually 60-100 km thick
–Is <5 km thick under MOR axial valley
–Is >300 km thick in some continental areas
Concentric shell in earth II
• Asthenosphere
–Warm, relatively ductile layer within mantle
–Has relatively low seismic velocity and high
attenuation of seismic waves
–Usually about 250 km thick; it occurs
between 60 & 350 km below surface
–Absent beneath some continental regions
–Ductility may result from partial melting;
compare geothermal gradient against solidus
temperature
Concentric shell in earth III
• Mesosphere
–Lower portion of the mantle
–Has relatively high seismic velocities & low
attenuation of seismic waves; velocity
increases with depth
–Rock is warm, but high ambient pressure
makes melting unlikely & increases strength
of rock
–We know little of the details of the
composition or behavior of the mesosphere
Concentric shell in deep in earth
• Outer core
–P wave velocities drop dramatically;
leads to P wave shadow zone
–S waves do not penetrate outer core;
leads to S wave shadow zone
–Infer that outer core is liquid, probably
composed of mixture of iron & nickel
Center of the earth
• Inner core
–P wave velocities increase significantly
–S waves (generated when P waves traveling
through outer core intersect inner core
boundary) travel through inner core
–Infer that outer core is solid mixture of
iron & nickel
Have two systems for characterizing
Earth’s interior
• Distinguish crust,
• Distinguish lithosphere,
mantle, & core on the asthenosphere,
basis of chemical
mesosphere, outer and
composition of rock
inner core on basis of
material behavior
• Outer layers more
silica-rich
• Have variations in
strength, ductility,
• Inner layers enriched
seismic wave speed,
in iron & magnesium
etc.
Earthquake locations - where do
earthquakes occur?
• Time intervals between the arrivals of
different components of a seismic disturbance
gives the distance from the earthquake focus
to a seismic recording station
• Measure the time (in seconds) between the
arrival of P & S components (S-P interval)
• S-P intervals from three or more recording
stations fix the location of earthquake focus
(hypocenter) or epicenter
Compilations of earthquake locations
indicate
• Earthquakes can occur anywhere on earth
• Most earthquakes occur in discrete zones
–In mid-ocean ridges, mainly beneath axial
valleys
–Along oceanic fracture zones, usually between
offset segments of axial valleys
–In Benioff-Wadati zones near deep-ocean
trenches
–Beneath active, modern mountain ranges
Compilations of earthquake locations
indicate
• Earthquakes usually within 100 km of surface
–Most of these shallow focus earthquakes
occur at depths <20 km
• Some earthquakes occur at depths >100 km
–Intermediate focus earthquakes occur at
depths of 100-300 km
–Deep focus earthquakes occur at depths of
300-680 km