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

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
All earthquakes occur in
lithosphere
•Intermediate & deep focus earthquakes
occur where slabs of lithosphere extend
to depth
•Recognize lithosphere by its relatively
high seismic velocity & the relatively
low attenuation of seismic waves
Seismic data indicate
• Lithosphere is split into several plates
– Plates are 60-100 km thick but may be 100’s or
1000’s of km across
– Plates are composed of oceanic crust, continental
crust, or combinations of oceanic & continental crust
– To fit on curved earth’s surface, lithospheric plates
have 3D shape that resembles a dinner plate
• Locate plate boundaries by occurrence of
earthquakes
– Other geological phenomena, such as volcanic activity
and mountain building, occur along plate boundaries
Earthquake first-motion studies
• Seismogram may indicate that a
rock mass initially moved toward
or away from a recording station
• If first-motion is toward
recording station, leading edge
of P wave is a region of
compressed rock
• If first-motion is away from
recording station, leading edge
of P wave is a region of dilated
rock
• Every fault slip event generates
a systematic pattern of
compressions and dilations
D
C
C
D
C
D
D
C
Each pattern of
compressions and
dilations is consistent
with fault slip on one
of two mutually
perpendicular faults
C
D
D
C
First motion studies indicate
•Plates move away from each other at MOR
ridge crest/axial valleys
–Creates new lithosphere
•Plates move past each other in oceanic FZs &
across continental transcurrent faults
–Lithosphere neither created nor destroyed
•Plates move toward each other at DOTs and in
active mountain belts
–Lithosphere consumed by subduction
Create new lithosphere at MOR
•Plate divergence causes mantle upwelling, which
promotes mantle melting by reducing pressure
•Melt migrates toward surface, & solidifies to
form oceanic crust
•Generates layered oceanic crust, with residual
mantle at base, gabbroic layer = intrusive
magma chambers, sheeted dike complex =
feeders of extrusive material, & basaltic layer
= extrusive carapace
•Oceanic crust cooled by infiltrated sea water
•Divergent plate boundaries are constructive
plate boundaries
Subduction
•Plate convergence causes slab of oceanic
lithosphere to subduct, i.e. bend & descend
into the underlying asthenosphere & eventually
the mesosphere beneath another plate
•Overriding lithosphere may be oceanic or
continental
•Subducted oceanic lithosphere is heated &
subjected to great pressure, i.e.
metamorphosed
Subduction II
•Metamorphism of subducted slab causes a
dramatic increase in its density, enhancing its
tendency to sink
•Metamorphism releases water from subducted
slab; water helps both the slab & the overlying
asthenosphere to melt, causing volcanism seen
on landward side of DOT
•Bending, shortening, shearing, & compression in
subducted slab causes seismicity unlike that
seen elsewhere in earth - intermediate & deep
focus earthquakes are not like shallow
earthquakes
Plate tectonics theory
• Geometrically precise description of relative motion
of lithospheric plates
– Data from Very Long Baseline Array, satellite telemetry, & GPS
surveys confirm earth movements consistent with the predictions of
plate tectonic theory
• Provides rational explanation for the volcanism,
plutonism, metamorphism, deformation, & seismicity
observed on earth
– Processes concentrated at plate boundaries
– Successfully predicts the relative likelihood of seismic activity,
volcanism
– Understand the formation of mountain belts through Wilson cycle;
now understand the geology of continents
• Plate movement is a process by which heat escapes
from earth's interior