Earthquakes and the Earth’s Interior
San Francisco 1906
Magnitude 7.8
Charleston 1886
California’s Notorious San Andreas Fault
fault trace
Earthquakes are the release of energy stored in rocks.
Most earthquakes are associated with faults.
The focal point is place of the first release of energy.
The epicenter is the point on the ground immediately
above the focal point. Locate a quake by specifying
lat. and long. of epicenter and depth to focal point
Energy travels outward from the focal point in
a series of spherical waves fronts.
Earthquakes are explained using
the elastic-rebound theory.
Elastic-Rebound Theory I
Energy is stored over a long period of time in rocks
undergoing elastic deformation along a fault plane.
No movement along fault, however, until frictional
resistance (Fr) along fault is overcome.
Elastic-Rebound Theory II
Sudden movement when Fr is overcome.
Stored energy released instantaneously
as seismic waves propagating
through the earth.
Slippage along fault after earthquake
Energy traveling
outward from the
focal point can be
detected around
the world by
recording
instruments
known as
seismographs.
Energy travels outward from the focal point as a series
of seismic waves. The waves are elastic disturbances.
The waves of differing energies are known as
Primary, Secondary and Surface waves
Characteristics of Seismic Waves
For each type of seismic wave you should know:
• the name and symbol
• the type of motion
• the approximate velocity
• the material it is propagated through
• the shadow zone (if applicable)
Primary Waves
• the name and symbol = primary or P-wave
• the type of motion = compressional body wave
change volume, not shape
• approximate velocity = 8-13 km/sec
480-780 km/min
28,800-46,800 km/hr
35,000 km/hr ≈ 22,000 mph
• travels through = solids, liquids and gases
• shadow zone =
direct arrivals 0°-105°
no direct arrivals 105°-140°
refracted arrivals 140°-180°
Motion of P-Waves
alternating compactions and rarefactions
A propagating wave is an elastic disturbance.
The material returns to its original shape
after the waves has passed through it.
Shadow Zone for P-waves
arrivals and their interpretation
Secondary Waves
• the name and symbol = secondary or S-wave
• the type of motion = shear waves
change shape, not volume
• approximate velocity = 5-7 km/sec
300-420 km/min
18,000-25,200 km/hr
20,000 km/hr = 12,428 mph
• travels through = travel only in solids
• shadow zone =
direct arrivals 0°-105°
no direct arrivals 105°-180°
Motion of S-Waves
particle motion is at right angle to wave motion
A propagating wave is an elastic disturbance.
The material returns to its original shape
after the waves has passed through it.
Shadow Zone for S-waves
arrivals and their interpretation
Surface Waves
• the name and symbol = surface or L-wave
• the type of motion = pitching and rolling
causes motion felt during quake (and damage)
• approximate velocity = 4-5 km/sec
240-300 km/min
14,400-18,000 km/hr
15,000 km/hr ≈ 9,000 mph
• travels through = travel in solids and liquids
earthquakes and tidal waves (tsunamis)!
Motion of L-Waves
tsunamis in the open ocean and shallow water
518 mph
211 mph
31 mph
Note how the tidal waves slow down, bunch up
and grow taller in shallow water. Does this
remind you of anything else we’ve studied?
Energy distributed through entire depth of
water column (unlike wind waves).
Seismic Sea Wave Associated with 1964 Alaska Earthquake
Distribution of Earthquakes
shallow quakes = focal point < 100 km
deep quakes = focal point > 100 km
NB: I will use “deep” to include both intermediate and deep on this diagram.
The magnitude and frequency of earthquakes are inversely
related. There are 1 or 2 earthquakes a year with a
Richter magnitude of 8 or higher. There are
approximately 1,000,000 earthquakes per
year with a magnitude of 3 or less.
Earth’s Interior
Known mostly from indirect
evidence, especially the
behavior of seismic waves.
Method is simple
in theory, but
complex in
application.
homogenous interior
ugly reality!
velocity increases with depth
B
decrease in speed
at greater depth
Variation in Seismic Velocity
within the earth
C
A
increase in speed
at shallow depth
D
The marked increase in speed at varying
depths beneath oceans and continents is
called theMohorovicic discontinuity (or Moho).
The Moho is taken to indicate the base of the crust.
It marks the crust-mantle boundary, which
is a change in chemical composition.
Continental Crust
• ≈ 35 km thick
• granitic, rich in Si, Al, K
• average density 2.8 g/cc
Oceanic Crust
• ≈ 7-10 km thick
• basaltic, rich in Si, Mg, Fe
• average density 3.0 g/cc
Taken together oceanic and continental crust
make up 1.4% of Earth’s volume and 0.7%
of its mass.
beneath the Moho is the mantle
• extends down to 2900 km
• ≈ chemical composition
• rich in Fe and Mg
(ultramafic)
• mineral and chemical
composition ≈ peridotite
• ≈ 50% Earth’s radius
≈ 83% Earth’s volume
≈ 67% Earth’s mass
The mantle contains the LVZ
at a depth of ≈ 100-200 km.
The asthenosphere lies
entirely within the
upper mantle.
beneath the mantle is the core
• extends from 2900 km to
the center of the earth
(6371 km)
• roughly similar chemical
composition throughout
• composed almost entirely
of Fe
• liquid outer core
solid inner core
• ≈ 50% Earth’s radius
≈ 16% Earth’s volume
≈ 32% Earth’s mass
Compositional layering: crust, mantle, core
Geothermal
Gradient
Melting T of
peridotite
Melting T of
iron
NB: melting T
increases with
increasing P
Behavioral Layering
Lithosphere
• solid, brittle slab
• ≈ 100 km thick
• crust and mantle
includes the Moho
• earthquakes!
Asthenosphere
• solid, nonbrittle
• from 100-660 km
• mantle material only
• includes LVZ
from 100-200 km
• no earthquakes!
Layering
Compositional
crust
mantle
core
Behavioral
lithosphere
asthenosphere