Earthquakes (chapter 11) and the earth's interior (chapter 12)
Earthquakes are episodes of ground shaking and transmission of shock
waves
through the earth's interior as a result of the release of stress when
there is slippage along a fault. They are important for several
reasons.
Most obviously, they pose a hazard to life and property, and geologists
and engineers are interested in learning how to predict them and how to
design structures to withstand them. They also tell us a lot about
tectonic
processes, because earthquakes are for the most part concentrated along
plate boundaries. We can learn a lot by looking at where they occur on
a map, and even by looking at how deep the earthquakes typically are in
a particular area of the globe. Perhaps of greatest significance for
our
study of the earth, earthquakes send out seismic waves that reverberate
back and forth in the earth's interior, and the pattern of
seismic
wave transmission is used much the way an x-ray of the human body is
used:
to provide a look at the earth's internal structure that reveals
features
we cannot see in any other way. Our discussion will concentrate first
on
some basic characteristics of earthquakes, how they are measured and
how
their locations are determined, and then on some aspects of seismology
that may be helpful in predicting earthquake risks. After this we will
look at the internal structure of the earth as revealed by available
seismic
data.
Chapter 11
Note: the CD-ROM that came with the textbook has some very useful
material
that will help here.
- What is an earthquake?
- Elastic rebound theory: stress and strain along a fault, followed
by
slip
and the release of stress in the form of seismic waves
- Definition of focus and epicenter
- Seismographs: what they are and how they work
- Types of seismic waves: P (compressional), S (shear), and surface
waves,
and their relative velocities
- Time-travel curves and their use in determining distance from the
epicenter;
how seismographs from three stations can be used to locate the epicenter
- Earthquake magnitude and the Richter scale; relative intensity of
ground
motion and energy released for each unit on the scale (multiply by 10
for
ground motion, multiply by 33 for energy)
- modified Mercalli intensity scale
- Relation between earthquake locations and plate tectonics
- Patterns of earthquake depth at different types of plate
boundaries
- Patterns of earthquake damage and types of hazards:
- collapse of buildings
- avalanches and landslides
- soil liquefaction and subsidence
- tsunamis
- Theory that may help in earthquake prediction:
- seismic gap theory, patterns of earthquake occurrence along
different
sections
of a fault, and the inverse relationship between earthquake magnitude
and
typical frequency or time interval between earthquakes
- blind thrust faults in southern California: a new pattern of
earthquake
occurrence (not in textbook; see bookmarks on this topic from online
syllabus.)
Chapter 12
- Paths of seismic waves in the earth's interior
- relationship between seismic-wave velocity and physical
properties of
rock
- velocity increases with density and with tighter packing of
the crystal
structure
- velocity decreases in layers that are close to melting and
that are
relatively
weak and plastic
- behavior of seismic waves at boundaries between rock layers
- waves are refracted or bent at boundaries where there is a
change in
seismic-wave
velocity
- S-waves disappear when they enter a layer that is molten
- the P-wave and S-wave shadow zones provide evidence of
physical changes at the core-mantle boundary
- Pattern of seismic-wave velocity with increasing depth in
different
layers
of the earth
- Layers of the solid earth:
- thickness and composition of the crust; differences between
continental
and oceanic crust
- the Mohorovicic discontinuity (Moho): boundary between crust
and mantle
- the upper mantle and the transition between lithosphere and
asthenosphere ("low-velocity zone")
- transition from upper to lower mantle
- changes in packing of minerals, phase changes (olivine to
spinel at ~410 km, spinel to perovskite at ~660 km) and increases in
seismic-wave velocity
with
increasing depth in the mantle
- evidence from seismic tomography showing structures
resembling
descending slabs of lithosphere extending almost to the core-mantle
boundary
- transition from mantle to core
- the D'' layer: a boundary with considerable relief and
variable physical properties
- molten outer core
- solid inner core
- inner core rotates indepdendent of the rest of the earth -
gains a lap about once every 400 years
- Heat flow from earth's interior
- processes: conduction (slow and relatively ineffective)
- convection (much faster and therefore the dominant process for
transmitting
heat from the interior)
- The earth's magnetic field and paleomagnetism
- magnetic field generated by currents in rotating outer core
composed of
liquid iron
- remanent magnetism as an indicator of the direction and
orientation of
earth's magnetic field at the time when a rock cooed below 500 degrees
C.