Chapter 10 Earthquakes
10.2 What causes Earthquakes?
An earthquake is a vibration caused by sudden breaking or frictional sliding of rock in the Earth.
Seismicity is earthquake activity.
The point within the Earth at which rock starts to rupture and slip on a fault, is called the hypocenter, or
focus, of an earthquake. From this point the energy, in form of vibration, begins to propagate. We call the
point on the surface of the Earth directly above the focus, earth’s epicenter.
Faults in the crust
Sometimes a fault cuts through a marker, such a distinct sedimentary bed
or a fence. The marker on one side of the fault is no longer adjacent to the
marker on the other side. The distance between the two ends of the
marker is called the fault’s displacement. When faults do not cut the
surface, we call them blind faults. (They may become visible if exposed
by erosion). Faults that do intersect the ground surface, sometimes
produce a step called fault scarp. We refer to the ground surface
exposure of a fault (also when it was exposed by erosion) as a fault trace.
The rock mass above a sloping fault is called the hanging wall, and the
mass below the fault is called the footwall. A fault whose hanging wall
slipped down the slope of the fault, is a normal fault, and a fault whose
hanging wall slipped up the slope is a reverse fault (if steep) or a thrust
fault (if shallowly sloping. A strike-slip fault is near vertical fracture on
which slip occurs parallel to an imaginary horizontal line called a strike
line, on the fault plane. *Note: In reality major faults commonly consist
of a zone including several smaller faults. Faults that have moved
recently or are likely to move in the near future are called active faults.
Most active faults occur along plate boundaries or in currently
developing collision zones (reversed fault) and rifts (normal fault).
Generating Earthquake energy: Stick slip
Slip along a new fault
Stress is a push, pull or shear. If you apply stress to a rock, it bends. If you stop applying stress before is
breaks, it returns into its original shape. We call such phenomenon elastic behavior. The change in shape
due to elastic bending, stretching or shortening is called elastic strain. If you bend the rock further, a
number of small cracks or breaks start to form in the rock, typically in diagonally zone. Eventually the
cracks connect to one another to form a fracture that cuts across the entire block of rock. The rock start so
slide. When this happens, any elastic strain that had built up in the rock gets released so the rock
straightens out. A new fault can’t slip forever. Due to friction, the force that resists sliding on a surface
existing because surfaces are not perfectly smooth, slows and stops its movement.
Slip along a pre-existing fault
This is a reactivation of sliding on pre-existing faults. Stress builds up I a rock until its sufficient to
overcome friction, by breaking off asperities or by having asperities plow a groove into the opposing fault
surface (oneffenheden “ploegen” een gleuf in de tegenovergestelde laag).The breaking or plowing of
, asperities causes the relaease of energy and therefore causes earthquakes. We refer to a stick-slip
behavior when there is an alternation between stress buildup and slip events.
We see that earthquakes happen because stresses build up, causing rock adjacent to the fault to develop
elastic strain until either intact rock breaks or a preexisting fault reactivates. When the movement takes
place, the once-bent rock adjacent to the fault goes back to their original unbent shape thereby relieving
the elastic strain. Geologists refer to this overall concept as the elastic rebound theory.
Foreshocks and aftershocks
A major earthquake, or mainshock, along a fault may be
preceded by smaller ones called foreshocks. These result
from the development of the smaller cracks in what later
becomes the major rupture. Smaller earthquakes called
aftershocks, follow a major earthquake (at least 10 times
smaller). Their distribution defines the overall area of the
fault that slipped during the mainshock. Aftershocks happen
because slip during the mainshock does not leave the fault in
a perfectly stable configuration. The sudden release of
energy resulting from the fracturing of rocks relieves much
of the stress at the earthquake’s focus; however, much of this
energy is transferred to nearby rock. This transference either
creates stresses where none existed before or increases the
stress within or between rocks. When the sudden buildup of
stress is great enough to fracture these rocks, thereby relieving the stress between them, a series of smaller
tremors are produced. In any cluster of earthquakes, the one with the largest magnitude is called the
mainshock; anything before it is called a foreshock and anything after it is called an aftershock.
Sizes and amount of slip on faults
Faults differ in size. Very big ones are at convergent-plate boundaries. They are thrust faults that extend
for the entire length of the plate boundary.
Can faults slip without earthquakes?
When material breaks along fractures or cracks and separates into pieces, we say that it has undergone
brittle deformation. Under certain conditions, a rock flows very slowly without breaking. We call such
behavior plastic deformation. Plastic deformation changes the shape of rocks an accommodates crustal
movements, therefore it does not cause earthquakes. Because the temperature of the Earth increases with
depth, most brittle deformation and therefore earthquakes generating faulting in continental crust
generally occur only in the upper 15 to 20 km of the crust. At greater depths shear and movement can take
place by plastic deformation without generating earthquakes. When movement on a fault in the crust
happens without generating earthquakes, we call the movement fault creep. It occurs in particularly weak
rock or when the fault surface has a coating of soft clay.
10.3 Seismic waves and their measurements
Earthquake energy travels through rock and sediment in the form of vibrations. We call these seismic
waves or earthquake waves. Body waves pass through the interior of the earth, whereas surface waves
travel along the Earth’s surface. The amplitude decreases when depth increases. Waves that cause
particles of material to move back and forth parallel to the directions in which the wave itself moves are
10.2 What causes Earthquakes?
An earthquake is a vibration caused by sudden breaking or frictional sliding of rock in the Earth.
Seismicity is earthquake activity.
The point within the Earth at which rock starts to rupture and slip on a fault, is called the hypocenter, or
focus, of an earthquake. From this point the energy, in form of vibration, begins to propagate. We call the
point on the surface of the Earth directly above the focus, earth’s epicenter.
Faults in the crust
Sometimes a fault cuts through a marker, such a distinct sedimentary bed
or a fence. The marker on one side of the fault is no longer adjacent to the
marker on the other side. The distance between the two ends of the
marker is called the fault’s displacement. When faults do not cut the
surface, we call them blind faults. (They may become visible if exposed
by erosion). Faults that do intersect the ground surface, sometimes
produce a step called fault scarp. We refer to the ground surface
exposure of a fault (also when it was exposed by erosion) as a fault trace.
The rock mass above a sloping fault is called the hanging wall, and the
mass below the fault is called the footwall. A fault whose hanging wall
slipped down the slope of the fault, is a normal fault, and a fault whose
hanging wall slipped up the slope is a reverse fault (if steep) or a thrust
fault (if shallowly sloping. A strike-slip fault is near vertical fracture on
which slip occurs parallel to an imaginary horizontal line called a strike
line, on the fault plane. *Note: In reality major faults commonly consist
of a zone including several smaller faults. Faults that have moved
recently or are likely to move in the near future are called active faults.
Most active faults occur along plate boundaries or in currently
developing collision zones (reversed fault) and rifts (normal fault).
Generating Earthquake energy: Stick slip
Slip along a new fault
Stress is a push, pull or shear. If you apply stress to a rock, it bends. If you stop applying stress before is
breaks, it returns into its original shape. We call such phenomenon elastic behavior. The change in shape
due to elastic bending, stretching or shortening is called elastic strain. If you bend the rock further, a
number of small cracks or breaks start to form in the rock, typically in diagonally zone. Eventually the
cracks connect to one another to form a fracture that cuts across the entire block of rock. The rock start so
slide. When this happens, any elastic strain that had built up in the rock gets released so the rock
straightens out. A new fault can’t slip forever. Due to friction, the force that resists sliding on a surface
existing because surfaces are not perfectly smooth, slows and stops its movement.
Slip along a pre-existing fault
This is a reactivation of sliding on pre-existing faults. Stress builds up I a rock until its sufficient to
overcome friction, by breaking off asperities or by having asperities plow a groove into the opposing fault
surface (oneffenheden “ploegen” een gleuf in de tegenovergestelde laag).The breaking or plowing of
, asperities causes the relaease of energy and therefore causes earthquakes. We refer to a stick-slip
behavior when there is an alternation between stress buildup and slip events.
We see that earthquakes happen because stresses build up, causing rock adjacent to the fault to develop
elastic strain until either intact rock breaks or a preexisting fault reactivates. When the movement takes
place, the once-bent rock adjacent to the fault goes back to their original unbent shape thereby relieving
the elastic strain. Geologists refer to this overall concept as the elastic rebound theory.
Foreshocks and aftershocks
A major earthquake, or mainshock, along a fault may be
preceded by smaller ones called foreshocks. These result
from the development of the smaller cracks in what later
becomes the major rupture. Smaller earthquakes called
aftershocks, follow a major earthquake (at least 10 times
smaller). Their distribution defines the overall area of the
fault that slipped during the mainshock. Aftershocks happen
because slip during the mainshock does not leave the fault in
a perfectly stable configuration. The sudden release of
energy resulting from the fracturing of rocks relieves much
of the stress at the earthquake’s focus; however, much of this
energy is transferred to nearby rock. This transference either
creates stresses where none existed before or increases the
stress within or between rocks. When the sudden buildup of
stress is great enough to fracture these rocks, thereby relieving the stress between them, a series of smaller
tremors are produced. In any cluster of earthquakes, the one with the largest magnitude is called the
mainshock; anything before it is called a foreshock and anything after it is called an aftershock.
Sizes and amount of slip on faults
Faults differ in size. Very big ones are at convergent-plate boundaries. They are thrust faults that extend
for the entire length of the plate boundary.
Can faults slip without earthquakes?
When material breaks along fractures or cracks and separates into pieces, we say that it has undergone
brittle deformation. Under certain conditions, a rock flows very slowly without breaking. We call such
behavior plastic deformation. Plastic deformation changes the shape of rocks an accommodates crustal
movements, therefore it does not cause earthquakes. Because the temperature of the Earth increases with
depth, most brittle deformation and therefore earthquakes generating faulting in continental crust
generally occur only in the upper 15 to 20 km of the crust. At greater depths shear and movement can take
place by plastic deformation without generating earthquakes. When movement on a fault in the crust
happens without generating earthquakes, we call the movement fault creep. It occurs in particularly weak
rock or when the fault surface has a coating of soft clay.
10.3 Seismic waves and their measurements
Earthquake energy travels through rock and sediment in the form of vibrations. We call these seismic
waves or earthquake waves. Body waves pass through the interior of the earth, whereas surface waves
travel along the Earth’s surface. The amplitude decreases when depth increases. Waves that cause
particles of material to move back and forth parallel to the directions in which the wave itself moves are
