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Unit 2
Earthquakes
The movement of tectonic plates creates powerful forces
that squeeze or pull the rock in the crust.
These forces are examples of stress, a force that acts on
rock to change its shape or volume.
The movement of rock beneath Earth’s surface results in the
shaking and trembling known as an earthquake.
Types of Stress
A fault is a break in Earth’s crust where slabs of crust slip
past each other.
Faults usually occur along plate boundaries, where the
forces of plate motion compress, pull, or shear the crust so
much that the crust breaks.
Types of Faults
Normal faults occur at divergent boundaries.
They are caused by tension and are at an
angle. The half above is called the hanging
wall. The half below is called the footwall.
Reverse faults occur at convergent
boundaries and are caused by compression.
They have the same structure as normal
faults, but the blocks move in opposite
directions.
Strike-slip faults occur at transform
boundaries and are caused by shearing.
They occur when the rocks slip past each
other sideways with little up-or-down motion.
fault-block mountain - forms when normal faults uplift a
block of rock
folds - bends in rock that form when compression shortens
and thickens parts of Earth’s crust
anticline - a fold in rock that bends upward into an arch
syncline - a fold in rock that bends downward into to form
(sink)
a bowl
plateau - large area of flat land elevated high above sea level
Some plateaus form when vertical faults push up a large,
flat block of rock.
To know where an earthquake was centered, you need to
know where it began.
Most earthquakes begin in the lithosphere, within 100 km of
Earth’s surface.
focus - the point beneath the surface where rock that is
under stress breaks, triggering an earthquake
epicenter - the point on the
surface directly
above the focus
seismic waves - carry the energy of an earthquake away
from the focus, through Earth’s interior, and
across the surface
The energy is greatest at the epicenter.
seismograph - instrument used to record ground movement
caused by seismic waves as they move
through the Earth
Types of Waves
1. primary waves (P waves)
• first waves to arrive
• compress and expand like an accordion
• cause buildings to contract and expand
• can travel through both solids and liquids
Types of Waves
2. secondary waves (S waves)
• second waves to arrive
• vibrate from side to side as well as up and down
• shake the ground back and forth
• shake structures violently
• can only travel through solids
Types of Waves
3. surface waves
• form when P waves and S waves reach the surface
• some make the ground roll like ocean waves
• some shake buildings from side to side
• move more slowly than P waves and S waves, but
produce the most severe ground movement
How Earthquakes are Measured
The Richter Scale
• a rating of the size of seismic waves as measured by a
particular type of mechanical seismograph
• provides accurate measurements for small, nearby
earthquakes, but it does not work well for large or distant
earthquakes
How Earthquakes are Measured
The Mercalli Scale
• developed to rate earthquakes according to their intensity
• uses a 12-step scale to describe how earthquakes affect
people, buildings, and the land surface
• not a precise measurement
How Earthquakes are Measured
The Moment Magnitude Scale
• a rating system that estimates the total energy released by
an earthquake
• can be used to rate earthquakes of all sizes, near or far
• this is the current system used by geologists
When seismic waves move from hard dense rock to loosely
packed soil, they transmit their energy to the soil. Loose soil
shakes more violently than the surrounding rock.
A house built on solid rock will shake less than a house built
on sandy soil.
liquefaction - occurs when an earthquake’s violent shaking
suddenly turns loose, soft soil into liquid mud
likely where soil is full of moisture
causes buildings to sink and pull apart
can trigger landslides
aftershock - an earthquake that occurs after a larger
earthquake in the same area
(could strike hours, days, or even months later)
tsunamis - large waves formed by displaced water from an
earthquake that occurs on the ocean floor
fixed-base building - tilts and cracks during an earthquake
base-isolated building - designed to reduce the amount of
energy that reaches the building
during an earthquake
To observe changes in ground movement, geologists have
put in place instruments that measure stress and
deformation in the crust.
creep meters - use a wire stretched across a fault to
measure horizontal movement of the ground
laser-ranging devices • use a laser beam to detect even tiny fault movements
• calculate any change in the time needed for the laser
beam to travel to a reflector and bounce back
tiltmeters - measure tilting of the ground
satellite monitors - bounce radio waves off the ground and
record the echoes
The time it takes for the radio waves to make their round trip
provides precise measurements of the distance to the
ground.