Transcript Earthquakes

Earthquakes
Focus
• The point beneath Earth’s surface
where rock breaks under stress and
causes an earthquake
Epicenter
• The point on Earth’s surface directly
above an earthquake’s focus
Seismic Waves
Seismic wave generation
• Seismic waves are released when rocks that
are under pressure bend and then break,
releasing energy
• The point at which the rock breaks is called
shear strengh
• Elastic rebound - When the rocks break and
go back to their original shape releasing
energy
Seismic Waves
• A vibration that travels through
Earth carrying the energy released
during an earthquake
What determines how much the
ground shakes during an
earthquake?
• How close a location is to the
epicenter
• The types of rock and soil
surrounding the epicenter determines
how much the ground shakes
What are the different
kinds of seismic waves?
• There are three types of seismic
waves:
• Primary Waves (P waves)
• Secondary Waves (S waves)
• Surface Waves
• An earthquake sends out two types of
waves P waves and S waves
• When the waves reach Earth’s
surface at the epicenter, surface
waves develop.
Primary Waves
Primary Waves (P Waves)
• A type of seismic wave that
compresses and expands the ground
• The first wave to arrive at a
seismomoter
P wave movement
• P waves compress and expand like an
accordion
• When P waves arrive they vibrate the
particles of the crust forward and
back along the path of the wave
Secondary Waves
Secondary Waves (S Waves)
• A type of seismic wave that moves
the ground side to side
How are P waves different
from S Waves?
• P waves compress and expand the ground
• These waves cause buildings to contract and expand
• P waves travel through solids and liquids
• S waves vibrate from side to side
• These waves vibrate from side to side and thrust the
ground up and down, or back and forth
• S waves can not move through liquids
Surface Waves
Surface Waves
• A type of seismic wave that forms
when P waves and S waves reach
Earth’s surface
• Surface (L-waves) cause the most
damage
Detecting Seismic Waves
Seismograph
• A device that records ground
movements caused by seismic waves
as they move through Earth
Seismometers Old School vs.
New School
Measuring Earthquakes
The Mercalli Scale
Mercalli Scale
• A scale that rates earthquakes
according to their intensity and how
much damage they cause
Magnitude
• The measurement of an earthquake’s
strength based on seismic waves and
movement along faults
The Richter Scale
Richter Scale
• A scale that rates seismic waves as
measured by a particular type of
mechanical seismograph
How are the Mercalli scale and
the Richter scale similar? How
are they different?
• Both measure the strength of an
earthquake
• The Mercalli scale measures the strength
in terms of extent people notice the
earthquake and the amount of damage
caused
• The Richter scale measures the size of
seismic waves
Locating the Epicenter
P-S interval
How do scientists calculate how
far a location is from the
epicenter of an earthquake?
• Scientists calculate the difference
between arrival times of the P waves
and S waves
• The further away an earthquake is,
the greater the time between the
arrival of the P waves and the S
waves
Steps to finding an Earthquakes
epicenter
• Step 1: Calculate the S-P interval
• This is how much time is between the 1st Pwave and the 1st S-wave
Step 2- determine distance to
epicenter using a time-travel
graph
Step 3- map circumference
showing where EQ could
• To do this, find the distance to the epicenter
and draw a circle at that distance around the
seismograph location using the scale on
your map.
The point where all three
circles intersect is the
epicenter
• Houston
• 800 Km
• Savannah
• 900 km
Direct vs. Indirect Damage
• Direct Hazards- damage caused by the earth
shaking
Collapse of buildings, broken power lines,
pipelines, roads and bridges
• Indirect Hazards- a side-effect of the ground
shaking
– Fire, landslides, tsunami
Tsunami
• Travel at great speeds which make
warnings difficult (1000km/hr)
• Not very high waves on the ocean,
only a m, but can be more than 30 m
high when crashing on shore
What effects the amount of
damage caused during an
earthquake?
• Duration of Shaking
• Direction of Motion
• Underlying Earth Materials
– Loose, wet soil will sustain more damage than
bedrock
Designing Earthquake Resistant
Structures