Earthquakes & Tsunamis
Speed of Plate movement….where
are they the fastest?
Where are the earthquakes? How does
depth compare to boundary types?
Earthquakes
• Sudden releases of energy through
the earth’s crust
• The point within the crust where
the quake originated is called the
focus.
• The point on the surface above the
focus is the epicenter
• Measured in three ways:
• Richter Scale
• Moment magnitude Scale
• Mercalli Scale
Earthquakes
• Richter Scale
• Based on the logarithm of
the amplitude of waves on
a seismograph
• Logarithmic scale:
• Each number on the scale
is 31.7 times more
powerful
Earthquakes
• Moment Magnitude Scale
• Based on physical properties of the
earthquake
– Area of fault affected
– Distance the earth moved
Not based on any seismic readings.
Can more accurately measure large
earthquakes.
Earthquakes
• Mercalli Scale
• Measures the
intensity based
on observations
and damage.
• Useful in areas
where data was
not recorded.
• Used worldwide
Earthquakes
• Occur at plate boundaries
• Type of boundary will
determine the fault type
• Three main types:
• Normal
• Thrust/Reverse
• Slip-strike
Earthquakes at Faults
Normal faults
• Land falls below normal
• Occur frequently at divergent boundaries
• Result in weak, shallow earthquakes
Reverse/Thrust faults
• Land is lifted above normal
• Occur frequently at convergent boundaries
• Cause strong, deep earthquakes
Slip-strike faults
• Land is shifted horizontally
• Occur at transform boundaries
• Cause moderate, shallow earthquakes
Hollister, CA: Aseismic creep
Seismic Waves
• Seismic Waves are waves of energy that
travel through the earth.
– Body waves: travel through the interior
– Surface waves: travel along the crust
Seismic Waves
• P waves
• P waves, or Primary
waves, are body waves
that travel through the
earth with compression.
• They are detected first in
seismographs
• A drop of water spreads
using compression.
Seismic Waves
• S waves, or secondary
waves, are body waves that
travel through the earth
using shearing
• Shearing means to stretch
something side to side
• They are the second waves
detected by seismographs
Seismic Waves
• Surface waves only travel along the
earth’s crust, not through the interior.
• They move the earth up and down
• They are only felt locally
Seismic Waves
• Seismic waves tell us
about the interior of the
earth.
• First, as waves pass
through different layers,
they will change speed
and bend
• Also, S waves cannot
travel through liquids. S
waves show us where
the boundary for the
liquid outer core is.
Earthquake links
• http://earthquake.usgs.gov/earthquakes/re
centeqsus/
Tsunamis
• A tsunami is a wave with a huge wavelength,
often several hundreds of miles long.
• Where surface wave’s particle rotation only
reaches a few meters below the surface, the
tsunami’s wave rotation can be felt as deep as
the bottom of the ocean.
Tsunamis
• Tsunamis originate at earthquake epicenters at plate
boundaries.
• Earthquakes result in the shifting of plate boundaries.
• When this occurs under the ocean, the plate and the
ocean have to move
Tsunamis
• A Tsunami is created
from an area of ocean
bottom being suddenly
lifted up or sinking.
• This will displace the
water above it, creating
the tsunami.
Tsunamis
As the tsunami nears land, the beginning
trough will be seen as water retreating
from the shoreline and heading out to sea.
After that, the water will begin to rise until
the wave crest arrives.
A tsunami acts more like a giant wall of
rising water than a wave.
Tsunamis
• There is a tsunami detection system in
place in the Pacific Ocean.
• It saves millions of lives every year.
• http://www.education.noaa.gov/Ocean_an
d_Coasts/Tsunami.html