Transcript ppt

Seismicity & Earthquakes
Questions
• 1) Where would you expect the higher
earthquake insurance rates: Florida or
Oregon?
• 2) Briefly explain.
• 3) Did you use inductive or deductive
reasoning ?
• 4) explain
What is an Earthquake?
• Vibration of the Earth
produced by the rapid
release of energy.
• Energy release due to
plate tectonics and failure
(fault zones) of the
Earth’s crust
• Energy radiates as waves
in all directions from the
focus (source)
• Epicenter – surface
expression of the focus
Elastic Rebound Theory
• Tectonic forces slowly deform the rock
• As the rock bends, it stores energy
• When the rock’s resistance to the stress is
overcome, the rock slips or breaks
• Slippage occurs at the weakest point (focus), rock
“snaps” back to original position
• Energy released produces the vibrations we know
as an earthquake
Seismogram
Seismic Waves
P-waves and S-waves are body waves, they travel
through the Earth’s interior. Surface waves travel
along the Earth’s outer layer.
P-waves – Primary Waves
• Compressional wave –
push and pull rock in
direction of movement
• Can travel through all
materials (solids,
liquids, and gases)
• Fastest moving wave
• Smallest wave
amplitude (lowest
energy)
S-waves – Secondary Waves
• Shear wave - Waves
shake material at right
angles to their direction
of movement
• TRAVEL ONLY
THROUGH SOLIDS
• Intermediate speed –
2nd to arrive as seismic
station
Surface waves
• Move up and down, similar
to ocean waves
• Can also move side-to-side.
• Very damaging waves
• Travel the slowest – last to
arrive at seismic station
• Greatest wave amplitude
(highest energy)
Locating Earthquakes
• Need three seismic stations.
• Use the time difference (tH) in the
arrival of the p-waves and the swaves.
• Distance (d) extrapolated from a
time-distance graph.
• A circle with a radius of d is drawn
around the seismic station.
• Earthquake occurred somewhere on
the circle.
• The intersection from three stations
shows the epicenter location.
Locations of Earthquakes
• By studying 1000s of earthquakes,
geologists learned most occur along plate
boundaries
Earthquake Intensity and Magnitude
• Mercalli intensity
scale
– Assesses the damage
from an earthquake at
a specific location
– Based upon human
observation
– Can vary from
location to location
Earthquake Intensity and Magnitude
• Richter Scale
– Describes the
earthquakes
magnitude (energy
released)
– Measures amplitude of
largest wave, adjusted
for travel time
– Is the same from
location to location
– A 1 digit increase in
Richter scale is a 30
fold increase in energy
released
Is the Earth Entirely Solid?
• What piece of the Earth is not solid?
• How do we know this?
– Seismic waves
– Different materials transmit waves at different speeds,
i.e. p-waves change speed as they move from the crust
to the mantle and from the mantle to outer core
Wave Movement Through
Homogeneous Earth
Movement through material of
different density
Velocity and Amplitude Are a
Function of the Material
Velocity Profile Through Earth
P-wave Shadow Zone
• Liquid causes p-waves to
bend (refract)
• Refraction of waves creates
a zone where no p-waves
are recorded (shadow zone)
• 103 to 143 degrees from
focus
S-wave Shadow
• S-waves cannot pass through liquid
• Waves refract at surface of outer core,
creating a zone where no s-waves are
recorded (shadow zone)
• 103 to 180 degrees from focus
Why did the 1895 EQ have a wider
radius of influence?
Earthquake Prediction
Time Interval Analysis
• Statistics is used
to generate a
“best-fit line”
which can be used
to predict the time
of the next
earthquake
• Not reliable
• This example has
not occurred
Seismic Gap
• Look at the location of earthquakes
• Areas (Gaps) where no earthquake has
happened is an area of accumulating strain
• Gaps represent locations of future
earthquakes
Seismic Gap Examples
• Alaska
California
Environmental Impacts
• Tsunami
– Giant tidal waves
created by earthquakes
– Move at speeds
between 500 and 950
km/hr (300 – 600
miles/hr)
– Waves reach height of
over 30 meters (100ft)
• Landslides and
ground subsidence
– Vibrations cause
unstable material to
slide down slopes
– Vibrations cause
stable material to turn
into fluid –
liquefaction