Download Earth`s interior volc eq2

How do scientists know the
structure of the Earth?
• Rock samples (direct evidence)
• Seismic waves (indirect evidence)
– Vibrations that travel through Earth
carrying energy released during an
earthquake.
– The speed and paths of waves reveal
the structure of the planet.
– We will learn more about these later.
Earthquakes
• Occur along faults
• Caused by stressed rock during plate
movement.
• This energy travels as seismic waves.
Earthquake Zones
• Along tectonic plate
boundaries where a
large number of
faults are located
• Example: San
Andreas Fault Zone
in California
Seismic Waves
• P waves: or primary waves
– Fastest, so they travel ahead of other waves
and are the first to be detected.
– Can travel through all media
• solid, liquid, AND gas!
Seismic Waves
• S Waves: or secondary waves
– Second-fastest waves
– Only travel in solids (think S = solid)
Seismic Waves
• Surface waves
– Move along the Earth’s surface (in the
upper few kilometers) of the earth’s
crust.
– Move the ground much like ocean
waves.
– Travel the slowest.
– The most destructive.
• S and P waves give an idea of the inside the Earth.
– P waves can travel in solid, liquid, and gas
• Crust - solid - YES!
• Mantle - solid / liquid - YES!
• Core (outer) - liquid - YES!
• Core (inner) - solid - YES!
– S waves can only travel in solids
• Crust - solid - YES!
• Mantle - solid / liquid - YES!
• Core (outer) - liquid - NO! Must be liquid!
• Core (inner) - solid - YES!
– Solid due to intense pressure
Earthquake Measurement
• Seismologist - a person who studies earthquakes
• Seismographs - instruments that record seismic waves
• Seismogram - the “picture” that is created by the
seismograph
Determining Time and
Location
• Seismologists look at the seismogram to
note the difference in arrival times of P
waves and S waves
• Seismographs also help find the epicenter
and focus
Measuring Strength and Intensity
• Charles Richter created the Richter scale in the
1930’s.
• Magnitude - a measure of the strength / energy
of an earthquake
– Each unit on the Richter scale represents motion that is
10 times larger than the previous unit.
• A magnitude of 5 is ten times stronger than a 4.
• A magnitude of 6 is 100 times stronger than a 4.
Modified Mercalli Intensity
Scale
• Intensity - a measure of the degree it is felt by
people and the amount of damage caused
• Uses Roman numerals I to XII
• I = not felt, XII = total damage of an area
• The number changes depending on where you
are (highest number is close to the epicenter).
Earthquake Forecasting
• The Gap hypothesis
– Sections of active faults that have had
few earthquakes are likely to be sites of
strong earthquakes in the future.
– These sites are called seismic gaps
Where Volcanoes Form
• Over plate boundaries
– Ex. Ring of Fire surrounding the Pacific
Ocean (contains 75% of active
volcanoes on land)
– 80% (on land) where plates collide
– 15% (on land) where plates separate