Download Earthquake Notes

Magnitudes of Earthquakes
• There are two main methods of measuring the
strength of an earthquake.
• Mercalli Scale: This scale is based on what people
felt, what objects moved, and how much damage
was done by the Earthquake.
• Richter Scale: This scale is based on the
equivalent kg of explosives needed to create the
same effect.
– This scale goes up by an order of ~10 every time.
• 1=56kg, 2=560kg, 3=5600kg,
• 9=56000kg, 10=560,000kg
Earthquake Waves
• By determining the time of arrival for both the P
and S waves at a location we can determine how
far away the Earthquakes epicenter from the
seismograph.
• This does not tell us what direction the waves are
coming from though.
Locating the Epicenter of an Earthquake
Part I: Finding the distance to the epicenter.
• Step 1:
– Determine the difference in arrival time for
your P-wave, and your S-wave.
Locating the Epicenter of an Earthquake
Part I: Finding the
distance to the
epicenter.
Step 2:
– Take out your ESRT.
Open to the Travel Time
Graph on page 11.
• Step 3:
– Use the vertical scale
(time) to mark off the
difference in arrival time
on a scrap sheet of paper.
Locating the Epicenter of an Earthquake
Part I: Finding the
distance to the
epicenter.
• Step 4:
– Make sure to keep your
scrap paper vertical!
Slide it along the curves
until you find the place
where your marks each
line up on one of the
curves.
Locating the Epicenter of an Earthquake
Part I: Finding the
distance to the epicenter.
• Step 5:
– Read off the distance from
the horizontal axis that
corresponds to this spot.
This is the distance between
the epicenter and your
seismograph location.
Locating the Epicenter of an Earthquake
Part I: Finding the distance to the epicenter.
• Step 6:
– Repeat steps 1-5 for at least 2 other locations.
– It is necessary to have at least 3 stations, if you
do not you can not be sure of the exact location
of the epicenter.
Locating the Epicenter of an Earthquake
Part II Locating the Epicenter
• Step 1:
– Find the location of your first seismographic
station on the map.
• Step 2:
– Use a compass or string to create a circle with
its center at your seismograph location, and a
radius equal to the distance you found.
Locating the Epicenter of an Earthquake
Part II: Locating the Epicenter
• Repeat this procedure around two other
seismographic stations.
• Where the three circles all intersect (cross) is
where your epicenter is located.
• If the circles do not all intersect, but form a small
triangle, the epicenter is the center of the triangle.
Locating the Epicenter
Locating the Epicenter of an Earthquake
Part III: Origin time of the Earthquake
• From the distances determined in Part I,
determine how long it would take a P wave
to travel that distance.
Origin Time of Earthquake
• Step 1:
• Find the distance on the
horizontal axis.
• Go up to the point where
you hit the P-wave travel
line.
• Go over to the vertical
axis and read off the
time.
Origin Time of Earthquake
• Step 2:
– Taking the time found in step 1, subtract this
from the arrival of the P wave and that is the
original time of the earthquake.
– (3:21pm)-(7 min 40 seconds)
Uses of Earthquake Waves
• We now know…(or should know) that Pwaves travel faster then S-waves.
• We also know that P-waves travel through
solids and liquids, and S-waves travel only
through solids.
• Scientist have found that as energy in waves
passes through material it can bend,
depending on the density of the material.
Bending Waves:
• Have you ever looked into a fish tank and
seen two images of the same thing?
• This is an example of waves of light being
bent.
• When P-waves change from traveling
through a liquid to a solid they also bend.
• This bending forces them to change
direction slightly.
The Effects of Bent waves.
What this means to seismograph
Stations
• Since S-waves can not travel through
liquids, they STOP when they hit a liquid
layer.
• Since P-waves slow down when they enter a
liquid, they bend.
• This creates a “Shadow Zone”, an area
where no waves from an earthquake are
picked up by a seismograph.
Wave Behavior
• These shadow zones occur at a distance of
105-140 degrees (360 degrees in a circle) from
the Epicenter of the quake.
• Between 140 degrees in each direction only Pwaves were seen.
• These wave behaviors let scientist know what
phase of matter the material in the earth, as
well as its composition.
Into the Earth
• This is what allows scientist to infer what
the interior of the Earth is like.
• Although the Earth appears to be made up
of solid rock, it’s actually made up of three
distinct layers
• The three main layers are the crust, the
mantle and the core.
Crust
• The crust is the thin, solid,
outermost layer of the
Earth.
• The crust is thinnest
beneath the oceans,
averaging only 5 kms
thick.
• It is thickest beneath large
mountain ranges.
• Continental crust varies in
thickness, but averages
about 30-35 km.
• Beneath large mountain
ranges, (Himalayas or the
Sierra Nevada), the crust
reaches a thickness of up
to 100 km.
(upper)
Mantle
• The layer below the crust
is the mantle
• The mantle has more iron
and magnesium than the
crust, making it more
dense
• The uppermost part of the
mantle is solid and, along
with the crust, forms the
lithosphere
• The rocky lithosphere is
brittle and can fracture.
This is the zone where
earthquakes occur.
• It’s the lithosphere that
breaks into the thick,
moving slabs of rock that
geologist’s call tectonic
plates.
Mantle
(lower)
• As we descend into the
Earth temperature rises
and we reach part of the
mantle that is partially
molten, the
asthenosphere.
• As rock heats up, it
becomes pliable or
‘plastic’. Rock here is hot
enough to fold, stretch,
compress, and flow very
slowly without fracturing.
(Silly Putty®)
• The plates, made up of the
relatively light, rigid rock
of the lithosphere actually
‘float’ on the more dense,
flowing asthenosphere!
Core
• At the center of the Earth lies
the super-dense core.
• It has a diameter of 3486 kms.
• The core is larger than the
planet Mars!
• The core of the Earth is made
up of two distinct layers:
– A liquid outer layer and a
solid inner core. Unlike the
Earth’s outer layers with
rocky compositions, the core
is made up of metallic ironnickel alloy.
– It’s hard to imagine, but the
core is about 5 times as dense
as the rock we walk on at the
surface!
Trends in the Earths Interior
• As you progress
further and further into
the Earth the more
dense the material
becomes.
• This is do in part to:
– the weight of material
overhead.
– Getting closer to the
Earths Core
Trends in the Earths Interior
• As you progress
further and further into
the Earth the hotter the
material becomes.
• This is do impart to:
– Boyles Law:
Compression causes
heating.
– Heat lost by the Earth’s
core
– Radio-active decay
giving off heat.