Transcript Magma Supply Vs Magma Plumbing

OCEAN/ESS 410
4. Global Seismology
William Wilcock
Lecture/Lab Learning Goals
• Understand the distribution of earthquakes on the
Earth and their relationship to plate tectonics (see
also Lab 1)
• Know what an earthquake is, how earthquake sizes
are classified, and the different types of body waves.
• Understand how seismic waves propagate through
the earth along many different paths and how this
constrains the internal structure of the earth.
• Be able to identify seismic body wave arrivals for a
teleseismic earthquake, interpret a seismic travel
time curves, and locate an earthquake using S-wave
minus P-wave arrival times and P-wave arrival times
- LAB
Tectonic Plates
Global Seismograph Network
What is an Earthquake
• “An earthquake is a sudden and sometimes
catastrophic movement of a part of the Earth's
surface. Earthquakes result from the dynamic release
of elastic strain energy that radiates seismic waves.
Earthquakes typically result from the movement of
faults, planar zones of deformation within the Earth's
upper crust. The word earthquake is also widely used
to indicate the source region itself.” - Wikipedia
• Earthquakes radiate two types of seismic waves body waves that travel through the earth and surface
waves that travel over it. There are two types of body
waves - P waves and S waves
Body Waves: P-waves
Primary Wave: P wave is a compressional (or longitudinal) wave in
which rock (particles) vibrates back and forth parallel to the direction of
wave propagation. P-waves are the first arriving wave and have high
frequencies but their amplitude tends not to be very large
Body Waves: S-waves
Secondary Wave: S wave is a slower, transverse wave propagated by
shearing motion much like that of a stretched, shaken rope. The rock
(particles) vibrate perpendicular to the direction of wave propagation. They
tend to have higher amplitudes and lower frequencies than P-waves. Swaves cannot travel through liquids (i.e., the outer core, the oceans)
Surface Waves
Surface waves
travel over the
surface of the earth.
They travel more
slowly than body
waves but tend to
have higher
amplitudes and
often are the most
damaging waves
from an earthquake
Surface wave
P-wave S-wave
aftershock
0
S-P
10
20
Time (min)
30
0
40
Depth (km)
670
Velocity (km/S)
4
8
12
Velocity Structure
of the Earth
•Upper mantle
P waves 8-10 km/s;
S-waves 4-6 km/s
•Lower mantle
P-waves 12-14 km/s
S-waves 6-7 km/s
2900
•Outer Core
P-waves 8-10 km/s
S-waves - Do not
progagate
5155
•Inner Core
P-waves 11 km/s
6371
S-waves 5 km/s
How do waves propagate through
the earth
1. Refraction - Snell’s Law
Waves bend back towards the surface
when traveling through regions
where the velocity increases with
depth
2. Interfaces
When a seismic P-wave propagates
across a sharp boundary a portion of
the wave will be reflected as P-wave
and a portion will be converted to
transmitted and reflected S-waves.
The same applies to an S-wave. 1
incoming wave gives rise to 4
outgoing waves.
Seismic Phase Names
Seismic
Travel Time
Curve
S minus P travel times constrain the
Earthquake Distance
This Figure is wrong in one respect - the seismograms do not show clearly that the Swaves are much lower frequency than P waves. You will see this in the exercise.
S-minus-P travel-times
will constrain the
distance from station to
earthquake
Note the lower
frequency for S-waves
compared to P-waves
Surface waves are lowfrequency and high
amplitude arrivals
Travel-times are
relative to P-wave
arrival time
Earthquake Location Exercise
In the next lab we are going to be doing an
earthquake location exercise which is courtesy of
Professor Larry Braile at Purdue University.
Professor Braile has developed an impressive array
of earth science education activities. His web site is.
http://web.ics.purdue.edu/~braile/