OCEAN/ESS 410
5. Seismology
William Wilcock
A. Earthquake Seismology
Lecture/Lab Learning Goals
• Understand the distribution of earthquakes on the
Earth and their relationship to plate tectonics (see
also lab1)
• 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.
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://www.eas.purdue.edu/~braile
B. Reflection Seismology
Lecture/Lab Learning Goals
• Understand what seismic impedance is and how it
controls the amplitude of seismic
• Know how seismic reflection data is collected
• Be able to explain how reflection data is stacked and
converted into a seismic record section
• Be able to interpret reflection profiles collected on midocean ridges in terms of oceanic crustal structure - LAB
Reflections from Interfaces
When a downgoing P-wave meets an interface, a portion of the wave is reflected.
Amplitudes of Reflections for vertical rays
Reflected Amplitude
A0
V2 r2 - V1r1
A = A0
V2 r2 + V1r1
V1, 1
V2, 2
Transmitted Amplitude
2V1r1
A = A0
V2 r2 + V1r1
The amplitude of the
reflected and transmitted
phase depends on the
seismic velocity, V and the
density,  in each layer.
Larger contrasts in the
product of velocity and
density (known as
impedance) result in large
amplitude reflections
Marine Reflection Seismology - Airgun Sources
Reflection data is relatively easy to acquire in the oceans. Seismic sounds (shots) can
be generated with arrays compressed air guns (airguns) towed behind the ship
Marine Reflection Seismology - Hydrophone
Streamers
The airgun shots are recorded by arrays of hydrophones towed behind the ship in a
streamer. The seismic streamers contain 1000’s of hydrophones and can be >10 km long.
A modern 3-D seismic ship will tow several (the records is 20) streamers.
Marine Reflection Seismology - Geometry
The streamer records waves reflected from interfaces
Marine Reflection Seismology - Data
The seismic data recorded for a particular shot will look display a geometric effect
termed “normal moveout” (NMO) which reflects the increased distance the wave
travels for as the source-receiver offset increases
0
Time
Offset X
Marine Reflection Seismology - Sorting Records
The records are sorted so that they all have the same mid-point (Common Mid-Point CMP)
Marine Reflection Seismology - Airgun Sources
The seismic records can be corrected for geometric affects and stacked (summed) to
produce a single record for the reflections below each each point
Before Geometric
Correction
After Geometric
Correction
Stacked
(summed)
Marine Reflection Seismology - Filled Wiggle Plots
Time, s
Stacked records are plotted on the same plot with the horizontal axis showing position
along the profile. Rather than showing lines for each record the plots often show
filled regions for positive (or negative) displacements
Position
Structure of a mid-ocean ridge crust
Depth, km
0
3
6
2A
molten
A reflection profile across the East Pacific Rise
Reflections come from the seafloor, the base of layer 2A (pillow basalts), the axial magma
chamber (AMC) and the Moho (M)
Intersecting Record Sections from the East Pacific
Rise