9 June 15
2
Summer Session
2015
Horst Rademacher
HH
Lect 4:
Seismic Waves
http://seismo.berkeley.edu/summersession/eps20/
Class Organization
Thursday’s class (June 11) 10am-12:30pm
My office hours: Tuesdays 3-5pm BSL Conference Room
Midterm: Next Tuesday (June 16),
during regular class hours
Seismicity Report
Seismicity Report
Depth in km
Seismicity Report
4 June 15, 23:15:43 UTC
Magnitude 6
Depth 10 km
18 people died in
Rock falls induced by EQ
Mount Kinabalu
highest mountain in Borneo
4095 m
Seismicity Report
Sacred to indigenous people, the Sabahans
Seismicity Report
Any
Questions?
HH
http://seismo.berkeley.edu/summersession/eps20/
Recap from last lecture I
Subduction = convergent margins
Transform = sliding plate margins
Spreading = divergent margins
Complex section of
Pacific Rim of Fire
Recap from last lecture II
Geodetic GPS
These faults split the
tectonic movement
between Pacific and
North American plates
Recap from last lecture III
Earthquake History
Seismic Waves: The Basics
What is an earthquake?
Common/laymen’s answer:
When the ground shakes where I am
Seismic waves
Seismic source
When the ground breaks along a fault
Scientific answer:
Both answers are correct!
Two different aspects of a seismic event
Seismic Waves: The Basics
What are seismic waves?
Acoustic waves:
What is necessary? Sound source
Medium
Generates acoustic energy
Transports the energy
Seismic Waves: The Basics
Parameters of a wave
Amplitude
A
Wavelength λ
Period
T
Frequency
f
Velocity
v
F=2π/T
[Hz=1/sec]
V= λ ∗ f [m/sec]
Seismic Waves: The Basics
Seismic Waves: The Basics
Where do sound waves travel?
much faster
through water
through air
As density
increases
sound speed
decreases
Seismic Waves: The Basics
Where do sound waves travel?
Sound also travels
through solids
….hence it travels through
the Earth
Elastic waves =
Seismic waves
Seismic Waves: The Basics
Two classes, four types
of seismic waves
P- or longitudinal
Body
waves
Surface
waves
S- or shear
Lord
Rayleigh
1842-1919
Augustus
Love
1863-1940
Seismic Waves: The Details
Same mechanism as sound waves in air
P-Waves velocity in Earth: 4 - 13 km/sec
P-Waves
Seismic Waves: The Details
Ground movement up/down or left/right
S-Waves velocity in Earth: 3 – 7 km/sec
S-Waves
Seismic Waves: The Details
Rayleigh-Waves
Rayleigh-Waves velocity in Earth: ~ 3 km/sec
Retrograde elliptical ground movement
Seismic Waves: The Details
Love-Waves
Love-Waves velocity in Earth: ~ 3 km/sec
Ground movement left/right
Any
Questions?
HH
http://seismo.berkeley.edu/summersession/eps20/
Ask these grumpy looking old men
How to measure seismic waves?
1900 Emil Wiechert (Göttingen) builds
How to measure seismic waves? world’s heaviest Seismograph
Mass 17 tons
Baryte (BaSO4)
How to measure seismic waves?
1909
Wiechert "lite"
horizontal, 80 kg
How to measure seismic waves?
Wish: record the Earth's movement
with high resolution
Problem: the reference frame
is also in motion
Requirement: to separate reference
frame from Earth
Tool: spring
Physics: inertia
How to measure seismic waves?
Inertia
Great Hanshin Earthquake, Kobe, Japan, 17 Jan 1995, M = 7.3
Security Camera in NHK Newsroom
NHK Video
How to measure seismic waves?
Inertia
Vertical
Pendulum
How to measure seismic waves?
Today we don’t use paper anymore
Electronic recording
How to measure seismic waves
How to measure seismic waves
Seismic Records =
Seismograms
Time in minutes
no
shaking
frequency gets lower
shaking begins
More shaking, lower frequency
shaking decreases
How to measure seismic waves
Different Seismic Wave types
travel with different velocities
3 Day Seismogram
How to measure seismic waves
local
regional
Further reading on seismograms:
http://seismo.berkeley.edu/blog/seismoblog.php
Search for seismogram
teleseismic
Any
Questions?
HH
http://seismo.berkeley.edu/summersession/eps20/
Using seismic waves
What can we do with seismograms?
1. Locate EQ
2. Explore the structure of the
Earth’s Interior
Locating earthquakes
Using seismic waves
Using seismic waves
Given the S-P time at 3
stations we can locate
the earthquake
Modern networks use
tens or hundreds of
waveforms
Locating earthquakes
Any
Questions?
HH
http://seismo.berkeley.edu/summersession/eps20/
Using seismic waves
Like all other waves
Seismic Waves are subject to:
Reflection
Refraction
Scattering
Exploring Earth’s Interior
We can X-ray (image) the
Earth with Seismic Waves
Exploring Earth’s Interior
If Earth had no
internal structure
EQ focus
Seismic waves
go straight through
Pressure, temperature
increases with depth
Seismic waves
are refracted (bent)
Exploring Earth’s Interior
Example I
Andrija Mohorovičić
(1857 – 1936)
discovered in 1910
the boundary between
Crust and Mantle,
the Moho
Thin under Oceans,
much thicker under continents
Exploring Earth’s Interior
Example II
Observation of shadow zones reveals liquid outer core
P-wave
S-wave
Exploring Earth’s Interior
complete Structure
of the Earth
Exploring Earth’s Interior
Naming of Seismic Phases
Exploring Earth’s Interior
Show seismic wave animation
Any
Questions?
HH
http://seismo.berkeley.edu/summersession/eps20/
Computerized Axial Tomography
Exploring Earth’s Interior..
CAT- Scan
in our own backyard
Seismic Tomography
Exploring Earth’s Interior..
in our own backyard
Necessary: dense network
of seismic stations
Earthquake of
known location
Stations recording
seismic waves from
earthquake
Seismic Tomography
Exploring Earth’s Interior..
in our own backyard
100 km
Necessary: preliminary Model
of the Earth’s interior
Stations recording
seismic waves from
earthquake
Example - in our Earth model, Pwaves travel at 5 km/s
If our station is 100 km away, we
would predict P-waves to arrive after
20 sec.
Seismic Tomography
Necessary: very good timing
Precise clocks
Exploring Earth’s Interior..
in our own backyard
SLOW!
FAST!
Stations recording
seismic waves from
earthquake
Example - On another path, we
observe waves traveling more quickly
than expected!
Example - BUT! We observe P-waves
arriving 1 sec later than they should
have!
Slow region
Fast region
Seismic Tomography
Exploring Earth’s Interior..
in our own backyard
Interpretation
Red - seismic waves travel more slowly than normal
Blue - seismic waves travel faster than normal
Chemistry - Chemical composition and mineralogy affects seismic properties
Example: Enrichment in iron can reduce shear-wave speed
Temperature - Increasing temperature causes material to soften and slows
down seismic velocities.
W
0 km
1000 km
coast
E
W
coast
0 km
600 km
1000 km
E
W
coast
0 km
1000 km
E
W
Yellowstone
Caldera
coast
0 km
Columbia
root
1000 km
E
W
E
coast
0 km
Columbia
root
410 km
660 km
?
1000 km
Exploring Earth’s Interior
East Asia: Himalayas to Japan
Min Cheng, Rice University
Exploring Earth’s Interior
Barbara Romanowicz, BSL
Any
Questions?
HH
http://seismo.berkeley.edu/summersession/eps20/
Things to remember I
Two classes, four types
of seismic waves
P- or longitudinal
Body
waves
Surface
waves
S- or shear
Lord
Rayleigh
1842-1919
Augustus
Love
1863-1940
Things to remember II
Different Seismic Wave types
travel with different velocities
Things to remember III
Inertia
Vertical
Pendulum
Things to remember IV
Given the S-P time at 3
stations we can locate
the earthquake
Modern networks use
tens or hundreds of
waveforms
Locating earthquakes
Things to remember V
We can X-ray (image) the
Earth with Seismic Waves
Seismic Tomography
W
coast
Things to remember V
0 km
600 km
1000 km
E
Class Organization
Visit classic Wiechert Seismograph now after
class
Thursday’s class (June 11) 10am-12:30pm
My office hours: Tuesdays 3-5pm BSL Conference Room
Midterm: Next Tuesday (June 16),
during regular class hours
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