Horst Rademacher HH Seismic Wavesseismo.berkeley.edu/~horst/Lect4-June9.pdf · Red - seismic waves...
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2 HH Horst Rademacher Lect 4: Seismic Waves Summer Session 2015 9 June 15 http://seismo.berkeley.edu/summersession/eps20/
Transcript of Horst Rademacher HH Seismic Wavesseismo.berkeley.edu/~horst/Lect4-June9.pdf · Red - seismic waves...
2
HH Horst Rademacher
Lect 4: Seismic Waves
Summer Session 2015
9 June 15
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
Depth in km
Seismicity Report
Seismicity Report
4 June 15, 23:15:43 UTC Magnitude 6 Depth 10 km
18 people died in Rock falls induced by EQ
Seismicity Report
Mount Kinabalu highest mountain in Borneo 4095 m
Sacred to indigenous people, the Sabahans
Seismicity Report
HH Any
Questions? http://seismo.berkeley.edu/summersession/eps20/
Complex section of Pacific Rim of Fire
Subduction = convergent margins
Spreading = divergent margins
Transform = sliding plate margins
Recap from last lecture I
These faults split the tectonic movement between Pacific and North American plates
Geodetic GPS
Recap from last lecture II
Earthquake History
Recap from last lecture III
What is an earthquake? Seismic Waves: The Basics
Scientific answer: When the ground breaks along a fault
Common/laymen’s answer: When the ground shakes where I am
Both answers are correct! Two different aspects of a seismic event
Seismic waves
Seismic source
Seismic Waves: The Basics What are seismic waves?
Acoustic waves: What is necessary? Sound source Generates acoustic energy Medium 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?
through air much faster through water
As density increases
sound speed
decreases
Seismic Waves: The Basics Where do sound waves travel?
Sound also travels through solids
Elastic waves = Seismic waves
….hence it travels through the Earth
Seismic Waves: The Basics
Two classes, four types of seismic waves
Body waves
P- or longitudinal
S- or shear
Surface waves
Lord Rayleigh
1842-1919
Augustus Love
1863-1940
Seismic Waves: The Details P-Waves Same mechanism as sound waves in air P-Waves velocity in Earth: 4 - 13 km/sec
Seismic Waves: The Details S-Waves Ground movement up/down or left/right S-Waves velocity in Earth: 3 – 7 km/sec
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
HH Any
Questions? http://seismo.berkeley.edu/summersession/eps20/
How to measure seismic waves? Ask these grumpy looking old men
1900 Emil Wiechert (Göttingen) builds 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?
NHK Video
Great Hanshin Earthquake, Kobe, Japan, 17 Jan 1995, M = 7.3 Security Camera in NHK Newsroom
Inertia How to measure seismic waves?
Vertical Pendulum
How to measure seismic waves? Inertia
Today we don’t use paper anymore Electronic recording
How to measure seismic waves?
How to measure seismic waves
no shaking
shaking begins shaking decreases
More shaking, lower frequency
frequency gets lower
Time in minutes
How to measure seismic waves Seismic Records =
Seismograms
Different Seismic Wave types travel with different velocities How to measure seismic waves
How to measure seismic waves 3 Day Seismogram
local regional teleseismic
Further reading on seismograms: http://seismo.berkeley.edu/blog/seismoblog.php Search for seismogram
HH Any
Questions? 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
Using seismic waves Locating earthquakes
Given the S-P time at 3 stations we can locate the earthquake
Modern networks use tens or hundreds of waveforms
Locating earthquakes Using seismic waves
HH Any
Questions? http://seismo.berkeley.edu/summersession/eps20/
Like all other waves Seismic Waves are subject to:
Reflection
Refraction
Scattering
Using seismic waves
We can X-ray (image) the Earth with Seismic Waves Exploring Earth’s Interior
Exploring Earth’s Interior
EQ focus
If Earth had no internal structure
Seismic waves go straight through
Pressure, temperature increases with depth
Seismic waves are refracted (bent)
Andrija Mohorovičić (1857 – 1936)
discovered in 1910 the boundary between Crust and Mantle, the Moho
Exploring Earth’s Interior Example I
Thin under Oceans, much thicker under continents
P-wave S-wave
Observation of shadow zones reveals liquid outer core
Exploring Earth’s Interior Example II
complete Structure of the Earth Exploring Earth’s Interior
Exploring Earth’s Interior Naming of Seismic Phases
Show seismic wave animation
Exploring Earth’s Interior
HH Any
Questions? http://seismo.berkeley.edu/summersession/eps20/
Computerized Axial Tomography CAT- Scan Exploring Earth’s Interior..
in our own backyard
Earthquake of known location
Stations recording seismic waves from earthquake
Exploring Earth’s Interior.. in our own backyard
Seismic Tomography
Necessary: dense network of seismic stations
Stations recording seismic waves from earthquake
100
km
Example - in our Earth model, P-waves 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: preliminary Model of the Earth’s interior
Exploring Earth’s Interior.. in our own backyard
Stations recording seismic waves from earthquake
Example - BUT! We observe P-waves arriving 1 sec later than they should have!
SLO
W!
Example - On another path, we observe waves traveling more quickly than expected! FAST!
Exploring Earth’s Interior.. in our own backyard
Seismic Tomography Necessary: very good timing
Precise clocks
Fast region
Slow region
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.
Exploring Earth’s Interior.. in our own backyard
Seismic Tomography
Interpretation
1000 km
0 km
W E coast
1000 km
0 km
W E coast
600 km
1000 km
0 km
W E coast
1000 km
0 km
W E Yellowstone Caldera coast
Columbia root
1000 km
0 km
W E
coast
Columbia root
660 km
?
410 km
Min Cheng, Rice University
Exploring Earth’s Interior East Asia: Himalayas to Japan
Barbara Romanowicz, BSL
Exploring Earth’s Interior
HH Any
Questions? http://seismo.berkeley.edu/summersession/eps20/
Two classes, four types of seismic waves
Body waves
P- or longitudinal
S- or shear
Surface waves
Lord Rayleigh
1842-1919
Augustus Love
1863-1940
Things to remember I
Different Seismic Wave types travel with different velocities Things to remember II
Vertical Pendulum
Inertia Things to remember III
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 IV
We can X-ray (image) the Earth with Seismic Waves Things to remember V
1000 km
0 km
W E coast
600 km
Things to remember V
Seismic Tomography
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
Visit classic Wiechert Seismograph now after class
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