horst rademacher hh seismic wavesseismo.berkeley.edu/~horst/lect4-june9.pdf · red - seismic waves...
TRANSCRIPT
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2
HH Horst Rademacher
Lect 4: Seismic Waves
Summer Session 2015
9 June 15
http://seismo.berkeley.edu/summersession/eps20/
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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
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Seismicity Report
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Depth in km
Seismicity Report
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Seismicity Report
4 June 15, 23:15:43 UTC Magnitude 6 Depth 10 km
18 people died in Rock falls induced by EQ
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Seismicity Report
Mount Kinabalu highest mountain in Borneo 4095 m
Sacred to indigenous people, the Sabahans
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Seismicity Report
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HH Any
Questions? http://seismo.berkeley.edu/summersession/eps20/
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Complex section of Pacific Rim of Fire
Subduction = convergent margins
Spreading = divergent margins
Transform = sliding plate margins
Recap from last lecture I
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These faults split the tectonic movement between Pacific and North American plates
Geodetic GPS
Recap from last lecture II
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Earthquake History
Recap from last lecture III
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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
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Seismic Waves: The Basics What are seismic waves?
Acoustic waves: What is necessary? Sound source Generates acoustic energy Medium Transports the energy
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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]
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Seismic Waves: The Basics
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Seismic Waves: The Basics Where do sound waves travel?
through air much faster through water
As density increases
sound speed
decreases
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Seismic Waves: The Basics Where do sound waves travel?
Sound also travels through solids
Elastic waves = Seismic waves
….hence it travels through the Earth
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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
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Seismic Waves: The Details P-Waves Same mechanism as sound waves in air P-Waves velocity in Earth: 4 - 13 km/sec
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Seismic Waves: The Details S-Waves Ground movement up/down or left/right S-Waves velocity in Earth: 3 – 7 km/sec
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Seismic Waves: The Details Rayleigh-Waves Rayleigh-Waves velocity in Earth: ~ 3 km/sec Retrograde elliptical ground movement
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Seismic Waves: The Details Love-Waves Love-Waves velocity in Earth: ~ 3 km/sec Ground movement left/right
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HH Any
Questions? http://seismo.berkeley.edu/summersession/eps20/
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How to measure seismic waves? Ask these grumpy looking old men
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1900 Emil Wiechert (Göttingen) builds world’s heaviest Seismograph
Mass 17 tons Baryte (BaSO4)
How to measure seismic waves?
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1909 Wiechert "lite" horizontal, 80 kg
How to measure seismic waves?
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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?
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NHK Video
Great Hanshin Earthquake, Kobe, Japan, 17 Jan 1995, M = 7.3 Security Camera in NHK Newsroom
Inertia How to measure seismic waves?
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Vertical Pendulum
How to measure seismic waves? Inertia
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Today we don’t use paper anymore Electronic recording
How to measure seismic waves?
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How to measure seismic waves
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no shaking
shaking begins shaking decreases
More shaking, lower frequency
frequency gets lower
Time in minutes
How to measure seismic waves Seismic Records =
Seismograms
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Different Seismic Wave types travel with different velocities How to measure seismic waves
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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
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HH Any
Questions? http://seismo.berkeley.edu/summersession/eps20/
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Using seismic waves
What can we do with seismograms?
1. Locate EQ
2. Explore the structure of the Earth’s Interior
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Using seismic waves Locating earthquakes
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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
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HH Any
Questions? http://seismo.berkeley.edu/summersession/eps20/
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Like all other waves Seismic Waves are subject to:
Reflection
Refraction
Scattering
Using seismic waves
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We can X-ray (image) the Earth with Seismic Waves Exploring Earth’s Interior
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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)
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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
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P-wave S-wave
Observation of shadow zones reveals liquid outer core
Exploring Earth’s Interior Example II
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complete Structure of the Earth Exploring Earth’s Interior
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Exploring Earth’s Interior Naming of Seismic Phases
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Show seismic wave animation
Exploring Earth’s Interior
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HH Any
Questions? http://seismo.berkeley.edu/summersession/eps20/
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Computerized Axial Tomography CAT- Scan Exploring Earth’s Interior..
in our own backyard
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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
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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
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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
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Fast region
Slow region
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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
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1000 km
0 km
W E coast
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1000 km
0 km
W E coast
600 km
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1000 km
0 km
W E coast
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1000 km
0 km
W E Yellowstone Caldera coast
Columbia root
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1000 km
0 km
W E
coast
Columbia root
660 km
?
410 km
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Min Cheng, Rice University
Exploring Earth’s Interior East Asia: Himalayas to Japan
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Barbara Romanowicz, BSL
Exploring Earth’s Interior
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HH Any
Questions? http://seismo.berkeley.edu/summersession/eps20/
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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
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Different Seismic Wave types travel with different velocities Things to remember II
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Vertical Pendulum
Inertia Things to remember III
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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
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We can X-ray (image) the Earth with Seismic Waves Things to remember V
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1000 km
0 km
W E coast
600 km
Things to remember V
Seismic Tomography
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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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X
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