Horst Rademacher HH Seismic Wavesseismo.berkeley.edu/~horst/Lect4-June9.pdf · Red - seismic waves...

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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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