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Modern seismometer. Three components of motion can be measured. east-west. north-south. up-down. If you speeded up any earthquake signal and listened to it with a hi fi, it would sound like thunder. Station 1. Station 2. Station 3. Station 4. Station 5.

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Presentation Transcript
slide3

Three components of motion can be measured

east-west

north-south

up-down

If you speeded up any earthquake signal and listened to it with a hi fi, it would sound like thunder.

slide4

Station 1

Station 2

Station 3

Station 4

Station 5

slide5

Different kinds of waves exist within solid materials

Body waves – propagate throughout a solid medium

slide8

Compressional Waves

in one- and two-dimensions

slide9

Shear waves

in one- and two- dimensions

slide10

Different types of waves have different speeds

Shear velocity

(just like waves on a string)

Compressional velocity

(a bit like a slinky)

  • = shear modulus = shear stress / shear strain (restoring force to shear)
  • k = bulk modulus = 1/compressibility (restoring force to compression)

P-waves travel faster than S-waves

(and both travel faster than surface waves)

slide12

As well as body waves, there are surface waves

that propagate along a surface

Rayleigh

Love

slide15

Different kinds of damage….

P-wave

S-wave

Sfc-wave

All

slide16

P-wave

arrival

Surface waves

arrival

S-wave

arrival

slide17

Difference between P-wave and S-wave arrival can be used to locate

the location of an earthquake more effectively…

= Hypocenter

slide20

The sense of motion can be used to infer the motion that caused it.

east-west

north-south

up-down

The “first-motion” of the earthquake signal has information about the motion on the fault that generated it.

slide24

Back to Snell’s Law

Any change in wave speed due to composition change with height

will cause refraction of rays….

SLOW

FAST

FAST

SLOW

This one applies to the crust

slide33

If the Earth were

homogenous in

composition…

slide34

But seismic velocities show great variety of structure

moho

crust

mesosphere

core

aesthenosphere

slide35

S waves cannot propagate through the core, leading to a huge shadow zone

S waves cannot propagate in a fluid (fluids cannot support shear stresses)

slide36

Shadow zones for P-waves exist

but less b/c propagation through

the core

slide40

Seismic “phases” are named according to their paths

P – P wave only in the mantle

PP – P wave reflected off earths surface so there are two P wave segments in the mantle

pP – P wave that travels upward from a deep earthquake, reflects off the surface and then has a single segment in the mantle

PKP – P wave that has two segments in the mantle separated by a segment in the core

slide61

Beneath subduction zones

Note the occurrence of deep earthquakes co-located with the

down-going slab

slide62

Beneath

subduction

zones

slide67

Earthquakes are dangerous

Chi-chi Taiwan, 1999

slide68

Earthquakes are dangerous

Seattle, 2003

Seattle, 1956

slide69

Earthquakes are dangerous

Sichuan, China, 2008

slide70

“Helicorder” record of the Sumatra Earthquake and aftershocks recorded in the Czech Republic (December 26, 2004)

slide71

Earthquakes are dangerous

El Salvador, 2001

slide77

U.S. Earthquakes, 1973-2002

Source, USGS. 28,332 events. Purple dots are earthquakes below 50 km, the green dot is below 100 km.

slide78

Earthquakes in California – different frequency in different sections

of the fault

1906 break

creeping

1857 break

slide79

USGS shake maps – 2% likelihood of seeing peak ground acceleration equal to given color in the next 50 years

Units of “g”

slide80

USGS shake maps – 2% likelihood of seeing peak ground acceleration equal to given color in the next 50 years

Close to home…

slide81

USGS shake maps –

10% likelihood of seeing this level of acceleration in

The next 50 years

slide82

USGS shake maps –

Shaking depends on what you’re sitting on.

slide86

Different ways of measuring Earthquakes – Part 1. By damage

1966 Parkfield

Earthquake

Notorious for

busted forecast

of earthquake

frequency.

slide87

Different ways of measuring Earthquakes – Part 1. By damage

I-80 Freeway collapse (65 deaths)

Loma-Prieta

Earthquake 1989

slide89

Different ways of measuring Earthquakes – Part 1. By damage

1906 San Francisco vs. 1811 New Madrid

slide90

Different ways of measuring Earthquakes – Part 1. By damage

Charleston, MO

Earthquake

Extent of damage varies widely

slide91

Different ways of measuring Earthquakes – Part 2. Richter Scale

  • quantifies the amount of seismic energy released by an earthquake.
  • base-10 logarithmic based on the largest displacement, A, from zero on a Wood–Anderson torsion seismometer output.
  • ML = log10A − log10A0(DL)
  • A0 is an empirical function depending only on the
  • distance of the station from the epicenter, DL.
  • So an earthquake that measures 5.0 on the Richter scale has a shaking amplitude 10 times larger than one that measures 4.0.
  • The effective limit of measurement for local magnitude is about ML = 6.8 (before seismometer breaks).
slide92

Different ways of measuring Earthquakes – Part 2. Richter Scale

  • Two pieces of information used to calculate size of Earthquake:
  • Deflection of seismometer,
  • b) distance from source (based on P & S wave arrivals)
slide93

Different ways of measuring Earthquakes – Part 2. Richter Scale

Equivalency between magnitude and energy

slide95

Different ways of measuring Earthquakes – Part 3. By energy released

a. Total energy released in an earthquake

Earthquake “moment”

= force/unit area · displacement · fault area

= shear modulus · displacement · fault area

= total elastic energy released

b. Only a small fraction released as seismic waves

Eseismic = M010 -4.8 = 1.6 M0· 10-5

c. Create logarithmic scale…

‘Moment Magnitude’

slide97

Different ways of measuring

Earthquakes

– Part 3. By energy released

  • Equivalence of seismic moment
  • and rupture length
  • Depends on earthquake size
  • Depends on fault type
slide98

Different ways of measuring

Earthquakes

– Part 3. By energy released

Distribution of slip

For various Earthquakes

slide102

More information can come from analyzing Earthquake

If you speeded up any earthquake signal and listened to it with a hi fi, it would sound like thunder.

This is the sound of the 2004 Parkfield 6.0 Earthquake

slide103

Narrow band filters

Amplitude

Frequency

A spectrum what you get when you listen to a signal through a series of narrow band filters

slide104

Amplitude vs. time for different frequency bands

Lower frequencies have larger amplitudes

slide107

But real earthquakes don’t do this

Log 10 Moment (dyne-cm)

Log10 frequency (hz)

1/f (for a box car)

1/f2

(in reality)

slide108

Instead there is a ramp-up time…

The time series of displacement looks very similar

slide109

Which fits much better with the velocity spectrum

  • The theoretical spectrum for a “box car” velocity function decreases as 1/f.
  • Observations show a 1/f2 behavior.
  • This can be explained as ramping (i.e acceleration) of the velocity at the start and end.
slide110

Get lots of useful information from a velocity spectrum…

Scaled moment

1/source duration

1/ramp time

slide111

Log 10 Moment (dyne-cm)

Log10 frequency (hz)

The maximum amplitude gives information about the

moment magnitude of the Earthquake

1/f2

To~ 30 seconds

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