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## PowerPoint Slideshow about ' Modern seismometer' - lamar-mathis

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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.

Different kinds of waves exist within solid materials

Body waves – propagate throughout a solid medium

in one- and two-dimensions

in one- and two- dimensions

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)

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

the location of an earthquake more effectively…

= Hypocenter

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.

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

Seismology can be used to infer the structure of the interior of the Earth

First, recall that wave paths are curved within the Earth due to refraction.

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)

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

Hawaii…

Bam, Iran, 2003

Chi-chi Taiwan, 1999

Sichuan, China, 2008

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

El Salvador, 2001

Kasmir, 2006

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

Earthquakes in California – different frequency in different sections

of the fault

1906 break

creeping

1857 break

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

Units of “g”

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

Close to home…

Shaking depends on what you’re sitting on.

Different ways of measuring Earthquakes – Part 1. By damage

1966 Parkfield

Earthquake

Notorious for

busted forecast

of earthquake

frequency.

Different ways of measuring Earthquakes – Part 1. By damage

I-80 Freeway collapse (65 deaths)

Loma-Prieta

Earthquake 1989

Different ways of measuring Earthquakes – Part 1. By damage

Northridge Earthquake, 1994

Different ways of measuring Earthquakes – Part 1. By damage

1906 San Francisco vs. 1811 New Madrid

Different ways of measuring Earthquakes – Part 1. By damage

Charleston, MO

Earthquake

Extent of damage varies widely

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).

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)

Different ways of measuring Earthquakes – Part 2. Richter Scale

Equivalency between magnitude and energy

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’

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

Earthquakes

– Part 3. By energy released

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

Earthquakes

– Part 3. By energy released

Distribution of slip

For various Earthquakes

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

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

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

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

Amplitude

Frequency

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

Amplitude vs. time for different frequency bands

Lower frequencies have larger amplitudes

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)

Instead there is a ramp-up time…

The time series of displacement looks very similar

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.

Log10 frequency (hz)

The maximum amplitude gives information about the

moment magnitude of the Earthquake

1/f2

To~ 30 seconds

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