Modern seismometer

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

Station 1 Station 2 Station 3 Station 4 Station 5

Different kinds of waves exist within solid materials Body waves – propagate throughout a solid medium

Compressional Waves in one- and two- dimensions

Shear waves in one- and two- dimensions

Shear velocity Compressional velocity  = shear modulus = shear stress / shear strain (restoring force to shear) k = bulk modulus = 1/compressibility (restoring force to compression) Different types of waves have different speeds P-waves travel faster than S-waves (and both travel faster than surface waves) (just like waves on a string) (a bit like a slinky)

P-waves get there first…

Rayleigh Love As well as body waves, there are surface waves that propagate along a surface

Different kinds of damage…. P-wave S-wave Sfc-wave All

P-wave arrival S-wave arrival Surface waves arrival

= Hypocenter Difference between P-wave and S-wave arrival can be used to locate the location of an earthquake more effectively…

Difference between p- and s-waves can be used to track location

Need 3 stations to isolate location (and the more the better)

The “first-motion” of the earthquake signal has information about the motion on the fault that generated it. east-west north-south up-down The sense of motion can be used to infer the motion that caused it.

The orientation of faults can be determined from seismic networks

Go to board for Snell’s law

FAST SLOW Back to Snell’s Law Any change in wave speed due to composition change with height will cause refraction of rays…. This one applies to the crust

Do this on the board

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.

If the Earth were homogenous in composition…

aesthenosphere crust core mesosphere But seismic velocities show great variety of structure moho

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)

Shadow zones for P-waves exist but less b/c propagation through the core

Animation of P wave rays

Animation of P wave fronts

The pathways from any given source are constrained…

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

Ray path examples…

Can be identified from individual seismograms (just about)

What do we know about the interior composition of the Earth?

How does seismology help?

Velocity beneath Hawaii…

Beneath subduction zones Note the occurrence of deep earthquakes co-located with the down-going slab

Beneath subduction zones

Earthquake number by Richter Scale – variations over time?

Earthquakes are bad for you….

Earthquakes are dangerous Bam, Iran, 2003

Earthquakes are dangerous Chi-chi Taiwan, 1999

Earthquakes are dangerous Seattle, 1956 Seattle, 2003

Earthquakes are dangerous Sichuan, China, 2008

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

Earthquakes are dangerous El Salvador, 2001

Earthquakes are dangerous Kasmir, 2006

Where, when, and how?

U.S. Earthquakes, 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 creeping 1906 break 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…

USGS shake maps – 10% likelihood of seeing this level of acceleration in The next 50 years

USGS shake maps – 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.

I-80 Freeway collapse (65 deaths) Different ways of measuring Earthquakes – Part 1. By damage Loma-Prieta Earthquake 1989

Northridge Earthquake, 1994 Different ways of measuring Earthquakes – Part 1. By damage

1906 San Francisco vs New Madrid

Different ways of measuring Earthquakes – Part 1. By damage Extent of damage varies widely Charleston, MO Earthquake

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. M L = log 10 A − log 10 A 0 (  L) A 0 is an empirical function depending only on the distance of the station from the epicenter,  L. 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: a)Deflection of seismometer, b)b) distance from source (based on P & S wave arrivals)

Equivalency between magnitude and energy Different ways of measuring Earthquakes – Part 2. Richter Scale

E seismic = M = 1.6 M 0 · ‘Moment Magnitude’ = force/unit area · displacement · fault area = shear modulus · displacement · fault area = total elastic energy released Earthquake “moment” a. Total energy released in an earthquake b. Only a small fraction released as seismic waves c. Create logarithmic scale… Different ways of measuring Earthquakes – Part 3. By energy released

Equivalence of seismic moment and rupture length a)Depends on earthquake size b)Depends on fault type Different ways of measuring 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

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 More information can come from analyzing Earthquake

Amplitude Frequency Narrow band filters 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

Theoretical shapes for earthquakes

And the resulting velocity spectrum

Log10 frequency (hz) Log 10 Moment (dyne-cm) 1/f (for a box car) 1/f 2 (in reality) But real earthquakes don’t do this

Instead there is a ramp-up time… The time series of displacement looks very similar

The theoretical spectrum for a “box car” velocity function decreases as 1/f. Observations show a 1/f 2 behavior. This can be explained as ramping (i.e acceleration) of the velocity at the start and end. Which fits much better with the velocity spectrum

1/source duration Scaled moment 1/ramp time Get lots of useful information from a velocity spectrum…

Log10 frequency (hz) Log 10 Moment (dyne-cm) 1/f 2 T o ~ 30 seconds The maximum amplitude gives information about the moment magnitude of the Earthquake