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Modern seismometer. Works via electromagnetic forces holding a mass in place, and measuring the current required to do so. Three components of motion can be measured. east-west. north-south. up-down.
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Modern seismometer Works via electromagnetic forces holding a mass in place, and measuring the current required to do so.
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
Different kinds of waves exist within solid materials Body waves – propagate throughout a solid medium Surface waves – propagate at the interface between media
Compressional Waves in one- and two-dimensions
Shear waves 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)
As well as body waves, there are surface waves that propagate at the interface (i.e., along a surface) Rayleigh Love
Different kinds of damage…. P-wave S-wave Sfc-wave All
A network of seismometers all recording an earthquake S-wave arrival P-wave arrival
Difference between P-wave and S-wave arrival can be used to locate the location of an earthquake more effectively… = Hypocenter
Difference between p- and s-waves can be used to track location
Need 3 stations to isolate location (and the more the better)
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.
The orientation of faults can be determined from seismic networks
The orientation of faults can be determined from seismic networks Plane B Plane A Orientation of the fault plane dictates first motions on an array of seismometers
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
An example with standing waves behind the direct wave (multiple reflections in a slow crust)
New section: seismology can be used to infer the structure of the interior of the Earth
Wave speed depends on pressure and temperature (increase with pressure, decrease with temperature, pressure term wins typically)
Since velocities tend to increase in the crust, wave paths are curved due to refraction. This is maybe not wrong- why? (Ken says so)
If the Earth were homogenous in composition…
But seismic velocities show great variety of structure moho core crust mesosphere aesthenosphere Note, shear waves (s waves) can’t propagate in the liquid core & big drop in p-wave velocity
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
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
Theoretical Arrival times of different waves
Actual arrival times compiled from global data
What do we know about the interior composition of the Earth?
