powerpoints Flashcards
earthquakes cause….
vibration of the Earth & permanent deformation
in what mediums do earthquakes exist?
They exist in space and time
Elastic Rebound Model
You begin with unbent rocks, over time stress builds up and rocks bend (strain) and store elastic energy. Eventually rock breaks sending seismic waves through the earth.
Earthquake strain
Abrupt movement occurs along a fault. (No movement across fault for many years, then as much as
several meters of plate movement in a few seconds)
tectonic creep
In some areas tectonic movement is slow + continuous. There is no earthquake in these areas.
Where do earthquakes begin?
beneath the surface at the hypocenter or focus.
Location directly above focus at Earth’s surface
epicenter
surface rupture
Slip that reaches all the way to the surface
Average location of
energy release
centroid
local magnitude
Determined by amount of ground movement recorded.
magnitude = log (Amp./Period) + f (depth, distance)
moment magnitude
Determined by size of fault and the amount of slip. More accurate for energy release that local magnitude.
Mw = (log (M0) / 1.5) – 10.73
What type of relationship is there between earthquake size and energy released?
logarithmic.
Besides local and moment magnitude what are other ways that earthquake sizes can be determined?
- Human reports
* Instrumental recordings
seismology
study of the origin of ground vibration
What are some important applications of seismology?
- Hazard mitigation
- Exploration of resources
- check for nuclear testing
seismometer
instrument that measures ground shaking.
seismoscope
Inverted pendulum with mechanical linkage. A historic way of measuring size of earthquakes.
Parts of a simple seismograph
- Metal frame + recording device connected to Earth
- Recording device is moving paper (on rotating drum)
- Heavy metal object with pen suspended from beam
- Inertia: Mass essentially fixed while frame moves
- Pen fixed while paper moves, producing written signal
modern electronic seismometer
- Suspended heavy weight now has a magnet
- Wire coil surrounding magnet connected to earth
- When earth moves, magnet induces current in coil
- Voltage = velocity * magnetic field * wire length
DAS
Data Acquisition System- a specially designed computer. . records time and voltage from modern electronic seismometers.
energy radiates away from an earthquake in the form of…
seismic waves
P waves
- body waves
- begin at earthquake focus and move away in all directions
- compressional waves (push-pull movement)
S-waves
- secondary waves
- slower than p waves
- shear waves
- usually larger than p-waves
- only travel through solids
Surface waves
- slowest waves
- last to arrive
- generate greatest amount of wave movement
- move only along earth’s surface.
How do we locate earthquake epicenters
- we use that fact that p and s waves travel at different distances.
- Get info from at least 3 stations and triangulate location.
S waves have what two orientations?
horizontal (SH) and
vertical (SV) polarization
How are waves propagated from an earthquake epicenter?
*travel in spherical wave fronts (circles) away from the source
What happens when seismic waves interact between layers?
reflection and refraction
fast velocities cause a bend towards
the surface
critical angle
the refracted wave is horizontal
how do wave velocities react within each layer of the earth?
they increase with each layer.
using the slope of a travel time curve to determine velocity
slope = 1/velocity
shadow zones
A low velocity zone causes a set of distances where
no arrivals are recorded. Caused by top of the asthenosphere and the outer core.
waveform modeling
Utilize full information of seismogram (waveform) to
interpret velocity structure, not just arrival times.
synthetic seismograms
- predicted from a velocity structure
* compare with recorded seismograms
Forward modeling of velocity
- Make educated guesses about velocity structure;
- compare predicted seismogram with observed. This can show areas we correctly knew what was going on, and others where we needed improvement.
Seismic travel time tomography
- CAT scan of earth using seismic rays.
- Divides earth into boxes, looks at travel times of boxes. Determine if each box is fast or slow.
- Create a 3d model of velocity anomalies.
When look at seismic vectors blue means
cooler and faster than average.
when looking at seismic vectors red means
hotter and slower than average
What do cool temperatures do to seismic velocities?
raise seismic velocities.
What do warm temperatures do to seismic velocities?
slow seismic velocities.
Issues with tomography
weakness of data set (not uniform coverage, unknown quality of arrival times)
weakness of technique (inversions are non-unique, do not give uncertanties, solutions are only for the earth’s interior at that particular time)
equation for stress
stress equals strain/area
normal stress
perpindicular to surface
shear stress
parallel to surface
strain
material deformation due to stress. (Change in shape of material)
Types of deformation
elastic, ductile, brittle
elastic strain
not permanent
brittle strain
permanent. stress exceeds strength of material.
ductile strain
some strain is permanent.
shaded areas on “beach balls” represent
compression quadrants
The double-couple model
- In an elastic body, fault slip (ie., EQ) causes a 4-quadrant pattern of compression-waves.
- Collect first motion measurements from EQ. Use this information to define compressional and dilational quadrants.
- allows you to find 2 nodal planes.
Lynn Sykes
- Slip of earthquakes between MOR
- Opposite of apparent offset between ridge segments
- Proof for Wilson’s transform fault
- Birth of seismotectonics
When the compression or shaded are of “beach ball” is in the center what type of faulting do you get?
How does this look in cross-section?
Thrust or pure reverse faulting.
Shaded quadrants on top and bottom
strike slip faulting in cross section
Only 3 quadrants visible
Oblique faulting
Mixture of strike slip and dip slip faulting
