Test 1 Flashcards
Displacement Hypothesis and Wegener Theory
Continents were together and glaciation occurred over a smaller area (most likely) Wegener also assembled paleoclimatic data showing the distribution of coal deposits(evidence of moist temperate zones), and salt, gypsum and desert sandstones(evidence of dry sandstones(evidence of dry climate) for several geological eras These reflected climatic belts like today’s, e.g. an equatorial tropical rain belt, two adjacent dry belts, two temperate belts and two polar ice caps
Weakness in Wegener Theory
Wegener failed to address a convincing mechanism for continental drift
he proposed that the continents slide over ocean floor
Paleoclimatology
Paleoclimatology is the study of climates for which direct measurements were not taken.
Paleomagnetism
determination of direction and strength of magnetic field in old rocks
Old pole positions can then be determined (magnetic dip or inclination)
Seafloor spreading and evidence to support it
Harry Hess(1962) proposed that besides continents, sea floor might also be moving
- Marine Magnetic Anomalies
- Fracture Zones and transform faults
- Measuring plate motion directly w/ GPS
Stages of Data Retrieval
- Data Acquisition
- Data Reduction
- Data Processing
- Data Interpretation
Data Acquisition
Taking Measurements
Target: Anomalies
Data Reduction
Converting Readings into a more useful form
Noise
Unwanted variations in the quantities being measured
Signal
The wanted quantities you want measured
S/N Ratio
Repeat readings and take their average, also called stacking
You want high S/N ratio
Inverse Modeling
to deduce the causative body directly from the results
Forward Modeling
To “guess” a model, calculate the values it would produce, compare them with the observations, and then modify the model until it matches the results sufficiently well.
“Trial and error”
Inversion Problem
Trying to deduce the form of the body from the anomaly
Not possible if more than one body could produce the results
Resolution
stations may not be close enough to reveal all the details of the signal
i.e. Trying to define in a picture details smaller than the size of the pixels
Data Interpretation
The physical model has to be translated into geological terms
Data Processing
Fourier analysis
Harmonic analysis
Digital Filtering
Fourier analysis
Sort features by their widths, from which we can then select the one we want
Harmonic Analysis
Only certain wavelengths are used, such that 1, 2, 3,… half-wavelengths exactly fit the length of the profile; these are called harmonics
As more harmonics are added the approximation improves
Harmonic analysis: residual anomaly
How well this recombined harmonics match the observed profile can be measured by showing their differences as a residual anomaly
Harmonic Analysis: Considerations
- usually contains a range of wavelengths, so separation of wanted/unwanted signals is only partial
- Since the length of the profile are rather arbitrary, so are the harmonics
- rejecting shorter wavelengths is often regarded as removing anomalies of bodies near the surface to leave deeper ones. (But whereas a narrow anomaly cannot be due to a deep body, the converse is not true)
Digital Filtering: Low pass
they let pass through all wavelengths longer than some value, but reduce those shorter
Digital Filtering: High pass
lets through only wavelengths shorter than some value
Digital Filtering: Band pass
lets through only a range of intermediate wavelengths
Aliasing
wavelengths that do not exist can appear to be that do not exist can appear to be present because of large intervals
Nyquist wavelength
The critical wavelength(twice the sampling interval)
Waves (Seismology)
Vibrations of Rocks
4 Parts of Seismology
- Global seismology and seismic waves
- Refraction
- Reflection
- Earthquakes and seismotectonics
Wave Fronts
compression travels spherically outwards (spherical surfaces)
Rays
conforms a small portion of the wave front always perpendicular to it v=km/s
Seismometers
More sensitive
Less Robust
used to measure weak signals (like distant earthquakes)
Geophone
Used in small scale surveys
where highest sensitivity is not needed
Earth symmetry analysis as based on epicentral angles
- Times to travel all paths with the same epicentral angle are nearly the same (for both small and large angles), so the Earth is indeed spherically symmetrical
- (gets faster with depth) Earth is not seismically uniform
Refraction
When wave fronts cross obliquely into a rock with a higher seismic velocity they speed up, which causes them to change direction
Snell’s Law
Determines degree of change of direction for refraction
Sin(i1)/v1 = sin(i2)/v2
P-ray parameter
sin i1 / v1= sin i2 / v2= sin i3 / v3 … = constant = p= ray parameter. Used to deduce ray paths through Earth.
Iasp91 Model
The model that displays P and S wave velocities throughout the depths of Earth
P-ray shadow zone
98-144 degrees, caused by core refraction
S-ray shadow zone
98-180 degrees, S-waves cannot travel through core
Attenuation
Amplitude of seismic waves changes for two main reasons:
1. the wave front usually spreads out as it travels away from the source.
- If wave energy is absorbed, such as if rock is not fully elastic or S-waves entering rocks with some liquid
Elastic rebound theory
explains how rocks store energy until it is suddenly released in an earthquake: b) strain accumulates (elastically); c) elastic limit is exceeded, the fault slips releasing the pressure; d) surface rupture and surface offsets releasing the pressure; d)surface rupture, and surface offsets
Earthquake location
Traces recorded at seismograms from seismometer stations used to deduce location or origin time
If we use only time of first arrivals If we use only time of first arrivals(P-waves) at different stations we can deduce location.
Fault-plane solutions
It is also possible to deduce the orientation of the fault plane and the direction of displacement in that plane and the direction of displacement in that plane
Beach ball example, like lab. Compress= +, Dilation= -
Strike strip = basic quadrant
normal fault = +,-,+
Thrust fault = -,+,-
Rupture and displacement Principles
The ruptured area of a fault due to a large earthquake has a length along strike that is usually much greater than its depth
Earthquakes have rupture lengths up to hundreds of km, widths up to tens of km, and displacements up to tens of m
Measures of earthquake size
Intensity, Mercalli Scale
Magnitude, Richter Scale, uses Log scale
Seismic moment (M0)
Value = to the product of shear force, displacement, and rupture area.
and/or
equals the product of the shear forces (F) and the perpendicular distance between them (b)
