Lab 6 Earthquakes. Concepts Diastrophism Types & causes of stress 5 types of folds 5 types of faults Earthquakes Focus, epicenter Types of energy waves.
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Concepts
Diastrophism Types & causes of stress 5 types of folds 5 types of faults
Earthquakes Focus, epicenter Types of energy waves Methods of measuring earthquakes Determining the magnitude of an earthquake Determining the epicenter of an earthquake Real-world example: New Madrid Fault
Diastrophism
Definition: deformation of earth’s crust
Deformation without movement Jointing: Fracture of rock without displacement Affects resistance of rock to erosion (weakens)
Deformation with movement Folding: bending rock without breakage Faulting: fracture of rock with displacement (either
vertical or horizontal movement)
DiastrophismTypes & Cause of Stress
3 Types of Stress Compressional: Rocks move together (convergent motion) Tensional: Opposite movement (divergent motion) Shearing: Tearing (transform motion)
Causes of stress Confining pressure Temperature
Extreme heat folds the rock without breakage Extreme cold fractures the rock
Strength/Composition of rock Time
1) Monocline: one-sided slope. Slight bend in otherwise parallel layers of rock.
2) Anticline: simple symmetrical upfold, resembles an arch. Due to compression.
3) Syncline: rock is warped downward – due to compression.
4) Overturned: upfold that has been pushed so vigorously from one side that it becomes over-steepened.
5) Overthrust: pressure was great enough to break the over-steepened area and cause a shear (a break).
Diastrophism5 types of folds
Diastrophism5 types of faults
Normal: One block is displaced up, the other down. Due to tension.
Reverse: A block is pushed up and over the other. Due to compression.
Strike-slip: Adjacent blocks are displaced laterally. Movement is entirely horizontal. Due to shearing.
Diastrophism5 types of faults (con’t)
Graben: Subsidence of one middle block (it drops down). Due to tension.
Horst: 2 reverse faults push a middle block up. Due to compression.
Earthquakes Sudden vibration within lithosphere from a quick
release of energy Result of rock moving due to folding or faulting
From point of origin (focus), energy is transmitted to surrounding rock by waves
Focus: Origin of stress and energy release.
Epicenter: Surface location of focus (directly above the origin).
EarthquakesTypes of Energy Waves
Body Waves Occur first. These are the initial waves emitted from the
earthquake. These occur in a specific order. 1st wave: Primary “P” wave. 2nd wave: Secondary “S” wave.
Surface Waves Occur after the body waves. These affect the surface of the
earth (we typically feel these). Type 1: Love wave. Type 2: Rayleigh wave.
Primary Wave (P wave)
Expansion & contraction of rockas wave moves through it
Fastest body wave Moves through solid rock and fluids
(e.g., ocean/water)
Secondary Wave (S wave)
Wave moves through rock up anddown and side-to-side
Slower than P wave Can only move through solid rock
EarthquakesTypes of Energy Waves: Body Waves
Love Wave Rolling/swaying effect
on surface Moves the ground from
side-to-side Fastest surface wave
Rayleigh Wave Rolls along ground like
an ocean wave Type most often felt
during quakes
EarthquakesTypes of Energy Waves: Surface Waves
Modified Mercalli Intensity Scale Measures “intensity” of earthquake (e.g., the amount of
shaking felt and the damage done). Very subjective: depends on the viewer’s description of the
earthquake event! Based on observations.
Richter Scale Measures the “magnitude” of earthquake (the energy
waves released). Based on readings from a seismograph, and examining the
actual energy waves.
EarthquakesMethods of measuring earthquakes
Modified Mercalli Intensity Scale
I Not felt
II Felt only by persons at rest
III/IV Felt by persons indoors only
V/VI Felt by all: some damage to plaster/chimneys
VII People run outdoors, damage to poorly built structures
VIII Well-built structures slightly damaged, poorly-built structures suffer major damage
IX Buildings shifted off foundation
X Some well-built structures destroyed
XI Few masonry structures remain standing, bridges destroyed
XII Damage is total: waves seen on ground, objects thrown into air
Richter Scale
Logarithmic Scale: Each increase in magnitude is 10x more energy released
• 5.0 is 10x greater than 4.0• 5.0 is 100x greater than 3.0• 5.0 is 1000x greater than 2.0
A seismologist reviews data taken by a seismograph.
Two important pieces of data to record: Lag Time: difference in time
between the P wave and the S wave (when each is picked up by the seismograph).
Designated as “S – P” Given in seconds.
Amplitude: the size of the largest S wave (the height of the wave).
Given in millimeters.
EarthquakesDetermining the magnitude of an earthquake
Take the information
from a seismogram (lag time and
amplitude)and
apply it on a nomograph.
EarthquakesDetermining the magnitude of an earthquake
Receive data from at least three seismographs…
Station 1: Eq occurred 10km from stationStation 2: Eq occurred 5 km from stationStation 3: Eq occurred 8 km from station
Plot them all together, and yourintersecting point is the epicenter!
EarthquakesDetermining the location of an earthquake: Triangulation
EarthquakesExample: New Madrid Fault
Earthquakes & aftershocks of 1811 and 1812
Tremors caused the Mississippi River to flow backwards
Caused Reelfoot Lake to be formed Felt far away - steeple bells rang in
Boston Sparse population in that region
accounted for a lack of serious damage Richter scale had not been established,
but would have registered 8.5 to 9.0 A projected 60% chance for a(nother)
damaging earthquake by 2020 and 90% by 2040
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