Lab 6 Earthquakes. Concepts Diastrophism Types & causes of stress 5 types of folds 5 types of faults Earthquakes Focus, epicenter Types of energy waves.

Lab 6

Earthquakes

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

New Madrid Fault

Atlanta

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

EarthquakesDetermining the magnitude of an earthquake

Result from a seismograph machine: Seismogram

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