Exam 3

Earthquakes, soils, mass wasting, streams, glaciers, and groundwater.

earthquake

vibrations of earth produced by the rapid release of energy

Energy released from the source of the earthquake (focus) radiated out in all directions as seismic waves

Where does an earthquake happen?

Caused by the sudden slip on a fault located in the Earth’s crust.

How can earthquakes be explained?

theory of plate tectonics

How do you locate an earthquake?

using the difference in the arrival times between P and S wave recordings, which are related to distance

centers of an earthquake

focus (hypocenter)

epicenter

focus

AKA hypocenter

the place within Earth where earthquake waves originate

epicenter

Point on the surface, directly above the focus

fault

a fracture or zone of fractures in the Earth's crust where rocks have moved past each other

parts of a fault

hanging wall block

foot wall block

hanging wall block

part of a fault

the block named after miners being able to hang lanterns in

usually, on top of the break

foot wall block

part of a fault

the block named after miners being able to step on it

usually, on bottom of the break

main types of faults

dip-slip

strike-slip

normal fault

A type of fault where the hanging wall moves down relative to the footwall due to tensional forces pulling the rocks apart.

normal fault movement

divergent (pulling)

extensional

dip-slip fault

a type of fault describing vertical movement up or down along the dip (the fault line)

types of dip-slip faults

normal

reverse

thrust

reverse fault

type of fault which the hanging walls moves up relative to the footwall

shortening of the crust occurs

reverse fault movement

convergent (coming together)

compressional

thrust fault

special kind of reverse fault

shallow dip angle (less than 45 degrees)

displaces horizontally

10s of kms

evidence of thrust faults

in sedimentary rocks is seen when a sequence of the same rocks are repeated

strike-slip fault

a type of fault with displacement in the horizontal direction

movement parallel to strike of the fault line

strike is direction of the line formed by the intersection of the fault plane with the Earth’s surface

strike-slip fault location

associated with plate boundaries

How can you tell if a strike-slip happened to the left or the right?

You look at the features being displaced on the other side - for example, if a river across the fault moved to the left, the crust shifted to the left.

strike-slip fault stress

shear stress

Earthquakes are often preceded by __________________ and followed by _____________.

foreshocks

aftershocks

foreshocks

precede a major earthquake

might be used to predict future earthquakes

after shocks

adjustments after a major earthquake

usually much weaker than the main quake

elastic rebound theory

explained by H. Reid

rock can only deform so far before it breaks

releases waves of energy that were stored in the rock due to the stress applied to the area

As part of the elastic rebound theory, earthquakes occur as rock elasticity…

returns to its original shape

San Andreas Fault

\~750 mile fracture that runs from the Salton Sea area of SE California to the Mendocino in NW California

Different portions of the San Andreas Fault system behave in different ways depending on the tectonic stresses

Average displacement of 1-2 inches per year

* In 30 million years, LA will be adjacent to San Francisco

seismology

study of earthquake waves

seismographs

a device put deep within the bedrock of the crust to detect earthquakes

vibrates left and right to write on a rotating drum

produces seismograms

seismograms

strips of paper recorded by a seismograph

can be used to read the different types of earthquake waves

types of earthquake waves

body waves

surface waves

body waves

radiate from the focus

three dimensional

types of body waves

P-wave

S-wave

P-wave

type of body wave

occur first (primary)

fastest

push-pull like a slinky

goes through all matter types

S-wave

type of body wave

occurs second (secondary)

second fastest

shear vertical movement

only goes through solid

surface waves

confined to / near surface

2D away from epicenter

are slower than body waves - cause more property damage (3rd to show up)

\

types of surface ways

Love waves

Rayleigh waves

Love waves

less velocity than S-waves

shearing horizontal motion

3rd to arrive (after S-waves, before Rayleigh waves)

only travel through solids

Rayleigh waves

1/10 velocity of s-waves

very damaging to property

up-and-down rolling motion *and* back-and-forth motion, like Eddy currents in the ocean

4th (last wave)

How do you locate an epicenter?

use triangulation:

* Take 3 seismograms from different recording stations
* Circle a radius equal to the distance from the epicenter from each station
* See where the three circles intersect - that is your epicenter

How do you find the distance from the recording stations to the epicenter?

You take the difference in time in seconds between the p-wave and the s-wave.

Plot the interval on a S-P time travel curve - shows you the distance from epicenter the earthquake is.

time travel curve

body waves (p and s waves) go through the earth at different times based on distance

surface waves mostly linear

can find out the distance away from the epicenter a recording center is

Earthquake zones are closely correlated with…

plate boundaries

Why do earthquakes occur at plate boundaries?

95% of all energy released by earthquakes occurs along a plate margin

ex. Pacific ring of fire, oceanic ridge system

An earthquake can be measured by its…

intensity

magnitude

earthquake intensity

A measure of the degree of earthquake shaking at a given locale based on the amount of damage

most often measured by Modified Mercalli Intensity Scale (qualitative)

magnitude

introduced by Charles Richter in 1935 (quantitative)

Modified Mercalli Scale

qualitative measure to describe intensity

12 different classes

has some drawbacks

* damage changes with EQ distance
* damage somewhat depends on type of bed rock
* quality of construction matters
* population needs to be seen - what if there’s no one there?

Richter scale

quantitative measure to describe magnitude

based on largest seismic wave

designed specifically for southern California

logarithmic scale

drawback: does not estimate adequately size of very large earthquakes with lots of energy released

moment magnitude scale

scale for very large, energetic EQs

derived from displacement amount that occur along a fault zone (not ground away from the site of the EQ)

based on its seismic moment

seismic amount

from moment magnitude scale

product of:

* average amount of slip on the fault that produced the EQ
* area actually ruptured
* strength (resistance to distortion)of the rocks that failed

regolith

The soft and loose rock particles that result from \n weathering

composed of silicate clay, quartz silt, sand and gravel

rarely seen

Why is regolith rarely seen in nature?

organic and hydrologic forces quickly act on it, by:

* soil formation
* erosion to create sed rx

soil

loose, unconsolidated material on top of bedrock that has been modded / acted upon via physical, chemical and biological processes so that it can sustain rooted plant growth

soil horizons definition

used to assess the factors which have gone into the formation of a specific soil

develop from the top down - each horizion have unique set of physical and chemical properties

All of the soil horizons together form a…

soil profile

soil horizon types from top to bottom

Horizon A

* A0
* A1
* A2

Horizon B

Horizon C

bedrock

A horizon

dark gray to black

rich in organic matter

leaching carries dissolved ions and fine particles

often divided into 3 sublayers

A0 horizon

surface layer of leaf litter, consists of \n decomposed and non-decomposed organic \n matter (humus)

A1 horizon

humus-rich, dark layer dark in color, layer \n rich in organic material.

Forms just below the \n surface vegetation.

humus

decomposed plant material found in top soil horizons (A1 horizon especially, that contributes to \n the formation of organic acids (producing slightly \n acidic water) that accelerate leaching

A2 horizon

light colored, bleached out layer, zone of \n leaching.

This is the area where water moves \n downward. The water carries dissolved materials \n to lower horizons

B horizon

\
Zone of accumulation \n Accumulates leached material from above \n Enriched in clay minerals and oxides

* Lighter in color, can be red if a lot of iron \n oxides

Harder

Not much in the way of organic material

C horizon

layer of somewhat weathered parent material

directly above unaltered bedrock

composed of parent rock in various stages of weathering

bedrock

right underneath the soil horizons of unaltered rock

not part of the soil horizons

forming soils

well developed, fertile soil can take 100s or 1000s of years

controlled by 5 main processes

5 main process of forming soils

parent rock

relief and landscape factors

climate

biosphere

time

parent rock of soil

original composition influences soil composition

can either be fine or coarse grained, mafic or silicic

granitic rock soil type

break down to quartz and feldspars

feldspars break down to clay

basaltic rock soil tpye

fine grained, no sand, lots of clay

soils may not drain well, but can be fertile

relief and landscape factors on soils

influences the amount and rate of erosion

soils tend to be thick on flat surface and thin on steep slopes, due to the force of gravity

climate factors on soil

affects soil thickness and type of soil found

usually can be sorted by wet climate and dry climates

wet climate soils

thick

great deal of water and acids move down the soil horizons, creating accumulations of metal oxides

chemical weathering dominant

dry climate soils

thin, rocky soils, very little organic matter, little leaching

can have salts form in their soils, due to evaporation of water

physical weathering dominant

biosphere factor on soils

adds organic matter and mixes soil

plants break up rocks,

burrowing animals bring soil to surface & aerates / hydrates underground

decaying organic material adds CO2 to soil → H2CO3 (carbonic acid)

other organic acids also form to speed up chemical weathering processes

\

time factor on soils

soil forming is slow

character of soil changes with time

as time goes on, soil thickness increases

types of soils

residual

transported

residual soils

developed from the weathering of rock directly beneath the soil

may take a long time to develop

ex. decomposed granite, engineers like to build on it because it’s very stable

transported soils

did not form from local rock, but instead formed from regolith transported from some other area

usually named depending on their agent of transport (usually water, ice, or wind)

mass wasting

down slope movement of soil and rock

process of erosion - consequence of weathering

especially problematic in areas of high relief

Mass wasting is mainly due to…

the force of gravity

Mass wasting can also be triggered by…

earthquakes

heavy rain

weakness in the rock

climate

vegetation loss from fires / land clearings

high relief

steep slope angle

water and/or ice

* both as a weight and a lubricant

amount of loose rock and debris along course

relief

vertical difference between high and low

higher relief causes higher speeds and more momentum in mass wasting

role of water in mass wasting

may act to increase/decrease cohesive strength

* dry regolith / soil has little / no cohesion
* damp regolith / soil is sticky
* saturated regolith / soil flows easily

may dissolve cementing material

Water acts as a lubricant in mass wasting by… because…

reducing shear strength

as amount ofwater in debris increases, rate of movement increases as welll

The types of mass movement are based on…

type of material involved

how it moves

water content of material (influence of nature of movement)

velocity