Geohazards Flashcards
geohazard def
a geological condition that is dangerous/ potentially to enviro or people who live within it
examples of geohazards
earthquakes
vol eruptions
tsunamis
landslides
subsidence
avalanche
cliff fall
what is the focus and epicentre of an earthquake
focus-where movement occurred
point within the earth at which the earthquake og as movement occurs along fault plane
sesmic waves radiate away from it in all directions
epicentre point on surface above focus
how is stored elastic strain energy released?
stress = forces
strain = reaction
relative movement of rock on either side of fault applies stress
rock undergoes strain
this process transfers energy and increases elastic strain stored in rock
energy releases and elastic energy decreases
energy released due to movement
released as heat and seismic waves (greater amp greater energy)
amplitude def
maximum extent of displacement of an oscillation from the position of rest
what does amplitude show
greater the amp = greater energy released
what happens to amp as waves move away from focus
reduces as energy released and transferred to surrounding rock
attenuation of seismic waves
loss of energy experienced by a wave shown as reduction in amp as it propagates through material
waves faster through rigid as transfer is easier so attenuation is reduced in more rigid rocks
amp decreases with distance from focus
Mercalli scale
magnitude scale for measuring earthquakes
empirical
gives 12 categories earthquake can fall into based on destruction and how people felt it
instrumental to cataclysmic
Mercalli scale pros and cons
pros-
don’t need specific equation
don’t need formal training to understand
cons-
empirical (based on observation/experience )
subjective
not comparable - e.g. all observation relate to building but they are diff in each country
may not remember experience
intensity decreases from focus but mag doesn’t
different earthquake measurement systems in time order
earliest to oldest
Mercalli- Richter- moment mag
Richter scale
measures energy output of the earthquake
1-10
use seismogram:
lag time - time diff between P and S waves tells us distance from focus
height of greatest S wave
plot on richter scale graph tells mag
should give same value no matter where
log so 1= 10x amp
Pros and cons of Richter scale
Pros:
Maths equ so less subjective
but also has desc for public
Cons:
assume same rock type
larger is underrepresented and less accurate the larger it gets (seismologists only measured at certain frequencies)
outdated
moment magnitude scale
What is used now
when waves arrive at seismometer measure:
lag time - tells us distance from focus
amplitude and waves
actual displacement of rock at quake site (new, not in richter)
Pros of moment mag
interpret broader range of seismic frequencies- determines more accurate e from large earthquake
accounts for rock type and rigidity
moment mag equ
(given to us)
Mw= 2/3logE-6.1
different wave types
P- primary
S- secondary
L-love
R-rayleigh
features of P waves and effect on buildings
SEE BOOKLET FOR DIAGRAM
features-
travel through L and S
body waves
arrive first- fastest
longitudinal
effect on building-
least dangerous
features of S waves and effect on buildings
SEE BOOKLET FOR DIAGRAM
features-
can travel through S
body waves
arrive second- second fastest
shear waves - transverse
effect on buildings-
S waves more destructive than P
as greater amp
horizontal worse than vertical
features of L waves and effect on buildings
SEE BOOKLET FOR DIAGRAM
Features-
Surface waves do the most damage
shear wave where shearing motion is confined to horizontal plane at earths surface (oscillate)
effect on buildings-
responsible for most damage
greatest effect- sideways movement is more effective
features of R waves and effect on buildings
SEE BOOKLET FOR DIAGRAM
features- surface waves are most destructive
slowest
earths surface moves in a vertical elliptical path parallel to wave movement
effect on building-
worse than P and S
most movement with R- up and down so less destructive
effect on ground movement on built environment
L waves cause most damage to buildings
lose objects thrown upwards
buildings fracture
pancake
freeways collapse
brick and stonework seperate along motar
sects of utility pipes separate
how does rock competency affect how e is transferred and effect on buildings
comp rocks allow easier propergation of seismic waves and move on
less e dissipated to rock
less movement
also moves quicker through comp rock
liquefaction and effect on buildings
saturated or partially saturated unconsolidated material losing strength and rigidity in response to applied stress
e.g. Earthquake
unconsolidated and incompetent rock holds more H2O in pore spaces
when rock compressed due to seismic waves, pore spaces reduce and H2O comes up and out
loss of strength makes buildings tip and sink