EQ 3: what are the main hazards generated by seismic activity?

what is an earthquake?

the shaking or trembling of the ground through the release of stress built up in the earth’s crust due to tension, compression or shearing of rocks

what are the four features of an earthquake?

fault - fracture in a rock which makes up the earth’s crust
epicentre - the point at the surface directly above the focus
focus - the point within the earth where an earthquake originates
seismic waves - transmit energy released by an earthquakes

what is elastic rebound theory?

explains how energy is stored in rocks
rocks bend until strength of rock is exceeded, rupture occurs and rocks rebound to underformed shape
energy is released in waves that radiate outwards from fault

what are earthquakes categorised based on?

depth of focus
shallow (0-70km) - greatest damage and most common (75%), tend to be lower magnitude
Intermediate (70-300km)
deep (300-700km)

what are the three types of seismic waves?

primary, secondary, and surface (love + raleigh)

what are features of primary waves?

travel fastest through the earth, longitudinal waves, move through solids and liquids

what are features of secondary waves?

travel at half speed of primary waves, transverse waves, travel through solids only but do more damage, first strong shaking felt by people

what are features of love waves?

shake ground in horizontal movement, only travels through surface of crust

what are features of rayleigh waves?

slowest, rolling motion that produces vertical movement

what are the three types of shocks caused by earthquakes?

main shocks, foreshocks, aftershocks

what are three ways of assessing an earthquake?

richter scale, mercalli scale, moment magnitude scale

how does the richter scale work?

measures magnitude of earthquake, logarithmic scale (10-fold increase in strength + 30-fold increase in energy released)

how does the mercalli scale work?

measures intensity of an earthquake, subjective, qualitative assessment and uses roman numerals

why is the richter scale no longer used by scientists?

underestimated size of large earthquakes

how does the moment magnitude scale work?

measures seismic moment, the force needed to generate the recorded waves
Mo = u (rock rigidity), D (distance that one block slips relative to other block), A (area that ruptured between blocks)
only used for large earthquakes

what are factors impacting the severity of earthquakes?

• construction techniques

• prediction, monitoring, evacuation

• emergency services

• time of day

• education

• spread of disease

case study evidence for construction techniques

chile 2010 earthquake

• many homes built in poor rural areas were constructed through inadequate building techniques

• over 500,000 homes were made uninhabitable, and 370,000 were destroyed

case study evidence for prediction, monitoring, evacuation

Tohoku Earthquake, Japan, 2011

• one minute before the earthquake was felt in Tokyo, the Earthquake Warning System (constituted by 1000 seismometers across Japan, sent out warnings to people via SMS on all major mobile phone services

• this gave at least 31 seconds of warning before the initial P wave hit- potentially saving hundreds of lives

case study evidence for spread of earthquake

Haiti Earthquake, 2010

• due to inadequate access to clean drinking water and poor sanitation, and poor distribution of health supplies, Haiti experiences a severe outbreak of cholera

• 2010- August 2013, 8321 Haitians were killed by the disease

• UN initiated a \$2.2 billion initiative to combat cholera

how do seismic events affect landscape?

it can cause faults and faulting

define ‘stress’

• the amount of force exerted over a given area

• in the lithosphere this is almost always compressional

define ‘fault’

• a break where slabs of the crust slip past each other, causing a break

• faults are usually along plate boundaries

what is the most significant earthquake landscape

rift valleys

what are rift valleys also known as?

graben and horsts

what are the 3 types of faults?

• normal faults

• strike-slip faults

• reverse faults

features of a normal fault

• created by tension

• at an angle, so one side of fault is above the other

features of a strike-up fault

• transcurrent fault

• created by shearing both sides of fault in a vertical motion

features of a reverse fault

• created by compressional forces

• similar to normal faults but reversed as hanging wall move upawards

what are megathrust earthquakes?

a very large reverse fault on a subduction zone

what is deformation (folding)

refers to the distortion (strain) expressed in rock- change in the shape/ form of the rock

folding creates mountain ranges, casn influence coastal geology

folds can be eroded to form ridge and valley landscapes

how are overthrust faults created

created by compressional forces

one side of the fault is thrust over the top of and along the other side

what are the additional types of hazards earthquakes can pose?

• tsunamis

• ground shaking and displacement

• liquefaction

• landslides and avalanches

explain the hazard ‘tsunami’

• means ‘harbour wave’

• a tsunami is a series of waves made in an ocean by an underwater earthquake (landslide, volcanic eruption)

• occur mostly in pacific ocean

• as tsunami waves travel to shallower water near the coast, it slows and grows in height

what are the impacts of tsunamis

• tsunamis cause huge destruction when they hit coastlines

• cause flooding hundreds of metres inwards, crush homes and buildings

which plate boundaries are most tsunamis generated at

convergent plate boundaries

how do earthquakes in subduction zones cause tsunamis

1. oceanic plate is forced down into the mantle by convection currents and slap pull

2. there is lots of friction between subducting and overriding plates

3. friction causes plates to become stuck, stuck plates descend into mantle, resulting in a slow distortion of the overriding plate

4. energy accumulates in the overriding plate until it exceeds frictional forces between the two stuck plates

5. then, the overriding plate snaps bacl into the unrestrained position

6. the sudden motion is the cause of the tsunami

Tsunami case study- Boxing day Tsunami 2004

• epicentre, west coast of Sumatra, Indonesia

• megathrust earthquake (since the Indian plate was subducted by Burma plate, killed 230-280,000 people)

• an estimate of 1600km of fault surface slipped about 15m along the subduction zone, where Indian plate slid under, overriding the Burma plate

what are the impacts of ground shaking and displacement

• the vertical + horizontal movement of the ground

• severity of the ground shaking depends on:

- earthquake magnitude

- distance from epicentr

- local geology

• ground movements that displace rocks along fault lines can rip apart pipelines and sewers causing buildings to collapse

what is liquefaction

• when earthquakes strike areas with surface materials, causing them to behave like liquids

• liquefaction was a major issue during the Kobe 2010 earthquake, where 200 berths in the port were destroyed, affecting trade globally

what are landslides and avalanches

• ground shaking and liquefaction→ slope failure

• e.g. Nepal 2015 earthquake→ triggered lots of landslides and avalanches on Mt Everest,

• landslides block transport routes in mountainous regions