Most from plate boundaries, hot spots and fault lines
Some can be non-tectonic, caused by human activity putting too much stress on
faults (e.g. reservoir construction)
Plates try to move, get stuck and stress builds. They break free but causes
pressure release of energy, sends seismic waves from focus.
Epicentre directly above focus.
Seismic waves strongest at epicentre but damage doesn’t always occur there
Foci occur in narrow zones along plate boundaries, but effects extend far beyond.
Types of seismic zone: destructive/convergent, collision, conservative and
constructive
Shaking: seismic waves sent out. Primary waves travel fastest, and secondary
waves follow. Surface waves are slowest. Long waves with large amplitude cause
most damage as most violent. Buildings destroyed by shaking and ground tilting as
shock wave passes.
Measuring: magnitude on Richter scale. Intensity on 12-point Mercalli scale.
Secondary earthquake hazards
Landslides: Nepal April 2015 7.9, ground shook at base camp as massive rock snow
and ice fall. 19 deaths. Frost shattered rocks broke free and fell. Blocked roads
which delayed aid arrival.
Soil liquefaction: unconsolidated/weak rocks act as liquid and flow, leads to sinking
and destroyed buildings. Occurs when groundwater is near surface and soft
sediment mixes with water.
Tsunamis: high long period waves in ocean, result from sudden sea bed
displacement. In open ocean, crests are small and lengths long so hard to monitor.
Crests slow at shore and rise in height. Waves retreat then hit coast with force.
30m high and flood coasts far away from focus. Travel rapidly so little warning.
Floods: more damage and death if dam wall collapses or large landslides fall into a
lake resulting in sudden landslide down valley.
Tohoku (Sendai) earthquake and tsunami in Japan 2011
9.0, 32km off NE coast of Honshu where Pacific subducts Eurasian. Thrust faulting
uplifted sea floors by 35m. Affected 2000km of densely populated lowlands.
Prepared but inadequate defences, 38m.
Effect on lives and property: fires in all cities and oil refinery. Millions of home
without power. Fukushima nuclear power station closed as severely damaged and
caused partial meltdown and radioactive leads, evacuation zone 3 years after. Most
deaths caused by tsunami, followed after EQ so not enough warning time. Washed
away planes at Sendai airport. Coast shape amplified tsunami as funnelled up
estuaries and into bays. Kikuzenta was totally submerged. Few buildings left
standing and everything swept up. Damage made rescue and recovery hard and
people still living in temporary housing 3 years after. Economic loss of $US 300
billion, costliest ever. Caused major industries to halt production.
Comparison of Haiti and Christchurch earthquakes
Port-au-Prince, Haiti. January 2010; 16:53. 7.0. 13km focus depth. 200,000 deaths.
Epicentre 25km from city centre. 2.5m pop. Caribbean plate moves with North
American plate.
Effects on lives and property: mass graves to reduce disease so death estimate
wrong. Shaking lasted 1 minute. Destroyed hospitals, government buildings,
250,000 residences and 30,000 commercial buildings. Disrupted power,
communications and water. Aid delayed as airport control tower and port
destroyed, many roads blocked. Looting and violence for relief workers trying to
keep food and water safe to distribute. 8600 died from cholera form poor sanitation
in tent camps.
Reasons for EQ severity: capital city. Liquefaction as city built on loose sediments
so seismic waves amplified. One runway so limited aid. Poor country without
building regs as cheap construction with no reinforcement or foundations. Heavy
concrete buildings collapse as no steel in support columns. Live in flimsy shacks on
steep unstable slopes. Unexpected as no recent EQ. No army and few emergency
services.
Recovery: poor resilience as depend on overseas aid. Only half of debris cleared
after 3 years and 280,000 still in camps with poor sanitation. 1/5 pop lost jobs as
factories destroyed.
Largest destroyed amount but small economic cost, $US9billion. Few insured
against loses so biggest cost was rebuilding.
Christchurch, population of 400,000. September 2010. 4:35. 7.1. No deaths.
February 2011. 12:51. 6.3. 185 deaths. Pacific subducts Australian.
All damage done by 2011 after shock EQ as higher magnitude 210 had weakened
and damaged structures so they’d easily collapse. Some parts damaged by
liquefaction. 2011 shallower focus, close to centre and in lunch so people out.
Effects on lives and property: vertical and horizontal shaking at same time
destroyed 1000 major buildings. Intensity x4 bigger than Haiti. Liquefaction
undermined building foundations and destroyed houses on soft sand, tallest hotel
dropped one side 1m, forced road surface up, slowed rescue efforts. aftershocks
made recovery hard. TV buildings collapsed and caught fire, 185 deaths and 1500
injured. Water and sewage systems damaged and power cuts affected many,
services restored in a fortnight and communications only down a short time.
Economic cost of $40billion, mainly rebuilding and insurance. Undamaged airport
so aid arrived fast.
Perception of risk: strict building codes to withstand EQs and max height of 28m.
earlier emergency planning, alert immediately, forces/agencies start rescue fast
aided by satellites.
Factors influencing amount of damage caused by shockwaves
Amount of energy released, indicted by the Richter scale
Focus depth, shallow have greater effect, rocks absorb energy of deep focus
Number/strength of foreshocks/aftershocks, buildings weakened/damaged by one
EQ may collapse when further damaged by another.
Distance from epicentre, increases, the shockwave strength decreases.
Bedrock nature, if solid, limited damage but where weak rocks/loose sands,
liquefaction occurs. Very destructive particularly on a slope.
Population and building density at or near epicentre
Strength/type of buildings. Modern in HICs are strengthened against EQ damage.
Time of day. Fewer deaths at night than rush hour when people out.
Death depends on risk perception, leads to amount of preparation. Less in LICs
where funding is a problem
Managing earthquake hazards
Prediction: not possible to predict exactly when or where. short-term predictions
can’t be made. Average movement calculated. Seismic gap theory for where a
strong EQ will occur by looking at which section hasn’t had an EQ for a long time.
Past records used. Longer the time, greater the stress
Monitoring zones: horizontal movement detected by measuring the change in time
it takes a laser beam to move between 2 points either side of a fault. slopes
changes detected by title metre. variations in earth’s magnetic field resulting from
changes in rock’s stresses, measured by a magnetometer. seismograph records
monitored, clusters of small EQs precede a large one. amount of radon gas rising to
the surface is measured as can increase before an EQ.
Hazard mapping: done to show areas of ground likely to liquefy, fault locations and
dates of past movements, epicentres, areas where landslides and tsunamis may
result.
Hard engineering: what infrastructures need replacing moving or strengthening.
cost effective to make changes rather than restore
Strengthening buildings: skyscrapers built in a pyramid shape in Japan. measures
add to the cost and builders are more profit focused than safety. poor rural areas
have thatch for roofs and mud/straw for walls.
Land use zoning: land uses that are potential fire/explosion risks are away from
home and built on solid rock. land likely to liquefy isn’t used for buildings
Increasing risk perception: EQ drills allow practice for what to do are regular in
tectonic areas. evacuation points in every neighbourhood. maps of routes to follow
and leaflets of how to dress and what to take. informed how to improve safety in
homes. survival pack near an exit and food and drink to last 3 days.
Prediction, mapping and monitoring tsunamis: tsunami watch starts when a
seismic event is detected that could cause one. sea level height detectors confirm
if one has been generated. seismograph stations and tidal stations across the
ocean send warnings. satellites monitor as it travels. communications systems at
coasts make people aware of danger by various methods.