9.1 - Earthquakes and Volcanoes

how does geology affect earthquake waves

ground amplification - can become amplified when passing from hard, solid rock into softer, less dense, or unconsolidated material

EXAMPLE - ground amplification affecting the severity of an earthquake

Mexico City, 1985

built on a drained lake → saturated clay and silt amplified waves → greater shaking

ground liquefaction

water saturated (e.g. near a body of water), loose sediments temporarily lose their strength during shaking and behave like a liquid

explain why ground liquefaction happens

water cannot escape and the shaking increases pore water pressure

saturated pore spaces expand, reducing internal friction

ground cannot support its own weight

EXAMPLE - ground liquefaction affecting the severity of an earthquake

Christchurch, New Zealand, 2011

• almost 200 deaths, over 20,000 homes damaged

• caused a residential red zone - building was prohibited

tsunami definition

large powerful sea waves generated by sudden displacement of water column at sea

factors causing tsunamis

submarine volcanic eruption

submarine landslide

submarine earthquake

define wave shoaling

less water to move through as approaches the coast

wave compresses - amplitude increases, wavelength decreases

EXAMPLE - tsunami

2004 Indian Ocean Boxing Day tsunami

• over 200k deaths, $15B economic loss

• primarily affected Indonesia, India, Sri Lanka, Thailand

• destroyed coastal buildings and businesses, and coastal ecosystems such as beaches and mangroves across many countries

list human factors affecting impact of earthquake

pop density

building quality/EQ resistance

preparedness for short term response

preparedness for long term response

how does time of year affect impact of an earthquake

winter - displaced people with no home face freezing temperature

summer - some places welcome large tourist populations in summer

2 examples of building quality exacerbating earthquake impact

Haiti 2010

• only M7.0, several others that year were stronger

• but many multistorey concrete buildings packed closely together, many shanty towns due to poor population → very low strength

Turkey-Syria 2023

• building regulations existed but very poorly enforced

• many soft storey buildings → pancake collapse → casualties have no way out

examples of earthquake resistant building features (with example)

Japan has strict building regulations for earthquake resistance

• base isolators - stacks of rubber and steel that can flex and separate the ground’s movement from the building (not skyscrapers)

• tuned mass dampers - large counterweights installed within skyscrapers (e.g. Taipei 101) which can move to opposite building’s swaying†

• steel or wood frames, less brittle than stone

• cross bracing

seismic gap theory

sections of a fault that have gone without earthquakes for a long time are more likely to experience one in the future due to the buildup of tension

define volcano

mountain with a crater or vent

decompression melting

plates move apart at divergent boundaries → reduced pressure on mantle → lowers m.p. of mantle → plumes of magma

flux melting

subduction plate at a convergent boundary releases water from hydrous minerals → lowers m.p. of mantle → plumes of magma

how do divergent plate boundaries affect depth of focus of earthquakes (with example)

divergent - only shallow focus

• earthquakes result from tensional forces and fracturing of new crust

• Mid-Atlantic Ridge

how do convergent plate boundaries affect depth of focus of earthquakes (with example)

convergent - range of depths

• oceanic plates subducts, friction and tension build up in the Wadati Benioff zone in the mantle, which eventually release as a massive amount of energy, causing an earthquake

• Peru-Chile trench

how do transform plate boundaries affect depth of focus of earthquakes (with example)

mainly shallow focus because plates slide horizontally without significant vertical movement into the mantle - stress builds up along faults and is released close to the surface

• San Andreas Fault, California

what is a hotspot

thermal plume causes mantle to rise slowly

EXAMPLE of landform caused by hot spot

Hawaii

define tephra

rock fragments thrown into the air by a volcanic eruption

effusive vs explosive eruptions

effusive

• runny often basaltic lava allows volcanic gases to escape easily → gas pressure doesnt build up

• creates shield volcanoes, found at fissure volcanoes

• common at hot spots and divergent boundaries

• e.g. Mauna Loa, Hawaii

explosive

• tephra and viscous magma traps gases → pressure builds and released suddenly

• tephra creates cinder cones, thick lava creates stratovolcanoes, eruptions cause calderas

• common at convergent boundaries

• e.g. Mt St Helens, USA

primary volcanic hazards

volcanic gases

lava flows

tephra

ash fallout

pyroclastic flows

dangers of volcanic gases

toxic

• CO2 displaces oxygen → suffocation

• SO2 is a respiratory irritant

• H2S is highly toxic and lethal

many are colourless and invisible

can contaminate water and crops → secondary impacts

e.g. Lake Nyos, Cameroon

• CO2 suffocated thousands of people and cattle

dangers of lava flows with EXAMPLE

rare to directly cause loss of life due to slow speed

destroy buildings, vegetations, and roads in path

Kilauea 2018 - homes destroyed and roads linking communities cut off, disrupting emergency access

list approaches to volcanic hazard management

• hazard mapping

• scientific monitoring

4 Ps of hazard management

Prediction

Protection

Planning

Preparedness

how is volcanic hazard mapping used to risk assess

• valleys and river channels = high risk for lahars

• close to volcano = pyroclastic flow

• downwind = ashfall

• restrict development

• identify safe areas and evac routes

• where to deploy aid

example where volcanic hazard mapping failed

Mt St Helens, 1980

• hazard map based on past eruptions

• lateral explosion not expected → exclusion zone didnt include that area

• death

types of scientific monitoring of eruption precursors

seismic

ground deformation

gas emission

temperature

hydrological