Geophysical Hazards Flashcards
Convection currents
Convection currents in the mantle occur due to density differences caused by variations in temperature and composition.
It consists of hotter and less dense mantle material rising under diverging zones while colder, denser material sinks in subduction zones
Cause the movement of the plate tectonics
Plate Boundaries
Transform boundaries - Plates move past each other, but build up stress due to friction which is released as an earthquake. Eg - NZ’s Alpine Fault
Divergent boundaries - Plates slide apart from each other, and the space that this creates is filled with new crust from the magma formed below
Convergent boundaries - Plates slide toward each other. When one plate moves underneath the other, it is known as a subduction zone
Hot spots
An area on Earth that exists over a mantle plume. A mantle plume is an area under the Earth’s crust where magma is hotter than surrounding magma. Heat from this magma causes melting and thinning of the rocky crust, leading to volcanoes forming above the plume. As the tectonic plates move but the mantle plumes do not, chains of volcanoes are created on the Earth’s surface.
Types of volcanoes
Shield Volcanoes
Thin lava flows
Gentle slopes as a result of low lava viscosity, allowing lavas to flow fast and far
Low angle slopes
Largest volcanoes
Lava lakes, Summit calderas, Rift zones, Lava flows, lava tubes
Composite Volcanoes
Made up of layers of lava, volcanic ash and fragmented rocks built up over time as the volcano erupts through a vent or group of vents at the summit’s crater
More viscous lava, forming tall peaks
Composed of lava flows, pyroclastic deposits and mudflow deposits as well as lava domes
Cinder Cones
Form after violent eruptions blow lava fragments into the air, which then solidify and fall around the volcanic vent
Form when low viscosity lava with lots of gas erupts
Short (seldom above 500m), forming steep slopes with a very wide summit crater
Volcanoes - secondary hazards
Pyroclastic flows - a dense, destructive mass of hot ash, lava fragments and gasses ejected from a volcano and flowing at great speeds of up to 200m/s
Lahars - A violent type of mudflow or debris flow composed of a slurry of pyroclastic material, rocky debris and water. Lahars can occur on Mt Ruapehu when the crater lake gets too high.
Landslides - A mass movement of material such as rock, earth or debris down a slope. Tall, steep volcanic cones weakened by rise and eruption of molten rocks are most likely to have a landslide.
Human triggers of earthquakes
Dam Building
Added weight of a reservoir adds stress to faults, causing them to rupture.
Changes the fluid pressure in micro cracks and fissures in the ground and by the water that seeps into cracks
The filling of the Zipingpu Dam was linked to the devastating 2008 Sichuan earthquake that killed approx. 80,000, triggered by the weight of 320 million tons of water collected in the reservoir over a well-known fault line
Resource Extraction
The removal of material from the earth can cause instability, leading to sudden collapses that trigger earthquakes
The 1989 Newcastle earthquake has been attributed to deep coal mining in the region
Earthquakes triggered by human activity can occur far from the edges of tectonic plates
Earthquakes - secondary hazards
Tsunamis
Caused by earthquakes in subduction zones, the two plates build up friction against each other, accumulating energy over decades and centuries, until the energy exceeds the frictional forces and the overriding plate snaps back. This sudden motion gives a big shove to the overlying water, causing a tsunami
Landslides
The more the ground moves, the weaker it gets. High stresses, delivered quickly from an earthquake will impact a material’s resistance to sliding
An earthquake may weaken the materials that make up a slope or apply inertial stress that makes the slope unstable.
Liquefaction
The process by which water-saturated sediment temporarily loses strength and acts like a fluid
Most likely to occur in wet, sandy solids
Mass Movement
Movement of surface material caused by gravity
Water adds weight to the soil, fills pore spaces and exerts pressure which tends to push apart individual grains, making it more vulnerable
Triggers of landslides
Rainfall
Steep slopes
Deforestation/lack of vegetation
Weak bedrock
Near earthquake fault zones
Erosion
How NZ monitors and prepares for earthquakes & volcanoes
How NZ monitors and prepares for earthquakes
Hundreds of seismographs and strong motion sensors monitoring shakes and quakes each year
Continuous GPS being used to monitor “slow” earthquakes, recording land movement
How NZ monitors and prepares for volcanic eruptions
GNS science has a volcanic monitoring group to monitor active volcanoes
Link rocks to past eruptions to see how far material travelled
Conduct geochemical analysis of material expelled to understand the state of the Volcano’s magma system
Vulnerability & Risk of natural disasters
Less developed areas are more vulnerable as they lack money and resources
MEDCs are more likely to have the resources and technology for monitoring, protection and response against natural disasters (developing plans and preparing beforehand, rebuilding and recovering afterward)
Buildings that aren’t haven’t considered the effects of a natural disaster, e.g not earthquake proof are more vulnerable
Being more educated around disasters makes you less vulnerable as you can be more prepared and knowledgeable
Places with higher population density are more at risk
Areas with lots of older people (e.g Japan 2011 tsunami) and young children more at risk
Governments with laws prioritising being well prepared for disasters are less at risk
Being prepared means you’re less at risk, e.g NZ has a search and rescue team ready to go at any minute that helped with the Christchurch earthquakes
Building regulations, Land use zoning, Early warning systems
Building regulations
Buildings in areas prone to earthquakes should be earthquake proof, some countries e.g NZ have building codes that require buildings to be able to withstand the forces expected during an earthquake
An area that had bad liquefaction from the Christchurch 2011 earthquake is now a park, and not allowed to be rebuilt on
Land use zoning
Development and activities should be restricted in areas prone to flooding, e.g floodplains, as well as activities that aggravate flooding
Minimising or preventing the exposure of people, infrastructure and valuable assets to volcanic hazards
Early warning systems
National Geohazards Monitoring Centre (NGMC) provides 24/7 monitoring of geohazards in NZ
Reliable early warning and emergency response systems have been set up to manage lahars on Mt Ruapehu
Personal resilience, Insurance, Adoption of new technology
Personal resilience
Create a plan with your family on what to do as communication can sometimes be cut off
Have an emergency kit
Be educated on what to do
Insurance
In NZ natural disaster insurance will pay up to $345,000 to repair/rebuild your home, included in your home and landlord insurance
Adoption of new technology
Android phones alert users a few seconds/minutes before an earthquake occurs
75 tsunami warning sirens were installed in Tonga as well as an evacuation system which played a crucial role when the Tonga volcano erupted. Tens of thousands of Tongas evacuated leading to only one death.
Lava diversions
Diverting/delaying lava flows can be attempted by the use of explosives,m water and physical barriers
1973 Icelandic eruption, billions of gallons of seawater was pumped through a network of pipes laid out across the lava to cool the flow and stop its advance
Slope stabilisation, Earthquake proof building designs, Tsunami defences
Slope stabilisation
Improving drainage, changing the geometry of the slope and reinforcing the soil
A major slip above SH1 from the 2016 Kaikoura earthquake required 3500 square metres of actively stabilised mesh with over 600 hollow bar anchors installed to 6-9m in depth
Earthquake proof building designs
Floors and walls can be constructed to transfer the shaking energy downward through the building and back to the ground
Cross-bracing to give added strength and prevent twisting
Foundations set deep into the ground
Rubber shock absorbers
Base isolator allows sideways motion
Tsunami defences
High concrete walls being built along parts of Japan’s coast
15m high, offering more protection and safely
However walls block the sea view meaning locals won’t be able to see the sea and tourism could be affected
Response - Before, During, After
Pre-hazard response
Preparation, prevention and education
Stockpiling emergency food and equipment
Nz people were aware and educated on what to do (taught in primary school)
Emergency responders had previous experiences from the 2010-12 Canterbury earthquakes, meaning they had developed processes for similar future events so were well-prepared
During and immediately after
Implementation of coordinated emergency measures by organisations such as the police, army, ambulance and emergency service teams
Evacuation to other areas
Establishing emergency road access due to blocked/broken roads
Long term response
Repair major damage, minimise effects for future similar events
Rebuilding buildings following seismic assessment guidelines
Stabilising slopes - Rockfall protection mesh, rock anchors, rockfall catch fences
