Hazards 2- Geophysical hazard risk Flashcards
Map of the distribution of volcanoes and earthquakes
Describe the distribution of volcanoes
-Most volcanoes are found at plate boundaries.
-Some exceptions - Hawaii, which occur over hotspots.
-75% of the Earth’s 550 historically active volcanoes lie along the Pacific Ring of Fire.
-Other areas of active vulcanicity include Iceland, Montserrat and Mt Nyiragongo in the DRC.
At what types of plate boundaries does volcanic activity take place?
-At convergent boundaries, where one plate is subducted under another.
-If both plates are oceanic -submarine volcanoes erupt to produce island arcs, like that of the Aleutian Island Arc off Alaska.
-Continental & oceanic plates produces significant volcanic mountain ranges such as the Andes, which forms from the subducting Nazca plate under South America.
-These are the result of high silicate magma rising upwards through the continental crust.
-Volcanoes also form at hotspots and divergent plate boundaries, where shield volcanoes are more common. Eg/ Mauna Loa
At what plate boundaries do earthquakes occur?
At all plate boundaries
Map of the distribution of earthquakes
What is the ocean fracture zone (OFZ)?
A belt of activity through the oceans along the mid-ocean ridges, coming ashore in Africa, the Red sea, the Dead sea rift and California.
What is the continental fracture zone (CFZ)?
A belt of activity following the mountain ranges from Spain, via the Alps, to the Middle East, the Himalayas to the East Indies.
Describe the earthquakes at conservative plate boundaries
-High frequency, low magnitude earthquakes are more associated with conservative plate boundaries such as the San Andreas Fault in California.
-These earthquakes tend to be shallow in nature and low magnitude but conservative boundaries can also produce much higher magnitude earthquakes but at lower frequency.
Map showing the distribution of landslides
Describe the distribution of landslides
-Landslides can occur anywhere in the world but there is a higher risk in steep mountainous regions with coarse soil.
-Western regions of the Americas, East Asia and South East Asia stand out as regions of high risk.
What physical and human factors increase the chance of landslides occurring?
-Landslide risk is greatest in populated regions with high rainfall events, such as tropical storms and lead to slope saturation and river flooding.
-In addition, regions experiencing population pressure, deforestation, road construction, mining and plantations all lead to greater risk of landslides.
How is earthquake magnitude measured?
-Energy release is measured by the logarithmic moment magnitude scale (MMS)
-Damaging effects are measured by the Mercalli scale (measures intensity of shaking)
What is the moment magnitude scale?
A logarithmic scale where each level has 10 times the energy of the level below it
Describe 3.0 earthquakes (MM scale)
-Minor
-Many a year
-All around the world
-Rarely cause damage
Describe 5.0 earthquakes (MM scale)
-Moderate
-Between 1,000 and 1,500 a year
-Damage caused over a small region
Describe 7.0 earthquakes (MM scale)
-Major
-Between 10 and 20 a year
-Serious damage over a large area
Describe 9.0 earthquakes (MM scale)
-Great
-One every decade (or longer)
-Serious damage over a large region
Describe the IV stage of the Mercalli scale for earthquakes
-Shock waves felt indoors
-Windows rattle
-People woken at night
Describe the VII stage of the Mercalli scale for earthquakes
-Slight to moderate damage to well-constructed buildings
-Aseismic designs may be slightly damaged
-Walls crack
-People are alarmed
Describe the X stage of the Mercalli scale for earthquakes
-Wooden structures and masonry and frame buildings destroyed
-Train rails bent
-Ground may crack
-Landslides caused on steep slopes
Describe the XII stage of the Mercalli scale for earthquakes
-Total destruction
-Lines of sight and level distorted
-Objects thrown into the air
-Topography and river courses changed
How is volcano magnitude measured?
-Measured by the volcanic explosivity index ( VEI) and based on ejections’ volume, duration, and column height.
-Can be related to the type of plate boundary the volcano is located on.
-Effusive eruptions of basaltic lavas with low VEI ( 0-3) are associated with constructive boundaries or plumes.
-Explosive eruptions with high VEI (4 to 7) of andesitic or rhyolitic lava are associated with destructive boundaries.
What is the VEI scale? (magnitude of volcanoes)
A logarithmic scale where each level has 100 times the energy of the level below it
Describe stage 1 of the VEI scale
-Small, gentle eruptions
-Less than 10^6 meters of ejected material
-Often small fissure eruptions
-Hawaiian type
Describe stage 3 of the VEI scale
-Moderate to large eruptions
-Can be severe
-Up to __ meters of ejected material
-Eruption column of ash up to 10.5 km high
-Eruption phases last up to half a day
-Strombolian type
Describe stage 5 of the VEI scale
-Very large eruption
-10^10 meters of ejected material
-Eruption column up to 28km high
-Can be cataclysmic
-Composite volcano type (Plinian or Peleean)
Describe stage 7 of the VEI scale
-Very large, colossal eruptions
-10 __ meters of ejected material
-Eruption column height over 47km
-Eruption phases last over half a day
-Composite cone type (ultra-Plinian)
Diagram of the volcanic explosivity index
Picture of the Pressure and Release (PAR) model
