Intro Flashcards
(24 cards)
What are the seismic hazards?
Seismic Hazards
a Ground shaking
b Surface rupture
c Tsunamis
d Landslides
e Liquefaction
What are the mass movement hazards?
Mass movement
a Landslides (falls, slides & flows)
b Creep, subsidence, settlements
What are the volcanic hazards?
a Lava flow, debris flow, pyroclastic flow
b Ash fall/cloud, volcanic gases
c Volcanic earthquakes
What is the risk function?
Risk = f(Hazard, Vulnerability, Exposure) - due to nature
Hazard - probability of observing ground shaking or deformation in time period
Vulnerability - tendency of something to suffer damage - due to engineering
Exposure - economic and social quantification of entity exposed to hazard - due to planning
What are the generation elements and resulting seismic hazards?
Faulting (generation), leads to surface rapture and tsunami (hazards) as well as seismic energy release (generation). SER leads to strong ground motion (hazard), which leads to landslides and amplified ground shaking if affected by topography or liquefaction and amplified ground shaking if affected by soil deposits.
What is a surface rupture?
Where a fault rises to the top of the ground, creating a relative displacement across the structure crossing the fault.
What are tsunamis?
Tsunamis are long ocean surface waves generated by rapid displacement of
large volumes of sea water resulting from sources associated with catastrophic
geophysical events.
What geophysical events lead to tsunamis?
a Submarine earthquakes in shallow ocean waters (mainly in subduction
zones, by reverse - move diagonally and together, or normal - move diagonally and apart, faults which create rel vertical displacement)
b Submarine slides & slumps
c Marine volcanic eruptions
d Storm waves (very rarely)
What are the effects of tsunamis?
a Flooding
b Wave-structure impacts
c Flotation & transport of heavy objects
d Scour and inundation
e Breaching of protective barriers sheltering hazardous materials
What scenarios should be considered to reduce threat to locals of tsunamis?
a Local tsunamis generated so near a community that the first wave
arrives within few minutes.
b Distant tsunamis generated so far from the community that the first
wave does not arrive for hours.
What is soil layer amplification?
Surface geology & geotechnical characteristics of soil deposits modify the
incoming wave-field for a given seismic event in terms of:
a Amplitude
b Frequency content
c Duration
Leads to higher structural damage in areas of amplification.
Clays can amplify, rock de-amplifies (stiffness incr with depth). If clay is in middle of soil it de-amplifies as energy is lost.
What are the types of earthquake induced landslide?
rock falls & slides, earth slumps & debris slides
What is topographic amplification?
Increase in ground motion intensity due to focusing of waves within
hillsides/ridges
What is liquefaction and what can it lead to?
Complete loss of shear strength due to generation of excess pwp under undrained
loading.
a Bearing capacity failures
b Flow failures
c Settlement
d Sand-boils & sinkholes
e Lateral spreading
f Uplift of buried structures
What do small volcanic eruptions lead to? What causes larger eruptions?
Predominately effusive eruptions result in relative quiet extrusion of lava flows & mild to moderate explosive activity.
Larger eruptions due to lava dome collapsing under pressure.
How are lava flows characterised and how can they be mitigated against?
They are low-velocities and rarely life-threatening, but can damage houses and infrastructure. They are gravity driven, and constrained by topography so hazard zonation maps can be made, and they can be stopped with artificially constructed earth mounds to protect downstream areas.
What causes pyroclastic flows? How are they characterised and how can they be mitigated against?
Causes: lava dome collapse, explosion, gravitational collapse of eruption column.
They are mix of hot volcanic gases, pumice, hot ash, and coarser debris. They travel fast, threaten lives, and can cause structural damage and inundation of areas. They can also trigger explosively generated secondary flows and/or reactivate later to form debris flows.
Best mitigation is hazard zoning map, monitoring, and then evacuation of affected area.
Characterise the landslides? (bear in mind for large landslides the form can change)
a Falls
involve immediate separation of the falling material from parent rock or
soil mass
b Slides
moving material remains in contact and movement takes place along
discrete shear surfaces
c Flows
material becomes disaggregated and movement occurs without
necessarily forming discrete shear surfaces
Explain flows?
Flows are mass movements, involve more internal deformation than slides. Slides can turn to flows.
They have large runout and high velocity.
Occur as soil acts as fluid or large no. of shear surfaces engaged.
Debris has high pore pressures due to collapse of loose grain structure, behaves like flow.
What are the types of flow? What characterises flows?
a Mudflows
b Debris Flows
c Rock Avalanches
d Volcanic Debris Flows (Lahars)
e Lava Flows
f Pyroclastic Flows
Flows unique based on the rock/soil material
How do mudflows and debris flows occur?
How is the grading of mudflows?
- Heavy rain swells streams down mountain crest.
- Sediments slump into slurry (debris flow) down water channel.
- Debris flow swells as picks up sediments and trees from channel and canyon walls.
- Debris flow emerges from canyon onto fan where it causes damage.
Mudflows well-graded (or poorly sorted) with large particles at front
How do volcanogenic debris flows develop?
During the eruption or the post eruption period volcanogenic debris flows can develop by reworking pyroclastic debris.
What does a landslide hazard assessment involve?
Hazard assessment involves the consideration of a number of factors that are used to estimate the magnitude and frequency of events
What are some landslide mitigations?
Ring nets - allow water and small soil particles but blocks large block sections
Barriers - induce hydraulic jumps which creates turbulence to slow flows.
Debris dams and catchment basins - provides barrier to collect flow
Shelters - allow flow over, protecting road
Landslide hazard assessments, planning and avoidance.
Monitoring/warning
