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samenvatting Natural Hazards (including econ.)
samenvatting van alle Natural hazards stof (chapters & economy)
[Meer zien]Voorbeeld 4 van de 38 pagina's
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https://youtu.be/TI8OJjcLSHg & Summary Natural HazardsCHAPTER 4: TSUNAMIS
Japan tsunami of 2011:
- Japan is most prepared nation, but people were not informed about the possibility of such a big
tsunami.
o Must be educated
- Tsunamis can have secondary effects.
o Reactors of Fukushima were not prepared
o Economics
- Communication between scientists and government about possibility of such a tsunami was not good
enough
- People must react quickly: if they feel an earthquake, they should know that there is a possible
tsunami coming.
Source of a tsunami is an earthquake often the thrust fault in subduction zones.
- Must cause a movement on the seafloor; up or down
- Sometimes the tsunami is more destructive than the earthquake
4.1 Introduction to Tsunamis
Tsunami: series of waves caused by displacement of large volume of water.
- Longer wavelengths than normal waves
- At shore, the waves will be 10 – 15 m high
- Runup: maximum horizontal and vertical distances that the largest wave reaches when moving land
inward.
Several types of events can trigger a tsunami:
- Earthquake
o An earthquake can cause a tsunami by displacing the seafloor or the floor of a large lake, or
by triggering a large landslide. Displacement of seafloor is probably the most common of
these mechanisms and occurs when a block of Earth’s crust moves rapidly up or down during
an earthquake.
o By displacement of seafloor (or triggering a large landslide)
o Generally need an earthquake of magnitude >7.5
o Stages:
Displacement of seafloor during earthquake; create oscillatory waves that transmit
energy outward and upward.
Deep ocean: waves move rapidly and are spaced far apart
v=√ gd
Speed up to 200m/s 720km/h
deep ocean: Spacing between > 200 km, Height of waves < 1 m
Tsunami near land: water depth and velocity decrease
Speed only about 45 km/h
Due to decrease in velocity: decrease in spacing, increase in height
Wave approaches shore transforms into turbulent, surging mass of water
Debris laden waters
Bore: when one wave overtakes another, a steep wall of water is created
Water returns back to the ocean: strong, turbulent flow
o Edge wave: waves along the shore, may interact with the following waves to become larger.
o Usually series of tsunamis take several hours.
, o Distend tsunami (tele-tsunami): travel thousands of km across the ocean and strike
shorelines with little energy loss.
o Local tsunami: affects shorelines near source of the earthquake.
Can be more deadly because of less time for warnings.
o Can recognize in sediment:
Layer of soil and mud
Layer of tidal sediments due to sinking of land
Coarse, sandy sediment (sometimes with shells etc.) due to tsunami
Layer of tidal sediments
Soil and mud
- Landslide
o Landslides on seafloor or in lake can produce tsunamis
o Landslides that fall from slopes into body of water can generate tsunamis
o Loses energy over distances of thousands of kilometers
- Explosive Vulcanic eruption
o The explosion triggers a tsunami
o Smaller tsunamis can be triggered by large volcanic mudflows into the sea
Rapidly decrease in size; less hazardous
- Asteroid or comet
4.2 Regions at risk
Some coasts more at risk due to their location with respect to earthquakes, landslides, and volcanoes.
- Coast nearby subduction zone capable of generating M 9 earthquake; highest risk
- Pacific Ocean most; many subduction zones that surround it
- Landslides and volcanoes less common, so less hazard
- Due to geologic dating, you can determine how often a tsunami takes place; more often = bigger
hazard
o Dating of tree rings destroyed by the tsunami
o Radiocarbon dating of tsunami deposits
o Computer modeling
4.3 Effects of tsunamis and links with other natural hazards
Tsunamis have two effects:
- Primary:
o Impact of the onrushing water and its entrained debris
o Resulting flooding and erosion
- Secondary:
o Occur in hours, days and weeks after the events E.g., fires, melting of reactors, outbreaks
of disease, pollution of freshwater, mental health problems, debris that washes ashore.
Tsunamis often linked to other natural hazards:
- Earthquakes, landslides, eruptions of volcanoes and asteroid or comet impacts
- Erosion of the coast
4.4 Minimizing the tsunami hazard
Damage can be reduced:
- Detection and warning
o Using network of seismograms to estimate magnitude and location of quake:
First warning: earthquake of > M 7.5 in offshore area
Pacific-wide system Also uses buoys to verify that a tsunami was produced
(measures water pressure, thus the wave)
, Regional systems
Local systems
o When source of tsunami is < 100 km away: no time for warning and evacuation
Must move to higher ground in reaction to the earthquake and receding water
- Structural control
o Larger structures and critical facilities can be engineered to reduce or minimize the impact of
a tsunami
o Problem: building codes do not adequately address the effects of a tsunami
o Dykes and walls can be constructed to prevent waves from reaching certain places
o Offshore barriers can deflect tsunami waves or lessen energy
- Construction of tsunami inundation maps
o Illustrates variability of run-up in a typical tsunami within an area
o Risks to communities assessed by determining the frequency and size of tsunamis in
history
o Hight of tsunami is estimated by models and deposits
o Aim: identifying evacuation routes
o Can be used as a guide of how to build and to educate people in highest risks
- Land use
o Vegetation, mangroves, or several rows of trees reduce the energy of the tsunami
Mostly cut away for housing of resorts
- Probability analysis
o Risk = probability of the event + consequences
o Aim: likelihood of tsunami, location, extent of run-up, possible severity of damage
o Approach for analysis:
Identification and specification of potential earthquake sources and their
uncertainties
Specification of factors that reduce tsunami waves as they travel from source area
Statistical analysis of past tsunamis
o Difficulty: tsunamis at a particular location are rare events
If too little tsunamis happened in the past: Monte Carlo simulation;
Determine tsunami return periods and probabilities for distant and local
sources
Random sample of quakes and determines the tsunamis that follow
Model constructed based on simulated events to estimate tsunami
amplitude or run-up along coast
- Education
o Rare phenomenon: recollections of people fade in time
o Should be given in school, also about warning systems
o Difference between tsunami watch (notification that a quake could cause a tsunami) and
tsunami warning (tsunami has actually been detected)
o Important that tsunami is more than one wave
- Tsunami readiness
o Have emergency operation center, 24-hour capability
o Have ways to receive tsunami warnings
o Have ways to alert the public
o Develop tsunami preparedness plan; includes emergency drills
o Promote community awareness of tsunami hazards through educational programs
, CHAPTER 3: EARTHQUAKES
Consequences of earthquake depend on magnitude, depth, direction of fault rupture, distance from populated
areas nature of local earth materials and engineering of constructions.
3.1 Introduction to earthquakes
Earthquakes result from rupture of rocks along a fault, when these rocks suddenly move.
- Seismic waves are released
- Epicenter of the quake is point of surface directly above fault rupture
- Focus/hypocenter: location of initial rupture along the fault
Magnitude: amount of energy that an earthquake releases.
- Logarithmic scale
- Body wave scale (Mb): strength of wave traveling through the earth (P wave)
- Surface wave scale (Ms): strength of waves traveling along surface
- Moment magnitude (Mw): determined from area that ruptured along a fault plane during the quake,
amount of movement (slippage) along the fault and rigidity of the rocks near the focus.
o Increase of one means 10 times more shaking and 32 times more energy released
Intensity: measure of the effects of earthquake on people and structures.
- Depends on magnitude, distance from epicenter and nature of ground at the site
- Modified Mercalli Intensity Scale: degree to which an EQ affects people, property, and the ground.
o 12 categories; each contain description of how people perceive the shaking and to the extent
of damage to buildings and other structures.
o Take days to weeks to conduct, so longer that magnitude maps
Becomes faster due to internet forms
When through internet: community internet intensity maps
Motions of the ground are measured on a seismograph.
- Can create a shake map; due to direct measurements from seismograph stations can create a map of
where shaking was most severe
o Information from seismogram: instrumental intensity
- Valuable: because they are fast, important for teams who must save lives or utilities (e.g. gas)
3.2 Earthquake processes
Most earthquakes happen at plate boundaries
Process of faulting:
1. Plates moving past each other are slowed by friction along boundaries
2. Friction results in strain or deformation
3. When stress on rocks exceed breaking point (strength): suddenly move along the fault
a. Rupture starts at focus and propagates along fault plane
b. Ruptures produce seismic waves: therefore, are seismic sources
Fault types: distinguished by direction of displacement of rocks or sediment bordering them:
- Strike-slip faults: horizontal displacement
- Dip-slip faults: vertical displacement
o Normal fault: hanging wall has moved downward relative to footwall
o Reverse fault: the hanging wall has moved up relative to the footwall
o Thrust fault: angle less than 45 degrees
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