Lecture 11: Geologic Hazards Flashcards
Exposed ground surface at an angle with the horizontal (natural or man-made)
UNRESTRAINED/UNPROTECTED SLOPES
Slope subject to different elements and forces
and may eventually fail
Unprotected slope
Often times, it is simply referred to as landslide
Slope Failure
Classifications of Slope Failure
- Fall
- Topple
- Slide
- Spread
- Flow
Sudden movement of material down a slope of cliff.
Fall
Main force affecting fall
Gravity
Tipping over or forward rotation of a mass about one of its points.
Topple
What initiates topple
Gravity, water/ice in cracks
Movement of mass along a rupture zone or zones of great shearing
Slides
Slide with a curved rupture zone
Rotational
Slide with a a planar rupture zone
Translational
The soil or rock extends and gets thinner and subsides into the softer material below.
SPREADS
a special kind of spread that happens on very gentle or almost flat terrain. The upper layer extends and “spreads” over the softer layer underneath.
Lateral Spread
Continuous movement of material such that the surfaces of failure are very close to each other and are not saved.
Flow
flows with loose soil and rock that creates a slurry flowing down, and as such, they are
wrongly referred to as “mudslides”
Debris Flow
How does a slide become a debris flow?
A slide may end up evolving to a debris flow if it
becomes faster and gets more water along the way (or the mass just “breaks apart”
making it become a slurry)
debris flow but with volcanic
materials (tephra) as the main materials rather than usual soil and rock.
Volcanic debris flow
Extreme debris flow (very fast)
Debris Avalanche
Flow but made up predominantly of finer soils
Earthflow
slower earthflow (<1 meter of movement per decade)
Creep
Classification of landslides based on material
Rock, debris, earth
translational landslide but with materials
being a single or a few units that move together
Blockslide
greatly affected by its geometry and the
material of the slope.
Slope stability
Benefit of using a software to analyze slope stability
to make analysis of hundreds-thousands of possible failure surfaces possible and fast.
Ways to improve stability of Slope
• Making a milder slope.
• Creating benches or parts in your slope that is horizontal (so
that it is not a continuous sloping material).
• Adding structural members to improve strength of the slope like
soil nails or rock anchors.
• Using geosynthetics.
the release of energy from the ground
Earthquake
What wave brings energy to different areas
Seismic Waves
The boundary between 2 rocks where motion is present.
Faults
This is where earthquakes are typically associated
Faults
How are faults created
Through earthquakes
How are faults evidenced
by offsets and other features ranging from sizes in order of mm to km.
Fault types
reverse, normal, strike-slip
special reverse faults with very low angle of
fault plane.
Thrust fault
difference between NORMAL and REVERSE fault
The difference between NORMAL and REVERSE fault is whether the HANGING WALL (side that is above the fault plane)goes up or
down relative to the FOOT WALL (side the is below the fault plane).
when the sides slide past each other. It is
called “STRIKE”-slip since the movement is along the STRIKE or length of fault.
Strike-slip fault
How to classify strike-slip fault
stand on one side of the fault, look at the other side, and see which way it moves – if it moves to the left, it is a left lateral strike slip fault, and vice ver
Hypocenter
the point where the earthquake starts
Epicenter
the point on the surface directly above the hypocenter
Classifications of Earthquakes from a fault
foreshocks, mainshock, and aftershocks
foreshocks vs mainshock vs aftershocks
The main shock is the largest
earthquake. Foreshocks happen before the main shock and the aftershocks are the ones after the mainshock.
Classification of Earthquake Factors
Source, path, site
related to the fault itself as the source of
earthquake (fault size, dip angle, etc.)
Source
factors due to the path the seismic waves travel through. It includes how the seismic waves get weaker as they propagate, etc.
Path
factors related to the site concerned. This includes,
among others, the effect of the soft soil underneath a site. Soil/rocks in the site may amplify/deamplify the seismic waves.
Site
2 types of seismic waves
1 Body waves
2 Surface waves
Seismic waves that go through Earth’s interior
Body wave
Seismic wave that travels along the surface of the Earth
Surface wave
Which has higher frequency surface wave or body wave?
Body wave
P wave
• transferred via motions alternating between compressions and
extensions (push and pull).
• First to arrive at a site.
• can travel through solids and liquids.
S wave
- Moves side to side or up and down.
- slower than P-wave and thus second wave to get to a site.
- Can travel through solids only.
Moves the ground from side to side
Love wave
Action is similar to an ocean wave as it rolls across the body of water.
Rayleigh wave
come after the body waves but are responsible for
majority of the destruction.
Surface wave
How to determine the distance of the earthquake
from a recording station.
From the time difference between arrival of P and S
wave
2 ways to characterize Earthquakes
magnitude, intensity
Earthquake characterization based on the amount of energy released so there is only 1 value of magnitude for a certain earthquake event.
Magnitude
Earthquake characterization based on the effect or shaking felt in a site, hence, an earthquake will have different intensities recorded for different sites.
Intensity
Magnitude scales
- Local magnitude (Richter Magnitude)
- Surface wave magnitude
- Coda magnitude
- Moment magnitude
Measures the size of earthquake in terms of the energy released using the seismic moment
Moment Magnitude Scale
Happens when a certain magnitude scale
fails to reflect the appropriate magnitude value at high
magnitude levels.
Magnitude Saturation
Used to eliminate the effect called magnitude saturation
Moment Magnitude
Most famous intensity scale
Modified Mercalli Scale
Philippine intensity scale
PHIVOLCS Earthquake Intensity Scale
Basis of intensity
mostly what is felt by people/witnesses as
well as damages seen in the site.
EARTHQUAKE HAZARDS
• Strong motion and Surface rupture
• Structural collapse (buildings, infrastructure, etc.)
• Liquefaction
• Earthquake-induced landslides
• Embankment and Retaining Structure failure
• Tsunamis and Seiche (Seiche is like a tsunami but for enclosed
waters like lakes)
Strong motion from an earthquake can be
the start of various
hazards like collapses and landslides.
when water-saturated granular soil is subjected to
earthquake motion and it starts to ACT LIKE A LIQUID and LOSE ITS CAPACITY TO SUPPORT STRUCTURES.
Liquefication
Related effects of Liquefication
flow failure, lateral spreads, bearing capacity failure (failure of soil under foundation), and settlements.
2 Kinds of Volcanic Hazard
- Directly related to eruption
* Indirectly related to eruption
Examples of volcanic hazard directly related to eruption
Lava, Pyroclasts, Pyroclastic Density Currents
Examples of volcanic hazard indirectly related to eruption
Lahar, Landslides, Tsunami/Seiche
Magma on the Earth’s surface.
Lava
Smooth looking and “ropey” lava.
Pahoehoe
It is the result of forcing lava to flow faster than it could
AA
Pertains to rock fragments broken down by fire
Pyroclastic Materials
any volcanic fragment
ejected into the air by an eruption
Pyroclasts/Tephra
Tephra Classifications based on size
- Blocks and Bombs – >64mm
- Lapili – 2-64mm
- Ash – <2 mm
Can cause irritation of the body (skin, lungs, eyes, etc.)
and lead to damages in your eyes and lungs.
Ashfall
Hot gas, ash, and pyroclastics mixed together rushing down the slopes.
Pyroclastic Density Current
deadliest direct volcanic hazard
Pyroclastic Density Current
Range of PDCs
from pyroclastic flow (denser and closer to the
ground as it flows) to pyroclastic surge (faster since it is less dense and has higher gas to rock ratio)
Slurry of water and other volcanic materials that are flowing down the slopes (sometimes called volcanic mudflow)
Lahar
occur due to earthquakes associated with volcanic
eruptions, the explosive eruption itself, or when magma intrudes a certain area.
Landslide
released by eruptions in large quantities
Volcanic Gases
Tsunami is formed when water is displaced by
- Material (PDCs or soil from landslides) getting dumped into the body of water from the volcanoes
- Underwater volcanic eruptions (for seas)
- Magma rising up and deforming the floor of a caldera lake
