Lecture 11: Geologic Hazards Flashcards by Ashley De Guzman

Exposed ground surface at an angle with the horizontal (natural or man-made)

UNRESTRAINED/UNPROTECTED SLOPES

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Slope subject to different elements and forces

and may eventually fail

Unprotected slope

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Often times, it is simply referred to as landslide

Slope Failure

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Classifications of Slope Failure

Fall
Topple
Slide
Spread
Flow

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Sudden movement of material down a slope of cliff.

Fall

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Main force affecting fall

Gravity

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Tipping over or forward rotation of a mass about one of its points.

Topple

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What initiates topple

Gravity, water/ice in cracks

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Movement of mass along a rupture zone or zones of great shearing

Slides

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Slide with a curved rupture zone

Rotational

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Slide with a a planar rupture zone

Translational

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The soil or rock extends and gets thinner and subsides into the softer material below.

SPREADS

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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

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Continuous movement of material such that the surfaces of failure are very close to each other and are not saved.

Flow

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flows with loose soil and rock that creates a slurry flowing down, and as such, they are
wrongly referred to as “mudslides”

Debris Flow

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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)

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debris flow but with volcanic

materials (tephra) as the main materials rather than usual soil and rock.

Volcanic debris flow

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Extreme debris flow (very fast)

Debris Avalanche

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Flow but made up predominantly of finer soils

Earthflow

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slower earthflow (<1 meter of movement per decade)

Creep

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Classification of landslides based on material

Rock, debris, earth

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translational landslide but with materials

being a single or a few units that move together

Blockslide

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greatly affected by its geometry and the

material of the slope.

Slope stability

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Benefit of using a software to analyze slope stability

to make analysis of hundreds-thousands of possible failure surfaces possible and fast.

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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

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