Geohazards 1 (edit from Dianne)

Geohazards

—Disasters and Geological Hazards

—Earthquakes

—Volcanic Processes

—Landslides and Land Subsidence

—Flooding

—Sea Level Rise & Climate Change

—Earth Materials

—Extraterrestrial Hazards

Geology and Society: Challenges

—Dwindling non-renewable resources

—Environmental degradation through mining and other human activities

—Human survival and the threat of geologic hazards

Geologic hazard (Geohazard)

—Potentially destructive process that could harm man and his resources

—Natural vs Man-made

Disaster

-No people & resources at risk → may have hazard but no disaster

-No hazard → no disaster

A function of hazard, people & resources at risk

Earthquake

The vibration of the earth, caused by the rupture and sudden movement of rocks that have been strained beyond their elastic limits.

-occur along plate boundaries

—Elastic strain is recoverable portion like an elastic band being pulled

—Rocks are subject to stress → rocks can rupture or slip and release elastic energy

—Slip on old fault occur when stress > frictional forces

—New fault occur when stress > strength 

Explain: Elastic Rebound Theory

Slip rate and recurrence interval of faults

a function of:

—Rate of motion

—Friction or strength

(and cohesive strength of rocks)

Tectonic Earthquakes

Earthquakes associated with:

-Plate boundaries (subduction zones, mid-oceanic ridges, transform faults)

-Active Faults or faults that show movement over the last 10,000 years

Parts of an Earthquake

-Fault line/scarp

-Focus: point of origin or main source

-Epicenter: point directly above the focus which lies on the surface of the earth

-subduction zones

-trenches

Earthquake Generators of the Philippines

Seismicity of the Philippines

-shallow earthquakes are the most destructive

-most stable part of the Philippines is Palawan

Seismicity of the Philippines

-shallow earthquakes are the most destructive

-most stable part of the Philippines is Palawan

Earthquake Hazards

1. Ground Rupture

2. Ground Shaking

3. Liquefaction

4. Fire

5. Landslides and related downslope movements

6. Subsidence

7. Flooding due to dam failure

Ground Rupture

the visible offset of the ground surface when an earthquake movement along a fault affects the Earth's surface

-Digdig Fault, Nueva Ecija; July 1990

Ground Shaking

The vibration of the ground during an earthquake

-collapse of structures

-“pan-cake” effect for multistory building

-Kobe, Japan; 1995

Liquefaction

a process by which water-saturated sediment temporarily loses strength and acts like a fluid due to Earthquake shaking

-occurs in underlain by soft and unconsolidated material containing water

-the sand acts like a quicksand

-Dagupan, Pangasinan; 1990

Fire

—Indirect effect

—Can be devastating

—Eg. Tokyo, 1923

Fire

—Indirect effect

—Can be devastating

—Eg. Tokyo, 1923

Landslides and related downslope movements

Effect of Earthquakes on sloping areas which are unstable

Subsidence

the gradual caving in or sinking of an area of land.
-in coastal areas underlain by soft, unconsolidated material are subject to this

-very similar in process to liquefaction

Flooding due to dam failure

Fire

Indirect Effects of Earthquakes

Earthquake-Generated Tsunami

Occurs when earthquakes earthquake epicenter is offshore, specifically when there's a vertical component in the sense of motion of the fault blocks

• Caused by vertical motion of seabed or submarine landslide

Travel times for tsunamis from Hawaii to/from other circum-Pacific spots

1998 Irianjaya

Caused by vertical motion of seabed or submarine landslide

Small wave at sea (< 1 m) but fast (® 800 km/hr)

As → shallow water slow to ~ 60 km/hr and height ® 20 m! 

1960 Chile earthquake → tsunami that hit Hawaii 15 hrs later and killed 61 people

Hawaii and other areas have tsunami warning systems and beach signs

NW:  Finding ancient tsunami deposits at coast and Puget Sound

        Olympic coast very dangerous (no news or escape)

Considering a system for Alaskan and Cascade earthquakes using times and tide gauges

Tsunami Propagation

one of the blocks you have vertical motion and displaces water upwards and then it is generated in all direction

• if you're at sea you will hardly notice this waves

• the problem is when this waves approach the shore

• you will observe that when a tsunami arrives there's a retreat of sea water and then it will come back as a tsunami inundating the land

Engineering solutions

How to prevent collapse?

-Shear wall and use of reinforced concrete & steel

Redundancy/regular shape in architecture

-Computer controlled dampeners

-Shock absorbers between buildings

-Design: natural ground vs building frequency (Avoid buildings with similar ground frequency)

  -(building “heart beat” and resonance)

Earthquake Prediction

—Preparedness is emerging as an alternative.

• Seismic risk map

• Strict building code

• Zoning and land use

Earthquake Prediction

1. Unusual Animal Behavior

2. Use of Side-Aperture Radar (SAR) interferometry and GPS

3. Precursors

4. Use of seismic gaps

Earthquake Prediction: Unusual Animal Behavior

Earthquake Prediction: use of SAR interferometry and GPS

when rocks are being stressed, there is strain and deformation of the ground which is measured by these tools

Earthquake Prediction: Precursors?

stage 1, 2, 3, and 5

Earthquake Prediction: use of seismic gaps

Using portions of Earth where Earthquakes have not occurred for a long time despite known frequent occurrence. Especially observed in the pacific ring of fire. Probability estimated from theorized stored elastic energy.

Seismic Gaps in the northern San Andreas Fault

A panel of experts concluded that there is a 67 percent chance for at least one earthquake of magnitude 7 or larger in the San Francisco Bay Area between 1990 and 2020 (but that a repeat of an M8.3 like the 1906 earthquake is unlikely in this period)

Issue #1: How to evacuate megacities?

 Transport for millions of people???

 To where? How far?

 Evacuation route?

 Traffic jams management?

 Food and drink for millions? For how long?

Issue #2: Who will be left behind?

 Security of properties?

Earthquake Prediction: Issues and Problems

Earthquake Intensity

• qualitative measure (descriptive); effects on the Earth’s Surface and on humans and their structures. (e.g., based on survey)

• for the same earthquake,  _______ varies with distance from the epicenter and type of underlying rock

Earthquake Magnitude

• quantitative measure, in terms of absolute amount of energy released

• for any one earthquake, there is only one ______(unique)

Magnitude vs Intensity

only 1 magnitude while intensity varies with distance

intensity decreases as distance increases

Intensity as a function of Underlying Rock

crystalline/hard rocks - less shaking of ground

loose and unconsolidated rocks - more shaking

water saturated sand and mud amplifies the shaking

Modified Mercalli Scale

Scale for Intensity

Earthquake Magnitude

absolute energy released during an earthquake

Volcanoes

-formed along plate boundaries (convergent and divergent)

-formed in the middle of plates (hotspot volcanoes, e.g., in Hawaii)

Plate Tectonics and Volcanism

-volcanoes form when tectonic plates collide and one plate is pushed beneath the other (convergent plate boundary)

-partial melting occurs when water from the subducted ocean sediment lowers the melting temperature of the mantle, giving rise to melt

-when tectonic plates move away from another to produce volcanoes (divergent plate boundary)

-the compression occurs when confining pressure is released due to blade separation hot magma rises from the mantel at the mid-oceanic ridges pushing the plates apart this molten rock then rises to the surface and erupts in volcanoes

-Hot spots can be either oceanic (Hawaii) or continental (CRB, Yellowstone)

Hotspots or Hotspot activity

• in the middle of the plates some areas of the mantle are anomalously of high temperature to cause melting

• some of the voluminous eruptions were formed these volcanoes for example the decan traps associated with the extinction of dinosaurs which occurred at the same time and the other one is the siberian traps found in siberia

Volcanism (Hot Spots)

Implications: Plates move, as can be traced from the direction of the Hawaiian volcanic region.

The hot spot remains stationary, the islands were once on top of it, supplied with magma from below

• model for hawaiian volcanism it's the most famous hotspot in the world

• the plates move on top of a stationary magma source

• a famous example of hotspot volcanoes in hawaii are those that form the chain of islands

• it is the northwest north trending mechanic chain that can be traced from the emperor seamount chain where volcanoes increase in age

Table of Selected historic volcanic events

—Internal factors

  - vesiculation or degassing of magma (allows escape of magma from the melt??)

  - influx of fresh magma supply and buoyancy of magma

—External factors

  - load pressing

  - tectonic pressures

  - ocean tides and earth tides

What Causes Volcanic Eruptions?

What comes out of a volcano?

—Lava - molten rock coming from magma (only when extruded on the surface)

—Pyroclast (fragmented magma) = tephra (fragments ejected through air by volcanic process)

- Example of pyroclast: pumice

—Volcanic gases - made up of silicon, oxygen, aluminum, and other elements

Volatiles

• gasses dissolved in magma are called ___

Eruption Products

1. Lava Flow

2. Pyroclasts: Pumice, Ash (Tephra/Ash fall)

3. Pyroclastic Flow and Deposits

4. Volcanic Gases

Lava Flow

• the surface of the lava flow is fragmental and this is quite common but the interior is normally coherent when this lava was being erupted

• Typically glows during an eruption and is visible at night

Pyroclast

• derived from the greek terms meaning fire and broken

• this mainly are derived from explosive eruption

• occur in different sizes such as volcanic bombs as large as cars or houses

• Pumice is produced when lava with very high water content is discharged from a volcano

A fragment of volcanic material that is ejected during explosive volcanic eruptions. These fragments can be molten, partially molten, or solidified magma.

Volcanic Ash

• very fine as clay and sand

• fine dust of volcanic rock

Pyroclastic Flow and Deposits

Pyroclastic flow is fast moving current of hot gas and volcanic matter collectively known as tephra that flows along the ground away from the volcano at high velocities of 100 kilometers per hour

Volcanic Gasses

• principle components are water vapor (most abundant), CO2, H2S, nitrogen, argon, helium, methane, carbon monixide and hydrogen

• H2S or hydrogen sulfide is the most toxic which can be fatal when inhaled in large quantities

• there are some cases when volcano releases gasses (e.g. carbon dioxide) without any accompanying eruption

Types of Eruption

1. Explosive

2. Effusive

Explosive Eruption

• characterized by gas-driven explosions that propels magma and tephra

• e.g. 1991 mount pinatubo erruption

Effusive Eruption

• characterized by outpouring of lava without significant explosive activity

• e.g. kilauea volcano in Hawaii

Volcano Explosivity Index or VEI

• a logarithmic scale from 0 to 8 that measures the relative size of explosive volcanic eruptions based on ejecta volume, eruption cloud height, and qualitative descriptions

Plinian Eruption

• most explosive among the types of volcanic eruptions characterized by a high-speed column of gas and ash that can reach tens of kilometers into the atmosphere (>25km above crater)

Caldera

• Morphological feature, product of a Plinian Eruption, (most explosive eruptions)

• largest volcanic landform on Earth

• large depression formed by the collapse of a wide area due to the emptying of the magma chamber

• Volcano with a crater greater than 2 km in diameter

• e.g. Laguna de Bay, Taal Volcano, Crater Lake, Oregon

Laguna and Taal calderas

Laguna caldera is much older than Taal caldera

Huge Rhyolitic Eruptions

A rhyolitic eruption is a powerful, explosive volcanic event driven by highly viscous, silica-rich magma that traps large amounts of gas. These eruptions are among the most destructive and can produce vast ash columns and extensive pyroclastic flows, sometimes leading to the formation of large calderas.

• Mount Pinatubo erruption in 1991 is the third largest eruption in the century with 5km³ magma volume

• Yellowstone Caldera (600,000 years ago) produced 2000km³ magma volume

• Long Valley Caldera (760,000 years ago) produced 600lm³ magma volume

Volcanic Hazards

—Lava flows

—Tephra fall /Ash fall & Ballistics

—Pyroclastic flows & Pyroclastic surges (PDCs)

—Volcanic gases

—Lahars

—Debris avalanches, landslides, and tsunamis

—Crater Lake/Mountain Lake breaching

Lava Flow: Burial

(Volcanic Hazards)

• render land useless for decades

• US Government turned the hazard into an opportunity in Hawaii by developing tourism

• eruptions also provided additional land and space

Pyroclastic Flow and Pyroclastic Surge (PDCs)

(Volcanic Hazards)

Pyroclastic flow is a fast-moving current of tephra (hot gas and volcanic matter) that flows along the ground at high velocities (ave: 100kph, up to 700kph)

• PDCs - pyroclastic density currents

• dubbed as killers as they are the most dangerous process related to volcanic eruptions

• contain hot solid (sometimes liquid) in gas dispersion

• classified as density currents

• main hazard is via Burial and Erosion

• Death by asphyxia

a fast-moving current of tephra (hot gas and volcanic matter) that flows along the ground at high velocities (ave: 100kph, up to 700kph)

Mayon Volcano Eruption

• June 24, 2001

• the flows flowed along the streams on the slopes of the volcano mostly at the southeastern side

• the pyroclastic flows are fed by glowing and collapsing pyroclast laden columns at the summit or at the crater

St. Pierre, Martinique: Mt. Pelee, 1902

• worst volcanic disaster of the 20th century

Augustus Cybari

• the lone survivor of the 1902 Mount Pelee eruption with 3rd Degree Burns

**

Tephra Fall/Ash Fall

(Volcanic Hazards)

• cause collapse of roofs via accumulation

• crop damage

• eye irritant

• respiratory problems

Ash and Hazards to Aviation

(Volcanic Hazards)

• weight of ash when wet increases weight 3 times, collapsing supposedly safe structures

• when an aircraft fly into an eruption cloud, engines can be damaged leading to engine failure mid-air

Global Cooling

(Volcanic Hazards)

• pinatubo eruption produced colorful sunsets due to the reflection of light on the ash particles

• aerosols including ash particles exert a cooling effect on the earth’s surface

Noxious Gas - CO2

(Volcanic Hazards)

• heavier than air

• invisible

Lake Nios, Cameroon, 1986

• Slow release by dormant volcano → bottom of lake in crater suddenly overturned and released CO2 which is heavy and hugged the ground suffocating

Limnic eruption in 1896 **

Lahar: Burial and Erosion

(Volcanic Hazards)

A fast-flowing torrent of (liquid) water and volcanic debris, such as mud, rock fragments, ash, and ice, that rushes down the slopes of a volcano

December 2006 Lahars, Mayon Volcano

• lahar flowed on the slopes of Mayon Volcano

• extreme rainfall due to super typhoon reming triggered lahars around Mayon Volcano

• houses were buried in lahar

Crater Lake / Mountain Lake Breaching

(Volcanic Hazards)

• Lake Maughan, Parker Volcano (September 6, 1995)

• Pinatubo Volcano (July 10, 2002)

• volcanic explosions as a trigger for flash floods

Debris Avalanche & Related Hazards

(Volcanic Hazards)

• 1980 Mount St. Helens Volcano was the most destructive in United States history

• largest terrestrial landslide or debris avalanche reduced the summit of the volcano by 400m

Debris Avalanche Deposit

• materials removed from the flank of a volcano

• characterized by mounds

Hummocky Topography

• signature topography of debris avalanche

• landscape characterized by an irregular, rolling, or undulating surface of low, rounded hills (hummocks) separated by depressions

Philippine Volcanoes

—23 active volcanoes and more than 400 inactive

—most active is Mayon Volcano with 49 historical eruptions

—7 active volcanoes are being monitored by PHIVOLCS

—most volcanoes are subduction related; exotic types also exist, e.g. Amoguis Volcano in Palawan

Volcanoes in the Philippines

Volcano Monitoring

Volcano Observatories are set up on all active volcanoes that threaten the human population.

These are designed to monitor and potentially to predict the eruptive behaviour of the volcano in question.

Precursors to an Eruption

• Increased Seismicity

• Deformation

• Gas Output

(on volcano and remote sensing techniques)

Seismic Activity

(Precursors to an Eruption)

Earthquake activity commonly precedes an eruption

• Result of magma pushing up towards the surface

• Increase volume of material in the volcano shatters the rock

• This causes earthquakes

Earthquake activity is measured by Seismographs

• Seismographs are stationed on the flanks of the volcano

• These record the frequency, duration and intensity of the earthquakes and report it back to the volcano observatory.

Deformation Monitoring

(Precursors to an Eruption)

“Tiltmeters” are used to measure the deformation of the volcano

The tiltmeters measure changes in slope as small as one part per million. A slope change of one part per million is equivalent to raising the end of a board one kilometer long only one millimeter!

Change in tilt indicates new magma entering magma chamber of the volcano (induces swelling). This is a precursor to eruption along with seismic activity/

Gas Monitoring

(Precursors to an Eruption)

Commonly gas output from a volcano increases or changes composition before an eruption.

—As magma rises to the surface it releases (exsolves) much of its gas content.

—This can be measured

— Gas samples are collected from fumaroles and active vents.

— Gas levels and types may also be monitored by remote sensing techniques indicating new magma in the chamber

Volcanism – Bad points

Hazards

1.Pyroclastic flow

2.Pyroclastic fall

3.Debris Avalanche

4.Lahar

5.Lava

6.Tsunami

7.Noxious gas

Volcanism – Good points

Benefits

—Fertile soil

—Valuable minerals

—Water reservoirs

—Geothermal energy

—Scenic beauty

Landslides

—general term for rapid downslope movements/failure

Mass Wasting

Is even more general and includes slow movements (creep, slow flows)

Mass wasting / slope failure / landslides – all downslope movement of rock and sediment that occurs at the surface of the earth (also underwater) in response to gravity.  Speeds range from extremely fast to extremely slow.

Elastic Rebound Theory

Explains the energy released by an Earthquak

Slip (on old fault)

From elastic rebound theory, the effect when stress between rocks is greater than friction

New Fault

From elastic rebound theory, the effect when stress between rocks is greater than rock strength

• Tectonic

• Volcanic

Two types of Earthquakes based on generators

Volcanic Earthquakes

Frequent, Low-magnitude Earthquakes from rising magma fracturing rocks

• Earthquake Intensity

• Earthquake Magnitude

Two ways of measuring the strength of an Earthquake

1815 Tambora Eruption, Indonesia

Largest recorded Holocene eruption 180 km^3 of magma expelled

Vesiculation or degassing of magma

Most important cause of volcanic eruption

1. In the deep, high pressures mix gas and magma together as homogenous matter
2. Less dense magma rises over denser rocks beneath Earth surface
3. Gas separates in shallower regions with lower pressure, expanding magma
4. When too much pressure is built up on the surface, an explosion occurs breaking a weak spot in the Earth's crust, releasing lava in an eruption

Process of Vesiculation (Degassing of Magma)

Lava

Coherent molten rock coming from magma (magma when extruded on the surface)
Normally made up of crystals, volcanic glass, and bubbles

Pyroclast
(fragmented magma) = tephra

Fragmented Magma
Any volcanic fragments ejected through air by volcanic process (Example: pumice)

Volcanic gases

Gases released from Volcanoes, filled with tiny particles suspended in air
Made up of silicon, oxygen, aluminum, and other elements

Volcano Observatories

These are designed to monitor and potentially to predict the eruptive behaviour of the volcano in question.