GEO 104 final terms

Magnitude

(energy release) o *Moment magnitude:* measure of the energy released - Measures amount of strain energy released - Based on amount of fault displacement

Intensity of shaking

(ground motion) o As measured by instruments

Stages of earthquake cycle:

1. Inactive and aftershock stage 2. Stress accumulation stage 3. Foreshocks 4. Main shock (major earthquake)

Effects of earthquakes

1. Shaking and ground rupture 2. Liquefaction 3. Landslides 4. Fires (shaking breaks electrical and gas lines) 5. Disease 6. Tsunamis 7. Regional changes in land elevation

Effects of earthquakes: Shaking and ground rupture

Fault scarp: surface rupture by vertical motion

fault zones

Faults almost never occur as a single rupture. Rather, they form fault zones, which are a group of related faults roughly parallel to each other in map view. They often partially overlap or form braided patterns. Fault zones vary in width, ranging from a meter or so to several kilometers.

earthquake segments

Most long faults or fault zones, such as the San Andreas fault zone, are segmented, with each segment having an individual history and style of movement. An earthquake segment is defined as those parts of a fault zone that have ruptured as a unit during historic and prehistoric earthquakes. Rupture during an earthquake generally stops at the boundaries between two segments; however, major to great earthquakes may involve several segments of the fault.

Directivity

Shaking intensity is amplified in direction of fault rupture

slip rate

Slip rate on a fault is defined as the ratio of slip (displacement) to the time interval over which that slip occurred. For example, if a fault has displacement of 1 m during a time interval of 1,000 years, the slip rate is 1 mm per year.

Runoff

Some of the water that falls on the land as rain or snow infiltrates soils and rocks; some evaporates; and the rest drains, or runs off, following a course determined by the local topography. This *runoff* finds its way to streams, which may merge to form a larger stream or a river .

Depth of focus

Strongly influences damage caused by earthquakes - The depth of focus of an earthquake varies from just a few kilometers deep to almost 700 km (435 mi) below the surface. The deepest earthquakes occur along subduction zones

Effects of earthquakes: Regional changes in land elevation

Uplift and subsidence

On what basis are Mercalli intensity values assigned to locations? a) Interpretation of seismograms b) Interpretation of the length of fault rupture c) Qualitative perceptions of and structural response to the shaking d) Proximity to the epicenter of the earthquake

c) Qualitative perceptions of and structural response to the shaking

Types of landslides: Slides

coherent block moves downslope - *Slumping* (type of sliding): rock or soil along curved slip plane

Role of Earth material type: Rotational slides (slumps)

curved slip surfaces - Soil or weak rock

Ground rupture occurs during an earthquake as a) Saturated sediments are liquefied b) Buildings pull away from their foundations c) California falls into the ocean d) A near-surface fault breaks the surface

d) A near-surface fault breaks the surface

Surface waves are produced by a) Faults rupturing the earth's surface b) The absorption of s-waves by a liquid medium c) The reverberating effects of buildings shaking in response to high frequency p-waves d) P- and s-waves reaching the surface

d) P- and s-waves reaching the surface

Volcanic Hazards: Secondary effects

debris flows, mudflows, landslides, debris avalanches, floods, fires

Volcanic features: Craters

depressions at top of volcanoes, formed by explosion or collapse of upper portion of volcanic cone

Paleoseismicity

determination of earthquake history along a fault, on basis of geologic record

Slopes

dynamic, evolving systems

Alluvial fan

fan-shaped deposit that forms when stream flows from mountain onto plain

Richter magnitude

formerly used; based on size of largest seismic wave produced; (about = to moment magnitude for large earthquakes)

Delta

forms as river flows into ocean and deposits sediments that extend into sea or lake

Types of landslides: Falls

free fall from face of cliff

Forces on slopes: Slip planes

geologic surfaces of weakness - Foliation planes, bedding planes, fractures

Volcanic features: Calderas

gigantic, circular depressions resulting from explosive ejection of magma followed by collapse of the volcanic cone

Volcanic features: Hot spring

groundwater contacts hot rock, becomes heated, and discharges at surface

Stage

height of water in river at any given time

Volcanic features: Geyser

hot groundwater produces steam and hot water

Frequency of seismic waves

how many waves pass in a given length of time

Volcanic Hazards: Primary effects

lava flows, pyroclastic activity, release of gases

Epicenter

location on Earth's surface above the focus

Slip rate

long-term rate of movement along fault (mm/yr)

Lava

magma that has reached surface due to a volcanic eruption (most is basalt)

Forces on slopes: Driving forces

move material downslope - Weight of slope material + anything on it

Volcanic Hazards: Pyroclastic activity (explosive volcanism)

o *Ash fall* - Rock and natural glass fragments blown into the air o *Lateral blasts* - Explosion of gas and ash from side of volcano o *Ash flows (or pyroclastic flows)* - Avalanches of very hot pyroclastic material move rapidly (up to 125 mph) down sides of volcano

Shaking hazards to buildings

o *Near epicenter ("jolting")* - Low buildings vibrate due to high frequency waves - Tall buildings vibrate due to low frequency waves o *Far from epicenter ("rolling")* - High frequency waves weakened - Tall buildings still damaged by low frequency waves o Important to consider shaking hazard for tall building design, even far from large faults

Case history: Mt. Pinatubo: Philippines, 1991

o 2nd largest eruption of the 20th century o 300 people died from ash fall, debris flow, mudflows, and typhoon o thousands saved by prediction and good communication o ash cloud cooled atmosphere during year following the eruption

Case history: Mt. St. Helens: Washington, 1980

o Awoke in March 1989 after 120 years dormant o Bulge grew on north flank o M 5.1 earthquake triggered large avalanche o Lateral blast o 1 hour after blast- large, vertical ash clouds rose to 19km o several mudflows o altitude of volcano reduced by 450m o eruption prompted extensive program to monitor volcanic activity

Channel type: Meandering

o Bends migrate back and forth across floodplain - *Cutbank:* outside of bend, bank erosion - *Point bars:* sediment is deposited on inside of curve - *Oxbow lake:* abandoned channel filled with water

Long term, probabilistic prediction

o Best possible method (right now) o Earthquakes can strike without warning o Regional hazard maps: show probability of event or shaking likely to occur

Sediment carried by Rivers: Total load: Bed load

o Bounces and rolls along channel bottom o (sand/gravel, <10%)

Shake maps

o Can be quickly generated if dense network of seismographs exists o Helps locate areas needing attention- rescues, locating damaged gas lines, etc.

Reasons for erosion or deposition are correlated to physical properties of the river:

o Change in channel geometry o Composition of channel bed and banks o Vegetation o Land use

P waves

o Compressional waves o Travel fastest through all types of media

Ground acceleration

o Damage to structures is related to two factors: - Amplitude of waves - Rate of wave velocity change with time (acceleration) o Waves accelerate ground vertically and horizontally o Buildings must be designed for strong accelerations

Short term prediction

o Depends on precursors (not always reliable) o Possible precursors: - Pattern and frequency of earthquakes, such as foreshocks - Preseismic deformation of the ground surface - Emission of radon gas - Seismic gaps along faults - Anomalous animal behavior

Buried faults

o Do not displace or rupture the ground surface o Typically associated with anticline and synclines

Forces on slopes

o Driving forces o Resisting forces o Slip planes o Safety factor

Floodplain

o Flat surface adjacent to river channel o Periodically inundated o Produced by flooding process

Volcanic Hazards: Lava flows

o Flow speed and characteristics vary - *Pahoehoe* • Fast moving (low viscosity), high temperatures • Ropey texture - *Aa* • Slower, lower temperature • Blocky texture o Methods to control lava flows have had mixed success

Role of time

o Forces change seasonally as the moisture content or water table position alters o Chemical weathering acts in presence of water

Fault

o Fracture or fracture system along which rocks have been displaced o Seismic source that must be ID'd to evaluate seismic risk

Effects of land use change from forest to agriculture:

o Increased soil erosion o Increase in sediment load o Increased channel slop

Role of climate

o Influences the amount and timing of water o Influences abundance of vegetation

Densely populated countries with many active volcanoes

o Japan o Mexico o Philippines o Indonesia

Composite volcanoes

o Known for beautiful cone shape o US examples: Mt. St. Helens and Mt. Rainier o Characterized by magma with intermediate silica content (andesite) o Produce alternating layers of pyroclastic deposits and lava flows o Most deadly/destructive

Role of water

o Landslides can develop during storms when slopes become saturated o Slumped or transitional slides can develop months or years after water infiltrates deeply o Water can erode base or toe of slope o *Quick clay:* spontaneous liquefaction of clay-rich sediment

Shield volcanoes

o Largest volcanoes o Common in Hawaii, Iceland, Indian Ocean islands o Shaped like a shield o Lava tends to flow down sides of volcano rather than exploding violently, because of low viscosity/ low silica content o Common rock type: basalt o Also produce tephra (pyroclastic debris) o Lave tubes often move magma underground for many kilometers o Typically have summit caldera

Impact on people and society

o Modified Mercalli

Volcanism is directly related to plate tectonics

o Most active volcanoes located near plate boundaries o Pacific "ring of fire"- 2/3 of active volcanoes - Belt of convergent plate margins (subduction zone)

Surface waves (R waves)

o Move along Earth's surface o Travel slowest, but cause most of damage

Channel type: Braided

o Numerous gravel bars o Islands divide and reunite channel o Tend to be wide and shallow o Often steep with coarse sediment

Interplate earthquakes

o Occur along plate boundaries o Most large US earthquakes

Intraplate earthquakes

o Occur within a single plate o New Madrid seismic zone (1811-1812) o Charleston, SC (1886) • Eastern US: stronger, less fractured rocks more efficiently transmit earthquake waves • Intraplate earthquakes = more damaging and felt over larger area (than similar magnitude quake in CA)

Volcanic Hazards: Debris flows and mudflows (lahars)

o Produced when large volume of loose volcanic ash and sediments become saturated with water and unstable - Debris flows - Mudflows

Role of vegetation

o Provides cover that slows raindrops, promoting infiltration and inhibiting grain-by-grain erosion o Root systems add cohesion o Adds weight

Modified Mercalli scale

o Qualitative severity measurement of damages and ground movements o Based on ground observations, perceptions, and responses, not instrumental measurements

Dip-slip

o Reverse o Normal o Thrust

Strike-slip

o Right lateral o Left lateral A strike-slip fault is a fault in which the sides of the fault are displaced horizontally; a strike-slip fault is called right-lateral if the right-hand side moves toward you as you sight, or look along, the fault line, and left-lateral if the left-hand side moves toward you.

Role of Earth material type

o Rotational slides (slumps) o Transitional slides o Creep o Earthflows o Shale slopes or weak volcanic rocks commonly fail

Material amplification

o Seismic waves travel differently through different rock materials - Propagate faster through dense and solid rocks - Intensity of ground shaking more severe in unconsolidated materials

Volcanic Hazards: Poisonous gases

o Several types emitted o Toxic concentrations rarely reach populated areas - Lake Nyos - Sulfur dioxide can produce acid rain - Vog (volcanic smog)

S waves

o Shear waves o Travel slower than P waves o Only travel through solids

Role of slope topography

o Slope greatly affects magnitude of driving forces on slopes o Debris avalanches and earthflows: steeper slopes o Creep: gentler slopes

Cinder cones

o Small volcanoes o Form when tephra piles up around a vent o May occur as parasitic cone (on flanks of larger volcano) o Ex: paracutin (central mexico)

Sediment carried by Rivers: Total load: Suspended load

o Transported above streambed by flowing water o (silt/clay, 90% of total)

Sediment carried by Rivers: Total load: Dissolved load

o Transported as chemical solution o From chemical weathering of rocks in drainage basin

Stability is determined by several variables:

o Type of earth material o Slope angle and topography o Climate o Vegetation o Water o Time

Volcanic domes

o Viscous magma (rhyolite) with relatively high silica content o Activity is mostly explosive o Usually small o Mt. Lassen

Effects of dam construction:

o Water slows at head of reservoir, causing deposition o Below dam, water released has less sediment o Channel erosion will happen below dam

Forecasting volcanic activity: volcanic alert or warning

o at what point should public be alerted or warned? o USGS warning system coded by color denoting increasing concern o Primary human adjustment: evacuation

Forecasting volcanic activity: monitoring geophysical properties

o hot magma in reservoir beneath volcano changes local conditions o changes in electrical conductivity, magnetic field strength, and the force of gravity also trace magma movement

Forecasting volcanic activity: monitoring gas emissions

o looks for changes in: - amounts of steam, carbon dioxide, sulfur dioxide - gas emission rates

Forecasting volcanic activity: seismic activity

o often provided earliest warning o caused by moving magma

Forecasting volcanic activity: topographic monitoring

o used successfully at Kilauea o summit tilts and swells before eruption

Forecasting volcanic activity: geologic history

o useful in predicting types of future eruptions o includes geologic mapping and dating of lava flows and pyroclastic deposits

Volcanic features: Volcanic vents

openings through which lava and pyroclastic debris are erupted at the surface of the earth o Circular conduits o Elongate fissures

Forces on slopes: Resisting forces

oppose downslope movement - Strength of material acting on potential slip planes

Role of Earth material type: Transitional slides

planar, inclined slip surfaces - Fractures, bedding planes, weak clay layers, foliation planes - *Soil slip:* shallow slide in soil over rock that occurs parallel to the slope

Focus

point at depth where rocks ruptured to produce quake

Forces on slopes: Safety factor

ratio of resisting forces to driving forces (>1=stable, <1=failure expected)

Drainage basin (watershed)

region drained by single river or river system

Recurrence interval

repeat time; can be determine by three things: 1) Paleoseismic data: looking at geologic record. 2) Slip rate 3) Seismicity: This method involves using historical earthquakes and averaging the time intervals between events.

Role of Earth material type: Earthflows

saturated materials

Effects of earthquakes: Tsunamis

seismic sea waves that originate when water is vertically displaced

Seismic waves

shock waves produced by sudden rupture of the rocks

Role of Earth material type: Creep

soil or rock near surface

Alluvial

stream-deposited soil

Hydrology

study of water transport in natural systems

Mass wasting

term for downslope movement of earth materials

Flooding

the natural process of overbank flow

Effects of earthquakes: Liquefaction

transformation of water-saturated sediment from solid to liquid state

Types of landslides: Flows

unconsolidated material moves downslope and particles mix within the mass - *Creep:* very slow flow - *Earthflow, mudflow, or debris flow:* rapid flow

Volcano

vent at Earth's surface through which magma and other volcanic materials are ejected from the Earth's interior

River gradient

vertical drop of channel over horizontal distance

Types of landslides: Subsidence

vertical movement

Earthquakes

violent ground-shaking phenomenon by the sudden release of strain energy stored in rocks • One of most catastrophic and devastating hazards • Concentrated along plate boundaries (>90%) • USGS estimate: ~1 million quakes/year

Preventing landslides

• *Drainage control:* divert water with surface drains or by covering with impermeable layer • *Grading:* cut and fill; benches on high, steep slopes • *Slop supports:* retaining walls along base • Landslides prevention can be expensive, but benefit-cost ratio is 10-2000:1

Human adjustments to flood hazards

• *Historically* o Create physical barriers (dams, levees) o Straighten, widen, or deepen stream • *More recently:* o Flood insurance o Control land use on flood plains

Volcano origins

• *Mid-ocean ridge volcanism produces basalt* o Wells up directly from atmosphere o Shield volcanoes from above hot spots - Ex: Hawaiian islands • *Composite volcanoes* o Andesitic rocks o Subduction zones: rising magma mixes with oceanic and continental crust o Most common volcanoes on Pacific Rim • *Caldera-forming eruptions* o Extremely violent and explosive - Typically rhyolitic magma produced when magma moves upward and mixes with continental crust

Volcano types: Why different types?

• *Viscosity of magma determines* o Type of volcano that forms o Associated style of activity • *Viscosity: resistance to flow* o Directly related to - Silica (SiO2) content (~50-70%) - Temperature

River Discharge (Q)

• *Volume of water moving by a particular location in a river per unit time* • Q = W x D x V - Q = discharge (cms or cfs) - W = width of flow - D = depth of flow - V = velocity of flow

Land subsidence due to groundwater action

• *sinkhole:* area of subsidence into a subterranean void (often circular) • *high water table:* buoyancy of water supports overburden

Coal mining

• 50% of coal is mined • 50% of left as pillars supporting the roof • over time, pillars weaken, weather, and collapse • subsidence • most common when mining is close to surface

What determines the types of river deposits and erosion?

• Alluvial fan • Delta • physical properties of the river

landslides

• Approximately 25-50 people per year killed in US • Damage = $billions/year in US

Correcting landslides

• Attack process that started the slide • Drainage programs the reduce water pressure

Sediment carried by Rivers: Total load

• Bed load • Suspended load • Dissolved load

What are some different types of channels?

• Braided • Meandering

Effects of land use changes

• Change in amount of water or sediment --> change in channel slope or cross-section

Volcanic features

• Craters • Calderas • Volcanic vents • Hot spring • Geyser

Earthquakes caused by human activity:

• Dam or reservoir - Loading of Earth's crust - Increased water pressure • Underground nuclear explosions • Disposing of liquid waste through disposal (injection) wells

Seismograph

• Device that measures ground shaking • Sensor (seismometer) + recorder • Seismogram: written or digital record of earthquake

Earthquake cycle

• Drop in strain after earthquake • Re accumulation before next event • *Elastic strain:* deformation that is not permanent • *Elastic rebound:* "snap" of rocks back to original shape as strain is recovered

Response to earthquake hazards

• Earthquake hazard reduction programs • Adjustments to earthquake activity o Structural protection, land-use planning, insurance • Earthquake warning systems • Perception of earthquake hazard • Personal and community adjustments: before, during, and after

Earthquake processes

• Faulting • Fault types • Active faults • Tectonic creep

Nature and extent of flood hazards

• Flooding: #1 type of disaster in 20th century in US • Factors that affect damage: o Land use on floodplain o Magnitude of flood o Rate of rise and duration of flooding o Season o Sediment load deposited o Effectiveness of forecasting, warning, and emergency systems

Tectonic creep

• Gradual displacement not accompanied by felt earthquakes • Slow damage to roads and structures

Urbanization and flooding

• Land use can increase magnitude and frequency of floods in small drainage basin • % of impervious cover and % of area served by storm sewers are measure of degree of urbanization • Runoff increases because infiltration decreases

Damage depends on:

• Location • Magnitude • Surface geology • Population density

Earthquake risk and prediction

• Long term, probabilistic prediction • Short term prediction

Earthquakes can be compared by their:

• Magnitude (energy release) • Intensity of shaking (ground motion) • Impact on people and society

Earthquake intensity scale

• Modified Mercalli scale • Shake maps

Types of seismic waves

• P waves • S waves • Surface waves (R waves)

Volcanic Hazards

• Primary effects • Secondary effects • Lava flows • Pyroclastic activity (explosive volcanism) • Poisonous gases • Debris flows and mudflows (lahars)

Caldera eruptions

• Produce rare but extremely violent eruptions o At least 10 caldera eruptions in last 1 million years o Three in North America • Can extrude up to 100km3 of pyroclastic debris, mostly ash • Most recent North American examples o Yellowstone, approximately 600 years ago o Long Valley, California, approximately 700 years ago • Main events in caldera formation can occur quickly (days to weeks), but lesser-magnitude activity can linger for a million years

Why do people like to live and work on floodplains?

• Rich alluvial soil • Water supply • Ease of waste disposal • Proximity to commerce

Earthquake magnitude scale

• Richter magnitude • Moment magnitude:

Salt Deposits

• Salt mining - Water is injected through wells into salt deposits - The salt dissolves - Water supersaturated with salt is pumped out • Removal of salt leaves a cavity in the rock and weakens support for overlying rock • Can lead to subsidence

Volcano types:

• Shield volcanoes • Composite volcanoes • Volcanic domes • Cinder cones

Types of landslides

• Slides • Falls • Flows • Subsidence Landslides are commonly complex combinations of sliding and flowage

River erosion and deposition

• Smaller cross-sectional area --> increased velocity • Faster flowing rivers --> greater erosional capacity

Fault types

• Strike-slip • Dip-slip • Buried faults

Volcanic Hazards: Pyroclastic activity (explosive volcanism): ASH FALL: hazards

• Vegetation destroyed • Temp contamination of surface water • Structural damage to buildings • Health hazards (respiratory, eye) • Damage plane engines

Warning of impending landslides

• Warning systems do not prevent landslides • Can provide time to evacuate people and possessions, stop trains, reroute traffic

Effects of land subsidence

• building foundations, roads, and underground pipes crack • earth fissures • "groundwater mining"

Impact of groundwater withdrawal

• excessive pumping from unconsolidated sediments

Case history: Nevado Del Ruiz, Columbia, 1985

• killed over 23,000 people and 15,000 animals

How do scientists study active volcanoes to predict when an eruption will occur and how extensive it will be? Forecasting volcanic activity

• seismic activity • monitoring geophysical properties • topographic monitoring • monitoring gas emissions • geologic history • volcanic alert or warning

Impact of groundwater action

• slightly acidic groundwater dissolves soluble rocks • caves enlarge over time lack of support for overlying rock causes collapse • case study: Winter Park, Florida (1981) - formed during record low groundwater levels