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
