Natural Hazards - Earthquakes and Volcanoes

Define an Earthquake?

A sudden release of stored energy (stress) caused by movement along a fault

• Can be caused by volcanic activity, meteorites, and manmade explosion for nuclear bombs

• Most common - along faults

Define a Fault?

A fracture in the Earth across which the two sides more relative to each other

What builds up enough to cause rocks to fracture and/or shift, sending off waves of seismic energy felt as earthquakes?

Stresses

We recognize faults beginning with rock relationship, formalized by what process

Steno’s Law Of Rock Relationship

Steno’s Law Of Rock Relationship is split into three sections. What are they? What do they state?

1) Law of original horizontality - Sedimentary rocks are in layers, layers were original horizontal. If we see rocks not horizontal, faulting is taking place

2) Law of superposition - In sedimentary rocks, each rock layer is younger than the layer beneath it and older than the layer above it

3) Law of original continuity - Sediment layers are continuous, ending only against a topographic high, by pinching out from lack of sediment, or by gradational change from one sediment to another

Faults are also known as what

Joints; which are fractures and cracks in brittle lithosphere rocks. However, if joints move due to large stress differential on either side of a fracture, they become faults

• Movement ranges from mm to hundreds of km, resulting in tilting/folding of layers

What two terms are used to describe location in 3D space of deformed rock layers/describing faults

Dip & Strike

What does “Dip” mean when talking about the location in 3D space of deformed rock layers/describing faults

The angle of inclination from horizontal of titles layer

What does “Strike” mean when talking about the location in 3D space of deformed rock layers/describing faults

Compass bearing of horizontal line in titled layer

What are the two parts of a fault; describe each

Footwall - the rock beneath the fault

Hanging wall - rock above the fault

List and describe all types of faults

1) Dip-slip faults - dominated by vertical movement

• Normal faults: hanging wall moves down due to extensional forces (pull apart); found in locations of seafloor spreading and where continents are pulling apart; results in zone of omission (layers are missing)

• Reverse fault: hanging wall moves up due to compression stress (pushing); found where plates converge in continental collision or subduction; results in zone of repetition (layers are repeated vertically) CREATE THE LARGEST EARTHQUAKES

2) Strike-slip faults - moves side by side; convention works in either direction

• Left Lateral: moves to the right toward you when you look from the side of the fault

• Right Lateral: moves to the left toward you when you look from the side of the fault

Faults are complex zones of breakage with what surfaces

Irregular

Stress builds up in along faults over years until what occurs at weak points and propagates along fault surface

Ruptures

The point where ruptures first occur is called what

Hypocenter/focus

The point directly above the hypocenter/focus on the earth’s surface is called what

Epicenter

Energy during an fault rupture or earthquake is released as what

Seismic waves

A left step in right-lateral fault or a right step in a left-lateral fault both results in what

Compression, uplifts, creates hills and mountains

A right step in right-lateral fault or a left step in a left-lateral fault both results in what

Extension, down-dropping, basins and valleys

What is a Seismograph

The instrument that records the passing energy waves, aka seismic waves

What is a Seismogram

The actual recording of the passing waves

We use what four terms to describe seismic waves; describe them

Wavelength - distance between successive waves

Amplitude - Displacement between highest recorded point and level

Period - Time between waves (=1/frequency)

Frequency - Number of waves in one second

What are the two categories from seismic waves

Body waves: those that can pass through entire earth

Surface waves; those that move near surface only

Describe more on Body waves, as well as list the two types and their attributes

Faster than surface waves, have short periods, most energetic near earthquake hypocenter

P (Primary) waves

• Fastest of all waves; always first to reach a recording station

• Move as a push pull - pulses of compression then extension (like the wave through a Slinky toy)

• Travel through solid, liquid or gas

• Velocity dependent on density and compressibility of substance they are traveling through

S (Secondary) waves

• Reach the station second after primary

• Exhibit transverse motion - shearing or shaking particles at right angles to the wave’s path (like shaking one end of a rope)

• Travel only through solids

• Velocity depends on density and resistance to shearing of substances

  • Up/Down and side/side motion causes severe damage to buildings

What is the Lag Time

The time difference between the arrival of the P waves and the arrival of S waves

Describe more on Surface waves, as well as list the two types and their attributes

Travel near the Earth’s surface, created by body waves disturbing the surface; longer period than body waves and travel more slowly; carry significant energy great

Love waves

• similar to S waves, except side to side in horizontal plane

• Travel faster than Rayleigh waves

• Do not move through air or water

Rayleigh waves

• Similar to ocean waves; backward-rotating, elliptical motion producing horizontal and vertical shaking; feels like rocking a boat at sea

• More energy is released as Rayleigh waves when earthquake hypocenter is close to the surface

In order to find the location of an earthquake, you need what

The distance of earthquake from three stations; find the Lag time, intersecting at 1 point all three circles

What is the magnitude of an earthquake

The measurement of strength of an earthquake; based on the largest amplitude and distance from epicenter

What is the Richter scale

A way to measure energy released in an earthquake; a log scale

Richter magnitude increases of one number the amount of energy increased by what and amplitude of what

32 times; 10

Large earthquakes are not just single event, but apart of what

A series of earthquakes over a number of years

In an earthquake series, the largest event in the series is known as

Mainshock

In an earthquake series, the smaller events preceding mainshocks in the series are known as

Foreshocks

In an earthquake series, the smaller events following mainshocks in the series are known as

Aftershocks

The velocity of seismic waves depend on what; and what happens in hard rocks/soft rocker

The material through which it is moving

Hard rocks - high velocity; lower amplitude

Soft rocks - lower velocity; high amplitude

Shaking is amplified when what happens

Tall buildings shake at the same period as the seismic waves

What is the Mercalli Intensity Scale

A system used to quantify what people feel during an earthquake and amount of damage; a scale from I to XII

P waves travel how much faster than S waves

1.7 times

The Mercalli intensity scale’s scaled amount of damage depends on what

1) Earthquake magnitude - Bigger earthquake, more likely death and damage

2) Distance from hypocenter - Usually (but not always), closer earthquakes —→ more damage

3) Type of rock or sediment - soft sediments amplified

4) Building style - amplified by ridged short buildings

5) Duration of shaking - longer shaking lasts, more buildings can be damaged; larger earthquakes longer duration

Most earthquakes can be explained by what theory

Theory of Plate tectonics

What are the three types of plate boundaries in relation to earhtquakes

Divergent plate boundaries - Small earthquakes and generally non-threatening

Transform plate boundaries - Generating large earthquakes

Convergent plate boundaries - Generating enormous (largest) earthquakes

At convergent boundaries, what method is used to help sceintistics predict future earthquake locations

Seismic Gap Method, which states if some segments of a fault have moved recently, it is reasonable to expect that unmoved portions will move next, to fill the gaps

Recount the Mexico City Earthquake and its significance

1985; created long distance destruction

• 350 km from Mexico City, but caused intense damage

• Earthquake was expected due to the Michoacán seismic gap

• 9000+ people were killed by building collapses

• Resonance between seismic waves, soft lake sediment foundations and improperly designed buildings

The western portion of the US has earthquakes due to what

Plate interactions (between the North American and Juan De Fuca Plate - Pacific Plate)

The San Andreas fault is what kind of plate boundary

Transform boundary (slide by to produce earthquakes); however its not a straight line so can get compressional stress and thrust faults

• i.e Big Bend near Los Angles

Damaging earthquakes can result from what type of faulting

normal faulting (tensional stress)

How are earthquake probabilities when looking at long term predictions and short term predictions

Long term - fairly good based on paleoseismology (looking at pasts events in the region)

Short term - no. detailed behavior of faults may too unpredictable to ever allow short term predictions of earthquakes

Human triggered earthquakes are happened from what

Pumping fluid underground & disposal wells of drilling can lead to small earthquakes

Describe the Western North America and its relation to earthquakes

• Has plate boundary zone earthquakes

  • Largely due to subduction of small plates and continued effects of the over Farallon plate which includes California into Utah, the eastern edge around Salt Lake City, and the Rio Grande Rift

Describe the “Stable” Central United States and its relation to earthquakes

• Intraplate earthquakes

• Clusters of earthquakes at few locations in Central U.S

• Away from active plate edges; represents ancient rifts similar to Rio Grande Rift

• Fewer earthquakes, but can be large
  

Future earthquakes will affect St. Louis, Memphis and Nashville, as those building are not designed for earthquake shaking, have soft sediments, and are of a large area/population

Most volcanoes are associated with what

Plate tectonics/plate boundaries; 73% at spreading centers, 15% at subduction zones, and the remaining 12% of volcanism occurs above hot spots

List how volcanoes interact with each plate interaction;

Spreading Center (divergent): hot, ductile mantle rock rises upward at ocean spreading centers where the lower pressure causes melting. When the magma erupts, it forms new oceanic crust in the form of shield volcanoes or underwater lava flows, contributing to ridge formation like the Mid-Atlantic Ridge. These have nonexplosive eruptions.

Subduction: carries oceanic plate (with water rich sediments) into hot mantle, where water lowers melting temp of rock. This process generates magma that is typically andesitic to rhyolitic in composition, and very explosive

Transform faults/continent-continent collisions: No volcanism

Variations of what things help determine whether eruptions are peaceful or explosive

Magma’s chemical compostion, ability to flow, gas content, and volume

What are the three V’s in discussing volcanoes

• Viscosity - resistance to flow, high silica content magma high viscosity

  • Lower viscosity —→ more fluid behavior

  • Higher viscosity —→ thicker

• Volatiles - more dissolve gases more explosive, as pressure decreased gases become bubble which expand rapidly leading to violent eruptions

• Volume - large volumes more hazard, spreading center volcanoes have relatively peaceful eruptions but subduction zone volcanoes explore violently

Describe how Basaltic and Rhyolitic magma are with viscosity and volatiles

Basaltic magma - lowest viscosity

Rhyolitic magma - highest viscosity

Basaltic magma - low dissolve gases = peaceful eruptions

Rhyolitic magma - highest dissolve gases = violent

Try and chart comparison of three types of magma

What are the three styles of volcanic eruptions and the name of the volcanoes

1) Nonexplosive - icelandic and Hawaiian

2) Somewhat explosive - Strombolian

3) Explosive - vulcanian and Plinian

Summarize how water and viscosity interact with the level of explosiveness

Low-water content, low-viscosity = nonexplosive eruptions

High-water content, high viscosity magma = explosive eruptions

List the 5 types of volcanic landforms and their explosiveness

1) Shield - low viscosity, low volatiles, large = peaceful

2) Flood basalts - low viscosity, low volatiles, verge large volume = connected to mass extinctions

3) Scoria (Cinder) Cone - medium viscosity, medium volatiles, small volume - explosive

4) Stratovolcano - high viscosity , high volatiles, very large volumes = explosive

5) Caldera - high viscosity, high volatiles, large large volume = explosive

Describe information on the volcanism at subduction zones

• Most of the world’s famous volcanoes are subduction zone volcanoes

• Many regions around subduction zones volcanoes are heavily populated

• Erupt directly into atmosphere (not underwater) so have direct on worldwide climate

• Ex. Cascade range, pacific coast US and Canada

Why do spreading centers have peaceful explosives, but subduction zones explode violently

Chemical Composition of the enviroment: Magma at subduction zones typically has a higher silica (SiO₂) content, which makes it more viscous compared to the basaltic magma found at spreading centers

• Aluminum

• Iron

• Calcium

• Sodium

• Potassium

• Silicon

• Oxygen

• Magnesium

• other

List the top volcanic process and killer events when eruptions happen

Pyroclastic flows - 29%, the superhot, high speed turbulent cloud of ash, gas, and air is deadly

Tsunami - 21%

Lahar - 15%, volcanic mudflows, Mount Rainier on alert

Indirect (famine) - 23%

What is the Volcanic Explosivity Index

A semi-quantitative estimate of magnitude of volcanic eruptions, 0-8 more explosive, using volumes erupted and eruption-column height

The frequency of different VEI magnitudes is of what

An Inverse correlation

• Bigger eruption, less frequent occurrences

• Smaller eruptions, more frequent occurrences

How effective is volcanic monitering and warning systems

Efforts met with both failures and success

List the three signs of impending eruptions of volcanoes

1) Seismic waves, small earthquakes

2) Ground deformation; surface rises and falls in response to magma movement

3) Gas measurements

Chart the comparison of three types of magma

Describe the origin of cascade mountains

Subduction

Describe the 1980 mount st Helen eruption

high visocity got trapped, produced bulge small earthquakes occurred and allowed gas to expand and to to blast outwards, first blast lateral horizontal, which lead to vertical blast

Describe Supervolcanoes

A volcanic center that has had an eruption of magnitude 8 on the Volcano Explosivity Index (VEI)

What is the difference between pyroclastic flows vs surges

Pyroclastic flows, the superhot, high speed turbulent cloud of ash, gas and air is deadly – they result in about 29% of volcanic deaths

Pyroclastic surges, more steam and less pyroclastic material; no controlled by topography

What are lahars

Mixtures of water, volcanic ash, tephra, rock fragments, and chunks of ice that can flow like wet concrete

Why is Mt. Rainer considered to be so dangerous

Great height Extensive glacial cap Frequent earthquakes Active hot-water spring systems

What are vogs

A mixture that includes gases but is predominately aerosols (tiny particles and droplets) formed when volcanic gas reacts with moisture