Rocks, Minerals, and Earth's History

Module 4: Earthquakes and Volcanoes

• Module 4, Part 1 discusses earthquakes and volcanoes

Aquila Earthquake, Italy, 2009

• In 2009, Aquila, Italy, experienced a magnitude 6.3 earthquake.
• The earthquake killed 309 people.
• A panel of scientists was sentenced to 6 years in prison plus fines for failing to provide adequate warning to citizens, the verdict is now on appeal.

Course Learning Objectives: Part 1

• Define these terms: strike, dip, hanging wall, and footwall.
• Be able to sketch and label the three major types of faults.
• Explain how rocks deform before and after fault movements that produce earthquakes.
• Discuss how displacements from individual earthquakes accumulate to account for 100s of km of movement between tectonic plates.

How Rocks Respond to Stress

• Small amounts of stress leave blocks essentially unchanged.
• Strain involves changes in size and shape (metamorphism).
• Displacement refers to rocks moved along faults.
• Rotation occurs when rocks are tilted, forming folds.

Rock Deformation

• At shallow depths where temperature and pressure are low, most rocks break, resulting in brittle deformation (faults).
• In deep crustal conditions with high temperature and pressure rocks flow, resulting in ductile deformation.

Describing Faults

• Dip is the inclination or slope of a fault surface measured from horizontal.
• A dip-slip fault involves fault movement parallel to the dip.

Dip-Slip Fault Movement

• The hanging wall is above the fault.
• The footwall is below the fault.

Types of Dip-Slip Faults

• In a normal fault, the hanging wall moves down due to tension.
• In a reverse fault or thrust fault, the hanging wall moves up due to compression.

Strike & Faults

• Strike is a horizontal line on an inclined surface.
• A strike-slip fault involves movement on the fault parallel to the strike.

Strike-Slip Faults

• Rocks move horizontally along strike-slip faults, with no vertical movement.
• Movement is defined by looking across the fault for offset features.

Lateral Faults

• To determine if the road facing across a fault is displaced to the left or right defines if it is a left or right lateral fault

Fault Motion and Plate Tectonics

• Fault movements result from stresses produced by plate tectonics.
• Friction along the fault surface is enough to cause most faults to "stick".
• Stress builds up, causing rocks closer to the fault to bend elastically.
• After decades or centuries, stress builds to levels sufficient to overcome friction and cause fault movement, or slip.

Faults and Plate Tectonics - Western US

• Faults with the potential to produce large earthquakes are in the western U.S. due to stresses from nearby plate boundaries.
• The San Andreas Fault is has a slip rate of ~2.5 cm/year due to plate motions.

Individual Earthquakes

• A stream channel has been displaced 130 meters by the San Andreas fault.
• At a slip rate of 2.5 cm/year (0.025 meters per year) due to plate motion, it would take 5200 years to offset this stream channel.

San Andreas Fault System

• The San Andreas fault system has had over 500 km of fault movement in 20 million years.
• Major faults are hundreds of kilometers long.
• Major faults break in segments, one part at a time.
• The longer the break, the bigger the earthquake.
• Big earthquakes on the same fault occur hundreds to thousands of years apart.

Module 4, Part 2: Learning Objectives

• Classify diagrams of fault types.
• Define basic terms such as focus and epicenter.
• Explain why and where earthquakes occur.

Learning Objectives: Part 2

• Explain how earth scientists use seismic waves to locate and measure earthquakes.
• Summarize the difference between measures of earthquake intensity and magnitude.
• Describe at least five different types of hazards resulting from earthquakes.
• Explain the geological conditions likely to result in greater risk from future earthquakes.

Seismic Waves and Earthquake Detection

• Seismic waves are vibrations caused by an earthquake that travel in all directions from the focus of the earthquake
• Slower surface waves travel along Earth's surface and cause the most damage.
• Faster body waves (P and S waves) travel through Earth's interior.

Seismic Waves cont.

• P waves arrive first at a seismograph station.
• They travel at ~6+ km/s in the crust. _ They compress material parallel to the travel direction.
• S waves arrive after P waves but before surface waves.
• They travel at ~4 km/s in the crust.
• They vibrate perpendicular to the travel direction

Seismogram

• A seismogram is a record of seismic waves.
• Seismograms record the arrivals and size of P, S, and surface waves.
• Closer earthquakes mean less time between waves.
• Bigger earthquakes mean bigger waves. Velocity of waves is not related to the size of the earthquake

Earthquake Location

• Earthquake location: -Earthquakes are recorded by a network of seismic instruments using P & S waves.
• Earthquake records are selected to measure the difference in P-S arrival times.
• The difference in P-S arrival time is converted to distance. Triangulation is used to find the epicenter.

Measurement of Earthquakes

• Earthquake size can be determined by measuring the amplitude (height) of the seismic waves.
• By plotting amplitude and distance the earthquake magnitude can be estimated.

Measuring Earthquakes

• Two methods for measuring earthquakes:
• Magnitude: A standard measure of shaking and/or energy released based on seismic waves.
• Intensity: Measures the effects of an earthquake on people and buildings.

Earthquake Magnitude

• Magnitude is measured on a logarithmic scale.
• Each division represents a 10-fold increase in ground motion and a 32-times increase in energy released.
• Example: a magnitude 5 earthquake exhibits 100 times more shaking and releases nearly 1,000 times more energy than a magnitude 3 quake

Earthquake Intensity

• Intensity measures effects on people and structures using the Modified Mercalli Scale.
• The scale is 12 points and uses Roman numerals.
• It can be applied to historical events and compared to magnitude scale.
• The Modified Mercalli Scale is useful for rapid collection of online data following earthquakes.

The USGS

• It generates Community Internet Intensity Maps (CIIMs, citizen science).
• Example: CIIM for 6.7 magnitude Northridge earthquake (1994).

Virginia Earthquake Details

• Virginia experienced a magnitude 5.8 earthquake in August 2011 at a depth of 6 km. with a reverse Fault Type and fault length: ~5-15 km
• The earthquake was measured Mercalli Intensity IV NCSU which was 245 km away,
• Factors that would affect intensity values includes number of stories, bedrock type, distance from epicenter.

Modified Mercalli Scale

• The Modified Mercalli Scale can be applied to historical accounts of earthquakes.
• Significant earthquakes occur in areas with little recent activity

Earthquake Hazards

• These hazards exist even with damage control measures and some structures were better constructed to withstand earthquakes after the 1994 Northridge, California, earthquake

Northridge Earthquake Hazards

• Landslides are a common event in steep slopes.
• Shaking of >.4g can collapse freeway overpasses.
• Buildings were damaged by shaking over a wide area (red dots)

More Earthquake Hazards

• Earth materials (rock/sand/mud) affect the degree of shaking.
• Weaker materials (clays vs. bedrock) may magnify shaking effect.
• Liquefaction occurs when water is released from saturated earth materials shaken to cause collapse.

Subduction and Earthquakes

• Subduction causes earthquakes.

Tsunami

• The 2004 Indian Ocean Tsunami wave heights up to 30 meters struck Sumatra.
• The waves lengths up to hundreds of kilometers, and traveling with speed of ~800 km/h

2004 Tsunami

• How long did it take the tsunami to reach: -India: 2 hrs -Africa: 7-11 hrs -South America: 20+hrs -North America: 29 hrs

EarthQuake Hazards conditions

Two cities with about the same population had an event a. Tarville with high Damage and Wolftown with Light damage, so list why each had light or high damage?

EarthQuake Hazards conditions.

• Two inland cities (Wolftown, Tarville) with the same population experienced two identical earthquakes

• (same magnitude, depth, and location relative to city).

• Tarville was devastated, while Wolftown suffered only light damage. Make a list of reasons to explain why Tarville was heavily damaged while Wolftown was not.

• Tarville was located on weak sediment that exaggerated the shaking. Tarville sediment was water-logged and liquefaction caused collapse.

• Tarville was constructed of buildings composed of poor grade materials.

• (Wolftown had better building codes to account for earthquake risk.)

• Tarville buildings were destroyed by landslides from nearby steep valley walls (Wolftown was located on a flat plain.)

• Tarville was located along the coast and was struck by a tsunami but Wolftown was much farther inland.

Module 4, Part 3: Learning Objectives

• Describe the differences among different types of volcanoes (shield, composite, volcanic domes, and cinder cones).
• Explain how viscosity is related to gas content and the nature of volcanic eruptions.

Volcanoes

• Most active volcanoes are located along convergent plate boundaries.
• The 1980 composite volcano eruption of Mount St. Helens will provide a great point of study.

Volcanic Eruptions

• View: Johnston Ridge - Mount st Helens
• After 123 years of quietness and earthquake activity it was followed by small eruptions with gas release then it ended with major eruption - May 18, 1980
• 57 deaths, $1 bill damages

Hazards

Main eruption products of composite volcanoes:

• Tephra (volcanic ash, lava bombs, debris)
• Lahars (volcanic mudflows)
• Pyroclastic flows Less commone eruption products:
• Lateral blast
• Lava (volcanic dome) Main eruption products of shield volcanoes:
• Lava (lava flows, cinder cones)

###Distribution Distribution around Mount St. Helens of the eruption products listed as 1 Tephra, 2 Lahar, 3 Pyroclastic and 4 Lateral Blast

Tephra Hazards

Volcanic Eruption Hazards - Tephra

• Volcanic Ash is crossed U.S. in 3 days.
• Plume up to 25km

Landform Hazards

• Lahars (volcanic mudflows) move in the range speeds with 10-20 mph- Debris mixes with water in streams or from melting iceshaking will change the volume or shape)

Volcanic Eruption

Hazards – Pyroclastic Flows consist of fast moving (60 m/s or 100km), very hot (~700 C), dense cloud of toxic gas and tephra and race down slope for days due to their large volume.

Hazards – Lateral Blast

With side buldge

Hazards - Lateral

Side Eruptions will Blast through hill side (instead of top of mountain).

Eruption - Lava

Little at Mount St. Helens -magma will be dome form.

Volcano type

• Shield Volcanoes - Is fluid from magma fissure (feeds with fractures)

(Bad) and (Good) Lava

(Bad) – High heat - Viscosity - easily coveres large area (Good) - dome - build

Prediction of eruption

How Do We Monitor Volcanic Activity is done with these methods.

• Earthquakes and Volcanic gases - with increasing gas activity.

Changes in Volcanic eruption

• Changes in shape can be read.

Part Module4

Course Learning Objectives I can identify the features of an ancient caldera I can sketch the formation of a caldera and explain why these eruptions are so destructive.

Activity study

To see if learn with I can describe the volcanic hazards that as associated with the eruption

• I can list the features and processes that geologists study when trying to predict an eruption

Assessment with different cone rocks. Viscosity of the silica effects the cones

Volcone with vents, viscous volcano or Caldera

Cone Viscosity and the different silica form

The difference plate tectonic setting in viscous cone form .

To form a vent. Can form as a chain volcanos

To effect the hazzards. Low silica is the viscous hazzards on a volcano

High Pressure on Volcanoes and the effect

Pressures on Volcanoes is 3 mild or effect Pressure on Volcanoes is 2 under pressure the gas form or effects. Pressure on Volcanoes is 3 when the gas is released

Understand the relationship between rocks and minerals. Learn the five characteristics of a mineral: solid, natural, inorganic, ordered internal structure, and specific chemical composition. Identify common elements and minerals in the Earth's crust such as oxygen and silicon.