# Chapter 3 Study Guide: Earthquakes

Prepare to define terms, identify fault types, explain how earthquakes and seismic waves form, compare measurement scales, interpret hazard examples, and explain how earthquake risk can be reduced.

## Chapter Learning Objectives

The chapter states these objectives. Each section below identifies the objective or objectives it supports.

- **LO:1:** Compare and contrast the different types of faulting.
- **LO:2:** Explain the formation of seismic waves.
- **LO:3:** Summarize the processes that lead to an earthquake and the release of seismic waves.
- **LO:4:** Differentiate between the magnitude scales used to measure earthquakes.
- **LO:5:** Identify global regions at most risk for earthquakes, and describe the effects of earthquakes.
- **LO:6:** Describe how earthquakes are linked to other natural hazards.
- **LO:7:** Discuss the important natural service functions of earthquakes.
- **LO:8:** Explain how human beings interact with and affect the earthquake hazard.
- **LO:9:** Discuss the United States governmental approach to minimizing the effects of earthquakes.
- **LO:10:** Propose ways to minimize seismic risk and suggest adjustments we can make to protect ourselves.

## 1. Essential Definitions (supports LO:1 through LO:10)

- **Fault:** A fracture or fracture system along which rocks have been displaced.
- **Stress:** Force applied to rocks, commonly due to tectonic movement.
- **Strain:** Deformation or change in shape or location produced by stress.
- **Fault plane:** The surface along which movement occurs during faulting.
- **Hanging wall:** The block of rock above an inclined fault plane.
- **Footwall:** The block of rock below an inclined fault plane.
- **Dip-slip fault:** A fault in which movement is mainly vertical, caused by tensional or compressional stress.
- **Strike-slip fault:** A fault in which movement is mainly horizontal, caused by shearing stress.
- **Normal fault:** A dip-slip fault in which the hanging wall moves down relative to the footwall. It is associated with tension and crustal extension.
- **Reverse fault:** A dip-slip fault in which the hanging wall moves up relative to the footwall. It is associated with compression and crustal shortening.
- **Right-lateral or left-lateral fault:** A strike-slip fault named for the apparent direction an offset feature moves when viewed across the fault.
- **Blind fault:** An active fault that does not reach Earth's surface. Folding, including anticlines and synclines, may reveal it.
- **Earthquake:** Sudden fault rupture and the resulting release of stored elastic energy as seismic waves.
- **Elastic rebound:** The process in which rocks deform elastically as stress builds, then rupture and rebound when their strength is exceeded.
- **Earthquake cycle:** The sequence of strain accumulation, possible foreshocks, mainshock, and aftershocks separated by periods of relative inactivity.
- **Focus or hypocenter:** The point within Earth where an earthquake rupture begins.
- **Epicenter:** The point on Earth's surface directly above the focus.
- **Seismic waves:** Vibrations produced by an earthquake that travel through Earth or along its surface.
- **P wave:** A fast, compressional primary wave that travels through solids, liquids, and gases.
- **S wave:** A slower, shear secondary wave that travels only through solids.
- **Surface wave:** A wave that travels along Earth's surface and commonly causes severe damage near the epicenter.
- **Tectonic creep:** Gradual fault movement that is not accompanied by perceptible earthquakes.
- **Paleoseismicity:** The prehistoric record of earthquakes, reconstructed from disturbed or faulted sediments and other evidence.
- **Seismograph or seismometer:** An instrument that records ground motion.
- **Magnitude:** A quantitative measure of the size or energy released by an earthquake at its source.
- **Intensity:** A measure of the shaking and damage experienced at a particular location.
- **Material amplification:** Increased ground motion caused when seismic waves pass through unconsolidated or water-saturated materials.
- **Resonance:** Especially strong shaking that occurs when the frequency of ground motion matches a building's natural vibrational frequency.
- **Liquefaction:** The transformation of loose, water-saturated sediment from a solid-like material into a fluid-like slurry during strong shaking.
- **Fault scarp:** A small cliff or step in the ground surface produced by fault rupture.
- **Megathrust earthquake:** A very large earthquake, commonly at a subduction-zone interface, where the largest earthquakes occur.
- **Seismic gap:** A segment of an active fault that has not experienced a large earthquake recently and may be storing strain.
- **Forecast:** A probabilistic statement about the likelihood of an earthquake in a particular area during a specified time period.
- **Prediction:** A more specific statement about the time, location, magnitude, and probability of an earthquake.
- **Early warning:** An alert issued after an earthquake begins but before damaging waves reach a location.
- **Retrofitting:** Strengthening an existing building or structure so it can better withstand earthquake shaking.
- **Microzonation:** Detailed mapping of local differences in ground response and earthquake hazard.
- **Tectonic framework:** The geometry, location, and spatial pattern of faults or other earthquake sources in an area.

## 2. Faulting and Earthquake Formation (supports LO:1 and LO:3)

Earthquakes occur when tectonic stress causes a fault to rupture. The basic process is:

1. Tectonic forces apply stress to rocks.
2. Friction temporarily locks the rocks along a fault.
3. The rocks deform and accumulate elastic strain.
4. Stress exceeds the strength of the rocks or the frictional resistance on the fault.
5. The fault slips, the rocks rebound, and stored elastic energy is released as seismic waves.

### Fault types and stresses

| Fault type | Main stress | Movement | Effect on crust |
|---|---|---|---|
| Normal dip-slip | Tension | Hanging wall moves down | Extension and thinning |
| Reverse dip-slip | Compression | Hanging wall moves up | Shortening and thickening |
| Strike-slip | Shearing | Blocks move horizontally | Lateral displacement |

For an inclined fault, remember: the hanging wall is above the fault and the footwall is below it. Strike-slip faults are usually near vertical, so hanging-wall and footwall terms generally do not apply.

Blind faults are important because they may produce damaging earthquakes without obvious surface rupture. They can sometimes be located by the anticlines and synclines formed above them. The 1994 Northridge earthquake occurred on a previously unknown blind fault.

## 3. The Earthquake Cycle (LO:3)

The earthquake cycle commonly includes:

1. A long period of relative inactivity.
2. Accumulation of elastic strain.
3. Possible small earthquakes as strain increases.
4. Possible foreshocks shortly before the main event.
5. The mainshock, when the major rupture occurs.
6. Aftershocks near the rupture, which may continue from minutes to about a year or so.

Not every earthquake has recognizable foreshocks. Aftershocks are smaller earthquakes that follow the mainshock; they are not simply unrelated events.

### Fault activity

- A fault is generally classified as **active** if it moved during the past 10,000 years.
- A fault that moved during the Pleistocene but not the Holocene may be classified as **potentially active**, using a time span of up to about 2 million years.
- A fault with no movement for about 2 million years is generally considered inactive, although determining the last movement can be difficult.

Paleoseismic trenches expose folded or broken sediments. Dating those sediments helps determine when past earthquakes occurred, how often they occur, and how much a fault has moved.

## 4. Seismic Waves (LO:2 and LO:3)

The sudden rupture of a fault releases elastic strain energy in all directions from the focus.

### Body waves

- **P waves:** Compressional waves that alternately push and pull material in the direction the wave travels. They are the fastest seismic waves and can move through solids, liquids, and gases.
- **S waves:** Shear waves that move material at right angles to the direction of travel. They are slower than P waves and cannot travel through liquids.

### Surface waves

When body waves reach the ground surface, they produce surface waves. These waves travel more slowly than P and S waves but often cause much of the damage near the epicenter because of their complex horizontal, vertical, and rolling motion. Love waves produce especially damaging horizontal shaking.

### Locating an earthquake

1. A seismograph records the arrival of P, S, and surface waves.
2. P waves arrive before S waves because they travel faster.
3. The time difference between P- and S-wave arrivals indicates the distance to the epicenter.
4. Distances from at least three seismic stations are used to draw circles around the stations.
5. The intersection of the circles locates the epicenter by **triangulation**.

## 5. Measuring Earthquakes (LO:4 and LO:5)

### Magnitude versus intensity

- **Magnitude** describes the size or energy released at the source. One earthquake has one magnitude.
- **Intensity** describes the effects at a particular location. One earthquake can have many intensities.

Intensity varies with distance from the epicenter, depth of the focus, rupture direction, local geology, and building quality.

### Richter or local magnitude

The Richter scale is based on the maximum ground motion recorded by a seismograph, especially the S wave. It is logarithmic and is useful for initial local estimates, but it is not an absolute measure of earthquake size and is not well suited for comparing very large earthquakes in different places.

### Moment magnitude

The moment magnitude scale is preferred for comparing earthquake sizes. It estimates the physical size of the rupture using:

- the area of the fault that moved;
- the amount of slip; and
- the strength of the rocks near the focus.

### Logarithmic magnitude scale

An increase of one whole magnitude unit represents approximately:

- 10 times greater ground motion; and
- 32 times more energy released.

For example, a magnitude 7 earthquake produces about 10 times the ground motion and about 32 times the energy of a magnitude 6 earthquake.

### Modified Mercalli Intensity scale

The Modified Mercalli scale describes observed shaking and damage from I to XII:

- I: Felt by very few people.
- V: Felt by nearly everyone; some objects may break or overturn.
- VII: Damage varies greatly with building design and construction.
- VIII to XI: Increasing structural damage, partial collapse, infrastructure failure, and destruction.
- XII: Total damage and severe ground distortion.

The scale is qualitative and location-dependent. It should not be confused with magnitude.

## 6. What Controls Earthquake Shaking? (supports LO:4 and LO:5)

Three major factors determine the shaking experienced at a location:

1. Earthquake magnitude.
2. Location relative to the epicenter and the direction of rupture.
3. Local soil, rock, and geologic conditions.

### Distance and depth

Shaking generally decreases with distance from the epicenter. A deeper focus usually produces less surface shaking because the waves lose energy before reaching the surface; this loss of energy is called **attenuation**.

### Rupture direction or directivity

Rupture may travel preferentially along a fault. Energy can be focused in the direction of greatest rupture, causing stronger shaking in one direction rather than directly above the focus.

### Local materials and material amplification

Seismic waves move quickly through consolidated bedrock but slow down and may grow in amplitude in unconsolidated sediment. Water-saturated sediment produces especially strong amplification. Important settings include:

- alluvium;
- artificial bay fill;
- coastal mud; and
- sedimentary basins.

The 1989 Loma Prieta earthquake caused intense damage in parts of the San Francisco Bay area because buildings, roads, and bridges were located on soft, water-rich bay fill and mud. The 1985 Mexico City earthquake similarly caused amplified shaking in ancient lake sediments.

### Resonance

Buildings have natural vibrational frequencies. When the frequency of ground motion matches a building's frequency, resonance increases the shaking. Tall buildings generally have lower natural frequencies than short buildings, so different building heights can be damaged by different types of ground motion.

## 7. Earthquake Effects and Linked Hazards (LO:5 and LO:6)

### Primary effects

Primary effects result directly from fault movement and shaking:

- violent ground motion;
- surface rupture and displacement;
- fault scarps;
- damage or collapse of buildings, bridges, dams, tunnels, pipelines, and levees; and
- other direct damage caused by fault movement and shaking.

### Secondary effects

Earthquakes can trigger:

- liquefaction;
- landslides and avalanches;
- regional uplift and subsidence;
- tsunamis;
- fires from broken gas and electrical lines;
- flooding from failed dams or levees; and
- disease from contaminated water, damaged sanitation systems, and disrupted public health services.

### Liquefaction

Strong shaking increases water pressure between grains in loose, saturated sand. The grains lose contact and the sediment behaves like a liquid. Possible results include:

- buildings tilting or collapsing;
- roads, bridges, and utilities failing;
- underground tanks and pipelines floating upward;
- ground subsidence; and
- sand and water erupting at the surface as sand volcanoes.

### Regional uplift and subsidence

Large earthquakes can raise or lower broad regions, changing shorelines, groundwater levels, streams, docks, and coastal communities. Subsidence can increase flooding, while uplift can expose and kill marine organisms.

## 8. Geographic Regions at Risk (LO:5)

Most earthquakes occur along tectonic plate boundaries:

- **Subduction zones:** Produce the largest earthquakes, including megathrust earthquakes greater than magnitude 9.
- **Transform boundaries:** Produce frequent earthquakes along faults such as the San Andreas.
- **Divergent boundaries:** Produce shallow earthquakes and volcanic activity.

Earthquakes also occur within plates. These are **intraplate earthquakes** and are less common but can be highly damaging because people may be less prepared and buildings may not be designed for strong shaking.

### Important regions and examples

- The chapter also identifies the St. Lawrence River Valley, the California-Nevada border, Puerto Rico, and the U.S. Virgin Islands as areas with important earthquake risk.
- Pacific coastal California: San Andreas and related faults.
- Pacific Northwest: Cascadia subduction zone.
- Alaska: Aleutian and other subduction-related faults.
- Hawaii: Earthquakes associated with volcanic and tectonic processes.
- Central Mississippi Valley: New Madrid seismic zone.
- South Carolina: Charleston seismic zone.
- Canada: British Columbia and Yukon have especially important earthquake hazards.
- Japan, Chile, Indonesia, Turkey, Nepal, and other regions around the Pacific and along active plate boundaries.

Eastern North American earthquakes can be felt much farther away than similar earthquakes in the West because stronger, less-fractured eastern rocks transmit seismic waves efficiently.

## 9. Human-Induced Earthquakes (LO:8)

Although natural earthquakes cannot currently be prevented or controlled, human actions can trigger or increase local earthquake activity. The chapter identifies three main mechanisms:

- **Reservoir loading:** The weight of water behind a large dam can fracture rock and increase groundwater pressure, activating faults.
- **Deep-well injection:** Injecting liquid waste under pressure raises fluid pressure in fractures and can lubricate faults. Earthquake activity near the Rocky Mountain Arsenal declined after waste injection stopped.
- **Underground nuclear explosions:** Explosions can release stored strain and trigger earthquakes, including earthquakes on other faults.

These examples show the connection between fluid pressure, fractures, stress, and fault movement.

## 10. Forecasting, Prediction, and Hazard Assessment (supports LO:5 and LO:10)

### What scientists can do

Scientists can forecast probabilities using:

- the history of earthquakes on a fault;
- paleoseismic evidence;
- fault geometry and tectonic framework;
- slip rates;
- recurrence intervals;
- seismic gaps;
- ground-motion models; and
- patterns of foreshocks or other possible precursors.

### What scientists cannot reliably do

Consistent short-term earthquake prediction specifying the exact time, place, and magnitude is not currently available. Proposed precursors include foreshocks, ground deformation, changes in groundwater, changes in gravity or magnetism, electrical resistivity, and geochemical changes. Many proposed signals, including unusual animal behavior and lunar tides, have not been shown to reliably predict earthquakes.

### Forecast versus prediction

- A **forecast** gives a probability over a longer time interval and broader area, such as the chance of a magnitude 6.7 or greater earthquake on a fault within 30 years.
- A **prediction** specifies a relatively short time period, limited area, estimated magnitude, and probability.
- An **early-warning system** detects an earthquake after it begins. Because radio signals travel faster than seismic waves, warnings may provide seconds to about one minute before strong shaking arrives.

### Seismic hazard assessment

Assessment includes mapping faults, determining slip rates and recurrence intervals, estimating likely ground motion, identifying local soil and rock conditions, and modeling possible losses. A design-basis ground motion may be selected for engineering purposes, such as ground motion with a 10 percent chance of being exceeded during a 50-year period.

## 11. Reducing Earthquake Risk (LO:9 and LO:10)

### Community adjustments (LO:10)

- Map active faults and areas prone to shaking, liquefaction, landslides, and surface rupture.
- Use microzonation to identify local differences in ground response.
- Locate hospitals, schools, fire stations, utilities, and communication facilities as safely as possible.
- Adopt and enforce earthquake-resistant building codes.
- Retrofit older buildings, bridges, pipelines, dams, and other infrastructure.
- Design flexible structures that can tolerate ground displacement.
- Maintain emergency plans, supplies, communication systems, and evacuation procedures.
- Educate the public and conduct drills.
- Use insurance and relief programs to support recovery.

### United States governmental approach (LO:9)

The United States National Earthquake Hazard Reduction Program, involving the USGS, universities, and other scientists, focuses on understanding earthquake sources, determining earthquake potential, predicting effects, and applying research to reduce losses.

### Personal adjustments (LO:10)

Before an earthquake:

- Secure water heaters, shelves, heavy furniture, and other objects.
- Check chimneys, foundations, and the structure of the home.
- Store water, food, medicine, flashlights, batteries, a radio, shoes, and cash.
- Make a family communication and emergency plan.
- Learn the local hazards, including liquefaction and landslide zones.

During an earthquake:

- **Drop, cover, and hold on.** Get low, take cover beneath sturdy furniture, and hold on until the shaking stops.
- Stay away from windows and falling objects.
- Do not assume that a doorway is the safest location.

After an earthquake:

- Expect aftershocks.
- Watch for fires, gas leaks, downed power lines, damaged roads, contaminated water, and unstable buildings.
- Use emergency supplies and follow official instructions.
- Be prepared for disruptions to water, electricity, communications, transportation, and medical services.

## 12. Important Case Studies and Examples (supports LO:5, LO:6, LO:8, and LO:10)

- **2011 Tohoku earthquake, Japan:** A great subduction-zone earthquake and tsunami. Japan's early-warning system, building codes, retrofits, and preparedness saved lives, but the earthquake and tsunami were larger than predicted. Liquefaction, landslides, subsidence, and infrastructure failures added to the damage.
- **1995 Kobe earthquake, Japan:** Many deaths resulted from collapsed buildings and infrastructure. The event led to stronger codes, retrofitting, improved monitoring, and more extensive preparedness.
- **2010 Chile earthquake:** A large megathrust earthquake and tsunami. Earthquake-resistant construction and evacuation to high ground reduced losses for many people.
- **1994 Northridge earthquake:** A magnitude 6.7 earthquake on a blind fault. It caused major damage but far fewer deaths than similarly sized earthquakes in places with weaker construction.
- **1989 Loma Prieta earthquake:** Demonstrated the importance of material amplification in bay fill and mud and damaged the Nimitz Freeway and Bay Bridge.
- **2002 Denali earthquake:** A magnitude 7.9 earthquake in a sparsely populated region. A flexible, carefully designed pipeline crossed the fault and survived several meters of displacement.
- **1811-1812 New Madrid earthquakes:** Large intraplate earthquakes felt across much of eastern North America and capable of widespread ground deformation and flooding.
- **1988 Armenia versus 1994 Northridge:** Similar magnitudes produced dramatically different death tolls, illustrating the importance of building design, construction quality, and preparedness.

## 13. Natural Service Functions (LO:7)

Earthquakes are hazardous, but they also contribute to long-term Earth systems:

- Fault zones can carry groundwater to the surface as springs.
- Fault gouge can form underground barriers that store or redirect groundwater, oil, and natural gas.
- Faulting can help create petroleum and mineral deposits.
- Uplift and faulting build hills, mountains, coastal terraces, and other landforms.
- Earthquake-related deformation helps shape valleys, rivers, and scenic landscapes.

## 14. Likely Comparison Questions

Be ready to answer these directly:

- How do stress and strain differ?
- How do normal, reverse, and strike-slip faults differ?
- How are the hanging wall and footwall identified?
- How do the focus and epicenter differ?
- How do P waves, S waves, and surface waves differ in speed, motion, and damage?
- How do magnitude and intensity differ?
- How do the Richter and moment magnitude scales differ?
- Why can one earthquake have one magnitude but many intensities?
- How do depth, distance, directivity, and local materials affect shaking?
- How do plate-boundary and intraplate earthquakes differ?
- How does an earthquake forecast differ from an earthquake prediction and an early warning?
- How do natural and human-induced earthquakes differ?
- How do community adjustments differ from personal adjustments?

## 15. Practice Questions

1. Define stress, strain, fault, elastic rebound, focus, and epicenter.
2. Identify the stress and movement associated with normal, reverse, and strike-slip faults.
3. Explain why blind faults create special problems for earthquake hazard assessment.
4. Describe the stages of the earthquake cycle.
5. What is paleoseismicity, and how can a trench reveal prehistoric earthquakes?
6. Explain how a fault rupture produces seismic waves.
7. Compare P waves, S waves, and surface waves.
8. Explain how seismologists use P- and S-wave arrival times to locate an epicenter.
9. Compare magnitude and intensity.
10. Why is moment magnitude preferred over the Richter scale for comparing large earthquakes?
11. What does a one-unit increase in magnitude mean for ground motion and energy?
12. Explain material amplification and resonance.
13. Why can a deeper earthquake produce less surface shaking than a shallow earthquake of similar magnitude?
14. Describe the primary and secondary effects of earthquakes.
15. Explain how liquefaction occurs and identify its possible consequences.
16. Why are subduction zones capable of producing the largest earthquakes?
17. Why can intraplate earthquakes be especially dangerous despite being less common?
18. Explain how earthquakes can trigger landslides, fires, tsunamis, flooding, and disease.
19. Describe three ways humans can induce earthquakes.
20. What information is used in an earthquake hazard assessment?
21. Why are accurate short-term earthquake predictions not currently reliable?
22. Distinguish a forecast, a prediction, and an early warning.
23. List community actions that can reduce earthquake losses.
24. List personal actions to take before, during, and after an earthquake.
25. Use the Haiti, Armenia, Northridge, or Tohoku examples to explain why human vulnerability affects earthquake losses.

## Quick Review Checklist

Before the quiz or test, make sure you can:

- identify the hanging wall and footwall;
- match fault types with their stresses, movements, and effects on the crust;
- explain elastic rebound and the earthquake cycle;
- distinguish the focus from the epicenter;
- compare P waves, S waves, and surface waves;
- explain triangulation using seismograph records;
- distinguish magnitude from intensity and Richter magnitude from moment magnitude;
- apply the 10-times ground motion and 32-times energy rules;
- explain attenuation, directivity, material amplification, and resonance;
- define liquefaction and identify its effects;
- identify primary and secondary earthquake hazards;
- distinguish plate-boundary from intraplate earthquakes;
- explain seismic gaps, paleoseismicity, and earthquake forecasting;
- distinguish prediction from early warning;
- describe human-induced earthquake mechanisms;
- explain microzonation, retrofitting, building codes, and the US earthquake-reduction program;
- list personal safety actions; and
- use case studies to explain how construction, geology, population, and preparedness affect losses.