Understanding Earth Sixth Edition Chapter 13: EARTHQUAKES © 2011 by W. H. Freeman and Company Grotzinger Jordan

Chapter 13 Earthquakes

About Earthquakes Earthquakes can be understood in terms of the basic mechanisms of deformation. Earthquakes can be understood in terms of the basic mechanisms of deformation. Most earthquakes occur at plate boundaries (convergent, divergent, and sliding). Most earthquakes occur at plate boundaries (convergent, divergent, and sliding). Earthquakes cannot yet be reliably predicted or mitigated. Earthquakes cannot yet be reliably predicted or mitigated.

Lecture Outline 1. What is an earthquake? 2. How do we study earthquakes? 3. Earthquakes and patterns of faulting 4. Earthquake hazards and risks 5. Can earthquakes be predicted?

1. What Is an Earthquake? ● Global forces at work ● stress ● strain ●strength ● strength

1. What Is an Earthquake? ● Earthquakes occur where rocks being stressed suddenly break along a new or pre-existing fault. ● Seismic waves are ground vibrations caused by rocks slipping along opposite sides of a fault.

1. What Is an Earthquake? ● Why earthquakes occur ● elastic rebound theory ● fault rupture ●epicenter ● epicenter ●focus ● focus

1. What Is an Earthquake? Example of Elastic Rebound

1. What Is an Earthquake? Fault Rupture

1. What Is an Earthquake? ● Local buildup and release of stress ● foreshock ● aftershock

2. How Do We Study Earthquakes? ● Seismographs are machines that record the seismic waves record the seismic waves generated by earthquakes. generated by earthquakes. ● vertical ground movements ● horizontal ground movements

● Seismic wave types ● P waves (primary waves) ● S waves (secondary waves) ●Surface waves ● Surface waves 2. How Do We Study Earthquakes?

● Locating the earthquake epicenter ● P- and S-wave arrival times from at least 3 seismographs from at least 3 seismographs ● Graph of distance traveled versus time elapsed versus time elapsed 2. How Do We Study Earthquakes?

● Measuring the size of an earthquake ● Richter magnitude ● Moment magnitude ●Shaking intensity ● Shaking intensity 2. How Do We Study Earthquakes?

● Magnitude and frequency ● many small earthquakes ● few large earthquakes 2. How Do We Study Earthquakes?

Modified Mercalli Intensity Scale

Mercalliintensity of the New Madrid earthquake, Dec. 16, 1811

● Determining fault mechanisms from earthquake data earthquake data ● pattern of ground shaking (first motion of P waves) (first motion of P waves) ● orientation of fault rupture ●direction of slip ● direction of slip 2. How Do We Study Earthquakes?

● Fault mechanism tells us whether the rupture was: the rupture was: ● normal ● reverse ●strike-slip (right- or left-lateral) ● strike-slip (right- or left-lateral) 2. How Do We Study Earthquakes?

Main Types of Fault Movement

2. How Do We Study Earthquakes? First Motion of P Waves

3. Earthquakes and Patterns of Faulting ● Earthquakes and plate tectonics ● divergent boundaries ● transform-fault boundaries ●convergent boundaries ● convergent boundaries ●intraplate earthquakes ● intraplate earthquakes

3. Earthquakes and Patterns of Faulting

3. Earthquakes and patterns of faulting: Regional fault systems Example: Fault system of southern California

3. Earthquakes and Patterns of Faulting ● Earthquakes and destruction ● loss of life ● property damage ●tsunami and landslides ● tsunami and landslides

Earthquake damage in Los Angeles, 1994

Earthquake damage in Kobe, Japan, 1995

Earthquake damage in Kashmir, 2005

Earthquake damage in Mexico City, 1985

Tsunami effects in Thailand, 2004

Tsunami effects in Sumatra, 2004

Thought questions for this chapter The belts of shallow-focus earthquakes, shown by the blue dots in Figure 13-15a, are wider and more diffuse in the continents than in the oceans. Why? Why are earthquakes with focal depths greater than 20 km infrequent in continental lithosphere? Why do the largest earthquakes occur on subduction megathrusts and not, say, on continental strike-slip faults? Why are great tsunamis, such as the Indian Ocean tsunami of 2004, so rare?

Thought questions for this chapter In Figure 13-3, the right-lateral fault offsets the fence line to the right. In Figure 13-15b, the mid-ocean ridge crest is also offset to the right. Why then is the transform fault in Figure 13-15b left-lateral?

4. Earthquake Hazards and Risks ● How earthquakes cause damage ● faulting and shaking ● landslides and ground failures ●tsunamis ● tsunamis ●fires ● fires

● Reducing earthquake risks ● land-use policies ● hazard characterization and proper emergency planning proper emergency planning ●earthquake engineering ● earthquake engineering (proper building codes) (proper building codes) 4.Earthquake Hazards and Risks

Example of poor land-use planning:Construction along the trace of the San Andreas fault zone, San Francisco

Example of emergency planning: Tsunami barrier, Taro, Japan

4. Earthquake Hazards and Risks: Planning

● Reducing earthquake risks ● mapping seismic hazards ● assessing seismic risks 4. Earthquake Hazards and Risks

4. Earthquake Hazards and Risks: World Seismic Hazard Map

U.S. seismic hazard map

U.S. seismic risk map

Thought questions for this chapter Would you support legislation to prevent owners from building structures close to active faults?

5. Can Earthquakes Be Predicted? ● Earthquake forecasting ● long-term ● intermediate-term ●short-term ● short-term

Thought questions for this chapter Taking into account the possibility of false alarms, reduction of casualties, mass hysteria, economic depression, and other possible consequences of earthquake prediction, do you think the objective of predicting earthquakes should have a high priority?

Aftershock Building code Earthquake Elastic rebound theory Epicenter Fault mechanism Fault slip FocusForeshock Intensity scale Magnitude scale P wave Recurrence interval S wave Seismic hazard Key terms and concepts

Seismic risk Seismograph Surface wave Tsunami Key terms and concepts