Understanding Earthquakes: Mechanisms and Impacts

1. Lecture 31 EARTHQUAKES

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

3. Basics • Earthquakes generate waves that travel through the earth • Earthquakes occur when the stress on a fault overcomes its strength and leads to slip along the fault plane • Faults are classified by the kinds of movement that occur along them • Earthquakes don’t kill people, buildings kill people • Difference between Magnitude and Intensity • Predicting earthquakes is not yet possible but we can make hazard maps

4. TYPES OF EARTHQUAKES • Tectonic Earthquake • Volcanic Earthquake • Collapse Earthquake • Explosion Earthquake CLASSIFICATION DISTANCE 1) Teleseismic Earthquake > 1000 km 2) Regional Earthquake > 500 km 3) Local Earthquake < 500 km

8. Largest earthquake in the world • More than 2,000 killed, 3,000 injured, 2,000,000 homeless, and $550 M damage in southern Chile • Tsunami caused 61 deaths • $75 million damage in Hawaii; • 138 deaths and $50 million damage in Japan; • 32 dead and missing in the Philippines; • $500,000 damage to the west coast of the US Chile : 1960 May 22 19:11:14 UTC Magnitude 9.5

9. Earthquake Terminology • Earthquake = slip on a fault plane resulting in the rapid release of energy • Seismic waves = Energy moving outward from the focus of an earthquake • Focus= location of initial slip on the fault; where the earthquake origins • Epicenter= spot on Earth’s surface directly above the focus

10. EarthquakeBasics • Earthquakesoccuralonga faultor faultsystem • Magnitude= a measure of how muchenergyis releasedinalldirections • Biggerfaultsmakebigger earthquakes • Biggerearthquakeslasta longer time

11. epicenter focus Why do earthquakes occur? • Rupture occurs when the stress of the rocks overcome the strength and the rocks • Energy is released in waves that radiate outward from the fault

12. Sources • ・ What is the Earthquake Source? • Fault slip • Elastic Rebound • - Magnitudes (1/quake, related to energy released, Richter or Moment scale) • Intensity (many for each earthquake, measure of effects • Mercalli scale) Measurement

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

15. 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.

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

19. 1. What Is an Earthquake? Example of Elastic Rebound

20. 1. What Is an Earthquake? Fault Rupture

21. Elastic Rebound 2. Earthquakes occur when rocks slip along faults

22. Seismic cycle

23. Elastic Fault Deformation Can divide fault zone based on how faultslips –Seismogenic Crustexhibits stickslip –AseismicCrustexhibits stablesliding •Crustal earthquakesinvolve slip of seismogeniccrust and possibly transitionalzone

24. Interseismic: Strain accumulation •Between earthquakes: –Shallow fault is locked –Deeper fault is creeping atlong-term sliprate –Stress builds up:elastic strainenergy stored in crust •During earthquake, shallowfaultslips –Stress on faultreduced •Cycle repeats forever Slip ? Depth Locking Depth •Between earthquakes –Fault does notslip from surface to locking depth –Fault slips continuously beneathlocking depth –May befinitetransition zone betweenlocked andslippingparts •During earthquake –Shallow faultslips

25. Interseismic: Strain accumulation •Between earthquakes: –Shallow fault is locked –Deeper fault is creeping atlong-term sliprate –Stress builds up:elastic strainenergy stored in crust •During earthquake, shallowfaultslips –Stress on faultreduced •Cycle repeats forever coseismic interseismic Slip Depth Locking Depth •Between earthquakes –Fault does notslip from surface to locking depth –Fault slips continuously beneathlocking depth –May befinitetransition zone betweenlocked andslippingparts •During earthquake –Shallow faultslips

26. Interseismic strain Coseismic strain Elastic Rebound Interseismic + Coseismic = Long-term Motion Interseismic No Strain

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

28. Measuring earthquakes • Seismometers: instruments that detect seismic waves • Seismographs Chang Heng ‘Seismometer’ AD132 Record intensity, height and amplitude of seismic waves

29. How do Seismometers work? 1. Earthquakes generate waves that travel through the earth

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

33. Rayleigh Love

34. Seismic Waves: properties • Velocity: function of the physical properties of the rock the wave is traveling through • Velocity increaseswith rock density • Velocity changes when passing from one material to another (increases/decreases) • Liquids: S-waves do not get transmitted through liquid; P-waves slow down • Why is this important? • If we know the velocity of the wave, we can infer the type of rock it traveled through- that’s how we map the interior of the Earth