# Three Fs of earthquakes: forces, faults, and friction. Slow accumulation and rapid release of elastic energy.

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1 Earthquake Machine Stick-slip: Elastic Rebound Theory Jerky motions on faults produce EQs Three Fs of earthquakes: forces, faults, and friction. Slow accumulation and rapid release of elastic energy.

2 Three Basic Types of Plate Boundaries Transform (e.g. San Andreas Fault) Strike-slip faulting Magnitudes generally < 7

3 Three Basic Types of Plate Boundaries Divergent (e.g. spreading ocean ridge) Normal faulting Magnitudes < 6

4 Three Basic Types of Plate Boundaries Convergent (e.g. subduction zone) Magnitudes up to 9 or larger Depths to 700 km

5 Epicenter: Location on Earth s surface directly ABOVE the EQ. Hypocenter or Focus: Point WITHIN Earth where EQ occurred.

6 Body Waves and Surface Waves Surface waves travel along Earth s surface. Body waves (P and S) travel inside Earth. While P- and S- waves radiate outward in all directions, surface waves travel along the surface of the earth and decrease in amplitude with depth.

7 Seismic Slinky P Waves Are Pressure Waves P waves are fastest seismic waves.

8 Seismic Slinky S Waves Are Shear Waves S waves are slower than P waves but faster than surface waves.

9 Surface Waves Amplitude greatest at surface, decreasing with depth. Slower than P or S waves. Often are largest amplitude and most damaging seismic waves.

10 Seismic Slinky Human Waves Modeling P, S, and Surface Waves

11 How do scientists detect earthquakes? When an earthquake occurs the seismic waves travel through the Earth to the seismic station where the information is transmitted to distant computers. A seismograph detects and records EQs. A seismogram is the EQ record.

12 M7.7 Earthquake off Northern Chile Nov 14, 2007 University of Portland AS-1 Seismometer P waves ~12 min ~720 sec S waves ~22 min ~1320 sec Surface waves ~42 min ~2500 sec

13 Seismic Waves from Chile Earthquake USArray Animations

14 Distance of EQ from seismometer? Determined from S arrival time - P arrival time.

15 Locating an Earthquake 1) Determine distance of EQ from three seismic stations by calculating the S minus P arrival times. 2) Plot them on the travel-time graph. 3) Intersection of the circles gives the location

16 Moment Magnitude Pasta Quake

17 Magnitudes and Energy of Earthquakes Annual Numbers of EQs MOST of the energy is released by around 20 magnitude-7 and larger EQs every year.

18 Earthquake Intensity: Violence and Effects of Ground Shaking. I. Not felt. II. Felt only by a few people. III. Felt noticeably by people indoors. IV. Felt indoors by many, outdoors by few people. V. Felt by nearly everyone; many awakened. Some dishes, windows broken. VI. Felt by all; some heavy furniture moved. VII. Considerable damage in poorly-built structures; No damage to well-built structures. VIII. Great damage to poorly-built structures; considerable damage to ordinary buildings. IX. Damage great in substantial buildings, with partial collapse. X. Some well-built wooden structures destroyed; most masonry and frame structures destroyed. XI. Few masonry structures remain standing; Bridges destroyed. XII. Damage total. Magnitude/Intensity Comparison Intensities typically observed at locations near epicenter.

19 Earthquake Intensity: Violence and Effects of Ground Shaking. A table of intensity descriptions with the corresponding peak ground acceleration (PGA) and peak ground velocity (PGV) values used in USGS ShakeMaps. USGS ShakeMap Scien.fic Background h6p://earthquake.usgs.gov/earthquakes/shakemap/background.php

20 Earthquake Intensity: Violence and Effects of Ground Shaking. What controls Intensity? Magnitude More energy released. Distance Shaking decays with distance. Depth Deeper EQ yields less shaking at the surface. Geology Weak sediments can amplify shaking. Building style Duration of shaking. Image courtesy of the US Geological Survey

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22 Three kinds of Cascadia EQs = Triple Trouble Magnitude Magnitude 9 Subduction 7 Deep Crustal Zone Earthquake Earthquake 20 Ground 30 seconds shaking for of ground 4 6 minutes! shaking.

23 Effects of Near-Surface Geology Amplitude of oscillation increasing

24 Liquefaction What happens to a structure built on a weak foundation when an earthquake strikes? Liquefaction M7.4 Niigata, Japan 1964 Source: National Geophysical Data Center

25 Ground Shaking Amplification Seismic waves are amplified as they pass from bedrock into basins filled with sedimentary rock.

26 Earthquake-Induced Landslides El Salvador January 2001 M7.6

27 Relative Earthquake Hazard Maps Relative Earthquake Hazards Relative EQ Hazard Seismic Wave Amplification Liquefaction Potential Earthquakeinduced Landslide Hazard

28 Build a Better Wall Resistance to shear is critical. Earthquakes don t kill people. buildings that collapse during earthquake ground shaking kill people.

29 Oregon EQ Building Codes

30 Seismic Retrofit 1942 North Portland House Wood frame NOT attached to foundation.

31 Seismic Retrofit Bolting frame to foundation on exterior.

32 Seismic Retrofit Post & beam reinforcement.

33 Drop, Cover, and Hold On!

34 Three Types of Plate Boundaries Divergent Transform Tsunami Convergent (subduction zones)

35 2011 Tohoku Earthquake and Tsunami 15,883 people killed; 6,145 missing; 2,671 injured; Recovery, Cleanup, and Rebuilding Cost = \$235 Billion.

36 Tohoku Earthquake Fault Displacement Model Modeling of seismic waveforms shows maximum fault slip over 30 meters! Maximum slip region is toward the surface from the hypocenter. Such large fault slip near the surface resulted in large displacement of the ocean floor that in turn generated a large tsunami. March 11, 2011 Japan Tsunami Image courtesy of the US Geological Survey

37 GPS Displacements During the Earthquake 4.42 meters (14.5 feet) of horizontal displacement 0.75 meters (2.5 feet) of subsidence

38 Some Coast Areas Now Below Sea Level Yuriage Before After Some areas that were above sea level on march 10 dropped below sea level on March 11, This also happened along the Washington - Oregon coast during the 1700 AD great Cascadia earthquake.

39 Tsunami Produced by Earthquake Important points: LONG period: minutes. Can flood on shore for minutes. LONG wavelength: >800 kilometers (520 miles) Moves seawater all the way to floor of ocean. In deep ocean, tsunami has small amplitude and travels with speed of jet airliner. When approaching land, speed slows and amplitude increases dramatically. A tsunami is a SERIES of waves, not a single wave.

40 Tsunami Produced by Subduction Zone Earthquake Why are tsunamis rarely produced by earthquakes along divergent and transform plate boundaries?

41 Lessons from Tohoku, Japan Earthquake engineering works. Few buildings collapsed during the Tohoku earthquake. Cascadia must construct earthquake-resilient built environment. Earthquake and tsunami hazards education improves public safety. Tohoku tsunami inundation zone had 95% survival rate while Banda Aceh inundation zone had 10% survival rate. Tsunami Stones on Hillsides Message: Do not build below this level.

42 Cascadia Great Earthquakes and Tsunamis After Field Trip

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44 Relative Earthquake Hazard Maps Relative Earthquake Hazards Relative EQ Hazard = Seismic Wave Amplification + Liquefaction Potential + Earthquakeinduced Landslide Hazard

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