Lecture Presentation Chapter 3 Earthquakes 2012 Pearson Education

Lecture Presentation Chapter 3 Earthquakes © 2012 Pearson Education, Inc.

Learning Objectives § Understand how scientists measure and compare earthquakes § Be familiar with earthquake processes such as faulting, tectonic creep, and the formation of seismic waves § Know which global regions are most at risk for earthquakes and why they are at risk § Know and understand the effects of earthquakes such as shaking, ground rupture, and liquefaction © 2012 Pearson Education, Inc.

Learning Objectives, cont. § Identify how earthquakes are linked to other natural hazards such as landslide, fires, and tsunami § Know the important natural service functions of earthquakes § Know human beings interact with and affect the earthquake hazard § Understand how we can minimize seismic risk, and recognize adjustments we can make to protect ourselves © 2012 Pearson Education, Inc.

Introduction to Earthquakes § There are many earthquakes in any given day § They are compared based on: § Magnitude, the amount of energy released § Intensity, the effects on people and structures © 2012 Pearson Education, Inc.

Table 3. 1 © 2012 Pearson Education, Inc.

Earthquake Magnitude § They are mapped according to epicenter § Focus is directly below the epicenter § Measured by moment magnitude § Determined from area of rupture along, amount of slippage, and the rigidity of the rocks § Richter scale was previously used Figure 3. 2 © 2012 Pearson Education, Inc.

Earthquake Magnitude, cont. § Both scales are logarithmic § Based on powers of 10 § Ground displacement for a magnitude 3 earthquake is 10 times that for a magnitude 2 § Ground motion is measured by seismograph § Related to magnitude, depth, and geologic setting Table 3. 2 © 2012 Pearson Education, Inc.

Table 3. 3 © 2012 Pearson Education, Inc.

Earthquake Intensity § Measured by Modified Mercalli Scale § Qualitative scale (I-XII) based on damage to structures and people’s perceptions § Modified Mercalli Intensity Maps show where the damage is most severe © 2012 Pearson Education, Inc.

Table 3. 4 © 2012 Pearson Education, Inc.

Shake Maps § Shake Maps use seismograph data to show areas of intense shaking Figure 3. 3 © 2012 Pearson Education, Inc.

Earthquake Processes § Earthquakes are distributed along faults § Places where rocks are broken and displaced § All plate boundaries are faults § Movement along faults are slip rates § Measured in mm/yr or m/1000 yr § Sudden rupture of rock produces seismic waves § Release of stored energy © 2012 Pearson Education, Inc.

Figure 3. 5 © 2012 Pearson Education, Inc.

Fault Types—Strike-Slip § Crust moves in horizontal direction Figure 3. 6 a © 2012 Pearson Education, Inc.

Fault Types—Dip-Slip § Vertical movement § Include two walls defined by miners as: § Footwall where miners put their feet § Hanging-wall where they put their lanterns © 2012 Pearson Education, Inc.

Fault Types—Dip-Slip, cont. § Normal fault § Hanging wall moves down relative to footwall § Reverse fault § Hanging wall moves up relative to footwall § If angle is 45° or less it is a thrust fault § Blind faults do not extend to the surface © 2012 Pearson Education, Inc.

Figure 3. 6 b Figure 3. 6 c © 2012 Pearson Education, Inc.

Figure 3. 7 © 2012 Pearson Education, Inc.

Fault Activity § Active fault § Moved during the past 10, 000 years of the Holocene Epoch § Potentially active faults § Moved during the Pleistocene, but not the Holocene Epoch § Inactive § Not moved during the past 2 million years § Paleoseismicity of the fault © 2012 Pearson Education, Inc.

Table 3. 5 © 2012 Pearson Education, Inc.

Tectonic Creep and Slow Earthquakes § Tectonic creep occurs when movement is gradual such that earthquakes are not felt § Can produce slow earthquakes § Also called fault creep § Can slowly damage roads, sidewalks and building foundations § Can last from days to months § Magnitudes can be between M 6 and M 7 © 2012 Pearson Education, Inc.

Seismic Waves—Body Waves § Caused by a release of energy from rupture of a fault § Travel through the body of the Earth § P waves, primary or compressional waves § Move fast with a push/pull motion § Can move through solid, liquid and gas § It is possible to hear them § S waves, secondary or shear waves § Move slower with an up/down motion § Can travel only through solids © 2012 Pearson Education, Inc.

Figure 3. 9 a, b © 2012 Pearson Education, Inc.

Seismic Waves—Surface Waves § Move along Earth’s surface § Travel more slowly than body waves § Move both vertically and horizontally with a rolling motion § Are responsible for most of the damage near epicenter § Love wave—horizontal ground shaking © 2012 Pearson Education, Inc.

Figure 3. 9 c © 2012 Pearson Education, Inc.

Earthquake Shaking § Shaking experience depends on: § Earthquake magnitude § Location in relation to epicenter and direction of rupture § Local soil and rock conditions © 2012 Pearson Education, Inc.

Distance to Epicenter § Both types of body waves are emitted from epicenter of quake § Seismographs record arrivals of waves to station site § Seismogram is the record of the waves § P and S waves travel at different rates and arrive at station at different times § Distance to epicenter can be found by comparing travel times of the waves © 2012 Pearson Education, Inc.

Figure 3. 10 c © 2012 Pearson Education, Inc.

Figure 3. 10 d © 2012 Pearson Education, Inc.

Location of Epicenter § At least three stations are needed to find exact epicenter § Distances from epicenter to each station are used to draw circles representing possible locations § The place where all three circles intersect is the epicenter § Process is called triangulation © 2012 Pearson Education, Inc.

Figure 3. 11 © 2012 Pearson Education, Inc.

Depth of Focus § Depth of earthquake influences the amount of shaking § Focus is the place within the Earth where the earthquake starts § Deeper earthquakes cause less shaking at the surface § Loss of energy is called attenuation © 2012 Pearson Education, Inc.

Figure 3. 12 © 2012 Pearson Education, Inc.

Direction of Rupture § Direction that the rupture moves along the fault influences the shaking § Path of greatest rupture can intensify shaking § Directivity © 2012 Pearson Education, Inc.

Supershear § Occurs when the propagation of rupture is faster than the velocity of shear waves or surface waves produced by the rupture § Can produce shock waves that produce strong ground motion along the fault § May significantly increase the damage from a large earthquake © 2012 Pearson Education, Inc.

Figure 3. 13 © 2012 Pearson Education, Inc.

Local Geologic Conditions § Nature of the ground materials affects the earthquake energy § Different materials respond differently to an earthquake § Depends on their degree of consolidation § Seismic wave move faster through consolidated bedrock § Move slower through unconsolidated sediment § Move slowest through unconsolidated materials with high water content § Material amplification § Energy is transferred to the vertical motion of the surface waves © 2012 Pearson Education, Inc.

Figure 3. 14 © 2012 Pearson Education, Inc.

The Earthquake Cycle § There is a drop in elastic strain after an earthquake and a reaccumulation of strain before the next event § Strain is a deformation § Elastic strain is deformation that is not permanent § Stage 1: Period of inactivity along a segment of fault § Stage 2: Period of small earthquakes where the stress begins to release causing strain § Stage 3: Foreshocks occur prior to a major release of stress § Doesn’t always occur © 2012 Pearson Education, Inc.

The Earthquake Cycle, cont. § Stage 4: Mainshock and aftershocks where the fault releases all pent up stress releases the major quake § Cycle is hypothetical and periods are variable § Stages have been identified for many large earthquakes © 2012 Pearson Education, Inc.

Figure 3. 19 © 2012 Pearson Education, Inc.

Figure 3. 20 © 2012 Pearson Education, Inc.

Plate Boundary Earthquakes § Subduction Zones § Site of the largest earthquakes § Megathrust earthquakes § Example: Cascade Mountains § Convergence between a continental and oceanic plate § Example: Aleutian Islands § Convergence between two oceanic plates § Transform Fault Boundaries § Example: San Andreas Fault in California, Loma Prieta earthquake § Boundary between North American and Pacific plates © 2012 Pearson Education, Inc.

Intraplate Earthquakes § Earthquakes that occur within plates § New Madrid seismic zone § Located near St. Louis, MO § Historic earthquakes similar in magnitude to West Coast quakes § Earthquakes are often smaller than plate boundary quakes § Can be large and cause considerable damage due to lack of preparedness and because they can travel greater distances through stronger continental rocks © 2012 Pearson Education, Inc.

Effects of Earthquakes § Ground rupture § Displacement along the fault causes cracks in surface § Fault scarp § Shaking § Causes damage to buildings, bridges, dams, tunnels, pipelines, etc. § Measured as Ground Acceleration § Buildings may be damaged due to resonance § Matching of vibrational frequencies between ground and building © 2012 Pearson Education, Inc.

Effects of Earthquakes, cont. 1 § Liquefaction § A near-surface layer of water-saturated sand changes rapidly from a solid to a liquid § Causes buildings to “float” in earth § Common in M 5. 5 earthquakes in younger sediments § After shaking stops, ground re-compacts and becomes solid § Elevation changes § Regional uplift and subsidence § Can cause substantial damage on coasts and along streams © 2012 Pearson Education, Inc.

Effects of Earthquakes, cont. 2 § Landslides § Earthquakes are the most common triggers in mountainous areas § Can cause a great loss of human life § Can also block rivers creating “earthquake lakes” § Fires § Displacements cause power and gas lines to break and ignite § Hard to put out because water lines are often broken § Disease § caused by a loss of sanitation and housing, contaminated water supplies, disruption of public health services, and the disturbance of the natural environment © 2012 Pearson Education, Inc.

Natural Service Functions § Water, oil, and natural gas may be rerouted due to faults § New mineral resources may be exposed § Scenic landscapes may form § Future earthquakes may be reduced due to release of energy © 2012 Pearson Education, Inc.

Human Interaction with Earthquakes § Loading Earth’s crust, as in building a dam and reservoir • The weight from water reservoirs may create new faults or lubricate old ones § Injecting liquid waste deep into the ground through disposal wells • Liquid waste disposals deep in the earth can create pressure on faults § Creating underground nuclear explosions • Nuclear explosions can cause the release of stress along existing faults © 2012 Pearson Education, Inc.

Minimizing the Earthquake Hazard § Focus of minimization is on forecasting and warning § National Earthquake Hazard Reduction Program Goals § § Develop an understanding of the earthquake source Determine earthquake potential Predict effects of earthquakes Apply research results © 2012 Pearson Education, Inc.

Estimating Seismic Risk § Hazard maps show earthquake risk § Probability of a particular event or the amount of shaking Figure 3. 29 © 2012 Pearson Education, Inc.

Short-Term Prediction § Pattern and frequency of earthquakes § Foreshocks § Deformation of ground surface § Changes in land elevation § Seismic gaps along faults § Areas that have not seen recent quakes § Geophysical and Geochemical changes § Changes in Earth’s magnetic field, groundwater levels. © 2012 Pearson Education, Inc.

Earthquake Warnings § Plan for issuing a prediction or warning § Earthquake warning systems Figure 3. 31 © 2012 Pearson Education, Inc.

Community Preparations for Earthquake Hazard § Critical facilities must be located in earthquake safe locations § Requires detailed maps of ground response § Buildings must be designed to withstand vibrations § Retrofitting old buildings may be necessary § People must be prepared through education § Insurance must be made available © 2012 Pearson Education, Inc.

Personal Reactions and Preparation § Do a home inspection to make sure that your home is structurally sound § Secure large objects § Make a personal plan of how to react to a quake § Leave buildings AFTER shaking stops § Turn off main gas line § Move to an open area © 2012 Pearson Education, Inc.

End Earthquakes Chapter 3 © 2012 Pearson Education, Inc.