Lecture Presentation Chapter 3 Earthquakes 2012 Pearson Education

Lecture Presentation Chapter 3 Earthquakes © 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.

Seismograph and inertia Notice that it is the mass that is moving, not the drum. A heavy mass is used (more inertia) and is suspended so that it is less effected by whatever it is attached to. See the animation next. © 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.

Figure 3. 20 © 2012 Pearson Education, Inc.

© 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.

Focus and Epicenter Focus – where the earthquake happens Epicenter – the spot on the surface above the focus © 2012 Pearson Education, Inc.

© 2012 Pearson Education, Inc.

Elastic Rebound Stress causes the ground begins to be deformed. An earthquake occurs when a rock passes its point of maximum elasticity. After it breaks, it regains its original shape (but not position. ) © 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 § Hanging wall © 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.

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 P(Primary) – body wave S(Secondary) – body wave L(Long) – surface wave © 2012 Pearson Education, Inc.

The shadow zone § Notice in this diagram that S waves are absorbed by the liquid outer core. P waves are refracted by the outer core, because waves refract (bend) when they enter a new medium at an angle. This is how geologists found out that the outer core is liquid. © 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.

Videos § http: //video. nationalgeographic. com/video/enviro nment/environment-naturaldisasters/earthquake-101/ § http: //video. nationalgeographic. com/video/enviro nment/environment-naturaldisasters/earthquake-montage/ § http: //video. nationalgeographic. com/video/enviro nment/environment-naturaldisasters/earthquakes/inside-earthquake/ © 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.

Seismogram- recording from seismograph P wave arrives first S wave then arrives The time between them tells you how far away they came from. Notice that it is 5 minutes from P to S wave. The graph on the next page shows how to do this 5 minutes © 2012 Pearson Education, Inc.

Distance Graph Another graph allows you to find the distance to the epicenter using a line that shows the difference in arrival time between P and S waves. The one shown can only be used for distances from 300 – 750 km. Simply read the distance off the line, as shown. Let us see how far an epicenter is if the difference between P and S waves is 60 seconds. About 580 km. © 2012 Pearson Education, Inc.

Finding the Epicenter Three seismograph stations are needed. Why? Each location gives us a distance, so we can draw a circle there. Then the intersection of 3 circles gives us the epicenter. © 2012 Pearson Education, Inc.

Finding Magnitude Use the distance you calculated from the seismogram and graph. Then find the amplitude of the S-wave on the seismogram. Put these two quantities on the opposite sides of this special graph called a nomogram. Connect them with a line, and this gives you the magnitude. © 2012 Pearson Education, Inc.

Let’s try one here. Use the time difference of 35 seconds to find the distance to the epicenter. The difference in arrival times between the P and S wave here is about 35 sec. (These graphs are all in seconds. ) The amplitude of the S wave is about 350 mm. About 340 km © 2012 Pearson Education, Inc.

Finding the Magnitude Now mark the distance and amplitude on the graph here - called a nomogram. (Watch the graph carefully as we click here) And then connect them with a line. Where the line intersects the magnitude line, you have found the magnitude. This one is about 6. 7, right? We will do some for practice here. © 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. 12 © 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.

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.

Effects of Earthquakes § Shaking and Ground Rupture § Liquefaction http: //www. youtube. com/watch? v=I 3 h. JK 1 Bo. Rak § Change in land elevation § Landslides § Fires § Disease © 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.

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.

Introduction § Tsunamis: The Japanese word for “large harbor waves”, produced by the sudden vertical displacement of ocean water § Can be triggered by any rapid uplift or subsidence of the seafloor, such as submarine earthquake, landslide, volcanism, and impact of asteroid or comet § Mega-tsunami, from asteroid impact, a wave about 100 times higher than the largest tsunami produced by an earthquake § Tsunamis produced by earthquakes are by far the most common © 2012 Pearson Education, Inc.

Figure 7. 3 © 2012 Pearson Education, Inc.

How Do Earthquakes Cause a Tsunami? § Cause a tsunami by movement of the seafloor and by triggering a vertical displacement/ landslide § M 7. 5 or greater earthquake create enough displacement of the seafloor to generate a damaging tsunami § A four-stage process that eventually leads to landfall of tsunami waves on the shore Figure 7. 7 © 2012 Pearson Education, Inc.

Figure 7. 7 © 2012 Pearson Education, Inc.

Tsunami Movement § When an earthquake uplifts the seafloor close to land, both distant and local tsunamis may be produced § Distant tsunami: Travels out across the deep ocean at high speed for thousands of kilometers to strike remote shorelines with very little loss of energy § Local tsunami: Heads in the opposite direction toward the nearby land arrives quickly following an earthquake § When the initial tsunami wave is split, each (distant and local) tsunami has a wave height about one-half of that of the original dome of water © 2012 Pearson Education, Inc.

Distant and Local Tsunami Figure 7. 8 © 2012 Pearson Education, Inc.

Landslides Cause a Tsunami § Submarine landslides can generate very large tsunamis § Large rock avalanches falling from mountains into the sea can also generate very large tsunamis § 1998, a M 7. 1 earthquake triggered a submarine landslide and caused a tsunami of 15 m (50 ft), leaving 12, 000 people homeless and over 2, 000 dead § Lituya Bay, Alaska, in 1958. The landslide set in motion by a M 7. 7 earthquake on a nearby fault. The huge mass of broken rock caused waters in the bay to surge upward to an elevation of about 524 m (1720 ft) above the normal water level © 2012 Pearson Education, Inc.

Regions at Risk § All ocean and some lake shorelines are a risk for tsunamis, some coasts are more at risk than others § Coasts close to a major subduction zone or directly across the ocean basin from a major subduction zone are at greatest risk § The greatest tsunami hazard with return periods of several hundred years § High risk regions: The Cascadia subduction zone, the Chilean trench, the subduction zones off the coast of Japan, parts of the Mediterranean, as well as the northeastern side of the Indian Ocean © 2012 Pearson Education, Inc.

Global Tsunami Hazard Ranks Figure 7. 11 © 2012 Pearson Education, Inc.

Effects of Tsunamis Primary § Damage to both the landscape and human structures from resulting flooding and erosion Secondary § Fires may start in urban areas from ruptured natural gas lines or from the ignition of flammable chemicals § Water supplies may become polluted § Damaged wastewater treatment systems § Disease outbreaks © 2012 Pearson Education, Inc.

Minimizing the Tsunami Hazard § § § Detection and warning Structural control Construction of tsunami runup maps Land use planning Probability analysis Education © 2012 Pearson Education, Inc.

Tsunami Warning System Figure 7. 14 © 2012 Pearson Education, Inc.

Detection and Warning § For distant tsunamis: Can be detected in the open ocean and accurately estimated their arrival time to within a few minutes § A tsunami warning system has three components: § A network of seismographs to measure submarine movements § Automated tidal gauges to measure unusual rises and falls of sea level § A network of sensors connected to floating buoys © 2012 Pearson Education, Inc.