1 Earthquakes and Earthquake Hazards Earth - Chapter 11 Stan Hatfield Southwestern Illinois College
4 What Is an Earthquake? An earthquake is the vibration of Earth, produced by the rapid release of energy. Energy released radiates in all directions from its source, the focus. Energy is in the form of waves. Sensitive instruments around the world record the event.
5 Earthquake Focus and Epicenter
6 What Is an Earthquake? Earthquakes and faults Movements that produce earthquakes are usually associated with large fractures in Earth s crust called faults. Most of the motion along faults can be explained by the plate tectonics theory.
10 What Is an Earthquake? Elastic rebound The mechanism for earthquakes was first explained by H. F. Reid. Rocks on both sides of an existing fault are deformed by tectonic forces. Rocks bend and store elastic energy. Frictional resistance holding the rocks together is overcome.
11 What Is an Earthquake? Elastic rebound Earthquake mechanism Slippage at the weakest point (the focus) occurs. Vibrations (earthquakes) occur as the deformed rock springs back to its original shape (elastic rebound). Earthquakes most often occur along existing faults whenever the frictional forces on the fault surfaces are overcome.
12 Offset Produced by Slippage Along Faults During Earthquakes
13 What Is an Earthquake? Foreshocks and aftershocks Adjustments that follow a major earthquake often generate smaller earthquakes called aftershocks. Small earthquakes, called foreshocks, often precede a major earthquake by days or, in some cases, by as much as several years.
14 San Andreas An Active Earthquake Zone San Andreas is the most studied fault system in the world. Displacement occurs along discrete segments 100 to 200 kilometers long. Some portions exhibit slow, gradual displacement known as fault creep. Other segments regularly slip, producing small earthquakes.
15 San Andreas An Active Earthquake Zone Displacements along the San Andreas Fault Still other segments store elastic energy for hundreds of years before rupturing in great earthquakes. Process described as strike-slip motion Great earthquakes should occur about every 50 to 200 years along these sections.
16 The San Andreas Fault System
17 Seismology The study of earthquake waves, seismology, dates back almost 2000 years to the Chinese. Seismographs are instruments that record seismic waves. Records the movement of Earth in relation to a stationary mass on a rotating drum or magnetic tape
18 A Vertical Ground Motion Seismograph
20 Seismology Seismographs More than one type of seismograph is needed to record both vertical and horizontal ground motion. Records obtained are called seismograms. Types of seismic waves Surface waves Travel along outer part of Earth
21 Seismology Types of seismic waves Surface waves Complex motion Cause greatest destruction Exhibit greatest amplitude and slowest velocity Waves have the greatest periods (time interval between crests). Often referred to as long waves, or L waves
22 Seismology Types of seismic waves Body waves Travel through Earth s interior Two types based on mode of travel Primary (P) waves» Push pull (compress and expand) motion, changing the volume of the intervening material» Travel through solids, liquids, and gases
23 Seismology Types of seismic waves Body waves Primary (P) waves» Generally, in any solid material, P waves travel about 1.7 times faster than S waves. Secondary (S) waves» Shake motion at right angles to their direction of travel» Travel only through solids
24 Seismology Types of seismic waves Body waves Secondary (S) waves» Slower velocity than P waves» Slightly greater amplitude than P waves
26 Locating the Source of Earthquakes Terms Focus the place within Earth where earthquake waves originate Epicenter location on the surface directly above the focus The epicenter is located using the difference in velocities of P and S waves.
27 Locating the Source of Earthquakes Locating the epicenter of an earthquake Three station recordings are needed to locate an epicenter. Each station determines the time interval between the arrival of the first P wave and the first S wave at their location. A travel-time graph is used to determine each station s distance to the epicenter.
28 Seismogram Showing P, S, and Surface Waves
30 A Travel-Time Graph
31 Locating the Source of Earthquakes Locating the epicenter of an earthquake A circle with a radius equal to the distance to the epicenter is drawn around each station. The point where all three circles intersect is the earthquake epicenter.
32 Finding an Earthquake Epicenter
33 Measuring the Size of Earthquakes Two measurements that describe the size of an earthquake are: 1. Intensity a measure of the degree of earthquake shaking at a given locale based on the amount of damage 2. Magnitude estimates the amount of energy released at the source of the earthquake.
34 Measuring the Size of Earthquakes Intensity scales The Modified Mercalli Intensity Scale was developed using California buildings as its standard. The drawback of intensity scales is that destruction may not be a true measure of the earthquake s actual severity.
35 The Modified Mercalli Intensity Scale
36 Measuring the Size of Earthquakes Magnitude scales Richter magnitude concept introduced by Charles Richter in 1935 Richter scale Based on the amplitude of the largest seismic wave recorded Accounts for the decrease in wave amplitude with increased distance
37 Measuring the Size of Earthquakes Magnitude scales Richter scale The largest magnitude recorded on a Wood Anderson seismograph was 8.9. Magnitudes less than 2.0 are not felt by humans. Each unit of Richter magnitude increase corresponds to a tenfold increase in wave amplitude and a 32-fold energy increase.
38 Earthquake Magnitude and Energy Equivalent
39 Measuring the Size of Earthquakes Magnitude scales Other magnitude scales Several Richter-like magnitude scales have been developed. Moment magnitude was developed because none of the Richter-like magnitude scales adequately estimate very large earthquakes. Derived from the amount of displacement that occurs along a fault
40 Locating the Source of Earthquakes Earthquake belts About 95% of the energy released by earthquakes originates in a few relatively narrow zones that wind around the globe. Major earthquake zones include the Circum- Pacific belt, Mediterranean Sea region to the Himalayan complex, and the oceanic ridge system.
41 Earthquake Belts
42 Earthquake Destruction Amount of structural damage attributable to earthquake vibrations depends on: Intensity and duration of the vibrations Nature of the material upon which the structure rests Design of the structure
43 Earthquake Destruction Destruction from seismic vibrations Ground shaking Regions within 20 to 50 kilometers of the epicenter will experience about the same intensity of ground shaking. However, destruction varies considerably, mainly due to the nature of the ground on which the structures are built.
44 Damage Caused by the 1964 Anchorage, Alaska Earthquake
45 Earthquake Destruction Destruction from seismic vibrations Liquefaction of the ground Unconsolidated materials saturated with water turn into a mobile fluid. Seiches The rhythmic sloshing of water in lakes, reservoirs, and enclosed basins Waves can weaken reservoir walls and cause destruction.
46 Mud Volcanoes Produced by Liquefaction
50 Earthquake Destruction Tsunamis, or seismic sea waves Destructive waves that are often inappropriately called tidal waves. Result from vertical displacement along a fault located on the ocean floor or a large undersea landslide triggered by an earthquake
51 Earthquake Destruction Tsunamis, or seismic sea waves In the open ocean, height is usually less than 1 meter. In shallower coastal waters, the water piles up to heights that occasionally exceed 30 meters. Can be very destructive Landslides and ground subsidence Fire
52 Formation of a Tsunami
53 Tsunami from the 2004 Indonesian Earthquake Near Sumatra
55 Can Earthquakes Be Predicted? Short-range predictions The goal is to provide a warning of the location and magnitude of a large earthquake within a narrow time frame. Research has concentrated on monitoring possible precursors phenomena that precede a forthcoming earthquake, such as measuring uplift, subsidence, and strain in the rocks.
56 Can Earthquakes Be Predicted? Short-range predictions Currently, no reliable method exists for making short-range earthquake predictions. Long-range forecasts Give the probability of a certain magnitude earthquake occurring on a time scale of 30 to 100 years, or more.
57 Can Earthquakes Be Predicted? Long-range forecasts Based on the premise that earthquakes are repetitive or cyclical Using historical records or paleoseismology Are important because they provide information used to: Develop the Uniform Building Code Assist in land-use planning
61 Earthquakes Evidence for Plate Tectonics A good fit exists between the plate tectonics model and the global distribution of earthquakes. The connection of deep-focus earthquakes and oceanic trenches is further evidence. Only shallow-focus earthquakes occur along divergent and transform fault boundaries.
63 Earthquakes Evidence for Plate Tectonics Earthquake depths Earthquakes originate at depths ranging from 5 to nearly 700 kilometers. Earthquake foci are arbitrarily classified as: Shallow (surface to 70 kilometers) Intermediate (between 70 and 300 kilometers) Deep (over 300 kilometers)
64 Earthquakes Evidence for Plate Tectonics Earthquake depths Definite patterns exist. Shallow-focus earthquakes occur along the oceanic ridge system. Almost all deep-focus earthquakes occur in the circum-pacific belt, particularly in regions situated landward of deep-ocean trenches.
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Chapter 7 Plate Tectonics Earthquakes Earthquake = vibration of the Earth produced by the rapid release of energy. Seismic Waves Focus = the place within the Earth where the rock breaks, producing an earthquake.
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Earthquakes http://thismodernworld.com/comic-archive Elastic rebound http://projects.crustal.ucsb.edu/understanding/elastic/rebound.html Elastic rebound Rocks store energy elastically When stored stress