Earthquakes Chapter 16. What is an earthquake? An earthquake is the vibration of Earth produced by...
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Transcript of Earthquakes Chapter 16. What is an earthquake? An earthquake is the vibration of Earth produced by...
![Page 1: Earthquakes Chapter 16. What is an earthquake? An earthquake is the vibration of Earth produced by the rapid release of energy Energy radiates in all.](https://reader035.fdocuments.in/reader035/viewer/2022062320/56649d4a5503460f94a27e15/html5/thumbnails/1.jpg)
EarthquakesChapter 16
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What is an earthquake?
An earthquake is the vibration of Earth produced by the rapid release of energy
• Energy radiates in all directions from its source, the focus
• Energy moves like waves • Seismographs record the event
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Earthquakes and faults • Earthquakes are associated with faults• Motion along faults can be explained by
plate tectonics
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Elastic rebound• Mechanism for EQ’s explained by H. Reid
– Rocks on sides of fault are deformed by tectonic forces
– Rocks bend and store elastic energy
– Frictional resistance holding the rocks together is overcome by tectonic forces
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Elastic rebound• Earthquake mechanism
– Slips starts at the weakest point (the focus) occurs
– Earthquakes occur as the deformed rock “springs back” to its original shape (elastic rebound)
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Elastic Rebound Movie in Mapview
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Reid elastic rebound cartoon
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Aftershocks
The change in stress that follows a mainshock creates smaller earthquakes called aftershocks
The aftershocks“illuminate” the that ruptured in the mainshock
Red dots show location ofaftershocks formed by 3earthquakes in Missouriand Tennessee in 1811/1812
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Normal Fault Quake - NevadaStrike Slip Fault Quake - Japan
Strike Slip Fault Quake - California
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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
• Most segments slip every 100-200 years producing large earthquakes
• Some portions exhibit slow, gradual displacement known as fault creep
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Fence offset by the 1906 San Francisco earthquake
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Seismology
Seismometers - instruments that record seismic waves
• Records the movement of Earth in relation to a stationary mass on a rotating drum or magnetic tape
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A seismograph designed to record vertical ground motion
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Motion of a seismograph
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Types of seismic waves • Surface waves
– Complex motion, great destruction – High amplitude and low velocity – Longest periods (interval between
crests) – Termed long, or L waves
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Types of seismic waves• Body waves
– Travel through Earth’s interior – Two types based on mode of travel – Primary (P) waves
– Push-pull motion– Travel thru solids, liquids & gases
– Secondary (S) waves– Moves at right angles to their
direction of travel – Travels only through solids
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P-Wave Motion
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S-Wave Motion
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Locating the source of earthquakes
Focus - the place within Earth where earthquake waves originate
Epicenter – location on the surface directly above the focus Epicenter is located using the difference in velocities of P and S waves
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Earthquake focus and epicenter
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Locating the epicenter of an earthquake• Three seismographs 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 then determines each station’s distance to the epicenter
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Graph used to find distance to epicenter
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Locating the epicenter of an earthquake• A circle with radius equal to distance to the
epicenter is drawn around each station • The point where all three circles intersect is
the earthquake epicenter
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Epicenter located using three seismographs
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Earthquake belts• 95% of energy released by earthquakes
originates in narrow zones that wind around the Earth
• These zones mark of edges of tectonic plates
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Locations of earthquakesfrom 1980 to 1990
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Depths of Earthquakes• Earthquakes originate at depths ranging
from 5 to nearly 700 kilometers • Definite patterns exist
– Shallow focus occur along oceanic ridges – Deep earthquakes occur in western
Pacific east of oceanic trenches
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Earthquake in subduction zones
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Measuring the size of earthquakes
Two measurements describe the size of an earthquake
• Intensity – a measure of earthquake shaking at a given location based on amount of damage
• Magnitude – estimates the amount of energy released by the earthquake
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Intensity scales • Modified Mercalli Intensity Scale was
developed using California buildings as its standard
• Drawback is that destruction may not be true measure of earthquakes actual severity
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Magnitude scales • Richter magnitude - concept introduced
by Charles Richter in 1935 • Richter scale
– Based on amplitude of largest seismic wave recorded
– Each unit of Richter magnitude corresponds to 10X increase in wave amplitude and 32X increase in energy
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Magnitudes scales• Moment magnitude was developed
because Richter magnitude does not closely estimate the size of very large earthquakes
– Derived from the amount of displacement that occurs along a fault and the area of the fault that slips
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Earthquake destruction
Amount of structural damage depends on • Intensity and duration of vibrations • Nature of the material upon which the
structure rests (hard rock good, soft bad) • Design of the structure
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Tsunamis, or seismic sea waves • Destructive waves called “tidal waves” • Result from “push” of fault block or
undersea landslide on water• In open ocean height is > 1 meter • In shallow coast water wave can be > 30
meters • Very destructive
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Formation of a tsunami
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Tsunami 1960, Hilo Hawaii
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Tsunami Model, Japan Earthquake
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Tsunami Model, Alaska Quake
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Can earthquakes be predicted
Short-range predictions • 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
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Earthquakes cannot be predicted
Short-range predictions • Currently, no method exists for making
short-range earthquake predictions
Long-range forecasts • Calculates probability of a certain
magnitude earthquake occurring over a given time period
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End of Chapter 16