Download Earth Science Chapter 19: Earthquakes Chapter Overview Section 1

Earth Science
Chapter 19: Earthquakes
Chapter Overview
Section 1: Forces Within Earth
Earthquakes are natural vibrations of the ground caused by movement along gigantic fractures in
Earth’s crust, or sometimes by volcanic eruptions
1. Stress and Strain
Most earthquakes occur when rocks fracture deep within Earth. Fractures form when stress,
the force per unit area acting on a material exceeds the strength of the material. There are
three kinds of stress that act on Earth’s rocks: compression, tension, and shear. The
deformation of materials in response to stress is called strain.
• Elastic deformation
Elastic deformation occurs what a material is compressed, bent, or stretched when under
low stress.
• Plastic deformation
When stress builds up past a certain point called the elastic limit plastic deformation
occurs. Plastic deformation results in the permanent deformation of of the material.
2. Faults
Many kinds of rocks that make up Earth’s crust fail when stress is applied too quickly, or
when stress is great. The resulting fracture or system of fractures, along which movement
occurs is called a fault
• Types of Faults
There are three types of faults:
ƒ Reverse faults are fractures that form as a result of horizontal compression
ƒ Normal faults are fractures caused by horizontal tension
ƒ Strike-slip faults are fractures caused by horizontal shear
3. Earthquake Waves
Most earthquakes are caused by movements along faults
• Types of Seismic Waves
The vibrations of the ground during an earthquake are called seismic waves. Every
earthquake generates three types of seismic waves:
ƒ Primary waves (P waves) squeeze and pull rocks in the same direction along which
the waves are traveling
ƒ Secondary wave (S waves) cause rocks to move at right angles in relation to the
direction of the waves
ƒ Surface waves are waves that move in two directions as they pass through the rock,
an up and down motion and a side-to-side motion.
P and S waves travel through the Earth’s interior and are called body waves. Body waves
originate from the point of failure or rocks at depths. This point, where an earthquake
originates, is the focus of the earthquake. The point on Earth’s surface directly above the
focus is the earthquake’s epicenter
Section 2: Seismic Wave and Earth’s Interior
The study of earthquake waves is called seismology. While earthquakes can cause tremendous
damage to structure and life, the study of seismic waves reveals a great deal of information about
the structure of the Earth
1. Seismometers and Seismograms
Vibrations sent out by earthquakes shake the entire globe. Although most of the vibrations
cannot be felt great distances from a quake’s epicenter, they can be detected and recorded
by sensitive instruments called seismometers. The record produced by a seismometer is
called a seismogram
• Time-Travel Curves
Over many years, scientists have collected data about the travel times of seismic waves
from earthquakes to different monitoring stations around the world. Using this data, global
travel-time curves for the initial P-waves and S-waves have been constructed. These
curves show that P-waves always arrive first from the epicenter, and that as the distance
increases, the time separation between the two types of waves increases.
2. Clues to Earth’s Interior
Most of the knowledge of Earth’s interior comes from the study of seismic waves, which
change speed and direction when they encounter different materials. P-waves that travel
through the mantle follow a fairly direct path, but are refracted when they enter the core. Swaves do not enter the Earth’s core because they cannot travel through liquids
• Earth’s Internal Structure
The travel times and behavior of seismic waves provide a detailed picture of Earth’s
internal structure. The waves also provide clues about the composition of the various
parts of Earth. The travel times of the seismic waves indicate that the lithosphere is made
up of primarily igneous rocks. Earth’s lower mantle is composed of simple oxides
containing iron, silicon, and magnesium. The core is very dense and is composed of a
mixture of iron and nickel
Section 3: Measuring and Locating Earthquakes
1. Earthquake Magnitude and Intensity
The amount of energy released by an earthquake is measured by it magnitude. An
earthquake’s rating on the Richter scale is based on the size of the largest seismic waves
generated by the quake. Each number on the scale represents an increase in seismic wave
amplitude of a factor of 10. The seismic-waves of a magnitude 8 quake are ten times larger
that those of a magnitude 7. Also, each increase in magnitude represents a 32-fold increase
in seismic energy. A magnitude 8 quake releases 32 times the energy of a magnitude 7
earthquake
• Moment Magnitude Scale
The moment magnitude scale takes into account the size of the fault rupture, the
amount of movement along the fault, and the rock’s stiffness. The values of this scale are
estimated from the size of several types of seismic waves produced by the earthquake
• Modified Mercalli Scale
Another way to assess earthquake is to measure the amount of damage done to the
structures involved. This measure, called the intensity of an earthquake, is determine
using the modified Mercalli scale, which rates the types of damage and other effects of
an earthquake
• Depth of Focus
Another factor that determines the intensity of an earthquake is the depth of the quake’s
focus. An earthquake can be classified as shallow, intermediate, or deep depending on
the location of the quake’s focus. The shallower the focus the greater the damage done
by the earthquake. Catastrophic quakes with high intensity values are almost always
shallow focus earthquakes
2. Locating an Earthquake
• Distance to an Earthquake
The distance from the epicenter of an earthquake to a monitoring station can be
determined by measuring the time interval between the arrival of the first P-wave and the
arrival of the first S-wave to the monitoring station. Because the travel times of P- andswaves are known, the time interval indicates the distance the waves traveled. To
determine the location of the epicenter of an earthquake the distance between at least
monitoring stations and the earthquake’s epicenter must be known
3. Seismic Belts
The majority of the world’s earthquakes occur in relatively narrow belts that separate large
regions with little or no seismic activity. Most earthquakes are associated with tectonic plate
boundaries. Almost 80% of all earthquakes occur in the Circum-Pacific Belt. 15% of all
earthquakes occur along the Mediterranean-Asian Belt.
Section 4: Earthquakes and Society
1. Earthquake Hazards
• Structural failure
Buildings can be destroyed by earthquakes because of such factors building materials
and building heights
• Land and soil failure
Soil liquefaction – occurs when sand saturated with water behaves as a liquid during an
earthquake
Tsunami – a large ocean wave generated by vertical motions of the seafloor during an
earthquake
2. Earthquake Forecasting
There is no completely reliable to forecast the exact time and location of an earthquake,
instead forecasting is based on calculating the probability of an earthquake.
• Seismic risk
Most earthquakes occur in long, narrow bands called seismic belts. The probability of
future earthquakes is much greater there than elsewhere.
• Recurrence rate
Earthquake-recurrence rates along a fault con indicate whether the fault ruptures at
regular intervals to generate similar earthquakes.
• Seismic gaps
Probability forecasts are also based on the location of seismic gaps—sections located
along faults that are known to be active but have not experienced significant earthquakes
for a long period of time.