Earth Science – Lesson 12

What is an earthquake?
movement or trembling of the ground that is caused by a sudden release of energy when rocks along a fault move

What is Elastic rebound?
the sudden return of elastically deformed rock to its undeformed shape

What is focus?
location along the fault at which the first motion of an earthquake occurs; depths vary.

What is shallow foci?
0-70 km; 90% of continental earthquakes; most damaging earthquakes

What is Intermediate foci?
70-300 km

What is deep foci?
300-650 km; usually by subduction zones

What is the Epicenter?
the point on the Earth’s surface directly above the focus

What are Seismic waves?
vibrations produced as energy is released by rocks as they slip into a new position

Name 2 main types of waves:
1. Body waves
2. Surface waves

What are Body waves?
waves that travel through the body of a medium

What are Surface waves?
waves that travel along the surface of a body rather that through the middle; Form from the conversion of energy when P and S waves reach Earth’s surface. Slowest-moving, cause the most damage

Name 2 types of Body waves-
1. P waves-primary/compression waves
2. S waves-secondary/shear waves

What are P waves?
primary/compression waves;
-Fastest;always the first waves detected
-Particles of rock move in a back and forth direction that is parallel to the direction in which the wave is traveling
-Can travel through solids, liquids or gasses (more rigid the material is, the faster the wave travels)

What are S waves?
secondary/shear waves;
-Second fastest waves; second waves detected
-Particles of rock move in a side-to-side direction that is perpendicular to the direction in which the wave is moving
-Can only travel through solid material

Name 2 types of Surface waves-
1. Love waves-
2. Rayleigh waves-

What are Love Waves?
Cause rocks to move side to side and perpendicular to the direction in which the waves are traveling

What are Rayleigh waves?
Cause rocks to move in an elliptical, rolling motion

How can waves tell us about the earth’s interior?
The different speed and direction S and P waves travel through the Earth’s interior can help scientists learn about the Earth’s interior

Who was Andrija Mohorovicic?
1909 Croatian scientist
Discovered the boundary between the crust and the mantle (10-30 km)

Name the 3 compositional layers
-crust, mantle, core

Name the 5 mechanical layers
-lithosphere, asthenosphere, mesosphere, outer core, inner core

What are Shadow zones?
locations on Earth’s surface where no body waves from a particular earthquake can be detected

Where do most earthquakes occur?
Most earthquakes occur at the 3 main tectonic environments because stress on rocks is greatest here

Describe earthquakes at Convergent Oceanic Environments-
Earthquakes occur as the overriding plate scrapes against the top of the submerging plate

Describe earthquakes at Divergent Oceanic Environments-
Spreading motion at the mid-ocean ridge causes earthquakes

Describe earthquakes at Continental Environments-
As continental plates converge, diverge, or slide past each other horizontally rocks undergo stress. Stress causes mountains to form and earthquakes to occur

What are fault zones?
-Regions of numerous, closely spaced faults
-Occur at plate boundaries where plates subduct, collide, or move past each other horizontally
-When enough stress builds up, movement occurs along one or more fault. This may cause major earthquakes

True/False: All earthquakes occur near plate boundaries
False: Not all earthquakes occur near plate boundaries
-Old, large fault zones may be located away from plate boundaries. These may cause earthquakes
-1811 and 1812 earthquakes in New Madrid, Missouri

What is Seismology?
the study of earthquakes

What is a Seismograph?
An instrument that detects and records vibrations in the ground
Modern ones measure vertical motion, and horizontal motion (east-west and north-south)

What does a Seismogram do?
Tracing of earthquake motion that is recorded by a seismograph
-P waves recorded first, S waves recorded second, Surface waves recorded last

How do scientists determine distance to an epicenter?
To determine distance to an epicenter, scientists analyze the arrival times of the P and S waves.
Longer lag time= further away the epicenter
Earthquake’s start time can also be determined this way

How do scientists determine the location of the epicenter?
To determine the location of the epicenter, scientists use computers to triangulate the epicenter based on information from several seismograph stations. Must have information from at least three seismograph

What are the two measurements of an earthquake
1) Magnitude (strength), and 2) intensity (damage)

What is Magnitude?
Measure of the strength of an earthquake
Due to amount of ground motion caused by an earthquake
Measurement expressed by using scales: Richter and moment magnitude scale

What is the Richter Scale?
measures magnitude based on ground motion

What is the Moment magnitude scale?
preferred by scientists, based on strength, size of the area of the fault that moves, average distance fault block moves, and rigidity of the rocks in the fault zone

What is Intensity?
In Earth science, the measure of the amount of damage caused by an earthquake.

How is intensity measured?
Measured in the Modified Mercalli scale (Roman numerals I-XII)

How do most injuries result from earthquakes (5 things)
Most injuries result from
-the collapse of buildings and other structures
-falling objects and flying glass
-Fires/explosions caused by broken electric/gas lines
-Floodwaters from collapsing dams

What are Tsunamis?
giant ocean wave formed after a volcanic eruption, submarine earthquake, or submarine landslide
Movement of the ocean crust causes a series of long, low waves that increase in height as they near the shore (tsunamis)

T/F: Most buildings are designed to withstand the swaying motion caused by earthquakes
False. Most buildings are not designed to withstand the swaying motion caused by earthquakes

How does the type of ground determine how a building responds in an earthquake?
The type of ground can affect the way in which the building responds to seismic waves
-Sand and clay vibrates violently

What are things you can do to prepare BEFORE an earthquake?
Keep canned food, bottled water, flashlights, batteries, portable radio, and first aid kit.
Plan how to react at home, in a car, or at school

What are things you can do to DURING an earthquake?
Stay calm!
At home, move to a safe position (under a doorway, under a desk/table) away from glass, heavy furniture.
At school, follow teacher/principal’s directions
In a car, stop away from tall buildings, trees, tunnels, power lines, or bridges. Remain in the car until the tremors stop

What are things you can do to AFTER an earthquake?
Be cautious. Wear shoes around broken glass. Avoid downed power lines and objects touched by downed wires

What can scientists do to predict earthquakes?
-Scientists study past earthquakes to predict where/when future earthquakes will occur
-Currently there is no reliable way of predicting exactly when or where an earthquake will occur
-Scientists are trying to detect changes in Earth’s crust that can signal an earthquake with instruments placed along faults

Name 3 methods scientists are using to predict earthquakes:
1) Seismic gap
2) Foreshocks
3) Changes in rocks

How are earthquakes predicted using Seismic gap?
An area along a fault where relatively few earthquakes have occurred recently but where strong earthquakes occurred in the past

How are earthquakes predicted using Foreshocks?
Sometimes earthquakes are preceded by little earthquakes called foreshocks. Can precede earthquakes by a few seconds or weeks

How are earthquakes predicted using changes in rocks?
Tilting of rock, magnetic and electrical changes in rock detected by water put in cracks, and natural gas seepage may indicate earthquakes

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