Faults and Earthquakes

Fracture - A crack or break in the Earth’s crust.

Fractures form when STRESS, the forces per unit area acting on a material, exceeds the strength of the rocks involved.

The deformation of materials in response to stress is called STRAIN.

Elastic Rebound

Stress and Strain ultimately lead to a fracture or a system of fractures, along which movement occurs, this is called a FAULT

Types of Faults Faults Are Classified According to the Kind of Motion That Occurs on Them

Strike-Slip - Horizontal Motion Dip-Slip – Vertical Motion

Strike-Slip Fault – Left Lateral

Strike-Slip Fault – Right Lateral

Normal Faults: Extension Reverse Faults: Compression Dip-Slip Faults Normal Faults: Extension Reverse Faults: Compression Reverse Faults are often called Thrust Faults

Dip-Slip Fault - Normal

Normal Fault Structures

Dip-Slip Fault - Reverse

Reverse Fault Structures

Earthquakes Most Earthquakes are caused by movements along faults Earthquakes Most Earthquakes are caused by movements along faults. Most Earthquakes occur along plate boundaries.

The location within the earth where a fault rupture actually occurs is called the FOCUS

The location on the surface directly above the focus is called the EPICENTER.

The vibrations of the ground during an earthquake are called SEISMIC WAVES.

Seismic Waves

Seismology - The study of Earthquake waves.

The sensitive instruments used to record the vibrations of earthquakes are called SEISMOGRAPHS or SEISMOMETERS.

The record produced by a seismometer is called a SEISMOGRAM The record produced by a seismometer is called a SEISMOGRAM. This shows the arrival times of the different seismic waves at that location.

Primary waves or P-waves squeeze and pull rocks in the same direction along which the waves are traveling.

Secondary waves, or S-waves causes rocks to move up and down at right angles in relation to the direction of the waves.

P-waves are the fastest seismic waves P-waves are the fastest seismic waves. Remember P comes before S in the alphabet.

Some Important Earthquakes 1755 - Lisbon, Portugal Killed 70,000 First Scientifically Studied Earthquake 1811-1812 - New Madrid, Missouri Felt over 2/3 of the U.S. Few Casualties

Some Important Earthquakes 1964 - Alaska Killed about 200 Wrecked Anchorage. Tsunamis on West Coast. 1976 - Tangshan, China Hit an Urban Area of Ten Million People Killed 650,000

Major Hazards of Earthquakes Building Collapse Landslides Fire Tsunamis (Not Tidal Waves!)

U.S. Seismic Risk

San Francisco and New Madrid Compared

Earthquakes (M.4) 1534-1994

U.S. Earthquakes 1973-2002 (28,332 events)

Seismology and Earth's Interior

Magnitude and Intensity How Strong Earthquake Feels to Observer Magnitude Related to Energy Release Determined from Seismic Records Rough correlation between the two for shallow earthquakes

Magnitude How Much Energy is Released

Magnitude is measured using the Richter Scale

Richter scale Each successive number in the scale represents an increase in seismic-wave size, of a factor of 10.

Ex. A magnitude 7 earthquake is 10 times larger than a magnitude 6 earthquake.

Each increase in magnitude corresponds to a 32-fold increase in energy

Ex. A magnitude 7 earthquake releases 32 times more energy than a magnitude 6 earthquake.

Intensity How Strong an Earthquake Feels to the Observer

Depends On: Distance to Quake Geology Type of Building The Observer Depends On: Distance to Quake Geology Type of Building The Observer! Varies from Place to Place

Intensity measured using the Mercalli Scale

Mercalli Scale - 1 to 12 a way of measuring earthquakes based on the amount of damage done.

Earth’s Interior as Inferred by Seismology

Discontinuity - any sudden change in the properties of a material

Earth’s Interior

CRUST- outer most layer; also known as the LITHOSPHERE

Two kind of crust: CONTINENTAL & OCEANIC

Comparing Continental to Oceanic Crust

Rocks made up of: Continental – Granite Oceanic - Basalt

Relative Thickness: Continental – Thicker Oceanic - Thinner

Relative Density: Continental – Less Dense Oceanic – More Dense

Relative Age: Continental – Older Oceanic - Younger

Moho – What is it? Boundary or interface between the CRUST and the MANTLE

The Moho was discovered by a change in seismic wave speed.

Mantle Region of dense material in the Earth’s interior between the CRUST and the OUTER CORE.

Asthenosphere Plastic-like part of the Upper Mantle

Core Broken into an Outer Core and an Inner Core

Outer Core It is a liquid… They know this because of a shadow zone in the P-waves

Shadow Zone Refraction of seismic waves creates an area on the Earth’s surface where no direct P-waves appear.

No S-waves pass through the outer core because they cannot pass through liquids.

Inner Core Solid - known because P-waves speed up

The inner core is believed to be composed of IRON and NICKEL

The overall structure of the Earth

Seismic Waves in the Earth

Inner Structure of the Earth

LOCATING EARTHQUAKES

Based on the time it takes seismic waves to reach a location, the distance of that location can be determined using a P and S wave time travel graph.

Locating Earthquakes

Doing this for two stations show you 2 possible epicenters of the earthquake.

Locating Earthquakes

Using 3 or more stations will show you the location of the Epicenter

Locating Earthquakes

Locating Earthquakes - Depth