Learning log

Describe what you think is happening during an earthquake.

Essential Question:What causes earthquakes, how do we know where they are, and how are they described?

Forces Within Earth• Earthquakes are natural

vibrations of the ground caused by plate movement in Earth’s crust or by volcanic eruptions

• In some instances a single earthquake has killed more than 100,000 people and destroyed entire cities.

• Anyone living in an area prone to earthquakes should be aware of the potential danger and how to minimize the damage that they cause.

Stress and Strain• Most earthquakes occur when rocks

fracture, or break deep within Earth

• Fractures form when stress (forces acting on a material) exceed the strength of the rock

• 3 kinds of stress:– Compression – decreases

the volume of a material– Tension – pulls a material

apart– Shear – causes a material to

twist

Strain is the deformation of materials in response to stress.

Rocks fail when stress is applied too quickly, or when stress is too great.

What is a fault?• the fracture or system of

fractures along which movement occurs.

• The surface along which the movement takes places is called the fault plane.

• 3 basic types of faults:– Reverse fault – form as a result of

horizontal compression– Normal fault – caused by horizontal

tension– Strike-slip fault – caused by

horizontal shearClip 0-13 on Engineering Earthquakes

Reverse Fault

Normal Fault

Strike-slip Fault

Earthquake Waves• The vibration of the ground

during an earthquake are called seismic waves.

• Every earthquake generates 3 types of seismic waves:–Primary waves (P-waves) –

causes rock particles to move back and forth as it passes

Earthquake Waves

Secondary waves (S-waves) causes rock particles to move at right angles to the direction of the wave

Earthquake Waves• Surface waves – causes

rock particles to move both up and down and side to side.

• Surface waves travel along Earth’s surface and produce the most serious damage.

• P-waves and S-waves (body waves) pass through Earth’s interior

Earthquake Waves• The first waves generated by an earthquake spread out

from the point of failure of rocks

• Focus – Point where an earthquake originates, usually several km below the surface

• Epicenter – Point on Earth’s surface directly above the focus is the earthquake’s.

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• Liquifaction – when earthquake vibrations make the loose soil behave like a liquid.

Measuring Energy Released and Locating Earthquakes

• More than one million earthquakes occur each year!

• More than 90% are not felt and cause little to no damage

• The ones that make the news are major seismic events that cause much damage.

Locating the Epicenter of an Earthquake

The Triangulation Method

Locating the Epicenter

In order to determine the location of an earthquake, the earthquake needs to be recorded on three different seismographs that are at significantly different locations.

The other piece of information needed is the time it takes for P-waves and S-waves to travel through the Earth and arrive at a seismographic station.

The Triangulation MethodTriangulation

A mathematical method for locating the epicenter of an earthquake using three or more data sets from seismic stations.

Seismograph - Earthquake monitoring

instrument that records the seismic waves of the earthquake.

A seismograph records earthquake activity by plotting vibrations on a sheet of paper to create a seismogram. Above are some sample seismograms:

Earthquake Magnitude & Intensity• The amount of energy

released during an earthquake is measured by its magnitude.

• Measured using the Richter Scale – based on the size of the largest seismic waves made by the quake

• 10-Fold (logarithmic) Scale: – meaning seismic waves of a

magnitude-8 earthquake on the Richter scale are 10 times larger than a magnitude-7 and 100 times larger than a magnitude-6.

Earthquake Magnitude & Intensity• Most seismologists use

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

– Moment magnitude values are estimated from the size of several types of seismic waves produced by an earthquake.

Earthquake Magnitude & Intensity• Another way to assess earthquakes

is to measure the amount of damage.

• Modified Mercalli Scale - measures the amount of damage done to the structures involved and is used to determine the intensity of an earthquake.

• uses the Roman numerals I to XII to designate the degree of intensity.

• Specific effects or damage correspond to specific numerals; the higher the numeral, the worse the damage.

Depth of Focus• Another factor that determines a quake’s intensity is

the depth of the quake’s focus– Can be classified as shallow, intermediate, or deep– Deep-focus = smaller vibrations at epicenter– Shallow-focus = larger vibrations at epicenter

• A shallow-focus, magnitude-6 quake will have greater intensity than a deep-focus, magnitude-8 quake

• Catastrophic quakes with high intensities are almost always shallow-focus quakes

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Seismic Belts• Seismologists have collected and plotted locations of epicenters.

– The global distribution of these epicenters reveals an interesting pattern. They are not randomly distributed

Most earthquakes are associated with tectonic plate boundaries• 80% occur along the Circum-Pacific Belt• 15% occur along the Mediterranean-Asian Belt• Most of the remaining occur at ocean ridges

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Locating an Earthquake• Initially, the exact location of an

earthquake’s epicenter and time of quake’s occurrence is not known

• But, it can be easily determined using seismograms and travel-time curves

Time of an Earthquake• The exact arrival times of

the P-waves and S-waves at a seismic station can be read from the seismogram.

• Using a travel-time graph, the time of occurrence can be determined by subtracting the travel time from the known arrival time of the wave.

Distance to an Earthquake• To determine the

location of the epicenter:– The locations of 3

seismic stations are plotted on a map

– A circle whose radius is equal to the epicentral distance is plotted around each station

– The point of intersection of these circles is the epicenter