Earthquakes

Section 1 How and Where Earthquakes Happen

There are over 3,000,000 earthquakes each year. That means that there is about 1 every 10 seconds.

Earthquakes are nature’s most destructive forces. They

are the shaking of the earth's crust caused by the release of energy. The plates move on convection currents in the mantle - stress is built up, must be released, a shaking of the earth happens. A minor cause of earthquakes is volcanoes.

Elastic rebound theory1. Stress between two moving plates increase.2. Plates bend with the stress.3. Stress becomes too great – the plates snap or slip past

each other.4. Plates will go back to the original form, only in different

positions.

The focus is the point source of an earthquake; the place where the earthquake happened. Even thought the depth of the focus varies, most continental quakes are classified as shallow; about 70km below, more than 70km is intermediate; 300-650 deep.

The ones that cause the most surface damage are the shallow ones since the energy is closer to the surface.

 The Epicenter is the point directly above the focus on the

surface.

Seismic waves Seismic waves are the earthquakes energies being transmitted through the

earth. They travel in all directions from the focus. They look like when you drop a pebble into a pond.

There are two types of waves:Body Waves: travel through the body of the medium (ground)Surface Waves: are on the surface rather than inside the earth

They travel through the interior of the earth. The rate that a wave travels depends on what they are traveling through. They bend depending on the density of the layer, similar to the refraction of light waves. Body waves are the first arriving tremors and aftershocks.

 1. P-Waves

- Primary waves; first waves- Compressional waves (like sound); they squeeze and stretch- Travel through all substances (gas, liquid, solids)

 2. S-Waves

- Sheer waves; secondary- Moves at right angles; perpendicular (up and down)- Cannot move through liquids (molten material) or gases below 2900km- S waves are about 60% the rate of P waves

 

They move similar to water waves and travel just under the surface. When P & S waves hit the surface sometimes areas very far away from the epicenter will be affected. These waves cause considerable damage because of their low frequency, long duration and large amplitude.

 1. Love Waves (L waves)

Move side to side perpendicular to the waves travel direction. Looks like a snake traveling (horizontal shearing)

Speed approx 90% that of S waves2. Rayleigh Waves

Move more slowly; 70% slower then S wavesMove matter elliptically (like pond ripples)

Also called ground roll,

Seismic waves are useful tools for scientists to explore. The type of material the waves travel through affects the speed and direction of their movement. By studying the speeds and directions of the waves, scientists can learn more about what makes up the interior of the Earth.

 

Earth’s Internal Layers3 Compositional layers

CrustMantleCore

5 MechanicalLithosphereAsthenosphereMesosphere (mantle)Outer CoreInner Core

The shadow zone is an area of the earth that receives no seismic waves; because of the refraction of the waves going through different densities of materials.

Refraction - the bending of waves because of density differences

 Studying Earthquakes gave us the inferred layers of the

earth. S waves can’t go through liquid, so we concluded a layer must be liquid since no S waves would occur in the shadow zone.

Convergent Oceanic Environments When either an oceanic and oceanic plate collides one will

subduct. An earthquake will occur. The same is true when an oceanic plate collides with a continental plate, an earthquake will also occur.

 Divergent Oceanic Environments The Mid-Ocean Ridge has earthquakes occurring all of the time

because the plates are pulling away from each other. Continental Environments When two continental plates converge, diverge or move

horizontally away from each other, they can build up pressure which will cause quakes to happen and even mountains to be built.

 

This is an area with numerous, closely spaced faults. They form at plate boundaries.

 

Earthquakes Away From Plate BoundariesNot all quakes result from movement along plate boundaries.

In 181-1812 a series of quakes occurred near New Madrid, Missouri. The waves were so strong that they were felt as far away is the Eastern Coast and New England.

http://www.ldeo.columbia.edu/LCSN/recenteqs/Quakes/quakes0.html

Seismology – Is the study of the earthquakes and the seismic waves

 

Recording Earthquakes

SeismographThe instrument used to measure seismic waves

SeismogramThe record that is made of the waves

Since P waves are the fastest moving waves, they are the first to appear on the seismograph. The S waves would be the second to arrive and the last would be the surface waves.

Locating the Earthquake

To determine the distance to an epicenter, scientists analyze the arrival times of the P and S waves. The longer the lag time between them, the further away the actual earthquake occurred.

Time Travel graph

It shows the relationship between the arrival times of the P & S waves and the distance to the epicenter.

How to locate the epicenter of an earthquake: You need three

seismographic stations to record the earthquake so you can determine through triangulation.

Locating an earthquake epicenter if I know the difference in arrival

times of P & S waves I can determine the distance to the epicenter

Scientists study the amount of energy release and the change that they caused. This is the study of the magnitude and intensity of an earthquake.

 Magnitude

It is the measure of the strength of earthquake. It is determined by measuring the amount of ground motion caused by the earthquake.

 Richter scale

It measures the ground motion from an Earthquake. It is used for smaller earthquakes now.

Movement MagnitudeIt measures the strength based on the size of the area of the fault that moves, the average distance that the fault block moves and the rigidity of the rocks in the fault zone. The larger the number, the stronger the earthquake is.

IntensityIntensity is the measure of the effect of an earthquake.

Modified Mercalli Scale:Expresses the intensity in Roman numeral from I to XI and provides a

description of the effects of each Earthquake Intensity.

Most injuries from earthquakes are a result from the collapse of buildings and other structures along with other objects crashing about. Other dangers can be explosions, floods, landslides and fires.

A tsunami is huge ocean waves that are formed y submarine earthquakes, volcanoes or landslides. The speed depends on depth of water; at 4500 meters (usual ocean dept) can travel 750km/hr. At the shore it slows down and reaches enormous height.

Foundation failure will happen due to the up and down and side to side motion of the ground.

Liquefaction:The ground will become liquefied due to the

friction and moisture content of the ground. This will make the ground unstable for foundations, weak and unsafe.

 After Shocks:They are smaller earthquakes that occur after larger

ones. They can cause further damage to areas like fires from damaged gas lines and power outages.

Earthquakes can happen anywhere. Some places will get more of them and some areas more destructive one because of their proximity to boundaries. By following safety measures some damages may not be as bad.

 Before an EarthquakeMake sure you have a plan to meet family members after the disaster.

Learn how and where to report damage. Make sure you have supplies and flashlights in an area that you can get to in case of an emergency.

 During the EarthquakeStay Calm.Take cover in a strong building under something sturdy. If you are

outside, move into an open area. Stay away from electrical wires and gas and pipe lines. Don’t move in an area that is prone to have them. Make sure you build structures that can withstand them. If you are in a car, stop the car in an area that is not by buildings or on bridges.

 After an EarthquakeWatch out for electrical problems or wires lying around. Don’t go back

into your home unless you are given an all clear to do so. Make sure that your area is safe and free from aftershocks.

 

Scientists have studied earthquakes for a long time trying to predict when they will occur. There is no reliable way to forecast them. Today scientists are looking for small changes in the earth’s crust to see if they can predict an earthquake.

Seismic GapsA seismic gap is an area along a fault that experienced only a few

earthquakes recently, but where strong ones have occurred in the past.

 ForeshocksForeshocks are little earthquakes that precede an earthquake by a

few seconds. Not a reliable source for earthquake prediction. Changes in RocksScientists look for stress and strain in rocks. They look for magnetic

changes, natural gas seepage and other related cracks in the rocks in an area.

 Reliability of Earthquake ForecastsVery unreliable, they can only continue to study them.

1906 - San Francisco 8.3

1964 - Alaska, unrecorded because of the severity, anywhere from an 8.5 to a 9.0 800 km of fault affected, vibrations continued for 18 months, Over 10,000 aftershocks

1960 - Chilean earthquake, 9.5

1995 – Kobe Japan, 7.2 one of the worst in Japan’s history, killing 6,433 people and causing more than $100 billion in damages.

2004 - Sri Lanka 9.0-magnitude earthquake between the Indonesian island of Sumatra and the Indian Andaman Islands

2008 - China earthquake, 7.9 devastated the country just before the Olympics  http://earthquake.usgs.gov/eqcenter/eqarchives/year/byyear.php  

Richter Magnitudes

Description Earthquake Effects Frequency of Occurrence

Less than 2.0 Micro Micro earthquakes, not felt. About 8,000 per day

2.0-2.9 MinorGenerally not felt, but recorded.

About 1,000 per day

3.0-3.9 MinorOften felt, but rarely causes damage.

49,000 per year (est.)

4.0-4.9 LightNoticeable shaking of indoor items, rattling noises. Significant damage unlikely.

6,200 per year (est.)

5.0-5.9 Moderate

Can cause major damage to poorly constructed buildings over small regions. At most slight damage to well-designed buildings.

800 per year

6.0-6.9 StrongCan be destructive in areas up to about 100 miles across in populated areas.

120 per year

7.0-7.9 MajorCan cause serious damage over larger areas.

18 per year

8.0-8.9 GreatCan cause serious damage in areas several hundred miles across.

1 per year

9.0-9.9 GreatDevastating in areas several thousand miles across.

1 per 20 years

10.0+ GreatNever recorded; see below for equivalent seismic energy yield.

Extremely rare (Unknown)