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Earthquakes

By Joe Murnane

Earthquakes

Myths

Myths

• Mongolia, China• A gigantic frog which

carried the world on its back would twitch periodically, producing what we now call earthquakes.

                                              

Myths

• Hindus of India• The Hindus believed

that eight huge elephants held up the world. Whenever one grew tired, it lowered it’s head and shook , causing an earthquake.

Myths

• Japan• A giant catfish lived in mud

beneath the earth. The catfish enjoyed playing pranks and could only be held at bay by Kashima,a god who protected the Japanese. As long as Kashima kept a mighty rock with magical powers over the catfish, the earth was still. But whenever Kashima got lazy, the catfish thrashed about, causing earthquakes.

Myths

• Russia• A god named Tuli

drove a sled filled with the Earth. The sled was pulled by dirty dogs covered with fleas. Whenever the dogs stopped to scratch, there was an earthquake.

Myths

• Peru• The Peruvian god would

periodically visit the earth to count how many people were there. When he walked about, his footsteps would caused earthquakes. To shorten his task, the people would run out of their houses, shouting such things as "I'm here, I'm here!”

Earthquakes

Modern Myths

Modern Myths

• Many people believe because their home is often shaken by small earthquakes that they are protected from larger ones. Many times the smaller quakes are actually forerunners to larger earthquakes.

Modern Myths

• Many People believe that when an earthquake happens, a chasm may open up and anyone who falls in it will be lost in the earth. Sudden movement along a fault may create a shallow crevice but there has never been any recordings of a person falling into such a place and dying.

Modern Myths

• It has been speculated that in some way Jupiter, Saturn, and Mars cause earthquakes and that tidal movements may also play a part. The most precise scientific evaluations show no significant relationship between earthquakes and tidal loading.

Modern Myths

• Many people believe that earthquakes cause volcanic eruptions. This is not true. Earthquakes will occasionally happen in the same area during, before, or after an eruption, but are a result of the forces which cause the eruption.

Modern Myths

• A theory was proposed that that earthquakes happened in primarily serene, cloudy conditions. They were then proceeded by gale force winds, meteors, and fireballs. This is not true. Earthquakes are causes solely by geologic processes and have no relation to weather conditions.

Modern Myths

• At the present there is no known way to prevent quakes. However, the amount of destruction caused can be lessened but quake resistant structures and proper education.

Earthquakes

Cause

Cause

• Tectonics- processes, structures, and landforms associated with deformation of the Earth’s crust.

- Continents and ocean basins are largest examples.

- Processes take place over hundreds of millions (oceans), millions (mountain ranges), hundreds of thousand (hills), and in seconds (scarps).

• Tectonic Cycle- All the processes that make up tectonics on a global scale

• Lithosphere- outer layer of Earth (crust and upper mantle)

- Strong and rigid

- Broken into several pieces lithospheric plates

- The plates move relative to eachother

Cause

• Asthenosphere- layer underlying lithosphere made up flowing, hot, weak rock.

- This layer is the cause for the movement of the lithospheric plates.

- Causes continental movement

Cause

• The boundaries where lithospheric plates meet are areas of great geological activity where earthquakes are most common.

• Three types of boundaries:1. Divergent- occur where new lithosphere is made and

plates are moving away from each other.2. Convergent- occur where one plate travels beneath the

edge of another plate “subduction.” (If both are made of low-density material such as granite, a continental-collision boundary can develop.) “Alps, Himalayas”

3. Transform- occur where one or more plates slide past one another, displacing ridges. “San Andreas Fault”

Cause

• Stress (force per unit area on a specified plane) develops from these processes.

• Strain (deformation) increases as these processes occur

• Elastic strain- deformation that is not permanent• Elastic rebound- when elastic strain is released• http://www.crustal.ucsb.edu/ics/understanding/el

astic/rebound.html

Cause

• When the stress caused by these processes grows greater than the strength of the surrounding rock, the rock ruptures, causing a massive release of energy and seismic waves in the form of an earthquake

• Because of this, this is why most earthquakes as a rule occur on faults.

• Fault- a fracture or system of fractures where rock has been displaced or deformed

Cause

• Intraplate earthquakes- earthquakes that occur far from the plate boundaries

• Relatively rare

• Caused by weak portions of lithosphere that break do to the stress caused by the distant plate boundaries.

Cause

• Intensity

• Focus- the area where the rupture is started and seismic energy first released.

• Epicenter- surface directly above the focus

• Deeper focus’ usually mean less damage done to the epicenter.

Earthquakes

Detection

Detection

• Long-term

• Paleoseismology — Study of the occurrence, size, and frequency of historical earthquakes lacking instrumental seismic records and prehistoric quakes.

• Two types of evidence:

Detection

• Geomorphic- deformed landform features (fold scarps, fault scarps, uplifted coastal terraces and deflected streams.)

• Stratigraphic- deformed sedimentary deposits (lake, beach, subsidence, sand boils, colluvial wedges, etc.)

Detection

• Common techniques• Trenching- observation of faults to find the

“piercing point” (offset channel, pipe etc.)• Seismic Reflection- shaking the ground via

explosions or electric vibrations to measure vibration “waves.” Data is compiled to make a model of possible underground deformities caused by fault offsets.

Detection

• Short-term

• Forecasting- specifies possible time period and probability.

• Foreshocks- smaller earthquakes often leading up to a single massive quake.

- Does not always occur

Detection

• Preseismic Uplift- Raising of earths crust for a period of time before the earthquake

• Seismic Gaps- Areas along active fault zones which are capable of producing large quakes but have not in recent time. Believed to store tectonic strain, thus having good potential for future quake sites.

Detection

• Electrical Resistivity- Before an earthquake, rocks dilate and there is an influx of water, decreasing electrical resistivity.

• Radon increases from deep wells

• Animal Behavior

Detection

• Instruments• Accelograph- Instrument used to measure

ground acceleration during seismic shaking• Seismograph- Instrument used to measure

seismic waves- Richter magnitude- determined by the largest

amplitude of seismic waves on a seismograph- Modified Mercalli Scale- http://www.seismo.unr.edu/ftp/pub/louie/class/10

0/mercalli.html

Detection

• From Space

• The ionosphere — an atmospheric region filled with charged particles that blankets the Earth between altitudes of about 75 to 1000 km.

• Fluctuations in the ionosphere= ionospheric scintillations= signal delays in GPS satellites

Detection

• From Space

• Ionosphere — works as a natural amplifier of seismic waves moving across the Earth's surface.

• Future use to detect seismic activity in deep ocean or island areas.