1 Introduction to Earthquakes مقدمة علم الزلازل. 2 Topics What are earthquakes ? Why...

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Introduction to EarthquakesIntroduction to Earthquakes

الزالزل علم الزالزل مقدمة علم مقدمة

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TopicsTopicsWhat are earthquakes ?What are earthquakes ?

Why do earthquakes occur ?Why do earthquakes occur ?

How do we describe earthquakes ?How do we describe earthquakes ?

Where do earthquakes occur ?Where do earthquakes occur ?

How often do earthquakes occur ?How often do earthquakes occur ?

How do we record earthquakes ?How do we record earthquakes ?

Can we predict earthquakes ? Can we predict earthquakes ?

What to do in an earthquake ?What to do in an earthquake ?

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What is an Earthquake?

An earthquake is a sudden movement of the ground that releases elastic energy stored in the rocks and generates seismic waves.

After the initial ground movement along the fault, seismic waves propagate outward and vibrate the ground.

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What is an Earthquake?

Other names for earthquakes include: event (or seismic event) teleseism - an earthquake from far away seismicity - the general pattern of earthquakes in a region mainshock, foreshock, aftershock - earthquakes that form

part of a sequence over time tremblor - mainly used by media

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Why Do Earthquakes Occur?

Short answer: Most earthquakes occur naturally as a consequence of the cooling of the Earth. Earth was very hot when it formed 4.5 billion years ago.

Long answer: involves plate tectonics and mantle convection, which we will thoroughly discuss later in the class.

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Why Do Earthquakes Occur?

Some earthquakes are actually induced by human activity.

These earthquakes are very rare and quite small.

The most common type of induced seismicity has to do with the injection of high pressure waste fluid into the Earth

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How are Earthquakes How are Earthquakes Described?Described?

By Location:By Location:– We use two numbers to describe where an We use two numbers to describe where an

earthquake has occurred: latitude and longitudeearthquake has occurred: latitude and longitude– Latitude varies from 90˚ S to 90˚ N, or -90 to +90Latitude varies from 90˚ S to 90˚ N, or -90 to +90– Longitude varies from 180˚ W to 180˚ E, or Longitude varies from 180˚ W to 180˚ E, or

-180˚ to + 180˚-180˚ to + 180˚– These two numbers together are known as the These two numbers together are known as the

earthquake’s epicenterearthquake’s epicenter

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By Location:By Location:– Distances from earthquake epicenters to seismic Distances from earthquake epicenters to seismic

stations are usually given in terms of degreesstations are usually given in terms of degrees– 1 degree is about 111 km. How do you figure this 1 degree is about 111 km. How do you figure this

out ? (Hint: Earth’s radius is 6371 km)out ? (Hint: Earth’s radius is 6371 km)– Distance is given along great circle paths (the same Distance is given along great circle paths (the same

paths that airplanes take)paths that airplanes take)– Why was Steve Fossett’s trip bogus ? Why was Steve Fossett’s trip bogus ?

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By Location:By Location:– It turns out that earthquakes often occur beneath the It turns out that earthquakes often occur beneath the

earth’s surface at various depthsearth’s surface at various depths– When we combine the earthquake’s epicenter with its When we combine the earthquake’s epicenter with its

depth we call it the hypocenterdepth we call it the hypocenter– Another word for hypocenter is focusAnother word for hypocenter is focus– The world’s deepest earthquakes occur at a depth of The world’s deepest earthquakes occur at a depth of

about 700 kmabout 700 km

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By Time:By Time:– The time when an earthquake starts is known as The time when an earthquake starts is known as

the origin timethe origin time– Origin times are reported in terms of Greenwich Origin times are reported in terms of Greenwich

Mean Time (GMT)Mean Time (GMT)– Sometimes the day and month are combined into Sometimes the day and month are combined into

the julian daythe julian day– Example: Example:

2002 073 20:23:14.2 , means March 14, 2002 at 2002 073 20:23:14.2 , means March 14, 2002 at 3:23:14.2 pm (St. Louis time)3:23:14.2 pm (St. Louis time)

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By size:By size:– The oldest method for describing the size of an The oldest method for describing the size of an

earthquake is its maximum intensity (amount of earthquake is its maximum intensity (amount of damage caused)damage caused)

– These methods were used extensively in the past These methods were used extensively in the past before seismometers had been invented before seismometers had been invented (seismometers have been around for only about 100 (seismometers have been around for only about 100 years)years)

– The most well known intensity scale is known as the The most well known intensity scale is known as the modified Mercalli intensity scale. It varies from I for modified Mercalli intensity scale. It varies from I for very small damage to XII for very extensive damagevery small damage to XII for very extensive damage

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By size:By size:

Definition of modified Mercalli intensity V:Definition of modified Mercalli intensity V:

Felt by nearly everyone, many awakened. Some Felt by nearly everyone, many awakened. Some dishes, windows, and so on broken; cracked dishes, windows, and so on broken; cracked plaster in a few places; unstable objects plaster in a few places; unstable objects overturned. Disturbances of trees, poles, and overturned. Disturbances of trees, poles, and other tall objects sometimes noticed. Pendulum other tall objects sometimes noticed. Pendulum clocks may stop.clocks may stop.

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By size:By size:– The more modern method of describing The more modern method of describing

earthquake size is by magnitudeearthquake size is by magnitude– Magnitudes are related to the size of the Magnitudes are related to the size of the

waves that are recorded by seismometerswaves that are recorded by seismometers– Magnitudes are much more accurate than Magnitudes are much more accurate than

maximum intensities in describing maximum intensities in describing earthquake sizeearthquake size

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By size:By size:– Magnitudes are defined with a logarithmic scaleMagnitudes are defined with a logarithmic scale– That means that 1 unit is really a factor of tenThat means that 1 unit is really a factor of ten– For instance, a magnitude 5.0 earthquake has For instance, a magnitude 5.0 earthquake has

10 times larger seismic waves than a magnitude 10 times larger seismic waves than a magnitude 4.0 earthquake4.0 earthquake

– In terms of energy (different than amplitude) a In terms of energy (different than amplitude) a 5.0 is 30 times bigger than a 4.05.0 is 30 times bigger than a 4.0

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By size:By size:– There are four main magnitude scales There are four main magnitude scales

used by seismologists.used by seismologists.– The Richter scale is never used anymore. The Richter scale is never used anymore.

It is strictly valid only for earthquakes in It is strictly valid only for earthquakes in southern California.southern California.

– It is impossible to convince journalists of It is impossible to convince journalists of this fact.this fact.

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By size:By size:– The four main magnitude scales are:The four main magnitude scales are:

Ml (local magnitude)Ml (local magnitude)

Mb (body wave magnitude)Mb (body wave magnitude)

Ms (surface wave magnitude)Ms (surface wave magnitude)

Mw (moment magnitude)Mw (moment magnitude)

– Each one has pluses and minuses. We will Each one has pluses and minuses. We will discuss them in detail later in this class.discuss them in detail later in this class.

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Where Do Earthquakes Occur ?Where Do Earthquakes Occur ?

Earthquakes occur in along geological Earthquakes occur in along geological structure known as faults.structure known as faults.

A fault is a fracture in a rock a that has A fault is a fracture in a rock a that has been offset.been offset.

Faults generally occur in groups known as Faults generally occur in groups known as fault zones.fault zones.

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Where do Earthquakes Occur ? Faults may be 100’s to 1000’s of km long,

10’s of km deep, but are generally relatively narrow.

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How Often Do Earthquakes Occur? Earthquakes are quite common:

Earthquakes occur in the outer 700 km of Earthquakes occur in the outer 700 km of Earth and most are located in well-defined Earth and most are located in well-defined belts.belts.

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How Often Do Earthquakes Occur?

Earthquakes seldom occur alone, but are generally part of a sequence of events consisting of a mainshock and aftershocks (occasionally foreshocks also occur).

Generally aftershocks are usually much smaller than the mainshock - but can still be damaging in large sequences.

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Larger mainshocks tend to have larger aftershocks and longer aftershock sequences

The decay rate of aftershocks is given by Omori’s Law:

earthquake frequency ~ 1/time

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Some earthquakes repeat on the same fault at periodic intervals.

These earthquakes are called characteristic earthquakes and the time between such events is called the recurrence interval.

Typical recurrence intervals are hundreds to thousands of years.

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How Do We Record How Do We Record Earthquakes?Earthquakes?

Devising systems that can accurately Devising systems that can accurately measure and record ground motions is a measure and record ground motions is a difficult task.difficult task.

Large amounts of time and effort are spent Large amounts of time and effort are spent on developing and improving seismic on developing and improving seismic recording systems because of the recording systems because of the enormous value of seismic data.enormous value of seismic data.

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seismoscopeseismoscope – an instrument that documents the – an instrument that documents the occurrence of ground motion (but does not record it occurrence of ground motion (but does not record it over time)over time)

seismometerseismometer – an instrument that senses ground motion – an instrument that senses ground motion and converts the motion into some form of signaland converts the motion into some form of signal

accelerometeraccelerometer – a seismometer that records acceleration, – a seismometer that records acceleration, also known as strong ground motionalso known as strong ground motion

geophonegeophone – another name for a seismometer, commonly – another name for a seismometer, commonly used in active source seismologyused in active source seismology

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seismographseismograph – a system of instruments that – a system of instruments that detects detects and recordsand records ground motion as a ground motion as a function of timefunction of time

seismogramseismogram – the actual record of ground motion – the actual record of ground motion produce by a seismographproduce by a seismograph

seismometryseismometry – the design and development of – the design and development of seismic recording systemsseismic recording systems

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•First seismoscope!

• Constructed by Chang Heng (a Chinese philosopher) around 130 AD.

• Ball would roll out of dragon’s mouth into frog’s mouth in the direction of ground motion.

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132 132 – first seismoscope (Heng, China)– first seismoscope (Heng, China)

17511751 – seismoscope which etched in sand (Bina, Italy) – seismoscope which etched in sand (Bina, Italy)

1784 1784 – first attempt to record ground motion as a function – first attempt to record ground motion as a function of time using a series of seismoscopes (Cavalli, Italy)of time using a series of seismoscopes (Cavalli, Italy)

18751875 – first true seismograph (Cecchi, Italy) – first true seismograph (Cecchi, Italy)

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18891889 – first known seismogram from a distant earthquake – first known seismogram from a distant earthquake is generated (Rebeur-Paschwitz, Germany)is generated (Rebeur-Paschwitz, Germany)

19141914 – first seismometer to use electromagnetic transducer – first seismometer to use electromagnetic transducer to sense ground motion (Galitzin, Russia)to sense ground motion (Galitzin, Russia)

19691969 – first digital seismograph (data recorded in discrete – first digital seismograph (data recorded in discrete samples on a magnetic tape) (U.S. researchers)samples on a magnetic tape) (U.S. researchers)

1990s1990s – broadcast of real time seismic data via internet – broadcast of real time seismic data via internet

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The first SLU seismometer was installed in 1909 (in The first SLU seismometer was installed in 1909 (in basement of DuBourg Hall)basement of DuBourg Hall)

It was part of the It was part of the firstfirst North American seismological North American seismological network – the network – the Jesuit Seismological NetworkJesuit Seismological Network

The first geophysics department in the Western The first geophysics department in the Western hemisphere was established at SLU in 1925, by J.B. hemisphere was established at SLU in 1925, by J.B. Macelwane, S.J.Macelwane, S.J.

At SLU in 1948 Florence Robinson became the first At SLU in 1948 Florence Robinson became the first woman geophysicist to obtain a PhD in North Americawoman geophysicist to obtain a PhD in North America

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Can We Predict Earthquakes ?Can We Predict Earthquakes ?

No!! There is no proven, reliable method No!! There is no proven, reliable method for predicting earthquakes.for predicting earthquakes.

A method must provide statistically A method must provide statistically significant results.significant results.

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Example:Example:– Every day I could predict a magnitude 5.5 Every day I could predict a magnitude 5.5

earthquake in Southern California and earthquake in Southern California and eventually I would be right and I will have eventually I would be right and I will have “predicted” the earthquake.“predicted” the earthquake.

– But this would not be statistically significant.But this would not be statistically significant.

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Because earthquakes cannot be predicted Because earthquakes cannot be predicted most of the work is directed toward most of the work is directed toward mitigating the hazard from earthquakes.mitigating the hazard from earthquakes.

However, there are legitimate scientists However, there are legitimate scientists who work on methods of predicting who work on methods of predicting earthquakes. earthquakes.

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Some of the things people study for Some of the things people study for possible use in predicting earthquakes:possible use in predicting earthquakes:

– Seismicity patternsSeismicity patterns– Electromagnetic precursorsElectromagnetic precursors– Variations in depth of the water tableVariations in depth of the water table– Variations in radon concentration in well water Variations in radon concentration in well water

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Why Study Earthquakes?Why Study Earthquakes?

Large earthquakes can be catastrophic to Large earthquakes can be catastrophic to cultures and societies:cultures and societies:

– Can this damage be mitigated in the future? Can this damage be mitigated in the future? (Yes)(Yes)

– Can earthquakes be predicted? (Probably not)Can earthquakes be predicted? (Probably not)

– Can earthquakes be prevented? (VERY unlikely)Can earthquakes be prevented? (VERY unlikely)

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Earthquakes are the dominant source of Earthquakes are the dominant source of seismic wavesseismic waves

Analysis of seismic waves gives us the Analysis of seismic waves gives us the best view into the interior of the Earth:best view into the interior of the Earth:

Seismologists use seismic waves to image the Seismologists use seismic waves to image the Earth in the same way that doctors use ultrasound Earth in the same way that doctors use ultrasound to image unborn babiesto image unborn babies

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Techniques used in studying earthquakes Techniques used in studying earthquakes can be applied to studying nuclear can be applied to studying nuclear weapons tests.weapons tests.

This is called This is called verification seismologyverification seismology and and aims to answer questions like:aims to answer questions like:

– Did nation X test a nuclear device?Did nation X test a nuclear device?– How big was the weapon nation Y tested?How big was the weapon nation Y tested?

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Who Studies Earthquakes ?Who Studies Earthquakes ?

• Earthquake science is broad and Earthquake science is broad and interdisciplinary and is studied by those in interdisciplinary and is studied by those in fields such as:fields such as:• Seismology, geophysics, geochemistry, Seismology, geophysics, geochemistry,

geology, and engineeringgeology, and engineering• Economics, history, and sociologyEconomics, history, and sociology• Students in EAS-A193 at SLUStudents in EAS-A193 at SLU

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What to do in an earthquake …What to do in an earthquake …

If you are in bed:If you are in bed:– Remain in bed until the shaking stopsRemain in bed until the shaking stops

– Hold onHold on

– Protect your head with pillowProtect your head with pillow

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If you are driving:If you are driving:– Slow downSlow down

– Drive to a clear area and stopDrive to a clear area and stop

– Remain in the car until the shaking stopsRemain in the car until the shaking stops

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If you are outdoors:If you are outdoors:– Find a clear spot away from tree, power lines, Find a clear spot away from tree, power lines,

and buildingsand buildings– Drop to the groundDrop to the ground

If you are near a tall building:If you are near a tall building:– duck into a doorway to avoid debris from duck into a doorway to avoid debris from

aboveabove

1 Introduction to Earthquakes مقدمة علم الزلازل. 2 Topics What are earthquakes ? Why...

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