Earthquake

Earthquakes occur when energy stored in elastically strained rocks is suddenly released. This release of energy causes intense ground shaking in the area near the source of the earthquake and sends waves of elastic energy, called seismic waves, throughout the Earth. Earthquakes can be generated by bomb blasts, volcanic eruptions, and sudden slippage along faults. Earthquakes are definitely a geologic hazard for those living in earthquake prone areas, but the seismic waves generated by earthquakes are invaluable for studying the interior of the Earth.

Origin of Earthquakes

Most natural earthquakes are caused by sudden slippage along a fault zone. The elastic rebound theory suggests that if slippage along a fault is hindered such that elastic strain energy builds up in the deforming rocks on either side of the fault, when the slippage does occur, the energy released causes an earthquake. This theory was discovered by making measurements at a number of points across a fault. Prior to an earthquake it was noted that the rocks adjacent to the fault were bending. These bends disappeared after an earthquake suggesting that the energy stored in bending the rocks was suddenly released during the earthquake.

Seismology, The Study of Earthquakes

When an earthquake occurs, the elastic energy is released and sends out vibrations that travel throughout the Earth. These vibrations are called seismic waves. The study of how seismic waves behave in the Earth is called seismology.

Seismographs - Seismic waves travel through the Earth as vibrations. A seismometer is an instrument used to record these vibrations and the resulting graph that shows the vibrations is called a seismograph. The seismometer must be able to move with the vibrations, yet part of it must remain nearly stationary.

This is accomplished by isolating the recording device (like a pen) from the rest of the Earth using the principal of inertia. For example, if the pen is attached to a large mass suspended by a spring, the spring and the large mass move less than the paper which is attached to the Earth, and on which the record of the vibrations is made.

Seismic Waves (freeware simulation 3.39Megs). The source of an earthquake is called the focus, which is an exact location within the Earth were seismic waves are generated by sudden release of stored elastic energy. The epicenter is the point on the surface of the Earth directly above the focus. Sometimes the media get these two terms confused. Seismic waves emanating from the focus can travel in several ways, and thus there are several different kinds of seismic waves.

Types of Seismic Waves

Body Waves - emanate from the focus and travel in all directions through the body of the Earth.

There are two types of body waves:

• P - waves - are Primary waves. They travel with a velocity that depends on the elastic properties of the rock through which they travel.

Vp = Ö [(K + 4/3m )/r ]

Where, Vp is the velocity of the P-wave, K is the incompressibility of the material, m is the rigidity of the material, and r is the density of the material.

S-Waves - Secondary waves, also called shear waves. They travel with a velocity that depends only on the rigidity and density of the material through which they travel:

Vs = Ö [( m )/r ]

• S-waves travel through material by shearing it or changing its shape in the direction perpendicular to the direction of travel. The resistance to shearing of a material is the property called the rigidity. It is notable that liquids have no rigidity, so that the velocity of an S-wave is zero in a liquid. (This point will become important later). Note that S-waves travel slower than P-waves, so they will reach a seismograph after the P-wave.

• Surface Waves - Surface waves differ from body waves in that they do not travel through the Earth, but instead travel along paths nearly parallel to the surface of the Earth. Surface waves behave like S-waves in that they cause up and down and side to side movement as they pass, but they travel slower than S-waves and do not travel through the body of the Earth.

The record of an earthquake, a seismograph, as recorded by a seismometer, will be a plot of vibrations versus time. On the seismograph, time is marked at regular intervals, so that we can determine the time of arrival of the first P-wave and the time of arrival of the first S-wave.

P-waves are the same thing as sound waves. They move through the material by compressing it, but after it has been compressed it expands, so that the wave moves by compressing and expanding the material as it travels. Thus the velocity of the P-wave depends on how easily the material can be compressed (the incompressibility), how rigid the material is (the rigidity), and the density of the material. P-waves have the highest velocity of all seismic waves and thus will reach all seismographs first.

Because P-waves have a higher velocity than S-waves, the P-waves arrive at the seismographic station before the S-waves.

Location of Earthquakes

Location of Earthquakes - In order to determine the location of an earthquake, we need to have recorded a seismograph of the earthquake from at least three seismographic stations at different distances from the epicenter of the quake. In addition, we need one further piece of information - that is the time it takes for P-waves and S-waves to travel through the Earth and arrive at a seismographic station. Such information has been collected over the last 80 or so years, and is available as travel time curves.

From the seismographs at each station one determines the S-P interval (the difference in the time of arrival of the first S-wave and the time of arrival of the first P-wave. Note that on the travel time curves, the S-P interval increases with increasing distance from the epicenter. Thus the S-P interval tells us the distance to the epicenter from the seismographic station where the earthquake was recorded. Thus at each station we can draw a circle on a map that has a radius equal to the distance from the epicenter.
Three such circles will intersect in a point that locates the epicenter of the earthquake.

Geology is the study of the Earth and its history

* Geologic Processes effect every human on the Earth all of the time, but are most noticeable when they cause loss of life or property. Such life or property threatening processes are called natural disasters. Among them are:
o Earthquakes
o Eruptions of Volcanoes
o Tsunamis
o Landslides
o Subsidence
o Floods
o Droughts
o Hurricanes
o Tornadoes
o Meteorite Impacts

# All of these processes have existed throughout Earth history, but the processes have become hazardous only because they negatively affect us as human beings. Important Point - There would be no natural disasters if it were not for humans. Without humans these are only natural events.

# Risk is characteristic of the relationship between humans and geologic processes. We all take risks everyday. The risk from natural disasters, while it cannot be eliminated, can, in some cases be understood in a such a way that we can minimize the hazard to humans, and thus minimize the risk. To do this, we need to understand something about the processes that operate, and understand the energy required for the process. Then, we can develop an action to take to minimize the risk. Such minimization of risk is called hazard mitigation.

Although humans can sometimes influence natural disasters (for example when road construction sets off a landslide), other disasters that are directly generated by humans, such as oil and toxic material spills, pollution, massive automobile or train wrecks, airplane crashes, and human induced explosions, are considered technological disasters, and will not be considered in this course.

# Some of the questions we hope to answer for each possible natural disaster are:

* Where is each type of disaster likely to occur and why?
* How often do these disasters occur?
* How can each type of disaster be predicted and/or mitigated?


The Earth in the Solar System

The Solar System

* The Earth is one of nine planets in the solar system

* In addition to the planets, many smaller bodies called asteroids, comets, meteoroids are present.

* All objects in the solar system orbit around the Sun.

* The four planets closest to the Sun (Mercury, Venus, Earth, and Mars) have high densities because they are mostly composed of rock, and are called the Terrestrial Planets.

The five planets outside the orbit of Mars (Jupiter, Saturn, Uranus, Neptune, and Pluto) have low densities because they mostly composed of gases, and are called the Jovian Planets.

Origin of the Solar System
o Original Solar Nebula
o Condensation of the Sun about 6 billion years ago
o Condensation of the Planets about 4.5 billion years ago.
o Process is continuing today, although at a much slower rate.

The Planet Earth

Comparisons Between Earth and the Other Planets
o Earth similar in size density and structure to the terrestrial planets (all have metallic core, high density, composed of rock, with thin to non-existent atmosphere.

o Earth is the only planet with an atmosphere composed of Nitrogen, Oxygen, Carbon Dioxide, and Water Vapor.

o Earth is the only planet that has a hydrosphere, a region on the surface where water can exist in liquid, vapor and solid forms. This is due to the Temperature on the Earth's surface that usually remains between the freezing point of water, 0oC, and the boiling point of water, 100oC. Temperature on the Earth is controlled by the distance from the Sun and by the atmosphere of the Earth, which tends to moderate temperature variation.

o Earth is the only planet with a biosphere, (life sphere) which is made up of all living matter. The biosphere exists because of the Earth's temperature, and because of the atmosphere. Oxygen is present in the atmosphere because of the biosphere.
o Earth is the only planet with a regolith. Regolith is a thin covering of loose rock debris that has formed as a result of a process called weathering. Weathering is the mechanical and chemical response of interactions between the rocks of the Earth and its hydrosphere, atmosphere, and biosphere. While other planets have something resembling regolith, most formed as a result of meteorite impacts which have mechanically broken the surface into loose fragments of rock. The Earth is unique in that other processes have occurred to produce a more varied regolith.
Interior Structure of Earth
o The Earth has a radius of about 6371 km, although it is about 22 km larger at equator than at poles.

o Density, (mass/volume), Temperature, and Pressure increase with depth in the Earth.

o The Earth has a layered structure. This layering can be viewed in two different ways (1) Layers of different chemical composition and (2) Layers of differing physical properties.
* Compositional Layering
o Crust - variable thickness and composition
+ Continental 10 - 70 km thick
+ Oceanic 8 - 10 km thick
o Mantle - 3488 km thick, made up of a rock called peridotite.


+ Core - 2883 km radius, made up of Iron (Fe) and small amount of Nickel (Ni)