Lower six

Ms Shalto

Figure showing the tectonic setting of earthquakes

Movement and slipping along plate boundaries can form an earthquake.

• Depending on the type of movement, the earthquakes occur in either a shallow or deep level in the crust.

• The majority of tectonic earthquakes originate at depths not exceeding tens of kilometers.

• Where old and cold oceanic crust descends beneath another tectonic plate, “Deep Focus Earthquakes” may occur at much greater depths (up to seven hundred kilometers!).

• These earthquakes occur at a depth at which the

subducted crust should no longer be brittle, due to the high temperature and pressure. A possible mechanism for the generation of deep focus earthquakes is faulting.

• Earthquakes may also occur in volcanic regions and are caused there both by tectonic faults and by the movement of magma (hot molten rock) within the volcano. Such earthquakes can be an early warning of volcanic eruptions.

The fastest wave, and therefore the first to arrive at a given location.

Also known as compressional waves, the P wave alternately compresses and expands material in the same direction it is traveling.

Can travel through all layers of the Earth.

Seismic Waves: Body Waves -

Primary (P)

The S wave is slower than

the P wave and arrives next,

shaking the ground up and

down and back and forth

perpendicular to the direction

it is traveling.

Also know as shear waves.

Seismic Waves: Body Waves -

Secondary Waves (S)

They follow the P and S waves. These waves travel along the surface of the earth

Also known as:

Rayleigh waves , also called ground roll, travel like ocean waves over the surface of the Earth, moving the ground surface up and down. They cause most of the shaking at the ground surface during an earthquake.

Love waves are fast and move the ground from side to side.

Seismic Waves:

Surface Waves

Intensity scales measure the amount of shaking at a particular location.

The intensity of an earthquake will vary depending on where you are.

Magnitude scales, like the Richter magnitude and moment magnitude, measure the size of the earthquake at its source.

Magnitude does not depend on where the measurement of the earthquake is made.

On the Richter scale, an increase of one unit of magnitude (for example, from 4.6 to 5.6) represents a 10-fold increase in wave amplitude on a seismogram or approximately a 30-fold increase in the energy released.

I. Not felt except by a very few under especially favorable conditions.

II. Felt only by a few persons at rest, especially on upper floors of buildings.

III. Felt quite noticeably by persons indoors, especially on upper floors of buildings. Many people do not recognize it as an earthquake. Standing motor cars may rock slightly. Vibrations similar to the passing of a truck. Duration estimated.

IV. Felt indoors by many, outdoors by few during the day. At night, some awakened. Dishes, windows, doors disturbed; walls make cracking sound. Sensation like heavy truck striking building. Standing motor cars rocked noticeably.

V. Felt by nearly everyone; many awakened. Some dishes, windows broken. Unstable objects overturned. Pendulum clocks may stop.

VI. Felt by all, many frightened. Some heavy furniture moved; a few instances of fallen plaster. Damage slight.

VII. Damage negligible in buildings of good design and construction; slight to moderate in well-built ordinary structures; considerable damage in poorly built or badly designed structures; some chimneys broken.

VIII. Damage slight in specially designed structures; considerable damage in ordinary substantial buildings with partial collapse. Damage great in poorly built structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furniture overturned.

IX. Damage considerable in specially designed structures; well-designed frame structures thrown out of plumb. Damage great in substantial buildings, with partial collapse. Buildings shifted off foundations.

X. Some well-built wooden structures destroyed; most masonry and frame structures destroyed with foundations. Rails bent.

XI. Few, if any (masonry) structures remain standing. Bridges destroyed. Rails bent greatly.

XII. Damage total. Lines of sight and level are distorted. Objects thrown into the air.

The vibrations produced by earthquakes are detected, recorded, and measured by instruments call seismographs.

The zig-zag line made by a seismograph, called a "seismogram," reflects the changing intensity of the vibrations by responding to the motion of the ground surface beneath the instrument.

From the data expressed in seismograms, scientists can determine the time, the epicenter, the focal depth, and the type of faulting of an earthquake and can estimate how much energy was released.

An oceanic spreading ridge is the fracture zone along the ocean bottom where molten mantle material comes to the surface, thus creating new crust.

This fracture can be seen beneath the ocean as a line of ridges that form as molten rock reaches the ocean bottom and solidifies.

Major earthquakes may occur along subduction zones.

The most recent sub-duction zone type earth-quake occurred in 1700.

Scientists believe, on average, one subduction zone earthquake occurs every 300-600 years.

A transform fault is a special variety of strike-slip fault that accom-modates relative horizontal slip between other tectonic elements, such as oceanic crustal plates.

Intraplate seismic activity occurs in the interior of a tectonic plate.

Intraplate earthquakes are rare compared to those located at plate boundaries.

Very large intraplate earthquakes can inflict very heavy damage.

Distribution of seismicity associated with the New Madrid Seismic Zone since 1974.

Buckled roads and rail tracks

Structural Damage

Landslides Avalanches

Alterations to Water Courses Fire resulting from an earthquake

Earthquake activity beneath a volcano almost always increases before an eruption because magma and volcanic gas must first force their way up through shallow underground fractures and passageways. When magma and volcanic gases or fluids move, they will either cause rocks to break or cracks to vibrate. When rocks break, high-frequency earthquakes are triggered. However, when cracks vibrate either low-frequency earthquakes or a continuous shaking called volcanic tremor is triggered.

Satellites can record infrared radiation where more heat or less heat shows up as different colors on a screen. When a volcano becomes hotter, an eruption may be coming soon.

New Madrid, Tennessee

San Andreas Faultline

Scientists consider seismic activity as it is registered on a seismometer.

A volcano will usually register some small earthquakes as the magma pushes its way up through cracks and vents in rocks as it makes its way to the surface of the volcano.

As a volcano gets closer to erupting, the pressure builds up in the earth under the volcano and the earthquake activity becomes more and more frequent.

Below is a digital seismogram. The data is stored electronically, easy to access and manipulate, and much more accurate and detailed than the analog recordings.

Tiltmeters attached to the sides of a volcano detect small changes in the slope of a volcano.

When a volcano is about to erupt, the earth may bulge or swell up a bit. Installing a tiltmeter

Hydrogeologic responses to large distant earthquakes have important scientific implications with regard to our earth’s intricate plumbing system.

The exact mechanism linking hydrogeologic changes and earthquakes is not fully understood, but monitoring these changes improves our insights into the responsible mechanisms, and may improve our frustratingly imprecise ability to forecast the timing, magnitude, and impact of earthquakes.

The cause of unusual animal behavior seconds before humans feel an earthquake can be easily explain-ed.  Very few humans notice the smaller P wave that travels the fastest from the earthquake source and arrives before the larger S wave. But many animals with more keen senses are able to feel the P wave seconds before the S wave arrives.

If in fact there are precursors to a significant earthquake that we have yet to learn about (such as ground tilting, groundwater changes, electrical or magnetic field variations), indeed it’s possible that some animals could sense these signals and connect the perception with an impending earthquake.

Tsunamis can be generated by:

Large Earthquakes (megathrust events such as Sumatra, Dec. 26, 2004)

Underwater or near-surface volcanic eruptions (Krakatoa, 1883)

Comet or asteroid impacts (evidence for tsunami deposits from the Chicxulub impact 65 mya)

Large landslides that extend into water (Lituya Bay, AK, 1958)

Large undersea landslides (evidence for prehistoric undersea landslides in Hawaii and off the east coast of North America)

Tsunami wave propagation characteristics – note that as water depth becomes smaller, waves slow down, become shorter wavelength, and have larger amplitude.

A tsunami warning system is a system to detect tsunamis and issue warnings to prevent loss of life and property.

It consists of two equally important components: (1) a network of sensors to detect tsunamis and

(2) a communications infrastructure to issue timely alarms to permit evacuation of coastal areas.

Tsunami Monitoring Buoy: Reports rises in the water column and tsunami events

Near the source of sub-marine earthquakes, the seafloor is "permanently" uplifted and down-dropped, pushing the entire water column up and down.

The potential energy that results from pushing water above mean sea level is then transferred to horizontal propagation of the tsunami wave (kinetic energy).

Within several minutes of the earthquake, the initial tsunami is split into a tsunami that travels out to the deep ocean (distant tsunami) and another tsunami that travels towards the nearby coast (local tsunami).

Several things happen as the local tsunami travels over the continental slope. Most obvious is that the amplitude increases. In addition, the wavelength decreases. This results in steepening of the leading wave--an important control of wave runup at the coast.

Tsunami runup occurs when a peak in the tsunami wave travels from the near-shore region onto shore.

Runup is a measure-ment of the height of the water onshore observed above a reference sea level.