Chapter 5: Earthquakes and Related Hazards Flashcards
How do earthquakes occur?
By definition, earthquake is the shaking or trembling of the surface of the Earth which is caused by the sudden release of energy stored in the rocks beneath the earth’s surface.
How did the scientist develop the theory that explains how earthquakes occur?
based on the elastic properties of rocks
this process is when rocks are subjected under a force (stress), they can become deformed and have a corresponding change in their shape (distortion) or volume (dilation).
Strain
change in volume or shape
rocks are considered to be this type meaning that if the force (stress) is removed they will return to their original shape.
Elastic
A property of all elastic materials which is the maximum amount of strain they can accumulate before either fracturing or undergoing plastic deformation.
Elastic Limit
What happens when brittle materials reach their elastic limit?
they will undergo permanent deformation by fracturing
A fracture is any separation in a geologic formation, such as a joint or a fault that divides the rock into two or more pieces. A fracture will sometimes form a deep fissure or crevice in the rock. Fractures are commonly caused by stress exceeding the rock strength, causing the rock to lose cohesion along its weakest plane.
What happens when ductile materials reach their elastic limit?
they will undergo permanent deformation by **flowing plastically **
Two types of Earthquake
- Volcanic
- Tectonic
type of earthquake due to volcanic activity (eruption or rising magma under a volcano)
Volcanic
type of earthquake due to movement of rocks past one another along faults. when a rock breaks, waves of energy are sent out or produced, known as seismic waves, causing earthquakes.
Tectonic
Will both iron and wooden rods will deform and return to their original shape? How?
Yes, as long as their elastic limit is not exceeded.
What happens when an iron rod exceeds its elastic limit?
It deforms permanently by bending.
What happens when a wooden rod exceeds its elastic limit?
it will suddenly break by fracturing, releasing energy in the form of vibrational waves.
Note: when fractured rods break, the separate pieces rebound and become straight again.
This theory holds that earthquakes originate whena a force (stress) acts on a rock body, causing it to deform and accumulate strain. Eventually the rock reaches its elastic limit, at which point it ruptures or fails suddenly, releasing the strain it had accumulated.
Elastic Rebound Theory
Where the release of energy generally begins at this point
Focus/Hypocenter
point on the earth’s surface directly above the hypocenter
Epicenter
What happens when rocks become more ductile (less brittle)?
they tend to accumulate less strain, and instead undergo plastic deformation.
this is the reason why earthquakes do not occur deeper than 435 miles (700 km) below the surface.
Why are there no earthquakes occuring at deeper than 435 miles (700 km) below surface?
because higher temperatures cause the rocks to become so ductile that they deform only by plastic flow, hence they do not rupture.
redistribution of strain commonly produces a series of smaller earthquakes
aftershocks
occurs for days or weeks after the primary earthquake
main shock
are earthquakes that precede larger earthquakes in the same location. Aftershocks are smaller earthquakes that occur in the same general area during the days to years following a larger event or “mainshock.”
Foreshock
An earthquake cannot be identified as a foreshock until after a larger earthquake in the same area occurs.
refer to vibrational waves that travel through solid earth materials which may be magmatic, tectonic, or aritifical in origin.
Seismic Waves
Two classification of Seismic Waves
- Body waves
- Surface waves
travel through the earth’s interior, spreading outward from the hypocenter in all directions (like sound in air). it is subdivided into primary and secondary waves.
Body waves
compressional waves; parallel to direction the wave is travelling, causing rocks to alternately compress and decompress as successive waves pass through
Primary (P) waves
transverse/perpendicular to direction of wave propagation
Secondary (s) waves
waves that travel on earth’s surface away from teh epicenter (like ripples on water); slowest wave (at speed of 10% slower than S-waves), can cause more property damage compared to body waves
Surface waves
two types of surface waves
- rayleigh
- love
known as ground roll, spread to the ground as ripples, similar to rolling waves on the ocean; move both vertically and horizontally in a vertical plane pointed in the direction in which the wave is travelling.
Rayleigh waves
move the ground from side to side in a horizontal plane but at right angles to the direction of propagation.
Love waves
Difference of P-waves to S-wave
- very fast at speeds of 4 to 7km/sec
- first wave to arrive at a station
- Can pass through solid and liquid
Difference of S-waves to P-wave
- slow, at 2 to 5 km/sec
- arrives at a later time than p-wave does
- can pass through solid but not liquid
particulary damaging to the foundations of structures
love waves
propagate through the ground as ripples
rayleigh waves
instrument used to detect seismic waves. determine the amount of ground motion, but does not record the motion.
Seismometer
a seismometer with a recording device that produces a permanent record of earth motion, usually in the form of wiggly line drawn on a moving strip of paper.
Seismograph
the paper record of earth vibration. different waves travel at different rates, so they arrive at seismograph stations in a definite order: first p waves, then s waves and finaly surface waves.
Seismograms
the importance of the analysis of seismograms
can reveal the location and strength of earthquake
Process of locating earthquakes
- p and s waves that start out from the hypocenter
- as they travel, they gradually separate because of their different speeds.
- the interval of the time of arrival between P and S waves increases with increasing distance of the seismic stations from the focus and epicenter; the longer the time, the greater the distance is.
True or False. A single station can record only the direction, not the distance to a quake.
False. A single station can record only the distance, not the direction to a quake.
pinpoints the location of the earthquake
the intersection of the three circles
classification of an earthquake with depth of focus at 0-70 km
Shallow
classification of an earthquake with depth of focus at 70-350 km
Intermediate
classification of an earthquake with depth of focus at 350-670 km
Deep
whereby earthquakes are ranked based on a set of observations most humans could report objectively, particularly the type of damage sustained by buildings.
Mercalli intensity scale
An italian seismologist who developed a means of comparing both modern and historical earthquakes through the use of firsthand human observations during earthquakes (1902).
Giuseppe Mercalli
is a seismic scale used and developed by the Philipine Institute of Volcanology and Seismology (PHIVOLCS) to measure the intensity of an earthquake.
PHIVOLCS Earthquake Intensity Scale (PEIS)
How was the PEIS developed?
developed as a response to the 1990 Luzon Earthquake (magnitude 7.7) and was adtoped in the Philippines in 1996 replacing the Rossi-Forel Intensity Scale.
one of the first seismic scales developed by Michele Stefano de Rossi and Francois-Alphonse Forel in the late 19th century, to reflect earthquake intensities and was used for about two decades by some countries until the introduction of the Mercalli Intensity Scale in 1902.
Rossi-Forel Intensity Scale
are useful because they quantify the amount of ground motion during an earthquake, and the energy that was released when the rocks ruptured.
Magnitude Scale
rates earthquaks based on the size of their seismic waves, as measured by seismographs; governed by amplitude (wave height) and distance.
Richter Magnitude Scale (named after the seismologist, Charles F. Richter)
is more acurate over a wide range of magnitudes and geologic conditions; based on the total amount of energy released and is determined by measuring the surface area of the ruptured fault and how far the land moved along the fault.
Moment Magnitude Scale
