Chapter 6: Earthquakes

What are the visible signs of an approaching tsunami on a beach?

(I) A sudden withdrawl of water back into the ocean. (II) A sudden darkening of the horizon because of the huge amount of water vapor formed by the churning of the ocean water.

What are the two events that can cause a tsunami?

(I) An earthquake with its epicenter on the ocean floor. (II) Underwater faulting and land slides.

List four other dangers associated with earthquakes.

(I) Landslides. (II) Fires. (III) Explosions caused by broken electric and gas lines. (IV) Flood waters released from collapsing dams.

What safety rules must be followed in each of the following three situations to prevent death, injury, and property damage?

(a) Before an earthquake- Keep on hand a supply of canned food, bottled water, flashlights, batteries, portable radio and learn how to turn off the gas, water, and electricity in your home. (b) During an earthquake- Run outdoors into an open space. If you are indoors protect yourself from falling debris by standing in a doorway or crouching under a desk or a table. If you are in a car stop in a place away from tall buildings, tunnels, power lines, and bridges. (c) After an earthquake- Put on your shoes, avoid downed power-lines, glass, and leaning structures. Switch on your radio for updates.

The following are the three major earthquake zones of the world. State the cause of earthquakes occurring at each of these three zones: Pacific Ring of Fire, Mid Ocean Ridge, and Eurasian (Melanesian Mountain Belt).

(a) Pacific Ring of Fire- Most plates are being subducted or they scrape past each other. (b) Mid Ocean Ridge- Oceanic crust is pulling away from both sides of the mid-ocean- ridge. (c) Eurasian - Melanesian mountain belt-The plates are colliding.

What are the three steps that scientists use to locate the epicenter of an earthquake?

(i) Scientists measure the distance between the epicenter and three seismograph stations. (ii) With this distance as the radius three circles are drawn on a map. (iii) The epicenter of the earthquake is located very near the point at which the three circles intersect.

On the Mercalli scale what is the intensity or amount of damage caused by earthquakes of the following ratings: II, X, XII.

-> II: Felt only by a few persons at rest, especially on upper floor of buildings. -> X: Some well-built wooden structures destroyed, most masonry and frame structures destroyed with foundations, ground badly cracked, and rails bent. -> XII: Total Destruction.

What is the magnitude of the following earthquakes on the Richter scale: Major, Moderate, Minor, and Miro-quake.

-> Major: 7 or above -> Moderate: Between 6 and 7. -> Minor: Between 2.5 and 6. -> Microquake: Less than 2.5.

Differentiate between the following: Shallow Focus, Intermediate Focus, and Deep Focus Earthquakes.

->Shallow focus earthquakes- Occur within 70 km of the earth's surface. -> Intermediate focus earthquakes- Occur at a depth of 70 km and 300 km. ->Deep focus earthquakes- Occur at a depth of 300 km to 650 km.

State seven early warning signs that can help us predict future earthquakes?

1. Slight tilting of the ground. 2.Strain and cracks in rocks. 3. Change in magnetic and electrical properties of rocks. 4. Changes in level of water in wells. 5. Seepage of natural gas from strained and fractured rocks. 6. A sudden decrease in speed of P- waves and then returning to normal just before the earthquake strikes. 7. Development of seismic gaps.

When a fault is said to be locked? What happens when the stress on the rocks in a locked fault becomes great?

A fault in an immobile state is said to be locked. When the stress exceeds a certain limit the rocks suddenly grind past each other or slip causing the trembling and vibrations of an earthquake.

What do you understand by the term magnitude of an earthquake?

A measure of energy released by an earthquake and is also described as the amount of ground motion.

What is the role of duration of an earthquake in causing damage?

A moderate earthquake that continues for a long time often causes more damage than an earthquake of higher magnitude that lasts only a short time.

What are seismic gaps? How do they help us in predicting future earthquakes?

A seismic gap is a place where the fault is locked and unable to move. Small movements along seismic gaps can indicate an inevitable catastrophe.

What are the steps in the loading of data by a seismograph into a computer?

A seismograph translates earthquake vibrations into electronic signals which are recorded on a magnetic tape or directly loaded into the computer that analyzes the seismic waves.

What are after shocks?

A series of smaller tremors that follow a major earthquake are called aftershocks.

What is a tsunami? How is it created?

A tsunami is a giant ocean wave created by an earthquake that has its epicenter on the ocean floor. An earthquake on the ocean floor may cause faulting and underwater landslides causing the ocean floor to drop or rise. A large mass of sea water also drops or rises with the ocean floor causing it to churn and adjust to the change in sea level. This violent water movement sets into motion a series of long, low waves that develop into tsunamis.

What is a fault zone?

A zone with a group of interconnected faults is called a fault zone.

If an earthquake occurs in the center of Brazil, what can you infer about the geology of that area?

An ancient fault zone deep within the crust might be located there.

How does animal behavior change just minutes before an earthquake?

Animals like dogs, cats, horses and birds get nervous and restless.

Why do P waves travel faster through the lithosphere than through the asthenosphere?

Because lithosphere is cold and is made up of more rigid rock than the asthenosphere which is hotter and hence more plastic.

Why loose soil as compared to solid ground is not a good choice for the construction of a building?

During an earthquake, the loose soil and rock can vibrate like jelly causing the buildings constructed on top of this kind of ground experience exaggerated motion, they sway violently and collapse.

What is the elastic rebound theory?

In a locked fault the stress goes on increasing and when it passes a certain limit, the rocks fracture, they separate at their weakest point, and spring back to their original shape, or rebound.

What is the safest place in the event of an earthquake?

In an open level field than in a city of skyscrapers.

What is PTWC? What is its role in the event of an earthquake?

It is the Pacific Tsunami Warning Center. It alerts scientists to the location and magnitude of earthquakes. It issues warnings regarding the estimated arrival time of a tsunami at different locations.

What is the major cause of injuries and death during an earthquake?

Most injuries and deaths result from the collapse of buildings and other structures or from falling objects and flying glass, rather than from the movement of the ground.

What caused the 1812 earthquake in New Madrid, Missouri?

Movement of rocks along an ancient, hidden and buried fault zone formed 600 million years ago in the Mississippi river basin.

Describe the three separate sensing devices of a seismograph.

One device records the vertical motion of the ground, the other two record the horizontal motion in the east-west and north-south directions.

Differentiate between the following three types of seismic waves: P waves, S waves, and Surface Waves (L waves).

P Waves are the fastest, travels through both solid and liquid, particles move together and apart along the direction of the waves, and they cause the least amount of destruction. S waves are slow, travels only through solid, particles move at right angles to the direction in which the waves are traveling, and cause a lot of destruction. Surface Waves are the slowest, travels through both solids and liquids, particles rise and fall like water droplets in an ocean waves and cause maximum destruction.

Name the scale that we use today to measure the magnitude of an earthquake.

Richter Scale.

What are seismic waves?

Rocks while fracturing and slipping along a fault, to new positions, release energy in the form of vibrations called seismic waves.

Give the name and location of a major fault zone in United States.

San Andreas Fault in California.

What type of earthquakes cause most damage and why?

Shallow focus earthquakes because they release most of their energy on the surface.

What type of building construction and location regulations should be included in the building code of a city located near an active fault?

The Materials and designs should be able to withstand the motion caused by earthquakes, and buildings should be located on firm ground.

Define focus and epicenter of an earthquake. Draw a diagram showing the focus and epicenter of an earthquake.

The area along a fault where slippage first occurs is called the focus. The point on the earth's surface directly above the focus is called the epicenter.

What is the use of a seismograph?

To detect and record earthquakes.

What are earthquakes? Why do they occur?

Vibrations of the earth's crust, they usually occur when rocks under stress suddenly shift along a fault.