Chapter 8: Earthquakes and Earthquake Hazards

An earthquake measuring 7 on the Richter scale releases about ____ times more energy than an earthquake with a magnitude of 6.

thirty

epicenter

The location of Earth's surface that lies directly above the focus of an earthquake.

For each increase of 1 on the Richter scale, seismic wave amplitude increases _____ times.

ten

Explain why P waves move through solids, liquids and gases. whereas S waves move only through solids.

P waves are transmitted through solids and fluids; the transmitting material need not have any shear strength. S waves are shear waves and can propagate only in materials with a finite, shear strength; thus S waves are transmitted only by solids, not by liquids and gases.

List the major differences between P and S waves.

P waves travel through all materials, whereas S waves are propagated only through solids. Further, in all types of rock, P waves travel faster than S waves.

Distinguish between the Mercalli scale and the Richter scale.

Richter scale is used to measure the degree of an earthquake, while Mercalli scale is used to measure the earthquake's strength. The degree of an earthquake is based on a scale of 1 to 10. The strength is measured by a scale of 1 to 7.

surface waves

Seismic waves that travel along the outer later of Earth.

foreshocks

Small earthquakes that often precede a major earthquake

Which type of seismic wave causes the greatest destruction in shallow focus earthquakes?

Surface waves have much higher amplitudes than body waves (P and S waves) and account for nearly all, dangerous, ground displacements and accelerations associated with nearby or more distant earthquakes. The horizontally-vibrating surface waves in general present more danger to buildings and other structures than surface waves with vertical motion.

tsunami

The Japanese word for a seismic sea wave.

List four factors that affect the amount of destruction causes by seismic vibrations.

The amplitude of the ground displacement or acceleration, the length of time that shaking occurs, and the character of the groundshaking. Stability of the foundation material, building design, and construction quality are also important factors

fault slip

The distance of the displacement between two blocks that occurs during an earthquake when elastic rebound causes the blocks on either side of a fault to spring back to their undeformed state.

Briefly describe how earthquakes can be used as evidence for the theory of plate tectonics.

The locations of Earthquakes mark plate boundaries. Studies show that there is a relationship between deep-focus earthquakes and deep-ocean trenches [where slabs of oceanic lithosphere are being subducted into the mantle]. Other studies show earthquakes happen at divergent boundaries because there are no deep-focus earthquakes along oceanic ridges and transform faults, it supports the theory of plate tectonics.

Wadati-Benioff Zone

The narrow zone of inclined seismic activity that extends from a trench downward into the asthenosphere.

Cite some reasons why an earthquake with a moderate magnitude might cause more extensive damage than a quake with a high magnitude.

The proximity of people and buildings to the location of the moderate magnitude may be closer. What rock type the buildings are on makes a difference. Sediment can lead to liquefication.

seismology

The study of earthquakes and seismic waves.

elastic rebound

The sudden release of stored strain in rocks that results in movement along a fault.

liquefaction

The transformation of a stable soil into a fluid that is often unable to support buildings or other structures.

focus or hypocenter

The zone within Earth where rock displacement produces an earthquake.

seismograms

The record made by a seismograph.

fault creep

Gradual displacement along a fault. Such activity occurs relatively smoothly and with little noticeable seismic activity.

Explain why the moment magnitude scale has gained popularity over the Richter scale among seismologists.

(1) it is the only magnitude scale that correctly estimates the size of very large earthquakes; (2) it is determined by the size of the rupture surface and the amount of displacement; thus it is a better reflection of the total energy released during a quake; and (3) it can be verified by both field studies (measurement of fault displacements) and by seismographic methods.

Who first explained the actual mechanism by which earthquakes are generated?

H.F. Reid

Using figure 11.14, determine the distance between an earthquake and a seismic station if the first S wave arrives 3 minutes after the first P wave.

1900 km

Modified Mercalli Intensity Scale

A 12 point scale developed to evaluate earthquake intensity based on the amount of damage to various structures.

fault

A break in rock mass along which movement has occurred.

How are faults, foci, and epicenters related?

A fault is a large fracture along which there is movement. When movement occurs, the zone within Earth where rock displacement occurs is termed the focus. The point on Earth's surface directly above the focus of an earthquake is called the epicenter.

Faults that are "locked" may be considered "safe." Rebut or defend this statement.

A locked fault has a high probability for hosting a damaging, future quake.

intensity

A measure of the degree of earthquake shaking at a given locale based on the amount of damage.

moment magnitude

A more precise measure of earthquake magnitude than the Richter scale that is derived from the amount of displacement that occurs along a fault zone.

seismic sea waves

A rapidly moving ocean wave, generated by earthquake activity, which is capable of inflicting heavy damage to coastal regions.

Richter scale

A scale of earthquake magnitude based on the amplitude of the largest seismic wave.

seismic gaps

A segment of an active fault zone that has not experienced a major earthquake over a span when most other segments have. Such segments are probable sites for future major earthquakes.

secondary or S, waves

A seismic wave that involves oscillation perpendicular to the direction of propagation. Only travels in solids. Slower than P waves.

body waves

A seismic wave that travels through Earth's interior.

aftershocks

A smaller earthquake that follows the main earthquake.

What is a tsunami? How is one generated?

A tsunami is a seismic sea wave formed by the displacement of the ocean floor during an earthquake

primary or P, waves

A type of seismic waves that involves alternating compression and expansion of the material through which it passes. The fastest earthquake wave.

Describe the principle of a seismograph.

A weight is freely suspended from a support that is attached to bedrock. When earthquake waves reach the seismograph, the inertia of the weight keeps it motionless while the bedrock and the support vibrate.

List some reasons why some earthquakes are much stronger than others.

Amount of pressure, amount of energy released, depth,

magnitude

An estimate of the total amount of energy released during an earthquake, based on seismic records.

seismograph

An instrument that records earthquake waves.

What prominent features on the ocean floor are associated with deep-focus earthquakes?

Deep ocean trenches

In addition to the destruction created directly by seismic vibrations, list three other types of destruction associated with earthquakes.

Fire, landslides and ground subsidence, and seismic sea waves (tsunamis) are all capable of adding to the destructive nature of earthquakes.

Define earthquakes in geologic terms.

Natural geologic phenomena caused by the sudden and rapid movement of a large volume of rock.

Are accurate short-range earthquake predictions possible using modern seismic instruments?

No, there are no reliable methods for short-range earthquake predictions.

inertia

Objects at rest tend to remain at rest, and objects in motion tend to stay in motion unless either is acted upon by an outside force.

What is the value of long-range earthquake forecasts?

They can be used as guides for where to build things such as dams, roads, etc. and help architects who build large structures to know what types of earthquakes the structure will go through.

Most of the very largest earthquakes occur in the zone on the globe known as the _____.

This is the circum-Pacific belt. It includes many earthquakes associated with active subduction zones around the Pacific rim where different parts of the Pacific oceanic crust are sinking into the mantle below island arcs or continental margins

Describe the process of liquefaction.

Unconsoilidated materials are saturated with water, earthquake vibrations can turn stable soil into a mobile fluid.

seismic waves

Vibrations that travel through Earth carrying the energy released during an earthquake

What factor contributed most to the extensive damage that occurred in the central portion of Mexico City during the 1985 earthquake? (see Box 11.1)

Weak foundation material and poor building design both contributed to the unusually severe damage at a site over 200 miles from the epicenter.

Explain what is meant by elastic rebound.

When stress is applied to crustal rocks, they respond by bending, and in doing so they store elastic energy much like a rubber band does when it is stretched. Once the strength of the rock is exceeded, the rock fractures and movement takes place along this fracture or fault. This slippage allows the deformed rocks to snap back to their original shape: a process called elastic rebound. It is the springing back of the rock that produces the vibrations we call an earthquake.