Chapter 9 Earthquakes and Earth's Interior: Geology 111

In addition to the destruction created directly by seismic vibrations, list 3 other types of destruction associated with earthquakes

1. Landslides and Ground Subsidence 2. Fire 3. Tsunami

We can determine the distance separating the recording station from the earthquake in two steps:

1. Using the seismogram, we determine the time interval between the arrival of the first P wave and the arrival of the first S wave 2. using the travel - time graph, we find the P - S interval on the vertical axis and use that information to determine the distance to the epicenter on the horizontal axis. Following this procedure, we can determine that the earthquake occurred 2100 miles from the recording instrument in Nagpur, India.

How is moment magnitude calculated?

1. by determining the average of amount of slip on the fault 2. area of the fault surface that slipped 3. strength of the faulted rock

Seismologists use a variety of methods to determine two different measures that describe the size of an earthquake: 1. 2.

1. intensity 2. magnitude

Two types of surface waves

1. one type causes Earth's surface and anything resting on it to move, much as ocean swells toss a ship 2. second type of surface wave causes Earth's material to move side to side. This motion is damaging to foundation of structures

P waves

1. push/pull waves 2. momentarily push (compress) and pull (stretch) rocks in the direction the wave is traveling. 3. travel through solids, liquids, and gases 4. solids, liquids and gases resist stresses that change their volume when compressed and, therefore, elastically spring back once the stress is removed.

S waves

1. shake the particles at right angles to their direction of travel 2. change the shape of the material that transmits them 3. because fluids (gases and liquids) do not resist stresses that cause changes in shape --meaning fluids do not return to their original shape once the stress is removed. 4. liquids and gases do not transmit S waves

Seismograms reveal that two main types of seismic waves are generated by the slippage of a rock mass

1. surface waves - travels in the rock layers just below Earth's surface 2. body waves - travels through Earth's interior

List 4 factors that affect the amount of destruction that seismic vibrations cause to human - made structures

1. the intensity 2. duration of the vibrations 3. nature of the material on which the structures rest 4. nature of building materials and construction practices of the region

Destruction from Seismic Vibrations The amount of damage to human-made structures attributable to the vibrations depends on several factors 1- 4

1. the intensity 2. the duration of the vibrations 3. the nature of the material on which structures rest 4. the nature of building materials and construction practices of the region

List two reasons a repeat of the 1811 - 18112 New Madrid, Missouri, earthquakes could be destructive in Memphis, Tennessee, metropolitan area

1. they do not have adequate earthquake provisions in its building code 2. because Memphis is located on unconsolidated floodplain deposits, buildings are more susceptible to damage than are similar structures built on bedrock

Thus, the intensity of ground shaking for a magnitude 5 earthquake is ____ times greater than produced by an earthquake having a Richter magnitude (ML) of 4

10 times In addition, each unit of increase in Richter magnitude equates to roughly a 32 - fold increase in energy released.

How much more energy does a magnitude 7.0 earthquake release than a magnitude 6.0 earthquake?

10 times more

What is the strongest earthquake on record?

1906 Chilian subduction zone earthquake with a moment magnitude of 9.5

Tsunami Warning System U.S. Coast and Geodetic Survey established a tsunami warning system for coastal areas of the Pacific that today includes ____ countries

26 1. seismic observatories throughout the region report large earthquakes to the Tsunami Warning Center in Honolulu, then scientists at the center use deep-sea buoys equipped with pressure sensors to detect energy released by an earthquake 2. tidal gauges measure the rise and fall in sea level that accompany tsunamis, and warnings are issued within an hour 3. although tsunamis travel very rapidly, there is sufficient time to warn all except those in the areas nearest the epicenter

In addition, as in the Richter scale, each unit increase in moment magnitude equates to roughly a __________-fold increase in the energy released.

32

An earthquake with a magnitude of 6.5 releases _____ times more energy than one with a magnitude of 5.5. An earthquake with magnitude of 6.5 releases ______ times more energy than a magnitude of 4.5.

32 1000 times (32 X 32)

Name another major concentration of strong earthquake activity.

Alpine-Himalayan belt that runs through the mountainous regions that flank the Mediterranean Sea and extends past the Himalayan Mountains Tectonic activity in this region is mainly attributed to collisions of the African Plate with Eurasia and of the Indian plate with Southeast Asia. These plate interactions createdmany thrust and strike-slip faults that remain active.

Why is the moment magnitude better than the Richter scale for measuring large earthquakes?

Because the moment magnitude estimates the total energy released

Damaging Earthquakes East of the Rockies When you think earthquake, what two places come to mind?

California and Japan

Why is the moment magnitude favored over the Richter scale for large earthquakes?

For measuring large earthquakes, seimologists have come to favor the moment magnitude. The moment magnitude measures the total energy released during an earthquake. Moment magnitude is calculated by determining the average amount of slip on the fault, the area of the fault surface that slipped, and the strength of the faulted rock.

___________ ________________in 1902, he developed a more reliable intensity scale, which is still used to day in a modified form.

Giuseppe Mercalli

What information is used to establish the lower numbers on the Mercalli scale?

I - IV is what people felt

Richter scale

In 1935, Charles Richter of the California Institute of Technology developed the first magnitude scale to use seismic records.

Japan Tsunami

Japan is located along the circum-Pacific belt and its expansive coastline, it is especially vulnerable to tsunami destruction Mw 9.0 2011 and resulted in 15,861 deaths 3000 missing 6107 injuries 400,000 buildings destroyed 56 bridges destroyed 26 railways destroyed or damaged tsunami reached a maximum height of 130 feet and traveled inland 6 miles in region of Sendai, Japan meltdown of three nuclear reactors in Japan's Fukushimi Daiichi Nuclear Complex Across the Pacific in California, Oregon, Perum and Chili, some loss of life occured, several homes destroyed several boats and docks were destroyed tsunami generated when a slab of seafloor located 60 km or 37 miles off the east coast of Japan was suddenly thrust up an estimated 5 to 8 meters (16 to 26 feet)

Alaskan Earthquake was the most violent earthquake ever recorded.

March 27, 1964 at 5:36 p.m. most violent earthquake ever recorded in North America 128 dead thousands homeless 28 aftershocks 10 of them exceeding magnitude 6

_____________Magnitude is used for measuring medium and large earthquakes

Moment Magnitude (Mw) measures total energy released during an earthquake.

The greater the interval between the arrival of the first ___ wave and the arrival of the first _____ wave, the greater the distance to the ___________.

P wave S wave epicenter

The method used for locating an earthquake's epicenter relies on the fact that _______ waves travel faster than ____ waves.

P waves S waves

Two types of body waves.

Primary waves (P waves) Secondary waves (S waves)

Most of the structures in the developing world are constructed of unreinforced concrete slabs and bricks made of dried mud.

Reason the death toll in poor countries such as Haiti is higher than for earthquakes of similar size in the United States.

The ______scale is considered saturated for major earthquakes because it cannot distinguish among them.

Richter

Despite its usefulness, the _____ scale is not adequate for describing very _____ earthquakes.

Richter large

______________used a ____________ scale to express magnitude, in which a ___________-fold increase in wave amplitude corresponds to an increase of ____ on the magnitude scale.

Richter logarithmic 10 - fold 1

6 major earthquakes have occurred in the central and eastern U.S since colonial times.

Richter magnitudes of 7.5 7.3 7.8 centered near the Mississippi River valley in southeastern Missouri on the following dates: Dec. 16, 1811 January 23, 1812 February 7, 1812 destroyed the town of New Madrid, Missouri, triggered massive landslides and caused damage over a 6 state area

The ____ ____ is calculated by measuring the __________ of the largest seismic wave (usually an S wave or a surface wave) recorded on a seismogram.

Richter scale amplitude

Fire

San Fransicsco, California earthquake of 1906 most of the city was reduced to ashes and ruins 3000 people died 40,000 homeless fires raged for 3 days water lines were broken into hundreds of disconnected pieces, which made controlling the fires impossible

fault creep

Some sections of the San Andreas Fault exhibit slow, gradual displacement and produce little seismic shaking

What is the value of long-range earthquake forecasts?

The data is useful for providing important guides for building codes so that buildings, dams, and roadways are constructed to withstand expected levels of ground shaking

Explain why an earthquake east of the Rockies may produce damage over a larger area than one of similar magnitude in California.

The reason is that underlying bedrock in the central and eastern United States is older and more rigid. As a result, seismic waves are able to travel greater distances with less attenuation than in the western United States.

Where does earthquake activity occur?

The zones of earthquake activity are founf along one of the three types of plate boundaries 1. convergent 2. divergent 3. transform fault These narrow belts of earthquake activity are located along fault surfaces where tectonic plates are scraping past one another.

seismologists began to plot the distribution of rupture zones associated with great earthquakes around the globe.

These maps revealed that individual rupture zones tend to occur adjacent to one another, without appreciable overlap, thereby tracing out a plate boundary

When researchers studied historical records, they discovered that some seismic zones had not produced a large earthquake in more than a century, or, in some locations for several centuries.

These quiet zones, called seismic gaps, are believed to be zones that are storing strain that will be released during a future earthquake.

Review: Describe triangulation method used to determine the epicenter of an earthquake.

This method involves using the distance obtained from three or more seismic stations to establish the location of an earthquake.

What is a tsunami? How is one generated?

a tsunami is a large undersea earthquake generated by displacement along a megathrust fault that suddenly lifts a large slab of seafloor

paleoseismology

another method of long-term forecasting involves paleoseismology, the study of the timing, location, and size of prehistoric earthquakes. studies are conducted by diging a trench across a suspected fault zone and then looking for evidence of ancient faulting, such as offset sedimentary strata or mud volcanoes. A large vertical offset of the layers of sediments indicates a large earthquake.

seismographs or seismometers

are similar to the instruments used in ancient china. It has a weight freely suspended from a support that is securely attached to bedrock.

How is moment magnitude also calculated?

by modeling data obtained from seismograms. The results are converted to a magnitude number, as in other magnitude scales.

What is the zone of greatest seismic activity?

circum-Pacific belt, encompasses coastal regiona of Chile, Central America, Indonesia, Japan, and Alaska, including the Aleutian islands

thrust faults

compressional forces associated with continental collisions that result in mountain building generate many __ ___.

What type of plate boundary is associated with Earth's largest earthquakes?

convergent plate boundaries, where one plate slides a low angle beneath another. The contacts between the subducting and overlying plates are megathrust faults, along which Earth's largest earthquakes are generated.

Amplification of Seismic Waves

differences are usually attributable to the nature of the ground on which the structures are built. ex: soft sediments amplify the vibrations more than solid bedrock

MAGNITUDE Scales To more accurately compare earthquakes around the globe, scientists searched for a way to describe the _______ released by earthquakes that DID NOT rely on factors, such as building practices, which vary considerably from one part of the world to another.

enegy

Section 9.3 starts here: When seismologists analyze an earthquake, the first task is to determine the ______, the point on Earth's surface directly above the focus.

epicenter

Using this technique, zones of intensity were identified, with the highest near the maximum ground of shaking and often (but not always) the earthquake __________.

epicenter

The system for locating earthquake _______ was developed by using seismograms from earthquakes whose epicenters could be easily pinpointed from physical evidence.

epicenters

Long-range forecasts

estimates of how likely it is for an earthquake of a certain magnitude to occur on a time scale of 30 to 100 years or more.

Transform faults and smaller strike-slip faults also run through continental crust, where they may generate large earthquakes that tend to occur on a cyclical basis.

ex: California's San Andreas Fault ex: New Zealand's Alpine Fault ex: Turkey's North Anatolian Fault, which produced a deadly earthquake in 1999

Strong earthquakes can occur away from plate boundaries, although less common.

ex: Gujarat, India earthquake of 2001 ex: 1811 and 1812 New Madrid, Missouri earthquakes ex: 1886 Charleston, South Carolina earthquake intraplate earthquakes occur when internal stresses build due to interaction of neighboring plates or because of loading or unloading as, for example, the melting of the glacial ice sheets following the last ice age. These stresses may cause failure along once-inactive fault zones

strike - slip faults

faults in which the dominant displacement is horizontal and parallel to the strike (direction) of the fault surface are called

Turnagain Heights

image

triangulation

image of triangulation

Liquefaction

intense shaking of an earthquake can cause loosely packed water-logged materials, such as sandy stream or fill, to be transformed into a substance that acts like fluids. the phenomenon of transforming a somewhat stable soil into mobile material capable of rising toward Earth's surface is known as ___. ex: underground storage tanks and sewer lines float toward the surface ex: contributed to damage inflicted on San Francisco's water system during the 1906 earthquake ex: 2011 Japan earthquake, liquefaction caused entire buildings to sink several feet.

Intensity Scales By systematically mapping effects of the earthquake, a measure of the _________ of ground shaking was established. The map generated by this study used lines to connect places of __________ damage and hence _______________ ground shaking.

intensity equal equal

earthquake

is a ground shaking caused by sudden and rapid movement of one block of rock slipping past another along fractures in Earth's crust called faults.

Review Questions What does the Modified Mercalli Intensity scale tell us about an earthquake?

is a scale based on verifiable physical evidence to quantify intensity on a 12 - point scale. However, it is hard to measure intensity if there are no buildings or people present at a location

focus

is the location where slippage begins; also called the hypocenter.

epicenter

is the point on Earth's surface directly above the hypocenter or focus

Review : What information does a travel-time graph provide?

is used to determine the distance to an earthquake's epicenter.

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

its aim is to predict earthquakes within a time frame of hours or days, therefore short-range earthquake prediction is not feasible one claim of a successful short-range prediction based on an increase in foreshocks, was made by the Chinese government after the Feb. 4, 1975 earthquake in Liaoning Province predictions can lead to false alarms

seismic waves

large earthquakes release huge amounts of stored energy as seismic waves-- a form of energy that travels through the lithosphere and Earth's interior

transform faults

large strike-slip faults that slice through Earth's lithosphere and accommodate motion between two tectonic plates ex: San Andreas Fault

What is the most obvious factor that determines the degree of destruction?

magnitude of the earthquake and its proximity to a populated area During an earthquake, the region within 20 to 50 kilometers of the epicenter tends to experience roughly the same degree of ground shaking. and beyond that limit, vibrations diminish rapidly

What is a Tsunami?

major earthquakes set in motion s series of large waves that are known by the Japanese name tsunami (harbor wave). Most tsunamis generated by displacement along a megathrust fault that suddenly lifts a large slab of seafloor

Tsunami Damage from the 2004 Indonesia Earthquake

massive undersea earthquake of Mw 9.1 occured near the island of Sumatra on Dec. 26, 2006, sending waves of water racing across th Indian Ocean and Bay of Bengal one of the deadliest natural disasters claiming more than 230,000 lives

intensity of earthquake

measure of the amount of shaking at a particular location, based on observed property damage.

slippage that occurs along faults can be explained by the __ __ ___,

plate tectonic theory, which states that large slabs of Earth's lithosphere are continually grinding past one another

List at least three reasons an earthquake with a magnitude of 7.0 results in more death and destruction than a quake with a magnitude of 8.0.

proximity to a populated area building material of the buildings nature of ground material where structures are built

On a globe, a circle is drawn around each seismic station. The _____________ of these circles is equal to the distance from the seismic station to the epicenter. The ____ where the three circles intersect is the _______________ of the quake.

radius point epicenter

first warning of an approaching tsunami is often the _____

rapid withdrawal of water from the beaches, the result of the trough of the first large wave preceding the crest inhabitants of the Pacific basin have learned to heed this warning and quickly move to higher ground

seismograms

records obtained from seismographs are called _, provide useful information about the nature of seismic waves.

magnitude

relies on data gleaned from seismic records to estimate the amount of energy released at an earthquake's source

Turnagain Heights

small area of destruction shown in picture

foreshocks

small earthquakes called foreshocks often but not always precede major earthquakes by days or, in some cases, several years.

Earthquakes vary enormously in ___________, and great earthquakes produce wave amplitudes that are _______________ of times larger than those generated by weak _______________.

strength thousands tremors

aftershocks

strong earthquakes are followed by numerous earthquakes of lesser magnitudes, which result from crust along the fault surface adjusting to the displacement caused by the main shock.

seismology

study of earthquake waves, dates back to attempts made in China almost 2,000 years ago to determine the direction from which these waves originated.

Landslides and Ground Subsidence

the greatest earthquake-related damage to structure is often caused by landslides and ground subsidence triggered by earthquake vibrations ex: homes were destroyed in Turnagain Heights when a layer of clay lost is strength and over 200 acres of land slid toward the ocean.

megathrust fault

the plate boundary between a subducting slab of oceanic lithosphere and the overlying plate form a fault referred to as __ __. Because these large thrust faults lie partially beneath the ocean floor, movement along these faults may displace the overylying seawater, generating destructive tsunamis. These megathrust faults have produced the majority of Earth's most powerful and destructive earthquakes.

elastic rebound

the sudden release of stored strain in rocks that results in movement along faults

Why are buildings built of blocks and bricks bad?

they are not reinforced by steel rods and are the most serious safety threats in earthquakes.

From these seismograms, ____________ - ________ graphs were constructed.

travel-time

We know the distance, but what about the direction? Using a method called ______________, we can determine the precise location when we know the distance from three or more seismic stations.

triangulation

Different recording stations often obtain slightly different magnitudes for the same earthquake as a result of the ___________ in rock types through which the waves travel.

variations

Modified Mercalli Intensity scale

was developed using California buildings as its standard. When some well-built wood structures and most masonry buildings are destroyed by an earthquake, the affected area is assigned a Roman numeral X or 10.

inertia

when vibrations from an earthquake reach the instrument, the __ of the weight keeps it relatively stationary, while Earth and the support move. Inertia is described by the following statement: objects at rest tend to stay at rest, and objects in motion tend to remain in motion, unless acted upon by an outside force.