Gates A.E., Ritchie D. Encyclopedia of Earthquakes and Volcanoes

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Segara Anak caldera, Lombok, Indonesia Located on the

island of Lombok, this caldera includes Gunung Rinjani, a

stratovolcano on its east rim, and Gunung Baru, a cone

within the caldera. A strong earthquake in 1884 preceded an

eruption of Gunung Rinjani, which is said to have expelled

smoke and fire for several days. Another earthquake in

1898 was suspected of having originated at Gunung Rinjani.

Two strong earthquakes occurred under Gunung Rinjani in

November 1903, and the volcano was reported smoking in

November 1915. Several weeks after an earthquake early in

1966, a climber reported seeing a flow of lava from Gunung

Baru and detecting a strong smell of sulfur. The most recent

eruption was in 1994 when a lahar killed 30 people. The

eruption produced ash clouds but was relatively mild.

segmentation Many faults are not a single continental-

scale plane of weakness but a series of small fault pieces or

segments that are stuck together. The slip on a fault to pro-

duce an earthquake typically occurs along a single segment.

Segmentation may form from bends in fault planes, intersec-

tions with other faults, or lateral changes in rock units.

seiche A periodic oscillation in an enclosed, or nearly

enclosed, body of water. Basically, seiches are large waves

that slosh back and forth in an enclosed basin. These waves

can be large (up to 100+ feet [30.8 m]) and cause considerable

destruction. Earthquakes may contribute to producing seiches.

seismic gap This expression refers to a recently “quiet” por-

tion of an active earthquake-generating fault. This section

forms a gap on a map of recent earthquake epicenters along

the fault and may be a likely spot for strong earthquakes in

the future because the forces that generate earthquakes may

be building up along the gap and “waiting” to be released.

seismic moment The seismic moment of an earthquake is

the measure of its strength based upon the area of the fault

rupture, the amount of slip on the fault in the event, and the

force required to overcome the friction or strength of the

rock opposing the movement. Seismic moment is usually cal-

culated from the amplitude spectra of seismic waves on a

seismogram.

seismic sea wave See tsunami.

seismic velocity The speed at which earthquake waves

travel through Earth. The speed is a function of the rock

through which the waves travel. It depends upon the elastic

moduli of the rock. There are measures of incompressibility

and resistance to shear and torsion as well as density that

determine the speed. Basically, seismic waves travel faster in

hard, tight rock than in sediments or soft (ductile) rocks

or liquids.

seismogram A seismograph records the arrivals of incom-

ing seismic waves through time. It prints those records on a

seismogram. The piece of paper showing the wiggly lines of

an earthquake’s, P-waves, s-waves, or surface waves is a

seismogram.

seismograph A device for detecting and recording the

vibrations of earthquakes.

See also seismology.

seismology The scientific study of earthquakes. Earth-

quakes generate several kinds of waves. These are sometimes

The USGS seismic station at Menlo Park, California. The three recorders

show vibrations in the three dimensions, records shown are those of the

1989 Loma Prieta earthquake. (Courtesy of the USGS)

Typical seismogram that is registered on a seismograph. The wiggly line

is what appears on a seismograph as an earthquake occurs. Each wiggle

represents a different kind of wave: P-wave, S-wave, or surface waves

(both Rayleigh and Love waves). The great delay between each arrival

means that the focus of the earthquake is very far away because each

wave travels at a different velocity. If the focus was closer, the arrivals

would be closer together. If the seismograph was at the focus, all waves

would be grouped together in one set of wiggles in which no specific

wave type could be discerned. Notice that the surface waves give higher

amplitudes and longer durations. They cause the most damage.

232 Segara Anak

classified in two categories: surface waves and body waves.

Surface waves, are confined to the surface and shallow sub-

surface of Earth like ocean waves are confined to the surface

and near surface of the water. They are very slow. There are

two kinds of surface waves, named Love waves and Ray-

leigh waves after their discoverers. Love waves move like a

Diagram showing the response of rock as different types of seismic waves pass through. No permanent deformation of the rocks occurs from the passage

of these waves. Body waves include P- and S-waves. A P-wave is like a low speed accident, where the bumper is compressed at impact but expands

(dilation) and recovers after the impact. An S-wave is like whipping a jump rope. In surface waves, the ground surface moves like an ocean wave in a

Rayleigh wave. In Love waves, the surface moves from side to side as the wave passes through. It causes structures to fall over sideways.

seismology 233

snake. They exhibit horizontal shear, meaning a side-to-side

or transverse shaking motion as they go forward. They cause

buildings and other structures to fall over sideways. Rayleigh

waves make the surface of Earth move like an ocean wave.

They involve an elliptical motion of the ground that is actu-

ally retrograde, or opposite to the direction of travel. In other

words, for a wave moving to the right, the movement of the

rock will be counterclockwise. Body waves are also called P-

waves and s-waves, named for the arrival times: P is the pri-

mary or first arrival, and S is the secondary arrival. P-waves

are the fastest of the seismic waves. They move by compres-

sion and subsequent rarefaction of the rock media through

which they move, in much the same manner as sound waves

traveling through the air or vibrations from a hammer strike

on a rock. S-waves move in a transverse motion like the wave

generated by whipping a jump rope. They also have a similar

motion to a Love wave and are slower than P-waves.

FACTORS AFFECTING DESTRUCTION FROM EARTHQUAKES

The extent of destruction from an earthquake depends on

several factors:

Magnitude and Intensity

Earthquake magnitude refers to how much energy the earth-

quake released. Magnitude is not always proportional to

damage. Under some conditions, such as high population

density in the affected area, a relatively minor earthquake

may do far more damage than a more powerful quake in a

sparsely populated area.

There are two scales that are most commonly used to

measure earthquakes: the Richter scale and the Mercalli

scale. The Richter scale, named after Dr. Charles Richter

of the California Institute of Technology, is a logarithmic

scale (expressed in powers of 10—see also magnitude) that

reflects the amplitude of ground movement. Several steps

are involved in establishing an earthquakes magnitude on

the Richter scale. First, the location of the quake must be

determined. It is done by noting the time a wave passed two

widely separated locations and inserting the data into a set

of equations to determine how far each station was from the

origin of the quake. The next step is to estimate the intensity

of ground motion at the earthquake’s origin by using a correc-

tion factor that introduces some error into the calculation of

Seismic risk map for the United States. Based on historical records of earthquakes, both intensity and recurrence interval, the country is divided into

zones of earthquake risk. Certain areas will be relatively low-risk for earthquakes over a 50-year interval, whereas others are high.

234 seismology

magnitude. Measuring magnitude on the Richter scale does

not yield precise results, but within certain limitations, it does

provide a useful estimate of the strength of the earthquake.

The Richter scale starts at zero and is open ended. An earth-

quake of magnitude 1 on the Richter scale probably would go

unnoticed by anyone but an observer with a seismograph.

Magnitude 4 earthquakes are mild but still perceptible, and

earthquakes of magnitude 8 and above are devastating if

they strike densely populated areas. The San Francisco

earthquake of 1906 is estimated to have been about 8.2 on

the Richter scale, and two other earthquakes of the 20th cen-

tury—one beneath the Pacific Ocean near Japan in 1933

and the other off the coast of Ecuador in 1933—are thought

to have ranked around 8.6.

The second common scale for measuring earthquake

intensity is the Mercalli scale and modified Mercalli scale.

It uses Roman numerals to avoid confusion with the Rich-

ter scale and is based on easily observable effects, such as

destruction of buildings.

Seismic moment is another measure of the size of an

earthquake. Seismic moment is the product of the average

amount of slip, the area of the rupture and the shear mod-

ulus, or strength, of the rocks affected. An expression of

magnitude related to seismic moment is moment magnitude,

which is approximately the same as Richter magnitude in

range 3–7.

Duration

Some earthquakes last only a few seconds, but others have

been known to last for a minute or two. As a rule, damage

increases with duration.

Local Geology

The character of underlying material does much to determine

the extent of damage from an earthquake. The safest mate-

rial on which to build, generally speaking, is solid rock, which

transmits vibrations from an earthquake but maintains its

structural integrity. More dangerous is unconsolidated sedi-

ment, which may exhibit liquefaction and extensive conse-

quent damage where groundwater rises close to the surface.

Fractured rock, as found in many parts of the western United

Energy Released and Equivalent Energy of Richter Magnitudes

Magnitude Energy Released (ergs) Energy Equivalent

-2 6 X 10

100 Watt-light left on for one week

-1 2 X 10

0 6 X 10

Seismic waves from one pound of explosive

1 2 X 10

A 2-ton truck traveling at 75 mph

2 6 X 10

3 2 X 10

Typical lightning bolt

4 6 X 10

Seismic waves from 100 tons of explosives

5 2 X 10

Average tornado

6 6 X 10

Hiroshima 1945 atomic bomb

7 2 X 10

One-megaton nuclear explosion

8 6 X 10

Average kinetic energy of a hurricane

9 2 X 10

Average annual seismic energy released on Earth

10 6 X 10

U.S. annual energy consumption

11 2 X 10

Earth’s annual internal heat flow

12 6 X 10

Earth’s daily receipt of solar energy

Mercalli

Intensity Observed Effects

I Not felt at all

II Felt by only a few individuals

III Felt indoors by many persons, but not neces-

sarily identified as an earthquake

IV Felt both indoors and outdoors and compa-

rable to vibrations produced by a passing train

or heavy truck

V Strong enough to awaken a sleeping person;

small objects fall off shelves, and beverages

may splash out of cups or glasses

VI Perceptible to everyone; pictures fall off walls,

and plaster may fall from ceilings.

VII Difficult to stand upright; ornamental masonry

falls from buildings; waves may occur in ponds

and swimming pools.

VIII Mass panic may occur; chimneys and smoke-

stacks may fall; unsecured frame houses slide

off their foundations

IX Heavy damage to masonry structures and

underground pipes; large cracks open in

ground; panic is general

X Numerous buildings collapse; water splashes

over riverbanks

XI–XII Virtually complete destruction

seismology 235

States, may absorb energy from earthquake waves and thus

reduce the geographical extent of damage from a major earth-

quake. By contrast, in areas where geological formations

stretch unbroken for hundreds of miles, as in portions of the

eastern United States, vibrations may travel great distances

with much of their initial destructive potential intact. The 1906

San Francisco earthquake, for example, was barely felt at all

outside California, but the New Madrid, Missouri, earth-

quakes approximately a century earlier were perceived over

much of the United States east of the Mississippi River. The

irony here is that the same processes that predispose a given

part of Earth’s crust to major earthquakes may also con-

fine damage from earthquakes through extensive fracturing,

whereas comparatively earthquake-free regions may harbor

much greater potential for destruction because of the relatively

undisturbed, contiguous rock that underlies them.

Time of Day

This is an important factor in determining the number of

casualties from an earthquake. Deaths and injuries from

collapsing buildings in rural areas, for example, are likely

to be more numerous if an earthquake occurs at night or in

the early morning when the population is indoors sleeping

or having breakfast. In cities, on the other hand, deaths and

injuries may be more likely during the working day, when

large numbers of pedestrians are exposed to harm from fall-

ing walls and ornamental masonry.

Architecture

Wood-frame buildings have a reputation for standing up

well to earthquakes because wood can bend and sway under

earthquake vibrations, thus improving a building’s chance of

survival. Buildings of brick and stone construction, by con-

trast, are more likely to crumble and collapse because they

lack wood’s resiliency, although careful design and con-

struction may increase the survivability of a non-wooden

structure. Possibly the worst building material in regard to

earthquake resistance is adobe, which was responsible for

numerous fatalities during earthquakes in the early years of

Euro-American settlement of the western United States. See

also Fort Tejon earthquake for an illustration of the dan-

gers of adobe construction.

Predicting Earthquakes

Earthquake prediction has been a goal of scientists for cen-

turies, and efforts in this direction have often been color-

ful, even when unsuccessful. Charles Mackay, in his 1841

book Extraordinary Popular Delusions and the Madness of

Crowds, tells how one man named Bell apparently used a

primitive mathematical model to predict the date of an earth-

quake in England during the 18th century:

In the year 1761, the citizens of London were

alarmed by two shocks of an earthquake, and the

prophecy of a third which [supposedly] was to

destroy them altogether. The first shock was felt

on the eighth of February and threw down several

chimneys in the neighborhood of Limehouse and

Poplar; the second happened on the eighth of March

and was chiefly felt in the north of London. . . . It

soon became the subject of general remark, that

there was an interval of exactly a month between

the shocks; and a . . . fellow named Bell, a soldier in

the Life Guards, was so impressed with the idea that

there would be a third in another month that he lost

his senses altogether and ran about the streets pre-

dicting the destruction of London on the fifth of

April [that is, four weeks after the previous earth-

quake].

Mackay tells how Bell’s prediction affected the public:

There were not wanting thousands who confi-

dently believed the prediction and took measures

to transport themselves and their families from the

scene of the impending calamity. As the awful day

approached, the excitement became intense, and

great numbers of credulous people [migrated] to all

the villages within 20 miles (32 km), awaiting the

doom of London. Islington, Highgate, Hampstead,

Harrow, and Blackheath were crowded with panic-

stricken fugitives who paid exorbitant prices for

accommodation to the housekeepers of these secure

retreats. Such as could not afford to pay for lodg-

ings at any of these places remained in London until

two or three days before the time [of the prediction],

then encamped in the surrounding fields, awaiting

the tremendous shock which was to lay their high

city all level with the dust. . . .

[The] fear became contagious, and hundreds

who had laughed at the prediction a week before

packed up their goods when they saw others doing

so and hastened away. The [Thames] river was

thought to be a place of great security, and all the

merchant vessels in the port were filled with people

who passed the night between the fourth and fifth

on board, expecting every instant to see St. Paul’s

totter and the towers of Westminster Abbey rock

. . . and fall amid a cloud of dust.

Even when the predicted earthquake failed to occur on

April 5, the public’s fears took days to subside completely:

The greater part of the fugitives returned on the fol-

lowing day . . . but many judged it more prudent

to allow a week to elapse before they trusted their

dear limbs in London. . . . [Bell] lost all credit in a

short time and was looked upon by even the most

credulous as a mere madman. He tried some other

prophecies, but no one was deceived by them; and

. . . a few months afterwards, he was confined in a

lunatic asylum.

Such “earthquake prophets” have remained active from

Bell’s day to ours, but only in recent years has knowledge of

earthquakes and their origins advanced to the point where

accurate prediction of earthquakes, or some quakes at least,

has become a possibility.

236 seismology

Determining the potential for future rupture along a

fault can be difficult because faults differ in their behaviors.

One fault may show no activity for many centuries and then

suddenly move 30 feet (9 m) or more, whereas another fault

may exhibit virtually continuous, gradual movement and

many small earthquakes. This variability in behavior means

that geologists cannot make accurate predictions of a fault’s

future activity merely by establishing that the fault is active.

Two selected faults, though both active, may move at greatly

different intervals and exhibit equally great differences in

rates of slip. Also, the rate of slip along an individual fault

may change. The slip rate along California’s San Jacinto

Fault Zone, for example, is believed to have fluctuated by

more than 1,200%.

Another source of uncertainty in determining slip rate

and history of activity is difficulty in estimating the ages of

offset deposits or other features along a fault. In the Los

Angeles area, for example, such estimates are unreliable in

many or most locations where faults are active. Yet another

problem in determining the true slip rate for a fault is the

partial nature of information on components of slip. Data

may be restricted to either the vertical or horizontal compo-

nent. Such limited data may yield an unreliable estimate of

slip along a fault, although the horizontal or vertical compo-

nent alone may prove useful if the ratio of vertical to hori-

zontal motion along a fault is known already. (In situations

involving a strike-slip fault, where motion along a fault

is primarily horizontal, the horizontal component alone may

provide a reasonably accurate measure of the slip rate. This is

the case along the San Andreas Fault in California. Vertical

component data reportedly have provided an approximation

of true slip rates along the reverse-slip faults in California’s

Transverse Ranges.) The best information that can be sup-

plied, in many cases, is an average figure for slip rate over a

long period of time. As a rule, however, the higher the aver-

age slip rate, the more active the fault and the more closely it

bears watching as a potential source of future earthquakes.

Only in a few areas—namely, boundaries between major

plates of Earth’s crust, as along the San Andreas Fault—do

faults show very high average slip rates, perhaps a half-inch

or more per year. Active faults in other areas generally show

less slip.

The future behavior of a fault may be inferred, to some

extent at least, from evidence including the rate of slip, the

size of earthquakes and intervals between them and the

amount of slip in each incidence of movement. No one set

of criteria exists, however, for determining how active a fault

may be in the future.

Complicating such analysis further is the fact that some

active faults do not extend to the surface of Earth and, on the

surface, may display only faint evidence of recurring seismic

activity. A highly damaging earthquake that struck Coalinga,

California, in 1983 involved such a blind fault similar to

the North Ridge earthquake of 1994. Moreover, simply

establishing that a fault is active does not allow geologists

to make accurate predictions of its future activity. Ongoing

measurements of seismic activity are useful tools for estimat-

ing the likelihood of earthquakes along a given fault in the

future. Seismic data may be misleading, however, because

ongoing earthquake activity, or the absence of it, along a

fault does not necessarily reflect the potential of that fault for

generating destructive earthquakes. An active fault may have

very little potential to cause destructive earthquakes because

it gradually releases its energy through creep, without actu-

ally generating earthquakes. Also, data on seismic activity

along a particular fault may span too short a time to provide

a useful means of predicting the behavior of earthquake-

generating faults over a long period. Even when historical

records are more extensive, as in parts of Asia, great uncer-

tainties remain in reconstructing the seismic history of a given

locality or region.

Despite these limitations, certain methods exist for estimat-

ing the size and frequency of possible future earthquakes along

a fault. These methods involve, but are not limited to, analyz-

ing the earthquake history of a region to find the biggest seis-

mic event linked to a given fault and comparing a given fault’s

history of earthquake activity with that of other faults similar

in structure and tectonic characteristics. Another approach is

to use empirically established relations between earthquake

magnitude and length of faults. It is assumed that the greater

the dimensions of the fault surface involved in an earthquake,

the greater magnitude the earthquake will have. One widely

used method for estimating the most powerful earthquake

that is likely to occur on a given fault rests on the assumption

that half the total length of the fault may rupture in a particu-

lar earthquake. This approach is not fully reliable, however,

because experience has shown that a major earthquake may

involve rupture of anywhere from only a small percentage to

almost the whole extent of a fault surface that existed prior to

the earthquake. Another drawback to this method is difficulty

in measuring the length of a given fault accurately. Much of

a particular fault may lie hidden under sediment and water.

In greater Los Angeles, the 1857 Fort Tejon earthquake is

thought to represent about the most powerful earthquake that

might affect that area, although an earthquake of still greater

magnitude is possible in theory. It has been suggested that the

whole San Andreas Fault might rupture in southern California,

producing a single gigantic earthquake; but there are questions

about whether or not the geology of the region would allow

such a single catastrophic event.

Semeru volcano, Java, Indonesia One of the world’s

most active volcanoes, Semeru has erupted at least 55 times

since 1818. The typical eruption has a VEI = 2–3 and pro-

duce lava flows and

NUÉE ARDENTEs on several occa-

sions. Ten of the eruptions produced fatalities including

some 600 death from lahars during the 1909 and 1981

eruptions. The current eruptive period began in 1967 and

typically produces Vulcanian-type eruptions. To date

nearly 500 people have been killed by pyroclastic flows

and avalanches.

Semisopochnoi volcanic island, Alaska, United States Semi-

sopochnoi Island is located in the western Aleutian Island

arc and consists largely of a single volcano, Pochnoi. It is the

youngest volcanic island in the western Aleutians. It is com-

posed of an older shield volcano. The central portion col-

lapsed and formed a caldera following the eruption of a

Semisopochnoi 237

great volume of pumice, although the date of the eruption and

collapse is uncertain. After the caldera formed, several new

cones formed. Accounts of eruptive activity from the 18th and

early 19th centuries are spotty, but there is reason to think the

volcano was active in or around 1772, 1790, 1792, and 1830.

An eruption appears to have occurred in 1873. A plume arose

from the island in 1987. Several days after this event, a pilot

reported seeing snow darkened by ash. That eruption was

explosive with a VEI = 2.

Serravalle di Chienti earthquake, central Italy On Sep-

tember 26, 1997, an earthquake of magnitude 5.7 of

occurred. A total of 11 people were killed, 100 people were

injured, and 80,000 buildings were damaged or destroyed.

The earthquake received extensive press coverage because of

the damage it did to the Basilica of St. Francis at Assisi. Foot-

age of a collapsing roof in a church was on the evening news

throughout the world.

Sete Cidades caldera, Azores The Sete Cidades caldera

on San Miguel Island has undergone eruptions on at least

eight occasions within historical times. An eruption occurred

around the year 1444 from a vent on the southwestern floor

of the caldera. In 1638 very strong earthquakes in May and

early June were followed in July by an underwater eruption

from a vent to the west of the caldera. A submarine eruption

in 1682 followed strong earthquake activity, and an earth-

quake swarm in 1713 may have accompanied an underwater

eruption west of Sete Cidades. Strong earthquakes in 1810 and

early 1811, including one that caused widespread destruction

on June 24, 1810, were followed in 1811 by a submarine erup-

tion several hundred yards southeast of the site of the under-

water eruption of 1638. This 1811 eruption produced an islet

about a mile and a half (2.4 km) off the western shore of the

island. The eruption subsided in early February but resumed

in June, when powerful earthquakes shook the island and a

strong odor of sulfur was detected. An underwater eruption

began on June 14, 1811, near the site of the eruption in Febru-

ary. Further submarine eruptions were recorded in 1861 and

1880. The caldera contains two crater lakes.

shadow zone On the opposite side of Earth from an

earthquake, there are no s-wave arrivals. S-waves move

with a shearing motion. This shearing motion cannot pass

through liquid. Therefore S-waves cannot pass through the

liquid core. Seismic waves that must pass through the liq-

uid core to reach the seismograph do not include S-waves.

Only P-waves arrive, and their path is so bent (refracted)

that there is even a small zone where they do not arrive. The

shadow zones are where there are no arrivals at all or no S-

wave arrivals. This phenomenon is the main way we can tell

that the outer core is liquid.

ShakeMap ShakeMap is a new tool developed for earth-

quake response in urban areas that is only available in Cali-

fornia at this point. When an earthquake strikes, emergency

management personnel must quickly decide where to con-

centrate their meager resources to best address the disas-

ter. An overwhelming amount of information on damage

is received and can only be processed in a cursory manner.

Because ground shaking causes the most damage to human-

made structures, knowing where the ground will shake the

most will determine the areas of highest damage. ShakeMap

is a map showing the distribution of relative shaking intensity

and thus the highest risk for damage. It is constructed using

geological bases as well as experience from previous earth-

quakes. As the seismic waves are still passing through, res-

cue teams can be mobilizing to the areas of highest damage

potential to shave precious minutes or to save the otherwise

victims of the disaster.

Shalla See Asawa.

shallow-focus earthquakes Earthquakes with foci of 44

miles (70 km) depth. By far and away, these are the most

common depth for earthquakes. Earthquakes are only pro-

duced during brittle deformation and brittle deformation

mainly occurs at depths of less than six to nine miles (10–15

km) in the crust. The most common deep-focus earth-

quakes occur in subduction zones.

Shasta, Mount Cascades, California, United States The

second tallest of the Cascade stratovolcanoes includes

a second cone, Shastina on its shoulder. Mount Shasta is an

Schematic diagram of the Earth showing travel of S-waves from

the earthquake focus to the surface, where they are detected by

seismographs. Because S-waves cannot pass through liquid, they are

absorbed by the liquid outer core. Seismographs on the opposite side of

the Earth from the earthquake do not receive any S-wave arrivals and

therefore a shadow zone is formed. A less extensive shadow zone for P-

waves is also formed based upon refraction of the waves.

238 Serravalle di Chienti

active volcano. It has erupted 11 times in the past 3,400 years

including three events in the past 750 years. The last eruption

was in 1786. There is a 300,000-year-old avalanche depos-

its that extend 27 miles (43.5 km) from the base. This event

deposited eight times the debris of the 1980 Mount Saint

Helens event. The explosion must have been huge. There is

pressure to build ever closer to Mount Shasta. Is it worth the

risk?

shear Shear is a type of stress in which force is transmitted

by sliding, like a hand sliding on a table, rather than simply

by pressing against the object. This motion cannot be trans-

ferred to liquid or gas. In seismology, it is defined as motion

at right angles to the direction in which an s-wave or sur-

face wave, from an earthquake is advancing (propagation

direction). Shear waves are also known as transverse waves.

shear wave (S-wave) An S-wave is named as such because

it arrives second after the primary wave. S stands for second-

ary, not shear. The S-wave, however, exhibits shear motion

as well. If you slide your hand across a table, you are exerting

shear on it. You can move a book on the table in this manner

or even the table if it is small enough. You are exerting shear

stress on it. This is the way shear waves travel. If you substi-

tute liquid or gas for the table, none of the shear stress will

be transferred. Your hand will just slide across the top. In the

same way, shear waves (S-waves) cannot pass through a liq-

uid or a gas.

Sheveluch volcano, Kamchatka, Russia A large strato-

volcano in north-central Kamchatka Peninsula. It has a

small internal cone with a large caldera where the recent

eruptions occur. After 150 years of dormancy, in 1854,

Sheveluch erupted violently. The explosion created a 1.25-

mile (2-km)-wide crater from which huge pyroclastic

flows were emitted. Some of the flows extend an unbeliev-

able 65 miles (105 km). This eruption was the largest in his-

toric times. It has erupted at least 28 times since then. The

two major eruptions were in 1944–50 and 1964. The 1964

eruption was the larger of the two and produced a huge

crater, an 11-mile (18-km) long and locally 160-foot (49-

m)-thick pyroclastic flow. It also produced a cloud with

impressive electrical output (thunder and lightning). Minor

eruptions occurred in 1879–83, 1897, 1905, 1928–29, 1980,

1984, 1985, 1986, 1988, 1989, 1990, 1991, 1993, and 1996.

shield volcano A volcano that is shaped like an ancient

shield. It is a relatively flat mound with gradual slopes,

formed by eruptions of basaltic lava. The basalt has such

low viscosity and lacking in ejecta that it cannot stack up

a steep slope. Olympus Mons on Mars and the Hawaiian

Islands are examples of shield volcanoes.

Shikotsu caldera, Japan The Shikotsu caldera appears to

have been the site of a great flow of ash and fall of tephra

in prehistoric times. About 30,000 years ago, the mod-

ern volcano was formed in a huge eruption that produced

approximately 30.5 cubic miles (125 km

) of ash (VEI =

6–7). Two of the three prehistoric eruptions were also large.

The eruptions in 6950 and 800 b.c. had VEI = 5. In the mod-

ern volcano, Lake Shikotsu occupies much of the interior of

the caldera. The volcano Tarumai (called Tarumae-San) is

located on the caldera’s southeast edge. In historical times,

the caldera has been active on at least 34 occasions. Major

eruptions occurred in 1667 and 1739 at Tarumai and left a

caldera approximately one mile (1.6 km) wide on the volcano

summit. A vigorous eruption of Tarumai in 1874 preceded

by about three weeks a powerful tectonic earthquake along

the northwestern coast of Hokkaido¯, the northernmost major

island in the Japanese archipelago. Small emissions of ash

occurred in 1894. Tarumai underwent a strong eruption in

1909, approximately a year after a major tectonic earthquake

off the north coast of Hokkaido¯. This 1909 eruption began

in late April with loud detonations, shaking of the ground,

and a tremendous column of dark smoke. Eruptive activ-

ity occurred again in May, apparently with great emissions

of smoke and vigorous seismic activity. Minor eruptions

occurred every few months from 1917 to 1923, and erup-

tive activity continued on an intermittent basis through 1936.

A powerful explosion occurred in 1920. Another eruption,

Diagram of a shield volcano with a summit caldera and flank eruptions

Shikotsu 239

on December 1, 1933, preceded a powerful earthquake on

December 5 and a renewal of volcanic activity on December

11. Explosive eruptions, and possible phreatic eruptions

as well, occurred between 1944 and 1955. earthquake

swarms started to increase in 1967, and minor swarms of

earthquakes were reported again in 1975, 1978, 1979, and

1981. Eruptions resumed in 1978, and small eruptions con-

tinued through 1981. A minor eruption in February 1981

marked the end of earthquake swarm activity that had been

occurring since 1967, although some earthquakes took place

after this eruption. Inflation appears to have occurred at the

caldera during the 1980s.

Ship Rock New Mexico, United States One of America’s

most spectacular and famous landforms, Ship Rock is a vol-

canic neck with dikes radiating from it. The ancient volca-

nic cone that once surrounded Ship Rock has been removed

by erosion. The dikes acted as feeders and probably fed flank

eruptions. Ship Rock is part of the Navajo Volcanic Field,

which lies in the Four Corners area (intersection of Utah,

Colorado, Arizona, and New Mexico) covering about

7,810 squares miles (20,000 km

). The igneous bodies have

an unusual composition and occur as either diatremes (kim-

berlites) or tuff pipes. The activity occurred from about 30

to 25 million years ago, and erosion has proceeded ever since.

Shiraz earthquake, Iran During the night of May 4, 1853,

a devastating earthquake struck the city of Shiraz, Iran, and

the surrounding towns and villages. The Richter magnitude

of this event was estimated at 6.5, and the damage was esti-

mated at VIII on the modified Mercalli scale. The earth-

quake caused extensive settling of the ground surface and

fissures and slump in the mountains surrounding the city.

There were strong aftershocks for months after the event.

Only the stronger masonry buildings survived the shaking; all

else was laid to waste. In all, some 12,000 people were esti-

mated to have perished in the event, although some sources

placed the number closer to 10,000.

Shishaldin volcano, Alaska, United States A beautifully sym-

metric, snow-covered stratovolcano in the eastern Aleutian

Islands (Unimak Island), Shishaldin is one of the most active

volcanoes in the Aleutians. It has erupted at least 29 times since

1775. The major eruptive periods were 1775–78, 1827–30,

1897–99, 1927–29, 1946–48, 1975–76, and 1986–87. The

most recent full eruption was in 1999, but there are constant

low-frequency rumblings and steam plumes from the volcano,

and it is constantly monitored. An eruption column 45,000

feet (13,716 m) high and an ash cloud that extended 500 miles

(805 km) was produced during this last eruption.

Sierra Negra volcano, Galápagos Islands, Ecuador A large

shield volcano, Sierra Negra occupies Isabela (Albemarle)

Island and has a shallow, elliptical caldera on its sum-

mit. The volcano is less than 4,500 years old. An explosive

eruption was reported in 1813, and other eruptive activity

of unknown character was recorded in 1817. A lava flow

occurred during an eruption in 1844, and an explosive erup-

tion was reported in 1860. Some kind of unrest appears to

have occurred at Sierra Negra in 1911 or 1912, but details

are unavailable. An explosive eruption and associated flow or

flows of lava were reported in 1948 and again in 1953–54.

Unrest of unknown character was reported in 1957. Explo-

sive eruptions with lava flows occurred in 1963 and again in

1979–80. An eruption in 1979 injected large amounts of sul-

fur dioxide, detectable on a global scale, into the atmosphere.

Sierra Nevada California, United States Approximately

350 miles (563 km) long and 60 miles (97 km) wide, the

Sierra Nevada range lies along the western edge of North

America’s Great Basin and is bordered on the north by the

Cascade Mountains. The Sierra Nevada comprises a huge

batholith of granitic rock with faults on the east side

and which is tilted westward. In some places, magma has

made its way to the surface and formed volcanoes atop the

pluton. The Sierra Nevada batholith forms the roots of an

ancient magmatic arc. A subduction zone existed under

California until about 25 million years ago when the tec-

Ship Rock is located in New Mexico. The main part is a volcanic neck

that remained after the rest of the volcano eroded away. The radial lines

are dikes that fed magma to the volcano. Ship Rock is nearly 1,400 feet

(470 m) high, and the dikes form large walls across the desert. (Courtesy

of NOAA)

The snow-covered stratovolcano Shishaldin on Unimak Island in the

Aleutian Islands, Alaska. This volcano has been compared with Mount

Fuji of Japan as one of the most perfect volcanic cones in the world. Only

volcanoes in polar regions are completely ice-covered. (Courtesy of the

USGS)

240 Ship Rock

tonics shifted around to the San Andreas transform margin

that exists today. Previously, California looked more like the

Andes with a high range of volcanic mountains.

The Sierra Nevada range is noted for seismic activity,

especially along its southern border, where a northward-

moving block of crustal rock encounters the deep roots of

the mountain range. This interaction generates numerous

earthquakes in southern California as the block of crust

grinds against, and fragments smaller pieces of crust around

the southwest edge of the Sierra Nevada. The transverse

ranges have arisen along the zone of collision. Veins of

gold were deposited in the rocks along the Sierra Nevada

and became known as the mother lode. Fragments of gold

eroded from the mother lode were laid down with sediments

in placer deposits that, when discovered during the early set-

tlement of California by Americans of European descent, led

to the gold rush of 1849. Alpine glaciation has done much

to sculpt the topography of the Sierra Nevada. The range

exhibits a curious inverted topography in places, where lava

flows that once filled canyons now form ridges because ero-

sion has worn away the granitic rocks on either side of the

lava flows, leaving the former valley (topped with lava) in

separate streams. The basins parallel to the Sierra Nevada on

the west side of the range contain thick deposits of sediment

eroded from the mountains. They formed as the basins sank

relative to the mountains, giving sediment an opportunity to

accumulate to great thickness.

silica Tetrahedra (pyramid-shaped) molecules of silicon and

oxygen. Silica is an important component of many minerals,

especially the silicates, quartz crystals are familiar exam-

ples of silica in its pure form.

silicates Silicate minerals incorporate a characteristic mole-

cule, which consists of a silicon atom and four oxygen atoms

arranged in a tetrahedral (pyramid) structure. Other atoms

such as potassium, iron, and magnesium may occupy spaces

within tetrahedra. Silicate tetrahedra may be joined together

in various arrangements. In minerals where tetrahedra occur

in isolated form, tetrahedra are packed together closely; gar-

net and olivine are examples of minerals with this structure.

An example of a mineral having rings of six tetrahedra is

beryl, commonly found in the form of six-sided crystals. Tet-

rahedra arranged in single chains (pyroxenes, for example)

and double chains (amphiboles—hornblende) are moder-

ately hard and tend to separate in a regular fashion (along

cleavage planes), parallel to the chains. In sheet form, tetrahe-

dra are linked together to form a planar structure; mica (mus-

covite and biotite) is a familiar example of a mineral with

this structure. In frameworks, where tetrahedra join to form

a three-dimensional lattice comparable to the framework of a

building, the best examples are feldspars (plagioclase and

k-feldspar), the most common mineral group, and quartz,

the most common mineral variety.

silicic Rich in silica.

sill A sill is a tabular body of intrusive igneous rock that lies

parallel to and along the layering of the country rock strata.

plutons that lie parallel to strata are said to be conformable

This photo is of a volcanic sill near Edinburgh, Scotland. The magma squeezed along the layering of the existing rock strata before solidifying. The

Palisades of New York and New Jersey formed by the same process. (Courtesy of NOAA)

sill 241