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

Подождите немного. Документ загружается.

Falcon Island Tonga Also known as Fonua Foo, Fal-

con Island has been emerging and disappearing beneath the

ocean surface for decades. HMS Falcon charted the island

in 1865 and described it as a shoal. Eruptions in 1885 pro-

duced a cone approximately 300 feet (91 m) tall, but erosion

destroyed much of the cone within months after eruptive

activity ceased, and nine years later the cone had been

reduced to a shoal again. Another eruption in 1896 raised a

small cone about 100 feet (30 m) above the sea, but this too

was destroyed in a matter of several years. Eruptions in 1927

and 1928 produced an island more than two miles (3 km)

long, but a quarter-century later, the island had vanished.

Fantale caldera, Ethiopia The Fantale caldera is located

in the main Ethiopian Rift near the point where the Great

Rift Valley joins the Afar Depression. Although Fantale

does not appear to have been an especially destructive vol-

cano in terms of fatalities and property damage, it appar-

ently has a notorious reputation in the surrounding area,

where parents are said to warn ill-behaved children that they

will melt like the volcano. An eruption in the 13th century

reportedly destroyed a village and a church. Several lava

flows occurred in the vicinity in 1820. Minor fumaroles

were observed at numerous places in the caldera in 1930,

and highly energetic fumarolic activity occurred in 1960, but

these observations are not thought to represent a great depar-

ture from normal activity.

Farallon crustal plate A lithospheric (tectonic) plate of

Earth that was believed to have occupied the space between

the Pacific crustal plate and the North American

crustal plate. It was located off what is now the coast of

California. The Farallon plate apparently was subducted

and consumed as North America advanced westward. The

subduction and destruction of the Farallon plate is thought

to have been responsible for widespread volcanic activity in

prehistoric California.

See also plate tectonics.

fault A break in Earth’s crust along which displacement of

rock occurs. Perhaps the world’s most famous fault is the San

Andreas Fault, which lies along California’s Pacific coast

for several hundred miles and is responsible for the earth-

quake that destroyed much of San Francisco and neighbor-

ing communities in 1906. faults are abundant in mountain

belts and deep basins, although they also may occur in some

flatlands. Some faults extend for hundreds of miles across

Earth’s surface and manifest themselves in dramatic ways.

Other faults may be comparatively small.

Faults are described as having strike (the compass direc-

tion relative to north) and dip (the angle of inclination or

slope relative to horizontal). Motion, or slip, can occur in

several directions along an active fault, and the various

kinds of fault are named accordingly. A strike-slip fault

exhibits horizontal or lateral movement (along strike direc-

tion), whereas a dip-slip fault displays vertical movement

(along dip direction). An oblique-slip fault shows characteris-

tics of both dip-slip and strike-slip faults. A normal fault is a

type of dip-slip fault characterized by the rocks moving nor-

mally with respect to gravity. If the fault is viewed as a hill,

the rock on it falls down the hill just as gravity requires. In a

reverse fault, rocks above the fault plane move up the hill

which is the reverse of gravity.

There are two types of strike-slip faults as well. The

faults are lines on a map or on the ground. The determina-

tion is made by standing on one side of the fault and looking

across to the other side. If the opposite side of the fault moves

to the left, it’s a left-lateral fault. If it moves to the right,

it’s a right-lateral fault. Oblique-slip faults can be named

by the components of movement.

Some faults are clearly visible on Earth’s surface. Evi-

dence of them may take such forms as displaced hills, offsets

in a shoreline across a fault, offsets in rivers, and elongated

lakes along faults (for example, Lake San Andreas near San

Francisco). Other faults are less evident, however, and in

many parts of the world—notably the eastern United States,

where bedrock lies buried under deep layers of sediment—

active faults may go unsuspected until movement along them

generates an earthquake. This was apparently the case with

the Charleston, South Carolina, earthquake of 1883,

which destroyed a large portion of the city, and the extremely

powerful earthquakes of 1811–12 in the New Madrid

Fault Zone in the Mississippi Valley.

There are various ways to identify an active fault and

estimate its potential for surface rupture in future earth-

quakes. One approach is to study the history of faulting and

movement. Another is to extreme ongoing earthquake activ-

ity and recurrence intervals (how often) for earthquakes.

Measurements of strain by geodetic surveys are also use-

ful. It is not always easy to differentiate between an active

and an inactive fault because a single fault may vary greatly

in its behavior over time. Whereas some faults are clearly

active because they display measurable creep or generate

frequent small earthquakes, not all faults are so consistently

active. A fault that has shown no activity for many centu-

ries may reactivate suddenly and slip 30 feet (9 m) or more

in a single event. The determination of a fault as active or

inactive may depend also, in part, on how much harm the

fault may cause if it should move significantly again. A fault

located near a nuclear power station, for example, may be

evaluated on a different basis than a fault that poses less of

a potential hazard to human activities. The evaluation of

faults as active or inactive is made harder by the fact that

not all active faults manifest themselves on the surface.

It is important to determine the slip rate along a fault in

projecting future activity. The slip rate is a measure of how

fast movement is occurring along a fault. It is also important

to know how often earthquakes occur along a fault and how

powerful the earthquakes are, as well how much slip occurs

per earthquake. The slip rate can be derived from a study of

recently formed features along a fault. The slip rate, however,

represents only an average over time of movement along the

fault and is not necessarily steady. Along a given fault, the

slip rate may represent steady activity over a certain period or

a few episodes of comparatively great and abrupt movement.

In other words, a fault may be very (even catastrophically)

active at some times and quiescent at others. As a rule, faults

along the boundaries of major plates of Earth’s crust exhibit

greater slip rates than faults elsewhere. In southern Califor-

nia, slip rates tend to be greatest along the San Andreas and

San Jacinto faults.

A fault’s potential for generating destructive earthquakes

may be judged partly on the basis of earthquake activity

recorded by instruments. If such activity is clearly associ-

ated with a fault on an ongoing seismological record, then

this information can help in associating the hypocenters of

earthquakes with a particular fault at depths of a few miles

or less below the surface. When numerous earthquakes can

be linked to a given fault in this way, with their hypocenters

on the fault at depth, then it can be concluded that the fault

is capable of generating more earthquakes in the future. This

approach cannot be used reliably to estimate a fault’s poten-

tial for destructive earthquakes in all cases, however, because

a fault can be quiescent but remain seismogenic, or capable

of earthquake activity. Moreover, even an active fault is not

necessarily dangerous to lives and property. A fault that gen-

erates many small earthquakes, for example, may pose lit-

tle or no danger to residents of the area. Even when a fault

exhibits surface rupture, its activity may be so gentle as to

pose no particular threat to the surrounding community.

Sometimes risk assessment for a seismogenic fault

requires estimating the maximum earthquake that the fault

could produce. Here, the historical record of seismic activ-

ity is important because the magnitudes of earlier earth-

quakes may provide guidance in estimating the possible

strength of other seismic events to come. (The strength of

the Fort Tejon earthquake, for example, has become a

benchmark to estimate the magnitude of future major earth-

Block models of the various types of faults. Strike-slip faults have

only lateral movement. Whether they are left- or right-lateral can be

determined by standing on one side of the fault and looking across to

the other. If the opposite side moves to the left, it is a left-lateral fault; if

it moves to the right, it is a right-lateral fault. Dip-slip faults only move

vertically and are classified based on gravity. It is normal for objects to

slide downhill and reverse for objects to slide uphill (the hill being the

fault plane).

84 fault

quakes in southern California.) Geologists also may find it

helpful to compare the history of seismic activity along a

particular fault with the history of another fault compara-

ble to it. The behavior of another fault under similar condi-

tions may provide clues to the behavior of the specific fault

concerned. The dimensions of a fault may provide some

clue to the magnitude of future major earthquakes because

big earthquakes are often associated with major faults. In

several studies, the length of a fault and especially its sur-

face rupture has been correlated with magnitude of earth-

quakes. A fault’s dimensions are not a completely reliable

guide to its seismogenic potential, however, because a fault

may not be traceable for its entire length at the surface

and because it is often difficult to foresee how far surface

rupture may extend along a fault in an earthquake. In that

event, the historical record may provide a better estimate of

how powerful an earthquake a given fault might deliver. (In

the Los Angeles area, a study of the history of earthquakes

and the geology of the region indicates that an earthquake

comparable in magnitude to the Fort Tejon earthquake of

1857 was about the most powerful seismic event that Los

Angeles may expect.) Because the maximum possible earth-

quake along a fault may occur only rarely, at intervals of a

century or longer on average, estimates of magnitude and

frequency may concentrate instead on the most powerful

earthquake that may be expected on a particular fault dur-

ing a shorter interval. Typical intervals are 50 to 60 years

because that is about the longest period many buildings are

expected to last.

One problem with using the historical record to esti-

mate the magnitude and frequency of potential earthquakes

is that the historical record does not always extend far

enough to make such estimates reliable. This is the case

in portions of western North America where records of

earthquake activity cover only a few decades. The histori-

cal record may be supplemented by studies of the geologic

record, which can reveal evidence of prehistoric earth-

quakes in the form of surface faulting and disturbance to

sediments. Such evidence may yield estimates of average slip

rate and the average interval between major earthquakes

along a specific fault. Studies of the San Andreas Fault in

southern California have indicated that major earthquakes

there have occurred at an average interval of approximately

50 years.

See also seismology.

fault plane solution When an earthquake occurs, it is

recorded on seismographs worldwide. seismologists

study the seismograph records (seismogram). By comparing

the delay between the arrivals of the waves, they can deter-

mine the location of the focus and epicenter. By studying

whether the needle on the seismograph first went up or went

down for each seismogram, they can determine what kind of

fault it was (strike-slip, dip-slip, etc.). By combining these

two pieces of information, seismologists can determine the

fault plane solution or focal mechanism for the earthquake,

that is, where and how the fault moved.

fault rocks There is a full classification system for fault

rocks, just as if they were sedimentary or igneous rocks.

Under shallow conditions 0–1.2 miles (0–2 km), the rocks

in a fault are broken into large to small angular fragments

with large gaps between them. This type of deposit is called

fault gouge. If the gouge is cemented together with minerals

precipitated from the groundwater, it becomes fault breccia,

a rock. Deeper in the ground 1.2–9.3 miles (2–15 km), the

pressure is too high to leave gaps. Instead, the rock is pul-

verized and ground together like coffee grounds. It is fine-

grained with no gaps and called cataclasite. Depending on

the amount of leftover uncrushed rock within the cataclasite,

the rock can be a protocataclasite (mostly uncrushed rock),

cataclasite (about half and half), and ultracataclasite (all

crushed rock). Below about 9.3 miles (15 km), rocks behave

in a ductile manner. They flow like silly putty in the fault

zone rather than crushing like glass as with cataclasites.

These rocks are called mylonites. They have the same classi-

fication as cataclasites. Fault rocks with mostly original rock

and very little stretched-out grains is called a protomylonite.

Half and half is a mylonite, and all stretched-out grains is an

ultramylonite.

fault system In contrast to a single fault or several parallel

faults, a series of interlinking faults is called a fault system.

Many of the major faults in the world are really fault sys-

tems. The San Andreas Fault is not a single fault but is part

of a large system of faults that form a braided pattern. The

Callaveras Fault and the Hayward Fault among many

others are really part of the San Andreas fault system. All of

the faults have similar orientations, they are all related, and

all are active at the same time (geologically speaking). Any

one of them can produce an earthquake at any time. During

the next 25 years or so they all will.

fault trace The surface expression of a fault. A fault is a 3D

planar to curvi-planar structure. The fault trace is the inter-

section of the fault with the ground surface. In other words,

if you look at a fault on a map or from an airplane, you are

looking at the fault trace. For very steep faults, like the San

Andreas, the trace is a linear feature. For shallow faults, espe-

cially in areas of rugged topography, the fault trace can be as

winding as a mountain road.

Federal Emergency Management Agency (FEMA) FEMA

is the official U.S. government agency that handles all natural

disasters, including earthquakes and volcanic eruptions. When

a disaster occurs, FEMA moves in quickly to coordinate rescue

and relief operations where possible and to distribute federal

aid to the affected communities all the way through the repair

and rebuilding processes.

feldspar The most common mineral group in Earth’s crust.

All feldspars are silicates in which the atoms are arranged in

a framework structure. There are two basic type of feldspar,

plagioclase and potassium (K)-feldspar (see k-feldspar).

feldspar 85

The main ions in plagioclase are calcium and sodium. Plagio-

clase occurs in probably 99 percent of all igneous rocks. Potas-

sium feldspar primarily occurs in granite and rhyolite.

felsic An igneous rock that is high in feldspar and

silica (fel from feldspar and si from silica). These are light-

colored rocks that form at the lowest temperatures possible

for igneous rocks (750–800°C). The most common rock

types include rhyolite (volcanic) and granite (plutonic).

These rocks must contain quartz and k-feldspar but

commonly contain plagioclase as well. These three miner-

als are commonly referred to as the felsic minerals. Felsic

rocks may also contain muscovite (white mica), biotite

(black mica), hornblende, and many other minerals in

small quantity.

Fernandina caldera, Galápagos Islands, Ecuador The

shield volcano Fernandina has a summit caldera whose

floor collapsed following eruptions in 1968. Two days after

a moderately strong earthquake about 200 miles (322 km)

north of the island, Fernandina began to erupt on May 20,

1968. This eruption lasted several days. Further earthquakes

on June 11 immediately preceded an explosive eruption that

morning, followed by another explosion later in the day. The

eruption continued for about one more day, and then the cal-

dera floor started to collapse. The greatest subsidence (more

There are ball-like symbols on earthquake maps that show the fault plane solutions or focal mechanisms of the earthquakes. The symbols show which

way the fault moved during the earthquake as shown by the diagram. The white stripes show compression, and the gray stripes show extension.

86 felsic

than 1,000 feet [305 m]) occurred at the southeast end of

the caldera. Several hundred moderately powerful earth-

quakes occurred during the collapse, which apparently took

place steadily over a period of several days rather than all at

once. The volume of the collapse was as much as 10 times

greater than the small volume of magma that erupted. Fer-

nandina erupted again in 1978, an hour following an earth-

quake. Another eruption occurred between late 1980 and

early 1982 but was so small that it went unnoticed until

later. Activity resumed briefly in 1984, between March 30

and April 11. The most recent eruption of Fernandina was

in 1995. lava flowed 2.5 to three miles (4–5 km) to the

ocean where it killed many birds and fish by scalding them

to death.

ferromagnesian Minerals and rocks that are high in iron

and magnesium. The term refers to silicate minerals. For

igneous rocks, these minerals include pyroxene, olivine,

hornblende, other amphiboles, and biotite. They are all

related in structure and chemistry. They are also referred to

as mafic minerals because they are abundant in mafic igne-

ous rocks. In ultramafic rocks, there are essentially no

other minerals. There are also a different group of metamor-

phic ferromagnesian rocks and minerals.

fiamme Dark-colored, glassy lens-like structures in pyro-

clastic deposits that have a flame shape. They are thought to

be formed by the collapse of pumice fragments during the

formation of a welded tuff by overlapping pyroclastic

flows. (Fiamme is Italian for “flames.”)

fire fountain A prolonged and high spray of lava above a

volcano’s vent. Also known as lava fountains, fire fountains

can be as high as 1,000 feet (305 m) but are more commonly

100 feet (30 m). They can fill lava lakes or feed flows.