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

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but was partially destroyed by an explosive eruption before

McCulloch Peak formed. McCulloch Peak was itself short-

lived; only 10 months after it appeared, the peak was demol-

ished by an explosion, leaving behind a heated lagoon almost

a mile wide. This eruption is said to have been spectacular,

with a thick black cloud and lightning reported. Further erup-

tions were reported in 1910, 1926, and 1931. The 1926 erup-

tion produced a new dome. An eruption of Bogoslof in the

1890s was one of several eruptions that occurred around the

same time (Bandai-San in 1888 and Vulcano in 1888–90).

Bolivia Located in the seismically and volcanically active

“Ring of Fire” that encircles the Pacific Ocean basin,

Bolivia is situated in northwest South America. A volca-

nic mountain in Bolivia, Cerro Rico, played an important

part in the economic exploitation of the Inca by the Spanish

Empire. The mountain contained rich veins of silver ore and

yielded great quantities of silver to be shipped to Europe. Vol-

canoes in Bolivia include Illimani, Parinacota, and Sajama.

On June 9, 1994, a massive earthquake of magnitude

8.2 occurred in a remote area of northern Bolivia. It triggered

many landslides and rockfalls, but damage to buildings

and roads was minor. The importance of this earthquake was

that it was felt throughout the United States and even in

parts of Canada. It is the first recorded incidence of an earth-

quake from this part of South America being felt in North

America. It is also the highest magnitude ever recorded in

this part of South America.

See also Andes Mountains.

Bolu-Duzce Turkey On November 12, 1999, an earth-

quake of magnitude 6.0 occurred. The earthquake resulted

in 834 deaths and 4,566 injuries. It triggered several major

landslides that blocked roadways. This earthquake was the

largest in a complex swarm of earthquakes that affected this

part of Turkey at the time.

bomb Fluid to blocky mass of lava that is shot out of a

volcano and solidifies or cools in midair before landing on

the ground. In general, volcanic bombs tend to be stream-

lined but may occur in many different shapes. If they are still

fluid, when they land they flatten. If solid, they tend to crack

and break apart. There are several different names for bombs

depending upon their shape, including bread-crust bombs

and cow-pie bombs.

Bonneville Slide Columbia River Gorge, Oregon and

Washington, United States The Bonneville Slide is a huge

quantity of soil and rock that fell from the sides of the

Columbia River gorge near Crown Point, east of Portland,

Oregon, some centuries ago, possibly around a.d. 1100.

Almost a half cubic mile of solid material was involved in the

Bonneville Slide, which may have occurred as a result of an

earthquake engendered by the Cascadia Subduction Zone

offshore to the west. The northern side of the gorge is espe-

cially susceptible to landslides because the uplift of the Cas-

cade Mountains has tilted the gorge toward the south and

because easily eroded sedimentary material underlying lava

flows along the northern side of the gorge undermines the

land. Landslides along the gorge are believed to have buried

more than 10 square miles (26 km

) of the gorge and created

a huge natural dam perhaps 200 feet (61 m) high and more

than 5,000 feet (1,500 m) wide. Other slides in the vicinity of

the Bonneville Slide include the Fountain Slide, Ruckel Slide,

Skamania Slide, Wind Mountain Slide, and Washougal Slide.

Boquerón volcano, El Salvador Located near the city of

San Salvador, Boquerón erupted explosively on June 6, 1917.

Though brief, the eruption destroyed most of San Salvador

and carried destruction over an area of some 20 miles (32 km)

around the volcano. A lake that had formed inside the cra-

ter of Boquerón spilled out in the form of a hot flood that

overwhelmed more than a dozen communities. lava from this

eruption collected to depths of 160 feet (49 m) in some loca-

tions, and ash accumulated in layers several feet in depth. One

curious effect of this eruption was hair loss among residents of

the area; evidently acid in an ashfall was responsible for this

occurrence. More than 400 people were reported killed in this

eruption of Boquerón. On more than a dozen previous occa-

sions, eruptions had destroyed the city of San Salvador.

bore Among the many meanings of the word bore, it also

describes a single wave in the ocean with a near vertical front.

These waves can result from seismic activity.

Bowens Reaction Series A theoretical model for the order

of crystallization of minerals in igneous rocks that explains

most rocks quite well. The diagram for crystallization is Y-

shaped with temperature decreasing downward. The left arm

of the Y represents the discontinuous reaction series (different

minerals) and the right arm represents the continuous reac-

tion series (one mineral). At about 1,400°C, olivine begins

to crystallize from the melt on the discontinuous side. As tem-

perature decreases, pyroxene, hornblende, biotite, and

muscovite will crystallize in progression, depending upon

composition of the melt. These minerals are largely ferro-

magnesian and dark in color. At 1,400°C, plagioclase

also begins to crystallize on the continuous side and it crys-

tallizes all the way down with decreasing temperature. It is

Breadcrust volcanic bomb named for the breadcrust-like cracked surface

of the bomb. (Courtesy of the USGS)

32 Bolivia

calcium-rich at high temperature and sodium-rich at low tem-

perature. The ratio of calcium to sodium depends upon the

temperature. At the stem of the Y are the lowest temperature

minerals k-feldspar and finally quartz. They crystallize at

about 750°C. This model also shows the mafic rocks crystal-

lize (and erupt) at the highest temperature (1,400°C) because

they are composed of olivine, pyroxene, and Ca-plagioclase.

intermediate rocks are composed of hornblende, biotite,

and Ca-Na plagioclase and form at intermediate temperatures

(1,000°C). felsic rocks are composed of quartz and feldspar

and form at the lowest temperatures (750°C).

bradyseism A long-continued, extremely slow vertical

instability in the crust near a volcano as in the volcanic dis-

trict west of Naples, Italy. The Phlegraean bradyseism

involves movements from 20 feet (6 m) above to 20 feet (6

m) below sea level during the last 2,000 years.

branching fault Branch off of a main fault. Also known

as a splay fault. These faults may take up some of the

movement that would have otherwise occurred on a main

fault. They may also relieve stresses that have built up but

which cannot be relieved by movement on a main fault. Most

branching faults are smaller than the main fault, but in some

cases the major movement can shift over to the branching

fault to the point of leaving parts of the main fault inactive.

bread-crust bomb Volcanic bomb with a surface that

looks like bread crust. The bomb has a rough surface that

develops cracks as it cools. It looks like old-fashioned or bak-

ery bread in which the crust is cracked as the result of expan-

sion during baking.

breccia A coarse-grained sedimentary rock that is made

up of angular clasts, or particles. They typically indicate that

the source of the rock fragments is nearby. There are also

volcanic breccias. In the case of volcanoes, a breccia is a rock

made up of angular clasts that are fragmented from explo-

sions at the vent of a volcano. They look similar to sedimen-

tary breccia but all of the fragments are volcanic rock and

there can be distinctive cementing material. There is also

fault breccia. Fault breccia is made of angular clasts that

were fragmented by fault movement during an earthquake.

The size of the clasts in fault breccia can vary widely.

British Columbia Canada The Canadian province of

British Columbia is located on the Pacific coast within the

circum-Pacific “Ring of Fire,” the belt of earthquake and

volcanic activity that encircles the Pacific basin. Although not

as noted for earthquake and volcanic activity as the coastal

United States (California, Oregon, and Washington)

to its south, British Columbia has a history of considerable

seismic and volcanic activity. This activity is due to the

Volcanic vent breccia produced during an explosive eruption. The white broken fragments are encased in lava. (Courtesy of the USGS)

British Columbia 33

subduction of the Juan de Fuca crustal plate along the

Cascadia Subduction Zone beneath the Pacific Ocean

to the west of the province. This subduction zone produces

the volcanoes of the Cascade Mountains, which reach into

southern British Columbia and include Mount Garibaldi, a

volcano north of Vancouver.

brittle A brittle response of a material to stress is to crack

and shatter like glass. Brittle rocks break into unconnected

pieces that are internally unchanged by the stress. For them

to break, stress must increase until it exceeds the strength

of the rock. The rock snaps, releasing sound and seismic

waves. The snapping is an earthquake. Therefore, all shal-

low faults are brittle. In contrast, ductile deformation

involves the slow and steady shape change as would occur in

squeezing gum.

brittle-ductile boundary In the shallow crust, rocks

deform in a brittle manner. They crack upon failure and

produce earthquakes. Deeper in the crust, rocks behave in a

ductile manner, distorting but not breaking. These rocks

typically do not produce earthquakes. There is a diffuse

boundary between rocks exhibiting each of these mechani-

cal behaviors that depends upon various factors. Typically,

this boundary lies at a certain depth below the surface as

a function of increasing temperatures and pressures. For

the dominant quartz-feldspar-rich rock, the transition

occurs between six and nine miles (10 and 15 km) of depth.

Under most crustal conditions, earthquakes are generated

at depths of less than six to nine miles (10–15 km). Rock

type is the primary determining factor. Clay is ductile at the

surface of the Earth, whereas ultramafic and mafic igne-

ous rocks can be brittle through the entire thickness of the

crust. mantle rocks are ultramafic, so they can be brittle

to much greater depths. Earthquakes can be generated at

depths of 240 miles (400 km) or more in a subduction

zone because the rocks are driven to great depths before

they can heat up. Normally, the transition from the lith-

osphere to the asthenosphere marks the brittle-ductile

transition in the mantle.

Bucharest earthquake, Romania A major earthquake struck

Bucharest, the capital city of Romania, at 9:20 p.m. on March

4, 1977. The earthquake had a Richter magnitude of 7.4

The 1977 earthquake in Bucharest caused the partial collapse of the reinforced concrete frames and masonry walls in office and apartment

buildings. (Courtesy of the USGS)

34 brittle

and was felt throughout the Balkans. Earthquakes in this area,

termed the Vrancea Zone, are typically from deep foci. The

recurrence intervals for seismic events are 30 years for mag-

nitude 7, and 120 years for magnitude 7.5. This earthquake

caused between 1,570 and 2,000 deaths. Damage was extensive.

Some 30,000 apartments, 32 eight-to-12-story buildings and

150 older six-to-eight-story buildings were destroyed, leaving

more than 35,000 families homeless. The total damage was esti-

mated at $2 billion.

Bulusan volcano, Philippine Islands Also known as Iro-

sin, the stratovolcano Bulusan is located in the southern

part of Luzon Island. Recorded explosive eruptions date

back to 1852 and have occurred every few years since then.

It erupted at least 13 times between 1886 and 1988. Seismic

records from 1978 to 1983 indicate that earthquake activity

increased before eruptions, and temperatures at hot springs

in the vicinity during this period have shown marked fluctua-

tions. earthquake swarms, however, have not always been

followed by eruptions. Earthquakes and hot-spring activ-

ity did not provide any warning of a small eruption of ash

in 1988, although an earthquake swarm was noticed some

hours following the eruption.

butte A geomorphic feature with steep sides and a flat top.

Buttes must be higher than they are wide. Otherwise, they

would be classified as a mesa or a plateau. Old weathered-

away volcanic necks like Devils Tower form buttes.

Buyin-Zahra earthquake, Iran The earthquake-ravaged

country of Iran was once again struck by a deadly temblor

in the Buyin-Zahra region in the northwestern part of the

country on September 1, 1962. The quake registered a 7.3 on

the Richter scale, and the focus was determined to be at

12 miles (20 km) of depth. Damage to buildings occurred in

Tehran, more than 100 miles (160 km) from the epicenter.

Some 12,220 people lost their lives in this disaster, and

more than 30,000 people were made homeless. More than

21,000 houses were destroyed, and heavy damage extended

60 miles (100 km) from the epicenter. Total property damage

was estimated at US$30 million in 1962 money, which made

it the most expensive disaster in Iran up to that point. It shut

down the Trans-Iranian railway for several days, stalling the

relief effort. The United States provided significant amounts

of aid that was distributed by the Iranian Red Cross.

b-value The b-value for an area is a calculated factor that

describes the ratio of small to large earthquakes within a

given time period. The b-value is commonly constant over a

large range of earthquake magnitudes. Mathematically, it is

the slope of the curve in the Guttenberg-Richter earthquake

recurrence relationship.

b-value 35

Cairo earthquake, Egypt Approximately two-thirds of

the great city of Cairo, Egypt, was reportedly destroyed by

a devastating earthquake on September 10, 1754. The esti-

mated Richter magnitude for this event was only 5.4, but

the epicenter was right under the city and the focus was at

a shallow depth. The damage rated X on the modified Mer-

calli scale. The final death toll was estimated at 40,000,

making it one of the most destructive in Egyptian history.

Calabria earthquakes, Italy The earthquakes that began in

Calabria in southern Italy in early February 1783 and con-

tinued for the rest of that year were felt over a wide area of

the Italian Peninsula and Sicily. The earthquakes began with

a powerful shock on February 5 and continued with another

strong event on March 28. The earthquakes appear to have

been centered near the town of Oppido and to have caused

extensive destruction for several miles around. Scarcely any

structure in Oppido was said to remain standing after the first

earthquake. Large chasms opened in the ground and report-

edly swallowed numerous houses. One chasm opened in these

disturbances measured some 500 feet (152 m) long and 200

feet (61 m) deep. landslides occurred along the shores of the

Straits of Messina and allegedly destroyed numerous villas.

Following the February 5 earthquake, residents of the

area recognized the danger of remaining on the coast and

evacuated the shoreline, either moving slightly inland to ele-

vated ground or putting out to sea on fishing boats. These

safety measures proved ineffective. The night after the evacu-

ation, a strong earthquake dislodged a large mass of earth

from Mount Jaci; part of the falling material landed in the

sea and generated a disturbance that sent a wave rolling over

the ground on which the refugees had gathered. The wave is

also said to have caused the destruction of the fleet of boats.

More than 1,400 people were reported killed. Much of the

city of Messina was destroyed in the earthquakes by the

shocks themselves and by subsequent fires.

The earthquakes in February and March are believed to

have killed some 40,000 people, not counting an estimated

20,000 who died later from exposure, disease, and other

causes. Reportedly, thermal springs burst forth from fissures

in the ground. One spectacular account says that some indi-

viduals fell into these fissures and then were cast out again

in outbursts of boiling water; although survivors of these

ordeals were reportedly rare, some did live through them but

were said to have been crippled by burns for the remainder of

their lives.

In another great earthquake at Calabria on December

16, 1857, whole villages reportedly fell into huge fissures in

the earth, and more than 10,000 people are thought to have

been killed. According to one estimate, some 111,000 people

in the vicinity of Calabria were killed in earthquakes between

the years 1783 and 1857—an average of 1,500 deaths per

year.

Calaveras Fault California One large splay fault off the

main San Andreas Fault. It is an active fault that produces

major earthquakes.

Calbuco volcano, Chile The stratovolcano Calbuco has

erupted several times since 1837. A 1929 eruption buried for-

ests in the vicinity under thick layers of ash.

calc-alkaline series A chemical index for volcanic rocks

that are produced in a subduction zone. The series usu-

ally begins with basalt of a composition distinct from that

produced in the mid-ocean ridge or in a hot spot. Later

volcanic rocks are andesite and possibly dacite. This suc-

cession contrasts sharply with that produced in a mid-ocean

ridge, which is referred to as a tholeiitic series.

Calcutta earthquake?, India The 1737 Calcutta earth-

quake was estimated to have caused some 300,000 deaths,

placing it among the top 3 or 4 earthquakes of all time.

Recent analyses of the event, however, not only cast grave

doubt on the number of casualties but also whether there

was an earthquake at all. The problem is that on the day of

the supposed earthquake, October 11, 1737, a major cyclone

struck the Calcutta area. It was such an intense storm that it

scuttled and sank eight of the nine British ships moored in the

Ganges River and virtually all of the boats operated by the

natives. Certainly, many deaths were attributed to this storm.

The British East India Company, however, did not compile

an official report of the disaster until the survivors reached

headquarters in London some six months later. Their official

report listed 3,000 fatalities, with no mention of an earth-

quake at all. In the following year, Gentlemen’s Magazine

and London Magazine featured stories that claimed 300,000

deaths as the result of an earthquake. Thus began the con-

fusion about the event. Historical records indicate that the

urban population of Calcutta in 1737 was probably less than

20,000. There were other settlements outside of Calcutta,

but it is doubtful that there were 300,000 in the whole area,

much less that many killed. Although there were numerous

small to moderate earthquakes in the Calcutta area, they were

not the huge temblors that struck the plate boundary area to

the north. It is unlikely, but not impossible, that such a huge

earthquake could even strike Calcutta. It is most likely that

the 1737 Calcutta earthquake is the earthquake that never

happened.

caldera A large, roughly circular depression commonly

formed by the explosion or collapse of a volcanic cone at

any elevation. A caldera may be miles in diameter and har-

bor lakes containing volcanic islands. A famous example of a

caldera is Crater Lake in Oregon. A caldera is much wider

than the volcanic vent(s) involved in its formation. A caldera

complex consists of the rock assemblage underlying a caldera

and may involve a wide variety of constituents, including, but

not limited to, tuff, breccia, dikes, stocks, and sills. A

“resurgent” caldera is one in which volcanic activity resumes

after the caldera has formed. Crater Lake again is a familiar

example of a resurgent caldera. Others include Creede Cal-

dera in Colorado and Valles caldera in New Mexico.

The formation of a caldera is a spectacular event. A tre-

mendous explosion or series of explosions may be involved,

as in the case of the destruction of Krakatoa, where explo-

sions demolished much of the island and left behind a fresh

caldera. Such an event may affect areas hundreds or even

Four-stage schematic model for caldera collapse and the formation of a crater lake at Mount Mazama. (A) Typical volcanic eruption with overlapping

stratovolcanoes. (B) Collapse of the caldera by gravity as magma is drained from the magma chamber. This results in extensive pyroclastic flows. (C)

Post-eruption fumarole activity. (D) A small cinder cone is built as water fills the collapsed caldera.

38 caldera

thousands of miles away, through mechanisms such as ash-

falls and tsunamis. The opposite process, collapse, rather

than explosion, also may be responsible for forming calde-

ras. Collapse may occur when magma underneath a volcanic

peak drains away through underground conduits or perhaps

through a parasitic vent on the flank of the volcano; as the

molten rock below it is removed, the peak loses its underpin-

nings, so to speak, and falls inward upon itself. This process

appears to have created Crater Lake in Oregon, one of the

most accessible and scenic calderas in the world. Calderas are

not always so easy to identify as those at Krakatoa and Crater

Lake; subsequent volcanic activity and/or erosion may cover

or erase many traces of a caldera. Over many years, several

calderas may form at a given location and form a complex

pattern of overlapping, roughly circular features.

Activity at calderas may take many forms. Ground

deformation may occur. In some cases, ground deforma-

tion is so slight that it can be measured only by using special,

sensitive instruments. In other cases, ground deformation

may be dramatic, measured in inches per day. In rare cases,

ground deformation has lifted structures at harbors many feet

out of the water. Giant shield volcanoes, such as those in

Hawaii, exhibit a special pattern of deformation. The entire

summit inflates until lava emanates from the volcano. Some-

times uplift at calderas can elevate the surface more than

1,000 feet (305 m), as happened during an eruption at Toya

caldera in Japan. A caldera floor may also drop suddenly by

hundreds of feet under the appropriate conditions. Perhaps

the most famous case in point occurred at Fernandina cal-

dera in the Galápagos Islands in 1968, when a portion of

caldera floor subsided up to 1,000 feet (305 m) or more in

little more than a week. As a rule, however, uplift and sub-

sidence at calderas tend to be more gradual.

Temperature changes are commonly seen when activity

occurs at calderas. fumaroles, lakes, hot springs, and the soil

itself may show marked increases in temperature. The water

level in crater lakes may change, as may the colors of lakes.

Chemical changes in groundwater, hot springs, and fumaroles

also may be observed. Another phenomenon observed in and

near calderas in connection with eruptions is change in Earth’s

magnetic field. Little is understood about these changes,

although they are thought to be linked to tectonic forces or to

demagnetization of country rock. Changes in gravity have

been noted at calderas and have been attributed to uplift or

subsidence, changes in density of rock beneath the surface, and

movement of magma or groundwater.

Unrest at a caldera may be brief or prolonged, depending

on circumstances at the site. Some calderas have been active

almost continually in the recent past, whereas others have

been quiet for many centuries. There is also a great range of

intensity in caldera unrest. Activity at some calderas has been

extremely violent, whereas other calderas have a history of

only mild activity. The most evident unrest may be confined

to the area of an active vent, although it is commonplace for

unrest at a caldera to involve the entire caldera in one way or

another; deformation and other signs of unrest may occur all

through a caldera during an episode of unrest.

Among the more exotic factors that appear to influence

activity at calderas are ocean loading and unloading, mean-

ing the increase and decrease respectively of the weight from

water piled atop the area of a caldera. Increased ocean load-

ing is thought to contribute to eruptions in some situations

by exerting pressure on a magma reservoir underground and

forcing magma toward the surface. This process is believed to

operate at Pavlof volcano in Alaska from time to time. On

the other hand, ocean unloading also is suspected of creating

conditions favorable to eruptions; a case in point is Rabaul,

where an eruption in 1983 and 1984 is believed to have been

connected with ocean unloading.

As a rule, earthquakes in or near calderas are small,

although there have been notable exceptions to this pattern,

such as the powerful earthquakes, greater than magnitude 7

on the Richter scale, that have occurred in the vicinity of

Kilauea in Hawaii on occasion. Earthquakes in a caldera

setting have various causes. Some are tectonic in origin,

meaning they result from the readjustment of shifting blocks

of Earth’s crust rather than directly from volcanic processes.

(A very strong earthquake in the vicinity of a caldera stands a

good chance of being tectonic rather than volcanic in origin.)

Some earthquakes at calderas are associated with intrusion of

molten rock underground, whereas others may result from

processes including tectonic activity and the draining away

of molten rock from beneath the caldera. Volcanic tremor

is the expression for earthquakes that are thought to result

from flow of magma and/or gases through narrow space

underground or other mechanisms such as the collapse of

bubbles in magma. There is no single pattern of seismic activ-

ity associated with eruptions at calderas; although, one pat-

tern often observed is for an eruption to occur as earthquakes

reach a maximum of energy release after a quick increase.

Another pattern sometimes seen is for an eruption to begin

in the lull after seismic energy has increased and then dimin-

ished sharply. There appears to be no connection between the

A large caldera on Kilauea, Hawaii. The white steam comes from boiling

groundwater. (Courtesy of the USGS)

caldera 39

intensity of earthquake activity before an eruption at a cal-

dera and the power of the eruption itself. Eruptions at calde-

ras may be classified as normal explosions, involving magma

and gases released from it, and phreatic explosions, involv-

ing groundwater turned into vapor. Phreato-magmatic explo-

sions, as they are sometimes called, are combinations of the

previous two varieties. Eruptive phenomena may include ash-

falls, lava flows, lava lakes, domes, crypto-domes, land-

slides, cinder cones,

NUÉE ARDENTEs, avalanches, and

submarine and subglacial eruptions.

caldera complex A group of multiple calderas on a single

or even small group of volcanoes. Caldera complexes are

common on shield volcanoes. There can be several calde-

ras that are concurrently active, or they can be intermittently

active. Typically, one of the calderas will be dominant both

in size and activity. In addition, new calderas may develop

while others may become inactive. A good example of a cal-

dera complex is Kilauea on Hawaii. Several calderas on the

same shield volcano have been active through recorded his-

tory. Each is named.

California United States California is known as the earth-

quake state because it has been the site of some of America’s

most powerful and destructive earthquakes, including the

famous San Francisco earthquake of 1906. The great San

Andreas Fault lies along the western shore of California, but

it is not the only active fault in the state. Numerous faults,

inactive and active, are found in California, and no part of the

state is completely free from seismic disturbances. Well-known

faults in California include the Hayward Fault, Calaveras

Fault Zone, Imperial Fault, Manix Fault, and Garlock Fault.

California is so active from a seismic standpoint that a com-

plete list of all earthquakes in its history is impossible in the

space available here. What follows is a partial list of major

California earthquakes during the past two centuries:

EARTHQUAKE HISTORY OF CALIFORNIA

Early Earthquakes

An earthquake on July 28, 1769, in the region of Los Ange-

les involved four very strong shocks followed by many oth-

ers over the next few days and possibly into the following

year. In October 1800, an earthquake at San Juan Bautista

left every dwelling there uninhabitable, and carts outdoors

were used for sleeping; the date of the principal shock is not

known, but it reportedly was accompanied by an extremely

loud noise. An earthquake in June 1838 in the area of San

Francisco is thought to have been comparable in power to the

catastrophe that destroyed the city in 1906; the earthquake

generated effects of Mercalli intensity X, and large displace-

ments were reported along the San Andreas Fault. The shock

was especially strong at Yerba Buena, and heavy damage was

reported at the missions in San Francisco, San Jose, and Santa

Clara, as well as at the Presidio in San Francisco.

Hayward, 1868

The October 21, 1868, earthquake at Hayward caused maxi-

mum damage there and at other locations along the Hayward

Fault. Virtually every building in Hayward experienced heavy

damage, and some were ruined completely. Civic buildings

were destroyed in San Leandro. San Francisco again experi-

enced damage to buildings on landfill, and property damage

appears to have exceeded $350,000. Some 30 persons are

thought to have been killed in this earthquake, which was fol-

lowed by aftershocks continuing into the following month.

The great San Francisco earthquake of April 18, 1906, is cov-

ered in detail elsewhere in this volume.

Santa Barbara, 1925

The Santa Barbara earthquake of June 29, 1925, caused an

estimated $8 million in damage in Santa Barbara, where

most buildings on the principal street in the commercial dis-

trict were damaged. Thirteen people were killed in the earth-

quake, and movement of approximately one foot was seen

along a boulevard built on a beach. An earth dam at a reser-

voir failed, but the freed water did little harm. Much of the

exterior of a hotel collapsed. An office building failed, and

the roof of a church collapsed. This earthquake served as a

reminder that sound design and construction can preserve a

Fault and major recent earthquake map of northern California, San

Francisco Bay area. The San Andreas Fault is a transform margin whose

actions control the activity of the other faults in the area. They are either

splays or compensation features. The star symbols are the locations of

major earthquake epicenters with the year of the event and the Richter

magnitude followed by an M.

40 caldera complex

building even in earthquake-prone territory and on less than

stable soil. For example, one concrete building, well designed

and given a good foundation, went through the earthquake

with little or no damage, even though the building was con-

structed in a marshy area. Reinforced concrete proved resis-

tant to the earthquake, except where construction was faulty.

Strong aftershocks were reported on July 3. Another earth-

quake at Santa Barbara on June 29, 1926, killed a child and

knocked down some chimneys.

Long Beach, 1933

The Long Beach earthquake of March 10, 1933, was not one

of the most powerful in California’s history (magnitude of

only 6.3), but it caused extensive damage because it occurred

in a heavily settled area. The epicenter was located slightly

offshore from Huntington Beach, but the shock was felt from

San Diego to Santa Barbara and as much as 50 miles (80 km)

inland. More than 120 people were killed, and hundreds

more were injured. Damage was estimated at approximately

$50 million. Most damage occurred in an area character-

ized by water-rich soil and structurally unsound buildings.

The most strongly affected area extended from southern Los

Angeles to Manhattan Beach, Anaheim, and Laguna Beach.

Destruction was especially notable in such places as Comp-

ton, Huntington Park, and Long Beach. Severe damage to

up to 75% of school buildings occurred, some to the point

of complete collapse. If school had been in session, it is esti-

mated that there would have been several thousand deaths.

No fault displacement was noted.

El Centro, 1940

One of the most famous and powerful earthquakes in Cali-

fornia history was the Imperial Valley earthquake of May 18,

1940. The earthquake was estimated at magnitude 7.1, gen-

erated effects of Mercalli intensity X, and had its epicenter

southeast of El Centro. Horizontal displacement up to 15 feet

(5 m) and vertical displacement of as much as four feet (1.3

m) were reported. Damage was widespread in the Imperial

Valley and was estimated at $6 million on the United States

side of the border alone, not counting indirect effects such

as those on agricultural irrigation systems. Surface ruptures

occurred over some 40 miles (70 km). About four-fifths of

the buildings in Imperial were damaged, and the city’s water

tanks were ruined. Almost half the buildings in nearby Braw-

ley were damaged, and a city water tank collapsed at Holt-

ville. The earthquake also was blamed for a fire at a hotel in

Mexicali, Mexico.

One of California’s largest earthquakes was in Kern

County, 1952. It and the San Fernando earthquake of

February 9, 1971, are described in detail elsewhere in this

volume, as is the 1989 Loma Prieta earthquake in the San

Francisco area and the 1994 North Ridge earthquake near

Los Angeles.

Tsunamis in California

California, having a long coastline, is subject to occasional

tsunamis, or seismic sea waves. These waves may be gen-

erated by earthquakes along the California coast or else-

where; the 1964 Good Friday earthquake in Alaska, for

example, produced a tsunami that caused extensive damage

in Crescent City, California, near the Oregon border.

One of the most remarkable tsunamis in California history

occurred on December 17, 1896, when a wave that may

have been seismic in origin struck Santa Barbara and carried

away a large portion of a boulevard that had been reinforced

especially to resist wave action. A large hill of sand between

the boulevard and the usual high-tide mark was also carried

away by the wave. There is a possibility, however, that this

wave was produced by a storm rather than by seismic activity

because there appears to have been no strong earthquake in

the vicinity on this date.

Because of the earthquake potential of southern Califor-

nia, the “tsunamigenic,” or tsunami-making, potential of faults

there has received considerable attention. Generally speak-

ing, faults along the southern California coast do not appear

to pose a great tsunamigenic threat, but faults thought to be

capable of producing tsunamis are found in some locations

off the coast of southern California, notably in the vicinity of

Point Arguello. In and near the Santa Lucia Bank, an elevated

area on the continental slope near the California shore, faults

are suspected of being able to produce powerful earthquakes

and the tsunamis associated with them. Another offshore area

with suspected tsunamigenic potential is located in the area

of Santa Barbara Channel and the nearby Transverse Ranges.

Powerful, destructive earthquakes have occurred in this area

(the December 21, 1812, earthquake that caused such heavy

damage to missions onshore, for example, is thought to have

involved fault rupture under the Santa Barbara Channel), and

future earthquakes of high magnitude might result in tsuna-

mis of considerable destructive power. Tsunamis produced

by submarine slides are another possibility along the southern

California coast, although this particular hazard is thought to

be small. The California coast remains vulnerable, however,

to tsunamis involved with seismic disturbances elsewhere

around the Pacific Basin, particularly in Alaska, where certain

areas contain the potential for very powerful earthquakes that

might produce large tsunamis.

California also has a history of recent volcanic activity,

some of it during the 20th century. Lassen Peak, a volcano

in northern California, erupted with great violence during

World War I. Volcanic formations may be seen also at Long

Valley caldera. geothermal energy plays a small but

interesting part in providing electric power for California.

Cameroon, Mount Cameroon A stratovolcano located

180 miles west of the capital city of Yaoundé. It is also

known as Mount Faka and “Chariot of the Gods.” It gen-

erally produces flank eruptions with lava flows from

small cinder cones. One eruption in the fifth century b.c.

and observed by a Carthaginian ship is one of the earliest

recorded volcanic eruptions in the world. Mount Cameroon

is very active, having produced six eruptions in the 20th cen-

tury alone. Its most recent activity lasted from March until

June 1999. Thousands of people were evacuated, and hun-

dreds of homes were destroyed during this eruption.

Campania earthquake, Italy On November 23, 1980,

an earthquake of magnitude 6.8 struck southern Italy. It

Campania 41