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

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resulting slumping and unequal settling. About one-third of

the city subsided some seven feet (2 m) as the result of com-

paction. The earthquake is famous for a group of apartment

buildings that were built to be earthquake-proof. They with-

stood the severe shaking by the seismic waves, but the foot-

ings around their foundations could not withstand the severe

liquefaction, and they fell over on their sides, still completely

intact.

Nisyros See Nysiros.

Niuafo’ou Island Tonga Niuafo’ou Island is also known

as Tin Can Island because of the way in which mail used to

be delivered to the island. Because the island had no harbor,

mail would be sealed in a metal container and dropped over

the side of a ship. A swimmer then would carry the can of

mail to shore. This method is said to have been discontin-

ued after a shark attacked a swimmer who was retrieving

the mail. The island is located in the northern portion of the

Tonga Archipelago and consists of a lava shield surmounted

by a composite volcanic cone. A large lake, called simply Big

Lake, occupies much of a caldera on the summit. A second,

smaller lake known as Little Lake is also located in the cal-

dera. Eruptions within historical times are recorded as early

as 1814, when an explosive eruption apparently was accom-

panied by phreatic activity and lava flows. This eruption

occurred in the caldera itself. Details of a reported eruption in

1840 are unavailable. Eruptions in 1853 and 1867 involved

fissures on the flanks of the volcano.

Observers of an eruption in 1886 left detailed accounts

of the event, which was heralded by earthquakes and an

apparent dramatic uplift in the seabed. The captain of a ship

anchored at Niuafo’ou discovered, after an earthquake on

August 12, that his ship was floating in only 48 feet (8 fath-

oms) of water, compared to some 120 feet (20 fathoms) the

previous day. A very loud noise was heard (compared after-

ward to the sound of a heavy gun), and some kind of dis-

charge occurred in the lake. Earthquakes stopped at this time.

Some subsidence appears to have occurred around the caldera

during this eruption.

A curious detail was noted in eruptions in 1919 and

1929: Coral was expelled along with lava, even though there

appeared to be no recently formed coral on the island. Very

little seismic activity was reported before the 1929 eruption.

An eruption in 1943 required the evacuation of the island,

and the inhabitants did not return for more than a decade.

Another eruption in 1946 was preceded by a very strong

earthquake just more than an hour before the eruption

began. Considerable uplift was reported in this eruption. One

account says that a reef on the northern side of the island was

raised 30 to 40 feet (9 to 12 m) out of the water in places.

Moderately strong earthquakes occurred in 1985.

normal fault See fault.

North America The North American continent is charac-

terized by localized earthquake activity and active volcanism.

According to the plate tectonic model and its dynamics,

North America occupies a plate of Earth’s crust that is mov-

ing slowly westward as new crust is formed along the mid-

ocean ridge that lies in the middle of the Atlantic Ocean.

The ongoing collision between the North American mainland

and the oceanic crust underlying the Pacific Ocean has

generated a complex geology along the Pacific coast of the

continent. It is marked by major and minor faults with asso-

ciated seismic activity, although there are other earthquake-

prone areas in North America. Much of North America’s

seismic activity is concentrated in the state of California,

where the San Andreas Fault, which lies along the coast,

has been associated with numerous strong earthquakes, such

as Fort Tejon in 1857, that which destroyed much of San

Francisco in 1906, and the less powerful but also highly

destructive San Fernando earthquake of 1971, the Loma

Prieta earthquake of 1989, and North Ridge earthquake

of 1994.

In both northern and southern California, but especially

in the Los Angeles area, urban development has created a

tremendous potential for destruction in the event of another

major earthquake comparable to that of 1906. Similar dan-

gers exist in other parts of the continent, such as the cities

surrounding Puget Sound. Other areas of North America

with a history of strong earthquake activity in the continen-

tal United States include New England, where earthquakes

have caused widespread damage to Boston, Massachusetts,

within historical times. In addition, Louisiana’s Gulf coastal

plain; upstate New York along the Saint Lawrence Val-

ley; the Appalachian Mountains, especially the southern por-

tion of the chain, from Virginia through Alabama; Utah,

which occupies part of a belt of pronounced earthquake

activity extending northward from Arizona to the Cana-

dian border; coastal South Carolina, site of the devastating

Charleston earthquake of 1886; and the Mississippi River

valley in the vicinity of New Madrid, Missouri, where the

most powerful earthquakes in the history of the continental

United States are believed to have occurred during the win-

ter of 1811–12. Virtually every part of the United States has

experienced seismic activity at one time or another since

settlement by Europeans began, although certain portions

of North America, such as the stable rocks of the Canadian

Shield, are all but free of substantial earthquakes.

The earthquake potential of western North America is

generally better understood than that of the eastern portion

of the continent, partly because earthquakes occur more often

in the west and partly because active faults in the east tend to

lie under thick layers of sediment that have hindered efforts

to study the seismology. Earthquake activity is also intense in

Alaska, the site of one of the greatest earthquakes of modern

times, the Good Friday earthquake of 1964, which was

accompanied by a powerful tsunami that caused great dam-

age as far away as Crescent City, California. Earthquake

activity in Alaska concentrates along the Aleutian Islands

and extends into the central portion of the state.

Mexico, too, has a long history of strong seismic activ-

ity. Earthquakes have caused widespread destruction in

Mexico over the centuries, notably in the Mexico City

earthquake of 1985. Western Mexico shares with the west-

ern United States and Canada

a potential for devastating

earthquakes, as smaller blocks of crust along the coast are

182 Nisyros

squeezed between the westward-moving continent and the

oceanic crust to the west. Especially susceptible to earth-

quakes are the northwest corner of Mexico, which is sub-

jected to the same set of geological conditions that make

southern California susceptible to major earthquakes, and

the extreme southern Pacific shores of Mexico, where earth-

quake epicenters are clustered almost as densely as in Cali-

fornia. The depth of earthquake foci differs greatly from one

portion of North America to another. Earthquakes in Alaska,

for example, extend much more deeply than those in south-

ern California and Nevada.

Volcanism has done much to shape the landforms of North

America. The western states of the United States exhibit numer-

ous signs of past and current volcanic activity. Great flows of

basalt in the northwestern states of the United States testify to

past volcanism in the Columbia plateau. A subduction zone

off the coast of the Pacific Northwest of the United States and

the Canadian province of British Columbia has given rise to

the volcanoes of the Cascade Mountains, including Mount

Hood, Mount Baker, Mount Rainier, Lassen Peak, and the

recently active Mount Saint Helens, as well as the photogenic

caldera of Crater Lake (Mazama) in Oregon. Volcanism

in Mexico has a long and colorful history; numerous volcanic

mountains stand near Mexico City and southward along the

Central American arc. The volcano Paricutín formed in a

spectacular eruption in a Mexican farmer’s field earlier in this

century. Others include El Chichón, whose massive eruption

in the early 1980s affected the weather patterns, Jorullo, and

Colima. Alaskan volcanoes have erupted on many occasions

in this century, sometimes with great violence, as in the Mount

Katmai-Novarupta eruption of 1912, which laid down a plain

of fumaroles, the Valley of Ten Thousand Smokes. Alas-

kan volcanism is tied to a subduction zone along the Aleutian

Island arc and includes many active volcanoes.

North American volcanism sometimes is extended to

cover that of the Hawaiian Islands because of their inclusion

in the United States but only for political reasons. Hawaiian

volcanoes include Kilauea and Mauna Loa, known for their

frequent but generally nonviolent and harmless eruptions.

Tsunamis pose a particular threat to the Hawaiian Islands

because of their position in mid-Pacific; the tsunami that

struck Hilo in 1946, for example, caused extensive damage

and loss of life.

The presence of large amounts of still-hot magma near

the surface in western North America has given rise to abun-

dant geothermal activity and hydrothermal activity in

certain locations, such as Yellowstone National Park,

where geysers have become tourist attractions. geothermal

energy has been exploited to generate electricity on a large

scale in northern California. There is also much evidence of

past volcanism like that which resulted from the separation

of North America and Africa. The Watchung Basalts of

New Jersey and Palisades Sill are a record of it.

North American crustal plate The North American

crustal plate generally underlies the continent of North

America and includes Canada and the 48 contiguous

United States in addition to Alaska and Mexico. The

North American plate is thought to be moving westward

relative to the Pacific crustal plate, with which it shares

a boundary along the western shore of North America. This

boundary between the North American and Pacific plate has

been the location of some of the most famous and destructive

earthquakes and volcanic eruptions in history. The Pacific

and North American plates are not the only ones involved in

generating seismic and volcanic activity there. The subduction

of the oceanic Juan de Fuca crustal plate, for example,

is the source for earthquakes and volcanism along the Cas-

cade Mountains of British Columbia and the northwest

United States, including portions of the states of California,

Oregon, and Washington. On the east, the North Ameri-

can plate is bounded by the mid-ocean ridge that traverses

the middle of the Atlantic Ocean. Here, new crust for the

North American plate is created as molten rock rises from

beneath and solidifies. To the southeast, the North American

crustal plate adjoins the plate underlying the Caribbean Sea,

along the borders of which have occurred some of the most

destructive eruptions and earthquakes in the recent history of

the Western Hemisphere. The Caribbean plate is bounded to

the north by a transform fault and to the east by a sub-

duction zone. The small Cocos plate, off the Pacific coast

of Central America, also shares a boundary with the North

American crustal plate. The boundary is a subduction zone

that yields the Central American arc.

See also plate tectonics.

North Carolina United States North Carolina is located

in a region of moderate seismic activity in the southeastern

United States. Although severe earthquakes have been rare

in North Carolina’s history, the state has a record of mild

seismic activity extending back into colonial times. An earth-

quake on March 9, 1828, probably centered in Virginia, was

felt as a severe shock at Raleigh, North Carolina, and was

associated with noises like thunder at Hillsborough. Another

earthquake on April 29, 1852, again probably centered in

Virginia, was felt in Raleigh, Greensboro, Hillsborough, and

Milton, North Carolina.

A notable series of earthquakes occurred between Feb-

ruary 10 and April 17, 1874, in McDowell County. Strong

shocks were followed by a curious rumbling sound. The

earthquakes shook buildings vigorously. Between 50 and 75

shocks are thought to have occurred altogether, each one

associated with rumbling. On some occasions, a noise like

artillery fire was also reported. The earthquakes affected an

area approximately 25 miles (40 km) wide.

North Dakota United States Although characterized by

low seismic activity, North Dakota is affected from time to

time by earthquakes in nearby states. Notable effects were

reported in North Dakota, for example, from the Montana

earthquake of 1959.

North Pagan caldera, Mariana Islands Located on Pagan

Island, North Pagan has erupted on several occasions since

the late 17th century. Details of eruptions are few, but a

large eruption appears to have occurred in 1872, and earth-

quakes reportedly accompanied other eruptions in 1917 and

1923. A huge eruption occurred in 1981 (VEI = 4) in which

North Pagan 183

the eruptive column was 11 to 12 miles (18 to 20 km) high.

The caldera appears to have been quiet until 1984, when a

small eruption followed an earthquake. Another minor erup-

tion occurred in 1987, and there is evidence of further erup-

tive activity in 1988.

North Ridge earthquake, southern California On January

17, 1994, an earthquake of magnitude 6.7 occurred. Some

60 people were killed, more than 7,000 people were injured,

and 20,000 were left homeless. More than 40,000 buildings

were damaged or destroyed in Los Angeles, Orange, Ven-

tura, and San Bernadino Counties. Services were disrupted

in many areas, and collapsed overpasses closed several major

freeways in the area. Total damage is estimated at $20 billion

dollars. rockfalls and liquefaction occurred throughout

the affected area. The fault that is responsible for the earth-

quake is a blind fault that was previously undetected.

Major aftershocks continued for nearly one year in the Los

Angeles area.

The famous points of destruction in this earthquake

included the Northridge Fashion Mall, which was essentially

destroyed. Many homes and apartment buildings had first-

floor carports and garages; during the earthquake, these bot-

tom floors collapsed and the buildings crushed down onto

the cars underneath. The California State University parking

structure was another spectacular sight as it bent over and

collapsed in on itself. The I–10 and I–5 Freeway collapses

and damage to the famous Fillmore Theater were also spec-

tacular.

nuée ardente A term from the French meaning “fiery

cloud,” nuée ardente refers to a violent and potentially highly

destructive phenomenon that occurs when a volcanic dome

collapses or a lateral blast occurs. A pyroclastic flow

emanates from the volcano along with an overlying cloud

of ash. The resulting cloud of ash, rock, and superheated

gas (800°C) flows along the ground at high velocity 40 to

70 miles per hour (64 to 113 km) and incinerates or melts

virtually anything in its path. A nuée ardente can devastate

the area around a volcano almost as effectively as a nuclear

explosion. Perhaps the most dramatic example of a nuée

ardente in the 20th century is the one that accompanied the

eruption of Mount Pelée in the Caribbean Sea. Mount

Saint Helens also produced a spectacular nuée ardente.

Nyamuragira volcano, Zaire Nyamuragira is a shield

volcano and has erupted frequently in the 20th century.

Since 1882, it has erupted 34 times. Only an eruption in

1912–13 caused fatalities. The latest eruption was in 1994–

95. A lava lake was active in the 1920s and 1930s.

Nyiragongo volcano, Zaire A stratovolcano, Nyiragongo

has erupted often in the late 19th and the 20th centuries. Some

100 people were killed by fast-moving 40 miles (64 km) per

hour lava flows from the volcano in 1977. In 1958, an expe-

dition to the volcano succeeded in measuring surface tempera-

tures within the lava lake of Nyiragongo. Measurements at the

surface indicated temperatures between 1,000° and 1,200°C.

The volcano was active in 1982 and again in 1994. Both pro-

duced fire fountains and phreatic eruptions.

Nyos toxic gas, Cameroon The village of Lower Nyos

was overwhelmed on August 21, 1986, by a cloud of toxic

gas that erupted from the bed of Lake Nyos and asphyxiated

some 1,200 people in that community, plus some 500 others

in the vicinity. Some 3,000 domestic animals also died. The

cloud consisted largely of carbon dioxide that had accumu-

lated at or near the bottom of the lake. This phenomenon

is thought to be common in volcanic lakes, although most

of those lakes undergo sufficient mixing (that is, overturn of

warmer upper waters with colder, gas-laden bottom waters)

that prevents excessive buildup of toxic gas in deep waters.

It is not known exactly what caused the gases to rise from

the bottom of Lake Nyos. According to one explanation,

the lake had gone so long without overturning that toxic

gas had built up to tremendous concentrations in bottom

waters; then, when something finally initiated mixing, the

asphyxiating gases rose to the surface and were released. An

earthquake has been suggested as the possible agent of this

overturn; and indeed, a rumbling noise reportedly was heard

from the vicinity of Lake Nyos just before the catastrophe. It

also seems possible that a minor volcanic eruption expelled

the gas from the lake bed, but this hypothesis has not been

confirmed.

Nysiros caldera, Greece The volcanic island Nysiros lies in

the Aegean Sea near Turkey. The caldera of Nysiros is about

2.5 miles (4 km) wide and occupies what was the summit of

an andesitic stratovolcano. The date of collapse is related

to one of two explosive phases that occurred about 25,000

years ago. Several rhyolitic domes have formed inside the

caldera. Unrest has been reported at the caldera on several

occasions since the early 15th century. In 1422 there was a

report of an eruption. Detonations and roaring noises, along

with emissions of vapor and hot water, reportedly occurred

A nuée ardente was suddenly produced from a lateral blast in the

previously dormant Lamington volcano, Papua New Guinea in 1951. The

hot gas cloud sped down the mountain at 65 miles (100 km) per hour, and

killed some 3,000 people. (Courtesy of NOAA)

184 North Ridge

at the caldera in 1830. In 1871 earthquakes were reported,

accompanied by “flames” and detonations; ash and lapilli

from this eruption are said to have wiped out gardens of fruit

on the floor of the caldera. A crater some 20 feet (6 m) wide

formed in 1873 following an earthquake; again on this occa-

sion, lapilli and ash were cast out from the volcano. A lake

formed in the caldera from hot, salty water, and eruptions of

dark mud lasted for several days. Also in 1873, the caldera

ejected lapilli on many occasions, along with saltwater and

mud. Eruptive activity had formed a crater approximately 75

feet (23 m) wide by the autumn of 1888. solfataric activity

was reported in 1956.

Nysiros 185

O-Akan See Akan.

oblique slip fault See fault.

obsidian A dark, glassy volcanic rock, obsidian is really a

supercooled liquid with no crystalline structure. Although it

is commonly black, obsidian is commonly of rhyolitic com-

position. If the quickly cooling lava gets drops of water in it,

small alteration spots formed by the devitrification process,

form white spots in the obsidian. This texture is called snow-

flake obsidian because the white spots look like snowflakes.

Obsidian was used as material for arrowheads by Native

Americans because it can hold a very sharp edge.

oceanic crust The crust that makes up the ocean floor.

Oceanic crust has a distinct three-layer pattern. The bottom

layer is composed of peridotite, which is really uppermost

mantle rather than crust. The next layer is dominated by

closely spaced sheeted gabbro dikes that acted as feed-

ers to the volcanoes. The top layer is composed of pillow

basalts overlain by deep-sea sediments. They are mostly

mud and chert but closer to the continents other sediments

are possible. The three-layered sequence is called Steinman’s

Trinity and coined for pieces of ocean crust that had been

broken and lifted onto continental crust through plate

tectonic collisions. Ocean crust on continental crust is

called an ophiolite.

Ocean crust is produced at the mid-ocean ridge. The

splitting apart of the ocean ridge is filled in with mafic

magma from the mantle. That forms the sheeted dikes. The

dikes feed underwater volcanoes that produce the top layer.

Therefore, new crust is constantly being formed at the mid-

ocean ridge. As the newly formed crust is in turn split, it is

pulled in both directions away from the ridge. Therefore, the

youngest oceanic crust is at the ridge, and it becomes pro-

gressively older as it is pulled away. It is like a conveyor belt.

Because the ocean crust is eventually destroyed at a subduc-

tion zone, it is all relatively young. The oldest ocean crust is

200 million years and the vast majority is much younger. In

contrast, continental crust can be 3.4 billion years old.

offset The amount of movement on a fault. Offset is a

quantitative measure of how far a fault has moved. It is used

for all fault types. There can be total offset for how much a

fault has ever moved or offset for the amount of movement

during a single earthquake. Offset can be both in the verti-

cal and horizontal sense. On a dip-slip fault, the vertical

offset is called the throw, and the horizontal offset is called

the heave.

Ohio United States Ohio undergoes significant earthquakes

on occasion. Several examples are listed here. The June 18,

1875, earthquake was felt strongly in Chicago, Illinois,

Columbus, and Cincinnati and was strongest at Urbana and

Sicily, Illinois. On September 19, 1884, an earthquake near

Columbus caused slight damage. This earthquake was felt

at the top of the then unfinished Washington Monument in

Washington, D.C. A September 20, 1931, earthquake at Anna

caused minor damage but was felt at great distances, in Ala-

bama, Arkansas, and Tennessee. In March 1937, a series of

strong earthquakes was reported. An earthquake on March 2

near Anna and Sidney caused alarm and slight damage. On

March 3, minor damage was reported from an earthquake

that was felt at Anna, Botkins, Jackson Center, and Sidney.

The March 8 earthquake in western Ohio caused minor dam-

age and was felt on upper stories of high buildings in Chicago,

Illinois; Milwaukee, Wisconsin; and Toronto, Canada.

Oklahoma United States Oklahoma is not one of the more

seismically active states of the United States, but strong

earthquakes do affect the state on occasion. A strong shock

on April 9, 1952, at El Reno, for example, was felt over an

area of 140,000 square miles (362,598 km

Okmok caldera, Aleutian Islands, Alaska, United States

The Okmok caldera is located on Umnak Island and has

187

two summit calderas overlapping each other. The Tulik vol-

cano is located several miles from the caldera. Eruptions have

been reported here since the early 19th century. A major

explosive eruption in 1817 deposited several inches of ash

over the surrounding area, and large rocks were cast out,

some landing more than 30 miles (48 km) away. Earthquake

activity and a tsunami were reported in 1878, although it

is not certain how this activity may have been related to the

Okmok caldera. Tulik emitted black smoke in 1931, and an

eruption took place on the island several weeks later. The

island was relatively quiet until 1944, when activity resumed.

In 1945 an earthquake was felt at the army base on the

island, and several days later pilots reported seeing an erup-

tive column. Later investigations showed that ash and a lava

flow had emanated from a cinder cone in the caldera. Air-

borne observations revealed another lava flow in 1958. From

1960 to 1961, explosive eruptions occurred, and a minor ash

eruption was observed in 1983. Several small eruptions of ash

took place between 1986 and 1988.

Okushiri earthquake, Japan On July 12, 1993, an earth-

quake of magnitude 7.8 occurred in the Sea of Japan just to

the north of the small island of Okushiri. A mere two to five

minutes after the earthquake, a tsunami with vertical run-up

of 50 to 100 feet (15 to 30 m) struck the island. The coastal

areas were devastated by these enormous waves. The epicen-

ter was too close for warnings and evacuations, so people

were taken completely off guard. Some 120 people were

killed, and there was more than $600 million in property

damage.

Old Faithful geyser, Yellowstone National Park, United

States The geyser Old Faithful has erupted approximately

once every 40 to 80 minutes since the 19th century. Water

from the geyser reaches a height up to 150 feet (46 m). Each

eruption is thought to release some 100,000 pounds of water.

olivine A common mineral in mafic and ultramafic igne-

ous rocks. Olivine is a glassy green to brown mineral that

in gem form is called peridot. It is a silicate mineral that is

formed at very high temperatures (1,400°C). It is commonly

found as crystals in basalt. On one beach in Hawaii, the

olivine has been preferentially weathered out of the basalt

and makes up the sand on the beach. It is bright green. There

are impressive olivine-rich layers in the Palisades Sill along

the Hudson River in New Jersey.

Olympus Mons volcano, Mars The largest-known shield

volcano in the solar system, Mars’s Olympus Mons stands

some 75,000 feet (22,860 m) high. Olympus Mons is one of

several large volcanoes in the Tharsis region of the northern

hemisphere. The Martian volcanoes appear to have played a

major role in shaping the topography of that hemisphere in

relatively recent times, although the existence of older vol-

canoes elsewhere on Mars indicates that volcanism has been

active on Mars for much of the planet history. The major

Martian volcanoes are larger by far than any on Earth. The

evident absence of plate tectonics on Mars may account

for the impressive size of its volcanoes. Unlike Earth, where

horizontal motion of crustal plates can prevent a volcano

from growing past a certain height because the volcano can

move out from its source, Mars appears to have a fixed, com-

paratively immobile crust. This lack of movement gives a vol-

cano time to reach tremendous size by terrestrial standards.

Because the processes that create volcanoes along crustal

plate margins on Earth are apparently lacking on Mars, it is

believed that radionuclides underground may supply the heat

required for volcanism there.

Onikobe caldera, Japan The Onikobe volcanic center is

located in the northern portion of the island of Honshu¯. The

volcano Kurikoma stands several miles northeast of the rim

of the caldera. The Narugo caldera and a group of vol-

canic domes are situated just southeast of the Onikobe cal-

The Martian volcano Olympus Mons is one of the largest in the solar

system. The volcano is more than 360 miles (600 km) across and 16 miles

(27 km) high. (Courtesy of NASA)

The summit of the Olympus Mons, Mars. It is composed of several

large collapse features that indicate a long period of activity. There

are also wrinkle ridges, and grabens suggest a complex tectonic

history. (Courtesy of NASA)

188 Okushiri

dera. Although the Onikobe caldera itself appears to have

been quiet in historical times, eruptions of Kurikoma were

recorded in 1946 and 1950. These eruptions do not appear

to have released large amounts of material, although a mud-

flow was associated with the 1950 eruption. Some unrest

was reported at the volcano in 1957, but there is no indica-

tion of an actual eruption.

On-Take volcano, Japan The On-Take volcano is located

on the island of Honshu¯, some 50 miles (80 km) northeast of

Nagoya. The volcano rests within a caldera and has a sum-

mit caldera more than one mile (1.6 km) wide. The volcano’s

first eruption within historical times occurred in 1979 follow-

ing several hours of seismic activity in the vicinity. A strong

earthquake in 1984 caused extensive landslides. fuma-

roles in the area of the 1979 vent appeared unaffected by

the earthquake, but changes were noted in a fumarole on the

summit of the volcano. Although it is not one of the more

active volcanoes in Japan, On-Take has received particu-

lar attention because of its eruption in 1979. The eruption

occurred after many hundreds of years of quiescence, and

the eruption is thought to be associated with earthquake

swarms in the area between 1976 and 1984 and with the

strong earthquake near the volcano in 1984.

Opala volcano, Kamchatka, Russia The Opala volcano

has been generally quiet over the last few centuries, although

an explosive eruption is reported for the year 1776. The

caldera was formed between 31,000 and 39,000 years

ago. About 1,500 years ago, explosions are believed to have

formed a crater about one mile (1.6 km) wide on the aver-

age within the Opala caldera, on the northern rim of which

the Opala volcano is located. Intense solfataric activity was

reported in 1827, 1854, and 1984 but no actual eruptions.

Öræfajökull volcano, Iceland Although not one of Ice-

land’s more active volcanoes, Öræfajökull has erupted on

two notable occasions within historical times. The first was

in 1362, when tephra and jökulhlaups caused considerable

damage in the vicinity of the volcano. Powerful earthquakes

are thought to have occurred just before this eruption. The

next eruption occurred in 1727 and was apparently similar to

the eruption of 1362.

Oran earthquake, Algeria On October 9, 1790, at 1:15

a.m., a major earthquake struck the northern part of Alge-

ria. The epicenter was in the town of Oran, which was com-

pletely destroyed. The intensity was estimated at X on the

modified Mercalli scale, and the maximum ground accel-

eration was calculated to have been 0.42 g. The recurrence

interval for earthquakes of this size is estimated at 500 years.

The source of the earthquake was the Oran Fault, which lies

east-west across the area with thrust and strike-slip move-

ment. The death toll for this earthquake was estimated at

3,000 people.

Oregon United States Located in the seismically active

Pacific Northwest of the United States, Oregon nonetheless

has not had a major earthquake since the territory was settled

by Americans of European descent in the 19th century. This

historical lack of seismicity does not mean that a powerful

earthquake in Oregon is impossible. However, parts of Ore-

gon would stand in risk of severe damage in any such event,

especially where numerous buildings are constructed of unre-

inforced masonry, a kind of construction that is extremely

vulnerable to damage from seismic events. Some notable

earthquakes in Oregon history are listed below.

On February 3, 1892, a strong shock caused buildings

to sway in Portland, and frightened residents of the city ran

into the streets. The earthquake was also felt strongly at

other locations, including Salem and Astoria. An earthquake

at McMinnville on April 2, 1896, involved three shocks and

awakened sleepers. The principal shock was felt also in Port-

land and in Salem. The earthquake of January 10, 1923, was

felt in Lakeview and Klamath Falls and probably was very

strong in an underpopulated area; minor damage occurred in

California.

Oregon shows abundant evidence of volcanic activity in

the recent geologic past, notably the peaks of several volca-

noes in the Cascade Mountains, which extend north-south

through the western and central portions of the state. Vol-

canism in Oregon is thought to be the result of the gradual

and ongoing destruction of the Juan de Fuca crustal plate

in the Cascadia Subduction Zone beneath the Pacific

Ocean immediately to the west of Oregon. As molten rock

from the fusion of the descending crustal plate rises back to

the surface along the Cascade range, the rising magma carries

with it a heavy load of dissolved gases thought to be derived

from ocean water carried downward into Earth’s mantle

with sediments from the seafloor. As the magma approaches

the surface and pressure on the molten rock is relieved, the

gases pass out of solution and into gaseous form, thus giv-

ing the Cascade volcanoes the potential for highly explosive

eruptions similar to that of Mount Saint Helens in Wash-

ington in 1980. Volcanoes in Oregon include Mount Hood,

Three Sisters and Crater Lake, the water-filled caldera left

from the collapse of Mount Mazama in prehistoric times.

origin time This is the exact time that seismic waves are

initiated at a seismic source. This time is calculated from the

arrival times of p-waves and s-waves at a seismograph.

Knowing the velocities of these two waves, seismologists

backtrack the waves to their focus to determine the exact

second that the rupture occurred. This origin time is most

commonly reported in universal time (UTC) rather than local

time.

Osaka earthquake, Japan Two closely spaced submarine

earthquakes struck Japan and caused great destruction in

December 1854. The epicenter of the first tremor was off

Totomi Sea, and the second was off Shikoku. The first of the

earthquakes struck Osaka at 8:00 a.m. on December 23, 1854,

and was felt strongly but produced only a moderate tsunami.

People were nonetheless nervous, and many decided to spend

the night outside their homes or on boats. The second event

struck at 5:00 p.m. on December 24, 1854, but the tsunami

did not arrive for two hours. It arrived in the outer harbor at a

height of 10 feet (3 m) but was 20 feet (6 m) high by the time

Osaka 189

it reached the port. It damaged or completely wrecked upward

of 1,500 boats, some of which were swept along by the wave

for a great distance, damaging at least 25 bridges. Between the

surface wave damage and the tsunamis, 60,000 houses were

destroyed as well. It was fortunate that only about 3,000 peo-

ple lost their lives because it could have been far worse. The

first moderate event likely forewarned people sufficiently that

they were better prepared for the second.