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

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180 people. A strong aftershock occurred approximately 90

minutes after the main shock, and aftershocks continued for

weeks afterward. Another earthquake, in Tabas in September

1978, killed 25,000 and was suspected of having some link to

the underground testing of a Soviet nuclear device some hours

before near Semipalatinsk (now Semey) in Siberia, approxi-

mately 1,500 miles (2,414 km) away. Believed to have yielded

10 megatons, the Russian bomb test was unusually powerful

and allegedly was set off at a shallow depth of only about

one mile (1.61 km). The coincidence between the test and the

earthquake was remarkable, as was the unusual set of char-

acteristics observed in the Iranian earthquake. It was very

shallow (centered only about 10 miles (16 km) underground)

and reportedly had no aftershocks. Also, a peculiar report

from Iran concerned access to the site of the earthquake. As a

rule, seismologists are admitted to be affected area follow-

ing a large earthquake to assess the damage; but in this case,

access was denied, leading some observers to wonder if there

was something especially sensitive and deserving of secrecy

that had occurred at Tabas. Other recent earthquakes in

Iran include Dasht-e Bayaz in 1968, in which some 12,000

were killed; Gilan in 1990, which had a death toll of

over 50,000; and the Bam earthquake of 2003, in which the

loss of life was 43,200. Although the death toll from these

recent earthquakes is far less than the hundreds of thousands

reported of those from antiquity, they are still among the

deadliest in modern times. With the frequency of these deadly

quakes, it is no exaggeration to dub Iran as one of the seismi-

cally most dangerous countries on Earth.

Irazu volcano, Costa Rica The stratovolcano Irazu

has a double crater and has undergone explosive erup-

tions some 23 times since 1723. Eruptions between 1963 and

1965, the most recent activity generated many ashfalls that

caused considerable harm to agriculture and affected the city

of San Juan. Resulting lahars killed at least 40 people and

destroyed 400 houses.

Irian Jaya earthquake, Indonesia On February 17, 1996,

an earthquake of magnitude 8.1 occurred. At least 108

people were killed and 423 were injured. There were 5,043

houses destroyed or damaged in the epicentral area. tsu-

namis generated from the earthquake reached heights of 22

feet (7 m).

Ischia volcano, Italy The volcanic island Ischia stands close

to Vesuvius, in the Bay of Naples, and has been active since

earliest historical times. Resurgent uplift at Ischia, similar to

that observed at Iwo Jima, had led some observers to specu-

late that Ischia has a caldera, although this interpretation

is not certain. There has been considered long-term ground

deformation at Ischia. One sign of this deformation is the

site of a Roman metal foundry on the northeast side of the

island, now about 15 to 20 feet (5–6.5 m) underwater. On

the islands southern side, a beach has been uplifted almost

100 feet (30 m) in places above the present water level. At

one point on the island, thermal baths built in the late 18th

or early 19th century have risen up to approximately 20 feet

(6.5 m), at a rate of more than an inch (2 cm) per year, since

their construction. In the 20th century, the south side of the

island appears to have started subsiding, thus reversing the

earlier trend of uplift. At the same time, Monte Epomeo, the

highest point on the island, appears to be undergoing con-

tinuing uplift.

Although uplift at Ischia may be volcanic in origin, it

also is possible that tectonic activity is involved. Subsid-

ence at Ischia, however, is more difficult to explain in terms

of volcanic activity. Major explosive eruptions in prehistoric

times appear to have laid down the Tufo Verde, or “Green

Tuff,” more than 3,000 feet (914 m) thick. If a caldera does

exist at Ischia, it may have formed during the eruption that

deposited the Tufo Verde. An eruption around 470 b.c.

drove away a Syracusan colony on the island. Residents of

the island had to flee yet again in an eruption that occurred

between approximately 400 b.c. and 350 b.c. This eruption

is said to have followed earthquake activity. A tsunami may

have accompanied an eruption, possibly of Monte Epomeo,

around 350 b.c. Another eruption may have taken place in

91 b.c., although there is some question whether this erup-

tion involved Ischia, Vulsini, or Roccamonfina. The erup-

tive history of Ischia over the next thousand years is sketchy,

but eruptions appear to have occurred around a.d. 80, 180–

222, and 284–305. Approximately 700 people were killed

in a rockslide in a.d. 1228. There was considerable earth-

quake activity in 1302 as well as an explosive eruption and

a flow of lava. In either 1557 or 1559, an earthquake caused

a church at Campagnano to collapse. Another church in the

same community collapsed during an earthquake in 1762,

and a church at Rotaro was demolished in an earthquake

in 1767. In the community of Casamicciola (formerly Cam-

pagnano), several houses and seven of their occupants were

destroyed in a 1796 earthquake. Strong earthquakes shook

the island in 1827 and 1828. After one earthquake, residents

of the island slept outdoors for more than two weeks. Earth-

quakes occurred again at Ischia in 1841, 1863, and 1867.

Seismic events between 1881 and 1883 may have accompa-

nied an increase in the temperature of wells on the island, but

this is not certain. A moderate earthquake occurred in 1961.

island arc This is an arcuate (in map view), or crescent-

shaped, string of islands that forms at an ocean-ocean con-

vergent plate boundary. Island arcs are subduction

zones where oceanic crust is going beneath other oceanic

crust. The downgoing crust melts at a certain depth pro-

ducing vast quantities of magma. The magma rises through

the overlying mantle and crust and forms volcanoes on

the ocean floor behind the trench. The volcanoes grow into

islands. Alaska’s Aleutian Island chain is a familiar exam-

ple of an island arc among many others.

isoseismal seismologists evaluate the modified Mercalli

intensity of each earthquake for the area around each epicen-

ter. The highest intensities usually occur near the epicenter

and the lowest farther away. intensity numbers, I to as high

as XII, are determined for the locations evaluated. Seismolo-

gists then put contour lines on the map to separate the areas

with similar intensity numbers. These lines represent equal

intensities and are called isoseismal lines.

122 Irazu

isostacy Lithospheric plates float on the soft, gumlike

asthenosphere. The depth at which the plates sink into the

asthenosphere is dependent upon the density and thickness of

the crust. These relations constitute isostasy. oceanic crust

is dense and sinks deeply into the asthenosphere. continen-

tal crust is light and floats higher. That is why the oceans

are on ocean crust—it is at a much lower elevation on the

planet and water goes to the lowest point. To support moun-

tain ranges, the continental crust must sink deeply into the

asthenosphere as well. Typical continental crust at sea level is

about 35 miles (56 km) thick. To support the 25,000–30,000-

foot (7,600–9,100 m) Himalayan Mountains, the crust thick-

ens to about 80 miles (129 km). Another example of isostasy

is glacial rebound. During the last ice age, a mile (1.6-km)

-thick sheet of ice extended from the Arctic to about New

York City. It was so heavy that it pressed the crust deeper

into the asthenosphere. After the ice melted and retreated the

land very slowly rose back up. Castles that were built on the

ocean in the Middle Ages are now one-quarter to a half-mile

(0.5–1 km) inland. Lake Champlain in New York is purely

freshwater, and yet there are whale bones in the sediments

from several thousand years ago. The lake was open to the

Saint Lawrence seaway after the glacier retreated but the

land rebounded and was elevated enough to cut off the lake

from the ocean.

Istanbul earthquakes, Turkey The great Turkish city of

Constantinople and its successor, Istanbul, were devastated

on several occasions by massive earthquakes. The city seems

to have been struck in blocks of earthquakes at various

points in its history. The first well-documented event was

on January 26, a.d. 447. It had an estimated Richter mag-

nitude of 7.3. and a modified Mercalli intensity of VIII.

This earthquake was responsible for extensive subsidence

and uplift to the point that ocean floor was exposed. There

was another large quake on September 25, a.d. 447, of esti-

mated magnitude 7.2 and intensity IX that was followed

by 90 days of strong aftershocks. Magnitude 6.5–7.0

events also occurred in 542, 554, and a.d. 557. All caused

great damage (VIII–IX on the modified Mercalli scale). The

a.d. 740 event on October 26 was a bit stronger, with a

Richter magnitude of 7.6 and modified Mercalli intensity

of IX and strong aftershocks for 12 months. There were

intermediate events on March 17, 780; May 23, 862; Janu-

ary 9, 869; October 26, 989; September 23, 1063; March

11, 1231; June 11, 1296; and October 18, 1343, for which

there is little information.

The earthquake of September 10, 1509, on the other

hand, was considerable. It was estimated to have had a mag-

nitude of 7.4 on the Richter scale and an intensity of X on the

modified Mercalli scale. It was said that not a single house

in Istanbul was left undamaged. The city’s fortifications were

also severely damaged, with 49 towers destroyed. Some 109

mosques were ruined. Great fissures opened in the city,

and sand boils were common. Subsidence along the coast

resulted in flooding, and reports of large waves suggest that

tsunamis may have formed. Within Istanbul, at least 5,000

people lost their lives and more than 10,000 were injured.

The death toll for the entire event was estimated at 13,000

people.

Istanbul was again struck on May 22, 1766, at 5:30 a.m.

The earthquake had an estimated magnitude of 7.3 on the

Richter scale and a maximum intensity of IX on the modi-

fied Mercalli scale. The quake was said to have produced a

significant tsunami in the Sea of Marmara that ravaged the

coast and especially the city of Izmit. The death toll for this

event was estimated at greater than 4,500 people.

Italy The Italian Peninsula and its nearby islands and

waters are home to a number of famous volcanoes, nota-

bly Vesuvius, which has erupted on numerous occasions

within historical times and caused widespread destruction,

including the ruin of the cities of Pompeii and Hercula-

neum. The other major and famous volcano is Mount Etna

in Sicily. It, too, is very commonly active. Eruption types

are named after two Italian volcanoes, Stromboli and

Vulcano. Earthquake activity is also frequently destructive

in Italy, which is located in the seismically active Medi-

terranean basin. Between two earthquakes in Calabria

in 1783 and 1857, more than 110,000 people were killed.

Because of the dense population and the large amount

of ancient ruins that are susceptible to being broken and

destroyed, earthquakes in Italy are typically well covered

by the press. geothermal energy has been harnessed in

Italy. The Messina earthquake of 1908 was perhaps the

worst natural disaster in European history. Some 160,000

people were killed by the terrible quake and subsequent

tsunami, leaving Messina to be dubbed “city of the dead.”

Illustrations of the principle of isostasy. (A) Shows two blocks of wood

of different sizes illustrating the large amount of block required below

the water surface to produce height above the water. (B) Shows the large

amount of continent required in the mantle to support mountain ranges.

Italy 123

The Avezzano earthquake of 1915 was also devastating,

with a death toll of 30,000–35,000. These frequent and

horrifying events make Italy, by far, the most dangerous

country in Europe in terms of natural disasters.

See also Ischia; Larderello; Phlegraean Fields;

Roccamonfina; Vulsini.

Iturup earthquake, Kuril Islands, Russia On October 4,

1994, an earthquake of magnitude 8.2 occurred. Only 10

people were killed, owing to the sparse population in the area.

However, resulting tsunamis affected coastlines throughout

the Pacific Ocean. Wave heights ranged from 12 feet (3.5 m)

to eight inches (17 cm), depending upon location.

Iwo Jima caldera, Japan This small island, a famous

World War II battleground, occupies a submerged caldera.

The name Iwo Jima means “Sulfur Island.” Much of the

island is taken up by Motoyama, a volcanic cone with a high

proportion of tuff to lava. Mount Suribachi, at the southern

tip of the island, is made up of tuff and lava. Iwo Jima has a

history of phreatic eruptions reaching back to 1889, and

the barrenness of certain pumice deposits seen on the island

during World War II indicated the deposits were reason-

ably fresh. fumaroles are abundant on Iwo Jima and tend

to be very hot; one test drilling had to be suspended because

the intense heat ruined the drill bits. The island appears to

have undergone dramatic uplift over the past several hun-

dred years. Estimates put the rate of uplift at perhaps seven

or eight inches (18–20 cm) per year. It is not clear whether

the uplift at Iwo Jima may represent an early indication of

another caldera-generating eruption in the future. Iwo Jima

has had at least 10 historical eruptions. The most recent erup-

tion was in 1982 and was small and phreatic.

Izalco volcano, El Salvador A young stratovolcano on

the south side of the Santa Ana volcano in the western part

of the country. It has had semicontinuous eruptions since

1770. In total, there have been at least 51 eruptive periods.

These eruptions consist of small explosions, ejection of cinder

showers and bombs and lava flows from lateral vents. As

the result of the activity, it came to be known as the Light-

house of the Pacific. However, right after a hotel was built

to accommodate tourists coming to see this attraction, activ-

ity waned and it is quiet today. Its last eruption was a small

flank eruption in 1966.

See also Grimsvötn.

Jorullo volcano, Mexico Jorullo is located only about 50

miles (80 km) from Paricutín and resembles that volcano

in many ways, although the birth of Jorullo was studied

less intensively than that of Paricutín because the area was

sparsely populated when Jorullo emerged in 1759. The erup-

tion that gave rise to Jorullo was preceded by earthquakes

that began in June, continued through late September, and

were strong enough to cause widespread distress among the

population and damage to a chapel. On September 29, ash

and steam erupted from a ravine. Condensation of steam

produced a muddy rain that fell on a nearby hacienda, as the

eruption cloud darkened the sky, and a strong odor of sul-

fur spread over the countryside. The eruption, which soon

required the evacuation of the hacienda, continued for two

days before lava (or what is believed to have been lava, on

the basis of a local plantation administrator’s written reports)

emerged from the volcano. The eruption destroyed forests in

the vicinity and, several days after it began, was reportedly

casting out large rocks, some of them the size of cattle. A new

vent, one of three that eventually formed appeared on Sep-

tember 12, Accumulating ash and contaminated water caused

great harm of flora and livestock during October and early

November. No written records of the eruption appear to

have been made from firsthand observation after late 1759,

although eruptive activity reportedly continued through early

1760 and recurred intermittently until 1775. Lava extruded

from Jorullo covered several square miles.

Juan de Fuca crustal plate A small plate of Earth’s

crust compared to the adjacent Pacific and North Ameri-

can crustal plates, and Juan de Fuca plate is nonetheless

important to the United States and Canada because its

ongoing collision with North America has created the Cas-

cadia Subduction Zone. This structure is the source of

seismic and volcanic activity affecting the Canadian province

of British Columbia and the states of Washington and

Oregon in the northwest United States as well as parts of

northern California. Subduction and consequent melting

of the descending crustal plate are thought to have generated

Map of the Japan region showing the plate geometry as well as the major

volcanoes and several cities that have experienced major earthquakes

128 Jawa

the magma that formed the volcanoes of the Cascade Moun-

tains, including Mount Hood, Mount Saint Helens, Mount

Rainier, and Mazama. The magma involved in eruptions of

these volcanoes has been rich in dissolved gases, as a result

of water-rich sediments being subducted along with the Juan

de Fuca plate. The water thus drawn into Earth’s interior

rises back to the surface dissolved in molten rock, generat-

ing highly explosive eruptions such as that of Mount Saint

Helens in 1980.

Although there is abundant evidence of volcanism con-

nected with the Juan de Fuca plate, there is no history of

highly destructive earthquakes such as those recorded fre-

quently in California. The reason for this relative absence

of strong earthquakes along the Cascadia subduction zone

is uncertain, but it has been suggested that the descend-

ing Juan de Fuca plate merely “slides” smoothly on its way

downward, without such powerful disturbances as threaten

portions of California. It is worth noting, however, that the

United States government considers the coast of the Pacific

Northwest an area of high earthquake potential and has

warned that a powerful earthquake remains a district possi-

bility for the future.

The Juan de Fuca crustal plate adjoins a subduction zone off the Pacific

Northwest coast of the United States. Also shown are the volcanoes of

the Cascade Range.

Juan de Fuca crustal plate 129

131

Kagoshima Bay See Sakurajima.

Kakuto caldera, Japan Located close to the Aira and

Kikai calderas, Kakuto caldera is about six by nine

miles (9.6 by 14.4 km) wide and is situated northwest of

Kirishima volcano. geothermal fields are situated nearby.

Although there have been no eruptions in Kakuto caldera

itself within historical times, Kirishima has a record of doz-

ens of explosive and phreatic eruptions dating back to the

eighth century a.d.

Activity at Kirishima and seismic activity at Kakuto

appear to be interrelated. In 1913–14, several earthquake

swarms at Kakuto were followed by minor eruptions at

Kirishima at about the same time as a major eruption of

nearby Sakurajima (although there may have been no con-

nection between eruptions at the two sites). This activity

coincided closely with a strong earthquake about 60 miles

(97 km) to the east. Earthquake swarms were noted again in

the caldera in 1915, but Kirishima did not erupt this time.

Several minor volcanic events in 1923, 1934, and 1936 did

not accompany earthquake activity in the caldera. Likewise, a

phreatic eruption in 1959 was preceded by only slight earth-

quake activity or none at all. Earthquake swarms occurred

after this eruption.

In 1961, a strong earthquake about 60 miles (97 km)

east of Kirishima preceded earthquake swarms in the vicin-

ity of the caldera. Earthquake activity, some of it moderately

strong, occurred between 1966 and 1969. Data collected

between 1967 and 1969 indicate perhaps a couple of feet

of uplift to the west, or possibly a comparable amount of

subsidence to the east. A minor phreatic eruption without

any outstanding seismic precursors occurred at a hot spring

at the base of Kirishima in 1971. An earthquake swarm

took place in the area later that year. fumaroles showed

increased activity in the early 1970s near Kirishima. Consid-

erable earthquake activity continued from 1974 to 1979.

In the winter of 1978–79, a small flow of sulfur, sev-

eral inches wide and about 150 feet (45 m) long, is thought

to have occurred. Between 1980 and 1983, fumaroles became

more active, another small sulfur flow occurred, and a pit

inside one crater at Kirishima cast out material resembling

tar. The most recent eruption of Kirishima was in 1992.

Kamchatka Peninsula Russia The Kamchatka Penin-

sula, on the east shore of Russia, is a magmatic arc in the

“Ring of Fire” around the Pacific Ocean basin. It con-

nects the Kuril and Aleutian Island arcs, but it is on a

peninsula of continental crust. The subduction zone at

Kamchatka involves the Pacific plate to the east being driven

beneath the Eurasian crustal plate to the west. The mar-

gin is relatively straight between the Kurils and Kamchatka,

but takes a nearly right angle turn at the boundary with the

Aleutian Islands. The surrounding landmasses may control

the shape of this bend, but it is also at the point where the

Emperor Seamounts are being subducted into the Kuril-

Kamchatka-Aleutian trench. The eastern margin of Kam-

chatka is quite mountainous, but there is a fault-controlled

valley down the middle of the island and parallel to the

trench. Much of the seismic and volcanic activity is con-

trolled by these faults.

Kamchatka is noted for its typical subduction zone vol-

canic activity, notably the eruption of Bezymianny in 1955

that closely resembled Mount Saint Helens. Kliuchevskoi

is another very active volcano that has made itself an avia-

tion hazard by spewing heavy eruption columns high into

the atmosphere on a regular basis. Other important volca-

noes of Kamchatka include Gorely, Karymsky, Koryak-

sky, Krasheninnikov, Kronotzky, Opala, Sheveluch,

and Toblachik among many others. Several of these volca-

noes are quite active. Most of the volcanoes are so remote

that there is very little information about their eruptive

histories.

Kamchatka is also seismically active. The faults bound-

ing the central valley produce regular moderate to strong

earthquakes. The Benioff zone on the downgoing Pacific

plate also produces regular deep earthquakes. Again,