EARTHQUAKES, VOLCANOES AND RELATED HAZARDS
Edited from http://pubs.usgs.gov/publications/text/tectonics.html
Over geologic time, plate movements in concert with other geologic processes, such as glacial and erosion by rivers and streams , have created some of nature's most magnificent scenery. The Himalayas, the Swiss Alps, and the Andes are some spectacular examples. Yet violent earthquakes related to plate tectonics have caused terrible catastrophes -- such as the magnitude-7.7 earthquake that struck the Chinese province of Hebei in 1976 and killed as many as 800,000 people.
Natural hazards
Most earthquakes and volcanic eruptions do not strike randomly but occur in specific areas, such as along plate boundaries. One such area is the Pacific Ring of Fire, where the Pacific Plate meets many surrounding plates. The Ring of Fire is the most seismically and volcanically active zone in the world.
Earthquakes
Because many major population centres are located
near active fault zones, such as the San Andreas, millions of people have suffered
personal and economic losses as a result of destructive earthquakes, and even
more have experienced earthquake motions. Not surprisingly, some people believe
that, when the "Big One" hits, California will suddenly "break
off" and "fall into the Pacific," or that the Earth will "open
up" along the fault and "swallow" people, cars, and houses. Such
beliefs have no scientific basis whatsoever. Although ground slippage commonly
takes place in a large earthquake, the Earth will not open up. Nor will California
fall into the sea, because the fault zone only extends about 15 km deep, which
is only about a quarter of the thickness of the continental crust. Furthermore,
California is composed of continental crust, whose relatively low density keeps
it riding high, like an iceberg above the ocean.
Aerial view, looking north toward San Francisco, of Crystal Springs Reservoir, which follows the San Andreas fault zone.
Like all transform plate boundaries, the San Andreas is a strike-slip fault, movement
along which is dominantly horizontal. Specifically, the San Andreas fault zone
separates the Pacific and North American Plates, which are slowly grinding past
each other in a roughly north-south direction. The Pacific Plate (western side
of the fault) is moving horizontally in a northerly direction relative to the
North American Plate (eastern side of the fault).
Movement along the San Andreas can occur either in sudden jolts or in a slow,
steady motion called creep. Fault segments that are actively creeping experience
many small to moderate earthquakes that cause little or no damage. These creeping
segments are separated by segments of infrequent earthquake activity (called seismic
gaps), areas that are stuck or locked in place within the fault zone. Locked segments
of the fault store a tremendous amount of energy that can build up for decades,
or even centuries, before being unleashed in devastating earthquakes. For example,
the Great San Francisco Earthquake (8.3-magnitude) in 1906 ruptured along a previously
locked 430 km-long segment of the San Andreas, extending from Cape Men-docino
south to San Juan Bautista.
Map of the San Andreas and a few of the other faults in California, segments of which display different behavior: locked or creeping (see text).
The stresses that accumulate along a locked segment of the fault and the sudden
release can be visualized by bending a stick until it breaks. The stick will bend
fairly easily, up to a certain point, until the stress becomes too great and it
snaps. The vibrations felt when the stick breaks represent the sudden release
of the stored-up energy. Similarly, the seismic vibrations produced when the ground
suddenly ruptures radiate out through the Earth's interior from the rupture point,
called the earthquake focus. The geographic point directly above the focus is
called the earthquake epicentere. In a major earthquake, the energy released can
cause damage hundreds to thousands of kilometres away from the epicenter.
A dramatic photograph of horses killed by falling debris during the Great San Francisco Earthquake of 1906, when a locked segment of the San Andreas fault suddenly lurched, causing a devastating magnitude-8.3 earthquake.
The magnitude-7.1 Loma Prieta earthquake of October 1989 occurred along a segment
of the San Andreas Fault which had been locked since the great 1906 San Francisco
earthquake. Even though the earthquake's focus (approximately 80 km south of San
Francisco) was centered in a sparsely populated part of the Santa Cruz Mountains,
the earthquake still caused 62 deaths and nearly £4 billion worth of damage.
The lesser known Hayward Fault running east of San Francisco Bay, however, may
pose a potential threat as great as, or perhaps even greater than, the San Andreas.
From the televised scenes of the damage caused by the 7.2-magnitude earthquake
that struck Kobe, Japan, on 16 January 1995, Bay Area residents saw the possible
devastation that could occur if a comparable size earthquake were to strike along
the Hayward Fault. This is because the Hayward and the Nojima fault that produced
the Kobe earthquake are quite similar in several ways. Not only are they of the
same type (strike-slip), they are also about the same length (70 km) and both
cut through densely populated urban areas, with many buildings, roads, and other
structures built on unstable ground.
On 17 January 1994, one of the costliest natural disasters in United States history
struck southern California. A magnitude-6.6 earthquake hit near Northridge, a
city located in the populous San Fernando Valley just north of Los Angeles, California.
This disaster, which killed more than 60 people, caused an estimated £20 billion
in damage, nearly five times that resulting from the Loma Prieta earthquake. The
Northridge earthquake did not directly involve movement along one of the strands
of the San Andreas Fault system. It instead occurred along the Santa Monica Mountains
Thrust Fault, one of several smaller, concealed faults (called blind thrust faults)
south of the San Andreas Fault zone where it bends to the east, roughly paralleling
the Transverse Mountain Range. With a thrust fault, whose plane is inclined to
the Earth's surface, one side moves upward over the other. Movement along a blind
thrust fault does not break the ground surface, thus making it difficult or impossible
to map these hidden but potentially dangerous faults. Although scientists have
found measurable uplift at several places in the Transverse Range, they have not
found any conclusive evidence of ground rupture from the 1994 Northridge earthquake.
Similar earthquakes struck the region in 1971 and 1987; the San Fernando earthquake
(1971) caused substantial damage, including the collapse of a hospital and several
freeway overpasses.
Not all fault movement is as violent and destructive. Near the city of Hollister
in central California, the Calaveras Fault bends toward the San Andreas. Here,
the Calaveras fault creeps at a slow, steady pace, posing little danger. Much
of the Calaveras fault creeps at an average rate of 5 to 6 mm/year. On average,
Hollister has some 20,000 earthquakes a year, most of which are too small to be
felt by residents. It is rare for an area undergoing creep to experience an earthquake
with a magnitude greater than 6.0 because stress is continually being relieved
and, therefore, does not accumulate. Fault-creep movement generally is non-threatening,
resulting only in gradual offset of roads, fences, sidewalks, pipelines, and other
structures that cross the fault. However, the persistence of fault creep does
pose a costly nuisance in terms of maintenance and repair.
Mid-plate earthquakes -- those occurring in the interiors of plates -- are much
less frequent than those along plate boundaries and more difficult to explain.
Earthquakes along the Atlantic seaboard of the United States are most likely related
in some way to the westward movement of the North American Plate away from the
Mid-Atlantic Ridge, a continuing process begun with the break-up of Pangaea. However,
the causes of these infrequent earthquakes are still not understood.
Above: Creeping along the Calaveras fault has bent the retaining wall and offset the pavement.
We know in general how most earthquakes occur, but can we predict when they will strike? This question has challenged and frustrated scientists studying likely precursors to moderate and large earthquakes. Since the early 1980s, geologists and seismologists have been intensively studying a segment of the San Andreas near the small town of Parkfield, located about halfway between San Francisco and Los Angeles, to try to detect the physical and chemical changes that might take place -- both above and below ground -- before an earthquake strikes. The USGS and State and local agencies have blanketed Parkfield and the surrounding countryside with seismographs, creep meters, stress meters, and other ground-motion measurement devices.
Volcanic eruptions
As with earthquakes, volcanic activity is linked to plate-tectonic processes.
Most of the world's active above-sea volcanoes are located near convergent plate
boundaries where subduction is occurring, particularly around the Pacific basin.
However, much more volcanism, producing about three quarters of all lava erupted
on Earth, takes place unseen beneath the ocean, mostly along the oceanic spreading
centers, such as the Mid-Atlantic Ridge and the East Pacific Rise.
Subduction-zone volcanoes like Mount St. Helens (in Americas Washington State)
and Mount Pinatubo (Luzon, Philippines), are called composite cones and typically
erupt with explosive force, because the magma is too stiff to allow easy escape
of volcanic gases. As a consequence, tremendous internal pressures mount as the
trapped gases expand during ascent, before the pent-up pressure is suddenly released
in a violent eruption. Such an explosive process can be compared to putting your
thumb over an opened bottle of champagne, shaking it vigorously, and then quickly
removing the thumb. The shaking action separates the gases from the liquid to
form bubbles, increasing the internal pressure. Quick release of the thumb allows
the gases and liquid to gush out with explosive speed and force.
In 1991, two volcanoes on the western edge of the Philippine Plate produced major
eruptions. On June 15, Mount Pinatubo spewed ash 40 km into the air and produced
huge ash flows (also called pyroclastic flows) and mudflows that devastated a
large area around the volcano. Pinatubo, located 90 km from Manila, had been dormant
for 600 years before the 1991 eruption, which ranks as one of the largest eruptions
in this century. Also in 1991, Japan's Unzen Volcano, located on the Island of
Kyushu about 40 km east of Nagasaki, awakened from its 200-year slumber to produce
a new lava dome at its summit. Beginning in June, repeated collapses of this active
dome generated destructive ash flows that swept down its slopes at speeds as high
as 200 km per hour. Unzen is one of more than 75 active volcanoes in Japan; its
eruption in 1792 killed more than 15,000 people--the worst volcanic disaster in
the country's history.
Mount Pinatubo plume
While the Unzen eruptions have caused deaths and considerable local damage, the
impact of the June 1991 eruption of Mount Pinatubo was global. Slightly cooler
than usual temperatures recorded worldwide and the brilliant sunsets and sunrises
have been attributed to this eruption that sent fine ash and gases high into the
stratosphere, forming a large volcanic cloud that drifted around the world. The
sulphur dioxide (SO2) in this cloud, about 22 million tonnes, combined
with water to form droplets of sulphuric acid, blocking some of the sunlight from
reaching the Earth and thereby cooling temperatures in some regions by as much
as 0.5 °C. An eruption the size of Mount Pinatubo could affect the weather for
a few years. A similar phenomenon occurred in April of 1815 with the cataclysmic
eruption of Tambora Volcano in Indonesia, the most powerful eruption in recorded
history. Tambora's volcanic cloud lowered global temperatures by as much as 3
°C. Even a year after the eruption, most of the northern hemisphere experienced
sharply cooler temperatures during the summer months. In part of Europe and in
North America, 1816 was known as "the year without a summer.
Apart from possibly affecting climate, volcanic clouds from explosive eruptions
also pose a hazard to aviation safety. During the past two decades, more than
60 airplanes, mostly commercial jetliners, have been damaged by in-flight encounters
with volcanic ash. Some of these encounters have resulted in the power loss of
all engines, necessitating emergency landings. Luckily, to date no crashes have
happened because of jet aircraft flying into volcanic ash.
Diagram showing the lower two layers of the atmosphere: the troposphere and the stratosphere. The tropopause, the boundary between these two layer, varies in altitude from 8 to 18 km (dashed white lines), depending on Earth latitude and season of the year. The summit of Mt. Everest and the altitudes commonly flown by commercial jetliners are given for reference.
Since the year A.D. 1600, nearly 300,000 people have been killed by volcanic eruptions. Most deaths were caused by pyroclastic flows and mudflows, deadly hazards which often accompany explosive eruptions of subduction-zone volcanoes. Pyroclastic flows, also called nuées ardentes ("glowing clouds" in French), are fast-moving, avalanche-like, ground-hugging incandescent mixtures of hot volcanic debris, ash, and gases that can travel at speeds in excess of 150 km per hour. Approximately 30,000 people were killed by pyroclastic flows during the 1902 eruption of Mont Pelée on the Island of Martinique in the Caribbean. In March-April 1982, three explosive eruptions of El Chichón Volcano in the State of Chiapas, southeastern Mexico, caused the worst volcanic disaster in that country's history. Villages within 8 km of the volcano were destroyed by pyroclastic flows, killing more than 2,000 people.
Mudflows (also called debris flows or lahars, an Indonesian term for volcanic mudflows) are mixtures of volcanic debris and water. The water usually comes from two sources: rainfall or the melting of snow and ice by hot volcanic debris. Depending on the proportion of water to volcanic material, mudflows can range from soupy floods to thick flows that have the consistency of wet cement. As mudflows sweep down the steep sides of composite volcanoes, they have the strength and speed to flatten or bury everything in their paths. Hot ash and pyroclastic flows from the eruption of the Nevado del Ruiz Volcano in Colombia, South America, melted snow and ice atop the 5,390m high Andean peak; the ensuing mudflows buried the city of Armero, killing 25,000 people.
Eruptions of Hawaiian and most other mid-plate volcanoes differ greatly from those
of composite cones. Mauna Loa and Kilauea, on the island of Hawaii, are known
as shield volcanoes, because they resemble the wide, rounded shape of an ancient
warrior's shield. Shield volcanoes tend to erupt non-explosively, mainly pouring
out huge volumes of fluid lava. Hawaiian-type eruptions are rarely life threatening
because the lava advances slowly enough to allow safe evacuation of people, but
large lava flows can cause considerable economic loss by destroying property and
agricultural lands. For example, lava from the ongoing eruption of Kilauea, which
began in January 1983, has destroyed more than 200 structures, buried kilometres
of roads. Because Hawaiian volcanoes erupt frequently and pose little danger to
humans, they provide an ideal natural laboratory to safely study volcanic phenomena
at close range. The USGS Hawaiian Volcano Observatory, on the rim of Kilauea,
was among the world's first modern volcano observatories, established early in
this century.
In recorded history, explosive eruptions at subduction-zone volcanoes have posed
the greatest hazard to civilizations. Yet scientists have estimated that about
three quarters of the material erupted on Earth each year originates at spreading
mid-ocean ridges. However, no deep submarine eruption has yet been observed "live"
by scientists. Because the great water depths do not allow easy observation, few
detailed studies have been made of the numerous possible eruption sites along
the tremendous length (50,000 km) of the global mid-oceanic ridge system.
Iceland, where the Mid-Atlantic Ridge is exposed on land, is a different story.
It is easy to see many Icelandic volcanoes erupt non-explosively from fissure
vents, in similar fashion to typical Hawaiian eruptions; others, like Hekla Volcano,
erupt explosively. (After Hekla's catastrophic eruption in 1104, it was thought
in the Christian world to be the "Mouth to Hell.") The voluminous, but
mostly non-explosive, eruption at Lakagígar (Laki), Iceland, in 1783, resulted
in one of the world's worst volcanic disasters. About 9,000 people, almost 20%
of the country's population at the time, died of starvation after the eruption,
because their livestock had perished from grazing on grass contaminated by fluorine-rich
gases emitted during this eight month-long eruption.
Tsunamis
Major earthquakes occurring along subduction zones are especially hazardous,
because they can trigger tsunamis (from the Japanese word tsunami meaning "harbour
wave") and pose a potential danger to coastal communities and islands that
dot the Pacific. Tsunamis are often mistakenly called "tidal waves"
when, in fact, they have nothing to do with tidal action. Rather, tsunamis are
seismic sea waves caused by earthquakes, submarine landslides, and, infrequently,
by eruptions of island volcanoes. During a major earthquake, the seafloor can
move by several meters and an enormous amount of water is suddenly set into motion,
sloshing back and forth for several hours. The result is a series of waves that
race across the ocean at speeds of more than 800 km per hour, comparable to those
of commercial jetliners. The energy and momentum of these transoceanic waves can
take them thousands of kilometres from their origin before slamming into far-distant
islands or coastal areas.
A giant wave engulfs the pier at Hilo, Hawaii, during the 1946 tsunami, which
killed 159 people. The arrow points to a man who was swept away seconds later.
To someone on a ship in the open ocean, the passage of a tsunami wave would barely
elevate the water surface. However, when it reaches shallower water near the coastline
and "touches bottom," the tsunami wave increases in height, piling up
into an enormous wall of water. As a tsunami approaches the shore, the water near
shore commonly recedes for several minutes before suddenly rushing back toward
land with frightening speed and height.
The 1883 eruption of Krakatau Volcano, located between the islands of Sumatra
and Java, provides an excellent example of an eruption-caused tsunami. A series
of tsunamis washed away 165 coastal villages on Java and Sumatra, killing 36,000
people. The larger tsunamis were recorded by tide gauges as far away as the southern
coast of the Arabian Peninsula-more than 7,000 km from Krakatau!
Travel times for tsunamis
Because of past killer tsunamis, which have caused hundreds of deaths on the Island
of Hawaii and elsewhere, the International Tsunami Information Center was created
in 1965. This center issues tsunami warnings based on earthquake and wave-height
information gathered from seismic and tide-gauge stations located around the Pacific
Ocean basin and on Hawaii.
Natural resources
Many of the Earth's natural resources of energy, minerals, and soil are concentrated
near past or present plate boundaries. The utilization of these readily available
resources have sustained human civilizations, both now and in the past.
Fertile soils
Volcanoes can clearly cause much damage and destruction, but in the long term they also have benefited people. Over thousands to millions of years, the physical breakdown and chemical weathering of volcanic rocks have formed some of the most fertile soils on Earth. In tropical, rainy regions, such as the windward (northeastern) side of the Island of Hawaii, the formation of fertile soil and growth of lush vegetation following an eruption can be as fast as a few hundred years. Some of the earliest civilizations (for example, Greek, Etruscan, and Roman) settled on the rich, fertile volcanic soils in the Mediterranean-Aegean region. Some of the best rice-growing regions of Indonesia are in the shadow of active volcanoes. Similarly, many prime agricultural regions in the western United States have fertile soils wholly or largely of volcanic origin.
Ore deposits
Most of the metallic minerals mined in the world, such as copper, gold, silver,
lead, and zinc, are associated with magmas found deep within the roots of extinct
volcanoes located above subduction zones. Rising magma does not always reach the
surface to erupt; instead it may slowly cool and harden beneath the volcano to
form a wide variety of crystalline rocks (called intrusive igneous rocks). Some
of the best examples of such deep-seated granitic rocks, later exposed by erosion,
are magnificently displayed in California's Yosemite National Park. Ore deposits
commonly form around the magma bodies that feed volcanoes because there is a ready
supply of heat, which convectively moves and circulates ore-bearing fluids. The
metals, originally scattered in trace amounts in magma or surrounding solid rocks,
become concentrated by circulating hot fluids and can be redeposited, under favorable
temperature and pressure conditions, to form rich mineral veins.
The active volcanic vents along the spreading mid-ocean ridges create ideal environments
for the circulation of fluids rich in minerals and for ore deposition. Water as
hot as 380 °C gushes out of geothermal springs along the spreading centers. The
water has been heated during circulation by contact with the hot volcanic rocks
forming the ridge. Deep-sea hot springs containing an abundance of dark-colored
ore minerals (sulphides) of iron, copper, zinc, nickel, and other metals are called
"black smokers." On rare occasions, such deep-sea ore deposits are later
exposed in remnants of ancient oceanic crust that have been scraped off and left
("beached") on top of continental crust during past subduction processes.
The Troodos Massif on the Island of Cyprus is perhaps the best known example of
such ancient oceanic crust. Cyprus was an important source of copper in the ancient
world, and Romans called copper the "Cyprian metal"; the Latin word
for copper is cyprium.
Fossil fuels
Oil and natural gas are the products of the deep burial and decomposition of accumulated organic material in geologic basins that flank mountain ranges formed by plate-tectonic processes. Heat and pressure at depth transform the decomposed organic material into tiny pockets of gas and liquid petroleum, which then migrate through the pore spaces and larger openings in the surrounding rocks and collect in reservoirs, generally within 5 km of the Earth's surface. Coal is also a product of accumulated decomposed plant debris, later buried and compacted beneath overlying sediments. Most coal originated as peat in ancient swamps created many millions of years ago, associated with the draining and flooding of landmasses caused by changes in sea level related to plate tectonics and other geologic processes.
Geothermal energy
Geothermal energy can be harnessed from the Earth's natural heat associated with active volcanoes or geologically young inactive volcanoes still giving off heat at depth. Steam from high-temperature geothermal fluids can be used to drive turbines and generate electrical power, while lower temperature fluids provide hot water for space-heating purposes, heat for greenhouses and industrial uses, and hot or warm springs at resort spas. For example, geothermal heat warms more than 70 percent of the homes in Iceland, and The Geysers geothermal field in Northern California produces enough electricity to meet the power demands of San Francisco. In addition to being an energy resource, some geo-thermal waters also contain sulfur, gold, silver, and mercury that can be recovered as a byproduct of energy production.
Geothermal powerplant, The Geysers
A formidable challenge
As global population increases and more countries become industrialized, the
world demand for mineral and energy resources will continue to grow. Because people
have been using natural resources for millennia, most of the easily located mineral,
fossil-fuel, and geothermal resources have already been tapped. By necessity,
the world's focus has turned to the more remote and inaccessible regions of the
world, such as the ocean floor, the polar continents, and the resources that lie
deeper in the Earth's crust. Finding and developing such resources without damage
to the environment will present a formidable challenge in the coming decades.
An improved knowledge of the relationship between plate tectonics and natural
resources is essential to meeting this challenge.
Farmer plowing a lush rice paddy in central Java, Indonesia; Sundoro Volcano looms in the background. The most highly prized rice-growing areas have fertile soils formed from the breakdown of young volcanic deposits.
The long-term benefits of plate tectonics should serve as a constant reminder to us that the planet Earth occupies a unique niche in our solar system. Appreciation of the concept of plate tectonics and its consequences has reinforced the notion that the Earth is an integrated whole, not a random collection of isolated parts. The global effort to better understand this revolutionary concept has helped to unite the earth-sciences community and to underscore the linkages between the many different scientific disciplines. As we enter the 21st century, when the Earth's finite resources will be further strained by explosive population growth, earth scientists must strive to better understand our dynamic planet. We must become more resourceful in reaping the long-term benefits of plate tectonics, while coping with its short-term adverse impacts, such as earthquakes and volcanic eruptions.
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