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

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Mazama volcano, Oregon, United States Mazama is the

name given to the volcano that collapsed and thus created the

basal wreck occupied by Crater Lake today.

Mazar-e Sharif earthquake, Afghanistan On September

18, 1994, an earthquake of magnitude 6.5 occurred. More

than 4,000 people were killed, and many thousands were

injured.

McDonald Island volcano, Australia McDonald Island

is a new volcano that was discovered in March 1997. It is

believed to have begun erupting in December 1996. It is the

first active volcano to be discovered in the Southern Hemi-

sphere in more than 50 years. It is also Australia’s second

active volcano. It is considered very dangerous for anyone on

the new island.

Me-Akan See Akan.

Medicine Lake volcano, California, United States Lo-

cated in northern California near Tule Lake, just south of

the Oregon border, Medicine Lake volcano has an area of

some 900 square miles (2,331 km

) and rises approximately

4,000 feet (1,219 m) above the surrounding land. Although

basaltic lava from comparatively quiet eruptions is found

on the flanks of the volcano, the mountain also has a his-

tory of violent eruption from its summit. One of these vio-

lent eruptions, believed to have occurred about 1,000 years

ago, deposited pumice on Mount Shasta, more than 30

miles (48 km) away. rhyolitic lava flowing from Medi-

cine Lake volcano formed Glass Mountain, and a separate

flow on the west rim of the caldera formed Little Glass

Mountain. The volcano has been generally quiet since then,

although strong seismic activity in 1910 fractured the sur-

face of the ground and was accompanied by a minor erup-

tion of ash. Earthquake activity around the volcano in 1988

indicated possible movement of magma underground. Lava

Beds National Monument occupies a portion of the Medi-

cine Lake volcano.

Mediterranean Sea The Mediterranean basin is the location

of numerous powerful earthquakes and volcanic eruptions and

has a complex geology involving both the northward move-

ment of Africa toward Europe and the interaction of numer-

ous smaller plates. Volcanoes in the Mediterranean include

Vesuvius, Etna, and Stromboli. The catastrophic eruption

of Thira, comparable in destruction to that resulting from the

1883 eruption of Krakatoa, is believed to have wiped out the

Minoan civilization in 1470 b.c. The Italian Peninsula espe-

cially is known for intense seismic activity as well as volca-

nism, and the presence of magma near the surface in portions

of Italy has enabled that country to satisfy part of its energy

needs by drawing on geothermal sources.

medium-rate ridge A mid-ocean ridge with an interme-

diate two to four inches (5–10 cm) per year rate of spreading.

The Juan de Fuca Ridge, 200 miles (322 km) west of Wash-

ington State, is the most studied medium-rate ridge in the

world. This ridge contributes to the formation of the Pacific

crustal plate. It has a small axial rift and is crossed by a

Map of the eastern Mediterranean Sea showing the plate boundaries and locations of volcanoes and several of the epicentral cities for major historical

earthquakes. Lines with triangles represent subduction zones, with triangles on the overriding plate.

162 Mazama

linear chain of hot spots. There is an axial volcano atop the

ridge where the two intersect.

Medvezhii caldera, Kuril Islands, Russia Explosive erup-

tions at the somma volcano Medvezhii are recorded in 1778

or 1779 and in 1883. The 1883 eruption reportedly involved

lava flows. phreatic activity was recorded in 1946, and

activity of undetermined nature was recorded in 1958. Erup-

tions are relatively small (VEI = 1–2).

megathrust A thrust fault is sometimes simply referred

to as a thrust. A megathrust is not simply a large thrust but

a thrust associated with the main motion on a subduction

zone. As such, these faults are capable of massive amounts of

movement in a single event in terms of length of the rupture,

amount of movement, and magnitude of the earthquake.

The Banda Aceh earthquake was the latest megathrust-type

event, though Kamchatca, Chile, and Alaska had such events

in 1952, 1960, and 1964, respectively, and Cascadia off the

northwest coast of the United States produced a megathrust-

type event (estimated at 9.0 on the Richter scale) in 1700.

Because these events are submarine and thrust in nature, there

is a strong potential for them to produce teletsunamis.

megatsunami A megatsunami is a grandiose description of

an excessively large tsunami. Sensationalistic television docu-

mentaries have cited the huge wave generated in Lituya Bay,

Alaska, in 1957 as evidence for a megatsunami. This wave,

which sloshed some 1,700 feet (510 m) up a slope was not a

tsunami but a seiche. There is no record of a wave even close

to this size having existed in a large ocean basin (such waves

tend to be restricted to small basins). An asteroid impact in

the open ocean might produce such a wave, but such large

impacts are extremely unlikely. Large marine volcanic erup-

tions in subduction zones could also generate a huge wave.

There is evidence that Thira generated a more than 200-foot

(60-m) wave and that Krakatoa generated a 135-foot (40-

m) wave. Although not as impressive as a seiche, these are

still very destructive waves.

meizoseismal region This is the area of particularly strong

shaking and significant damage in an earthquake. It is typi-

cally located near the epicenter. These regions are best dis-

played in a ShakeMap analysis.

Mendeleev caldera, Kuril Islands, Russia Mendeleev is

thought to have erupted in or around 1880, killing some trees

on the island through solfataric activity. phreatic activity

may have occurred in 1900, although this appears uncertain.

Some earthquakes in recent years may have been associated

with a geothermal energy facility on the island.

Merapi volcano, Java, Indonesia The stratovolcano

Merapi (Mountain of Fire) has a lava dome on its summit

and is believed to have erupted on more than 68 occasions

since a.d. 1548. It has produced more

NUÉE ARDENTEs than

any volcano on Earth (32 of the 68 eruptions). Merapi is

located just 15 miles (24 km) north of the city of Yogyakarta

with a population of 3 million, making it a very dangerous

volcano. The eruptions are very destructive (VEI = 3). One

eruption in 1006 was so destructive that the Hindu rajah of

the island moved to nearby Bali and Java fell under Islamic

influence. Other large eruptions occurred in 1786, 1822,

1872, and 1930.

The current eruptive phase began in 1987. At one point,

it was generating 40 nuée ardentes per day. In late 2006, the

lava dome collapsed sending pyroclastic flows some five

miles (8 km) from the volcano. This event killed 43 people

and forced the evacuation of 6,000. In 1998, evacuations of

many thousands of people was still a common occurrence.

See also seismology.

mesa A flat-crowned hill that is slightly broader than a

butte but smaller than a plateau. Mesas are formed in areas

with flat-lying sedimentary or volcanic and sedimentary

strata. Erosion proceeds through these layers rapidly until

it encounters a resistant layer. Erosion eventually breaks

through the resistant layer locally and quickly cuts through

the underlying layers. The area still containing the remaining

resistant layer remains elevated relative to the eroding sur-

roundings. It is a flat-topped, table-like feature because the

strata are flat. Mesas are common geomorphic features in the

western United States, where erosion is more physical than

in the wet and temperate east, where chemical weathering

dominates.

Mesa de los Hornitos volcanic formation, Mexico As-

sociated with the volcano Paricutín, Mesa de los Hornitos

(hornitos means “little ovens,” or spatter cones) was the

site of major lava flows from 1944 to 1947. One flow in

1944 overwhelmed the village of San Juan Parangaricutiro

but allowed enough time to evacuate the community.

Messina earthquake, Italy The most destructive earth-

quake in modern European history struck the port city of

Messina in Sicily, Italy, at 5:25 a.m. on December 28, 1908.

The earthquake was estimated to have been 7.5 on the Rich-

ter scale and came in a series of shocks ranging from 10 to

45 seconds in duration—the main shock lasting for about

30 seconds. The strong surface waves flattened everything

in a 120-mile (192-km) radius. Strong aftershocks continued

for well over one month.

Immediately following this tragic earthquake, a 40–50-

foot (12–15-m) tsunami roared across the Messina Straits,

destroying many coastal towns and cities. The loss of life

Messina 163

from this combined onslaught was staggering. The official

death toll was 160,000, but other estimates placed it as high

as 250,000. Messina began with a population of 150,000 and

was reduced by well over half, with more than 90% of the

city flattened. Beautiful architectural treasures, such as the

Norman Cathedral of the Annunziata dei Catalani (Annun-

ciation of the Catalans) and the Munizone and Vittorio

Emmanuele theaters, were lost. There were so many bodies

in the rubble that Messina was dubbed “Citta di Morte” (city

of the dead). In the nearby province of Reggio di Calabria, up

to 50,000 people were also killed. Although there had been

many devastating earthquakes in southern Italy, it was the

Messina earthquake that finally spurred the government to

institute strict seismic construction codes. Messina was recon-

structed under these regulations. Hopefully, they will prevent

another tragedy.

metamorphic rock Rock that has been physically and/or

chemically changed through recrystallization under the influ-

ence of changes in temperature, fluid content, and chemistry

and/or pressure. The changes are mostly mineralogical and tex-

tural, but chemical changes are possible as well. There are sev-

eral types of metamorphism, the most common being regional,

contact, and dynamic. contact metamorphism and a subset

called pyrometamorphism are most related to igneous activity.

The metamorphic rock is the result of contact with hot magma

or lava which bakes the country rock. The rocks produced

by this process include hornfels and granofels with minerals

formed by high temperature processes.

See also igneous rock; sedimentary rock.

methane A simple hydrocarbon molecule consisting

of a carbon atom with four hydrogen atoms attached to it

(CH

), methane is the centerpiece of a hypothesis by physicist

Thomas Gold (one of the contributors to the “Steady State”

model of the universe) that presents release of methane from

underground as an explanation for many curious phenom-

ena associated with earthquakes. Gold’s theory, known as

the “deep gas” hypothesis, suggests that large quantities of

methane (also known as marsh gas), left over from the for-

mation of the planet, still exist inside Earth and are vented

to the surface from time to time in outgassing phenomena.

The methane alternatively could be from petroleum sources.

The aforementioned phenomena, coinciding with earth-

quakes, might account, for example, for the fish kills and

incidents of “boiling” water that have been seen to accom-

pany earthquakes along the shore. Release of methane into

the water would give the sea the appearance of boiling, and

a sufficient quantity of the gas could asphyxiate fish, espe-

cially if the methane contained other gases such as hydrogen

sulfide. (Analysis of gas bubbles rising from the ocean floor

off Malibu Point following the San Fernando earthquake of

1971 showed the gas to be 93% methane, with small percent-

ages of nitrogen, carbon dioxide, oxygen, and argon.) Meth-

ane outbursts accompanying earthquakes also might account

for the strange phenomenon of earthquake light, a glow

that has been seen in the night sky during and near the time

of earthquakes. Methane gas escaping from the earth and

igniting near the surface could burn with enough luminosity

to explain earthquake light, although there appears to be no

proof that earthquake light originates in this manner. Meth-

ane eruptions from the seabed have even been suggested as a

possible mechanism for generating tsunamis. A large emis-

sion of methane gas from the seabed, approaching the sur-

face, might lift water above it into a dome, which then would

collapse and generate a major disturbance in the sea, with a

consequent tsunami.

Metis Shoal volcano, Tonga Islands In June 1995, this

normally submerged volcano produced a small island. This

is not the first time an island was produced by this volcano.

In 1851, it produced another island, but ocean waves quickly

eroded it back below the surface. In 1969, another vent pro-

duced an island nearby, but it met with the same fate. This

volcano produces huge amounts of pumice that form rafts

hundreds of kilometers across. People traveling in ships are

sometimes awakened by a squealing sound, only to come on

deck to see pumice as far as the eye can see. It feels as if the

ship is on land. The sound comes from the pumice scraping

along the sides of the ship.

Mexico As part of the westward-moving landmass of

North America, Mexico collides with the oceanic crust of

the Pacific crustal plate to the west of the continent. This

collision generates numerous earthquakes along the west coast

of Mexico, especially in Baja California, which is part of

a northward-moving block of crust that is grinding against

California and generating numerous earthquakes there. A

special set of geological conditions in the Mexico City area

has made that metropolis highly vulnerable to damage from

earthquakes, even those originating along the coastline far

to the west. Mexico City’s vulnerability to earthquake dam-

age was demonstrated clearly in the great earthquake of Sep-

tember 19, 1985, when an earthquake originating along the

Mexican Pacific shore struck Mexico City during rush hour

and caused numerous fatalities and extensive property dam-

age. The capital has been constructed atop unconsolidated

sediment that once formed a lake bed. Buildings atop this

kind of soil are especially susceptible to damage from sur-

face waves in earthquakes. Moreover, buildings in Mexico

City were designed and built largely without consideration

for earthquakes and consequently experienced more damage

than newer, quake-resistant buildings such as those required

in California. Severe damage and fatalities were reported in

four states of Mexico: Colima, Jalisco, Guerrero, and Michoa-

can. Several vessels at sea were reported missing following the

earthquake, and the crew of a trawler reportedly witnessed

waves some 100 feet (30 m) high rising from the sea.

Mexico also has some very active volcanoes. The erup-

tion of El Chichón in 1982 had a VEI of 5 and produced

beautiful sunsets worldwide for the next year. It is also said

to have been responsible for the unusual weather patterns.

An eruptive period of Paracutín from 1943 to 1952 also

produced some spectacular events and features. Colima is

another active volcano that produced explosive eruptions

in the 1980s and 1990s as is Popocatépetl. Other volca-

noes and volcanic features include Iztaccíhuatl, Jorullo, La

Reforma, and Pínacaté.

164 metamorphic rock

Mexico City earthquake, Mexico One of the most famous

earthquakes of the 20th century, the Mexico City earth-

quake had an estimated Richter magnitude of 8.1, making it

approximately as powerful as the San Francisco earthquake

of 1906. The earthquake occurred early on the morning of

September 19, 1985, and originated along the Pacific coast of

Mexico. This timing helped reduce the number of deaths from

the earthquake, which killed more than 8,000 people and

injured some 30,000. Had the earthquake occurred later in

the day, the number of deaths might have been much higher

because many public buildings vulnerable to damage would

have been occupied. Approximately 500 buildings in Mexico

City were destroyed or experienced heavy damage. Damage

was estimated at about $4 billion, but serious damage was

confined to a small total portion of the city. The character of

underlying material did much to determine how much damage

buildings in the city experienced from the earthquake. Damage

was worst in areas underlain by moist, unconsolidated mate-

rial deposited in the bed of what once was Lake Texcoco. The

lake was drained by Spanish settlers after the conquest of the

indigenous Aztec civilization. Construction began afterward

The volcanic crater and slopes of San Benedicto Island along the

northern Pacific coast of Mexico. (Courtesy of the USGS)

An eight-story frame structure with brick walls was broken away from

its foundation and broken in half during the 1985 Mexico City earthquake.

(Courtesy of the USGS)

Rayleigh waves caused the crush and collapse of the upper floors of

the Lasca Building during the 1985 Mexico City earthquake. Most of the

windows were popped out and turned into missiles. (Courtesy of the

USGS)

Mexico City 165

atop the sediments of the lake bed. This kind of soil is espe-

cially susceptible to ground motion in a strong earthquake,

and any structure built atop such soil is vulnerable to damage

from the ground motion. The earthquake’s destructive poten-

tial was reduced by the distance (some 200 miles [320 km])

between Mexico City and the earthquake’s point of origin. An

earlier earthquake in Mexico City, in 1957, produced a pat-

tern of damage similar to that from the 1985 earthquake.

mica A mineral group of sheetlike, smooth, and shiny

layers that are adhered together in a stack. The stacks are

referred to as books. They include white mica, muscovite,

black mica, biotite, as the main varieties. Biotite can occur in

andesite, dacite, latite, and rhyolite and their plutonic

counterparts. Muscovite is mostly restricted to rhyolite, and

even there it is rare. It is more common in granite, the plu-

tonic counterpart. Even if present, mica constitutes a minor

component in igneous rocks.

Michigan United States Although it generally lies within a

zone of minor seismic risk, the state of Michigan is subjected

from time to time to strong earthquakes originating in the

Saint Lawrence Valley. An earthquake at Calumet on July

26, 1905, is thought to have been induced by conditions asso-

ciated with mining operations. There was reportedly a great

explosion. Chimneys toppled, and windows were broken.

The earthquake was felt throughout Michigan’s Keweenaw

Peninsula. On May 26, 1906, some kind of seismic event at

a mine on the Keweenaw Peninsula affected an area as wide

as 40 miles (64 km) and resembled in its effects a powerful

earthquake; subsidence was reported, and rails were twisted.

microearthquake An earthquake with a magnitude less

than 2 on the Richter scale. Microearthquakes are com-

mon on all active faults and can number in the hundreds

to thousands per year without foreshadowing any major

activity. They are even common in areas of low seismic risk.

A radical increase in the number of microearthquakes in an

area, however, is commonly considered a warning signal that

increased major seismic activity is possible. Microearthquakes

also may foretell of volcanic eruptions. As magma forces its

way upwards through the crust to the volcano, it does so by

cracking the rock in its way. This cracking yields an earth-

quake. There are swarms of earthquakes present in advance

of an eruption in most volcanoes. These earthquakes usually

start out as microearthquakes but as the eruption nears, they

can be much larger.

microzonation The practice by which a town or county is

subdivided into smaller subareas according to the variation

in seismic hazards. Depending upon physical features as well

as underlying geology, different seismic hazards will be domi-

nant. For example, hills are prone to landslides, whereas

water may yield tsunamis or seiches.

mid-ocean ridge These structures are undersea moun-

tain chains that mark zones where magma rises from the

mantle and solidifies, generating new ocean crust. This

newly formed rock moves laterally outward from fracture

down the middle of the ridge. Midocean ridges are part of

a worldwide system of divergent boundaries that is not

confined to oceans but may extend into continents as well.

Mid-ocean ridges are sites of extensive earthquake and volca-

nic activity. Not only are there numerous normal faults form-

ing grabens along the peak of the ridge, but there are also

regularly spaced transform faults that break the ridge into

offset segments. These transform faults are seismically active

and spread several hundred kilometers across the ocean basin

thus extending earthquake activity to a much larger area.

Large transform faults are fracture zones. The rocks pro-

duced at mid-ocean ridges are the key evidence that geolo-

gists used to prove the existence of plate tectonics. New

ocean crust is constantly being formed at the center of the

mid-ocean ridge and moving away toward the edges of the

basin like a conveyor belt. The igneous rock that solidifies

along mid-ocean ridges preserves a record of the orientation

of Earth’s magnetic field at the time the rock was formed.

Because Earth’s magnetic field changes radically on a regular

basis, the ocean crust acts like a piece of magnetic recording

tape and preserves a history of the magnetic field. The mag-

netic signatures (patterns) of the ocean crust are exact mirror

images across the mid-ocean ridge as are the ages, elevations,

and sediment thickness. The theory of how plates are formed

and moved are based on this important discovery.

A black smoker at a mid-ocean ridge hydrothermal vent from the Atlantic

Ocean. (Courtesy of NOAA)

166 mica

Exotic chemical and biological environments are found at

certain points along mid-ocean ridges. For example, explorers

from the Scripps Institute of Oceanography and the Woods

Hole Oceanographic Institution examined the mid-ocean

ridge near the Galápagos Islands in 1977 and discovered

a dense concentration of undersea life around volcanic vents

on the seabed at depths of about 9,000 feet (2,743 m). Fauna

found at this site included giant clams more than a foot long,

and huge polychaete worms that lived in tubes. At another

site along the mid-ocean ridge off the western coast of Mex-

ico, mineral-laden hot water rising from vents in the seabed

has produced peculiar “chimneys” made of minerals precipi-

tated from the heated water. This process of precipitation may

have created extensive deposits of rich metal ores on the sea-

bed along certain portions of the mid-ocean ridges, but such

deposits do not appear to be commercially exploitable.

Mihara-yama See Izu-O

shima.

Minnesota United States Minnesota does not have a

long history of strong earthquake activity because the state

occupies a zone of minor seismic risk. A strong earthquake

reportedly occurred in Minnesota in 1860, but details are

unavailable. Another strong earthquake may have occurred

in Minnesota between 1860 and 1865, but this is not cer-

tain. On September 3, 1917, an earthquake at Staples caused

considerable damage to buildings and knocked objects off

shelves. Other, minor earthquakes were recorded in 1909,

1925, 1935, and 1968.

Mino-Owari (Nobi) earthquake, Japan The greatest inland

earthquake experienced in Japan occurred in the provinces of

Mino and Owari on October 28, 1891, at 9:38 p.m. The esti-

mated Richter magnitude for this event was 8.4. The area of

damage was over 4,300 square miles (11,150 km

). The epi-

center was on the island of Honshu, on one of a series of

northwest-to-southeast faults (now called the Mino-Owari

Fault Zone) that crosses the area, and the focus was less than

9.5 miles (15 km). It was therefore not one of the more com-

mon subduction zone–related earthquakes, which are much

deeper. The sense of movement on these faults was a combina-

tion of left-lateral strike-slip and reverse. The surface rup-

tures from this event were extraordinary. They could be traced

on land continuously for 40–70 miles (24–42 km). They exhib-

ited horizontal offsets up to 13 feet (4 m) and vertical offset up

to 20 feet (6 m), producing spectacular scarps. Later geodetic

surveys showed widespread uplift of some 28 inches (70 cm) in

some areas and subsidence of 12–16 inches (30–40 cm) in oth-

ers. Over the two years following the main shock, there were

some 3,365 aftershocks—10 of which were classified as violent

and 97 of which were classified as strong.

In all, some 7,273 people lost their lives in this event,

and more than 17,000 were injured. Officially, 142,177

houses were destroyed. In addition to destruction by shaking,

some 10,000 landslides occurred throughout the area. One

blocked a river, causing a flood and producing an earthquake

lake. There were also reports of tsunamis along the coast.

The 1891 earthquake spurred the government to change

building codes to more earthquake-resistant designs.

Mississippi United States The state of Mississippi is not

known for frequent and strong earthquake activity and lies

largely within an area of modest seismic risk. Nonetheless,

strong earthquakes have occurred in Mississippi. One of these

took place on December 16, 1931, in the northern portion of

the state. Walls and foundations were damaged and some chim-

neys destroyed at Charleston, South Carolina. Damage

also was reported at Belzoni, Water Valley, and Tillatoba. This

earthquake was felt over an area of about 65,000 square miles

(168,349 km

) and was rated at Mercalli intensity VI–VII.

Missoula, Lake northwest United States The landscape

of northwest United States was reshaped extensively by the

floods that resulted from the draining of Lake Missoula, which

apparently occupied much of western Montana 15,000 to

20,000 years ago. Glacial in origin, Lake Missoula is thought

to have begun draining in catastrophic fashion when a dam of

ice in the Clark Fork Valley in Idaho gave way and let several

hundred cubic miles of water flow westward, carrying away

soil and even eroding the underlying bedrock. One result

of this assault by water is the Channeled Scabland of Wash-

ington, an area of some 2,000 square miles (5,180 km

)

partly denuded by the flood. This same flood is believed to

have invaded Oregon’s Willamette Valley and reached south-

ward to the location of present-day Eugene. Other floods also

originated from Lake Missoula. The floods made the walls of

the Columbia River gorge steeper and thus contributed to

the conditions that are thought to have resulted in the great

landslides along that gorge, notably the Bonneville Slide,

which may have been set off by a powerful earthquake.

Missouri United States Missouri and its neighboring states

have undergone some of the most powerful earthquakes in

Diagram of a mid-ocean ridge showing newly formed ocean crust moving

away from the axis in both directions at the same speed. Circulation of

the mantle is shown by the arrows and generates magma beneath the

ridge. The magma fills the fissures left as the two plates are pulled apart.

The ridge is higher than the surrounding ocean floor both because the

mantle is pushing it up and because the new crust is hot and therefore

less dense and more buoyant. A mid-ocean ridge is an example of a

divergent margin.

Missouri 167

the history of the United States. Strong seismic activity in the

Mississippi Valley region combines with large areas of uncon-

solidated, moist soil to produce conditions that are highly

favorable for liquefaction. The most famous earthquakes

in Missouri history are thought to have been also the most

powerful recorded in North America since European settle-

ment began. These earthquakes occurred between December

1811 and February 1812 in the New Madrid area along the

Mississippi River and are thought to have affected an area

estimated at a minimum of 2 million square miles (5,179,976

). The earthquakes are believed to have altered topogra-

phy over an area of as many as 50,000 square miles (129,499

). Although the earthquakes have been estimated at Mer-

calli intensity XI, casualties were few because the area was

settled sparsely at the time.

The first earthquake in this series began in the early

morning of December 16, 1811, shaking houses and knock-

ing down chimneys. This earthquake had dramatic effects on

the landscape: It caused landslides, generated waves in the

river, and made entire islands vanish. Lesser shocks occurred

over the following days. Another shock, comparable in many

ways to the initial earthquake, occurred on January 23, 1812.

A third great shock, apparently even more powerful than the

first two, took place on February 7, 1812, and was followed

by occasional aftershocks during the next two years or

longer. Considerable uplift and subsidence occurred in these

earthquakes. Some areas were uplifted 15 feet (5 m) or more;

one such area, the Tiptonville Dome, is some 15 miles (24

km) long and as much as eight miles (13 km) wide. Submer-

gence at Reelfoot Lake in Tennessee occurred to depths of

some 20 feet (6 m) or even greater.

“Moho” See Mohorovicic discontinuity.

Mohorovicic discontinuity A thin transition zone

between Earth’s crust and uppermost mantle constitutes the

“Moho” for short. The Moho was first identified by a rapid

change in the velocity of seismic waves at a depth ranging

from about 22 to 50 miles (35 to 80 km), depending on the

elevation of the land. It was later determined that this transi-

tion marked the top of the mantle. The Moho occurs midway

through the lithosphere and marks the boundary with the

rigid mantle. In continent-continent collisions, sometimes the

rigid mantle is stripped off of the crust approximately along

the Moho in a process called delamination.

moment magnitude The moment magnitude, Mw, is the

measured magnitude of an earthquake using seismic moment.

The seismic moment is a measure of earthquakes based on

the leverage of forces across the area of rupture on the fault

plane. In mathematical terms, it is equal to the rigidity of the

rock times the area of faulting times the amount of slip on

the fault.

Momotombo volcano, Nicaragua A stratovolcano

in western Nicaragua. Momotombo has erupted at least

15 times since 1524. Its last major eruption was in 1905. Its

most famous eruption was in 1605–06, when it destroyed the

city of Léon, the former capital of Nicaragua.

Mono Lake California, United States Located in Cali-

fornia near the Nevada border and immediately north of

the Long Valley caldera, Mono Lake is considered one of

the most likely sites for future volcanic activity in the United

States and has undergone moderate earthquake activity in

recent years. Extending approximately north-south between

Mono Lake and Long Valley caldera is a set of volcanic land-

forms known as the Mono Craters. vents in the vicinity of

Mono Lake appear to have been active within the past two

millennia and to have become active as a series of vents along

a zone several miles in length, rather than erupted at a single

point, as is the case with many volcanoes. Two major series

of eruptions at Mono Lake are thought to have occurred in

the 14th century. This activity is believed to have begun when

molten rock moved along a fracture to the surface, came

in contact with groundwater there, and generated steam

that caused a powerful explosion. A string of new vents some

four miles (6.4 km) long formed during this eruption, which

expelled large amounts of ash over what now is central Cali-

fornia and western Nevada. pyroclastic flows followed

this initial eruptive activity but affected only the area imme-

diately around Mono Lake. Next highly viscous lava emerged

from the ground and formed domes and coulees, including

Panum Dome. Soon after these eruptive events, another series

of explosions commenced at Inyo Craters, immediately to the

south of Mono Lake, under circumstances apparently simi-

lar to those which led to the first eruptive cycle. (These two

outbursts may have been separated by only a few weeks.)

This eruption also produced large quantities of ash and steam

explosions, followed by formation of domes and flows of

obsidian. A series of earthquakes estimated up to 6.0 on the

Richter scale of magnitude occurred in 1980, and earth-

quakes of comparable magnitude happened again in 1986

along the nearby White Mountain fault, reminding observers

that the area around Mono Lake maintains a considerable

potential for seismic and volcanic activity. Depending on the

intensity of a future eruption at Mono Lake, as well as other

factors including wind direction and velocity, ashfalls might

affect areas hundreds of miles downwind.

Monowai Seamount Kermadec Islands, Pacific Ocean It

is a submarine volcano that is located about 932 miles

(1,500 km) northeast of the north island of New Zealand. It

has erupted at least eight times in the past 20 years including

a 1977 eruption that lasted for 18 months.

Montana United States The state of Montana is mod-

erately active from a seismic standpoint, with much of the

earthquake activity concentrated in the southwestern portion

of the state. Earthquakes centered in Montana have affected

vast areas of the western United States.

One of the greatest earthquakes in Montana’s history,

estimated at Mercalli intensity VII and Richter magnitude

6.75, occurred on June 27, 1925, east of Helena. This earth-

quake was felt very strongly over an area of some 600 square

miles (1,554 km

), and the total area affected was more than

300,000 square miles (776,996 km

). On an east-west axis,

the earthquake was felt from the border of North Dakota

to Washington State, and on a north-south axis from the

168 “Moho”

border of Canada to the central portion of Wyoming. after-

shocks continued for months following the earthquake. Dam-

age was concentrated at Logan, Lombard, Manhattan, and

Three Forks. A large school building at Manhattan was demol-

ished. Chimneys were seen to fall, not in one prevailing direction

but rather in many different directions; this phenomenon led one

observer to speculate that some kind of “twist” was involved in

this particular earthquake. Railroad tracks shifted, and rockfalls

interfered with service on the Northern Pacific Railroad and the

Chicago, Milwaukee, and St. Paul. A powerful roar accompa-

nied the earthquake at Bozeman. A church and a schoolhouse

were damaged at Three Forks.

The earthquake of August 17, 1959, near Hebgen Lake,

Montana, was of Mercalli intensity X and was estimated at

7.1 on the Richter scale. This earthquake affected an area of

some 600,000 square miles (1.535 million km

) and was felt

at locations as distant as Seattle on the west, North Dakota

on the east, Utah in the south, and Banff, Alberta, Canada

to the north. Twenty-eight people were killed, most of them

in landslides. One spectacular landslide occurred along the

south wall of Madison River Canyon, where a huge mass of

earth, rock, and trees fell and blocked the Madison River.

Within several weeks, this landslide had created a lake with a

depth of almost 175 feet (53 m).

Monte Negro earthquake, Croatia At 7:00 a.m. on April

6, 1667, the Monte Negro area of Croatia along the Adriatic

coast was rocked by a powerful earthquake. The estimated

magnitude of the quake was 7.2, and the intensity was

estimated as high as IX in the town of Budva. It was reported

that the shock was accompanied by strong winds in the same

east-west direction. The sea withdrew four times that day,

and tsunamis struck in many areas. It was said that the town

of Ragusa (Dubrovnik) lost 5,000 people, but there was also

near–total destruction on the island of Mozzo as well as in

the towns of Kastel Novo, Budva, and Kataro. The death

toll for the event was likely much higher.

Monte Pelato See Campo Bianco.

Montserrat volcanic island, Caribbean Until recently, there

had been no eruptions at Montserrat within historical times.

However, the recent and current eruption of Soufrière Hills

changed that. Previously, the island exhibited considerable

earthquake and solfataric activity related to intrusions of

magma under the Soufrière Hills at the southern end of the

island.

Moon Instruments placed on the Moon have reported large

numbers of seismic events, but the pattern of lunar seismicity

differs from that of Earth in important ways. Although sev-

eral thousand moonquakes are believed to occur each year,

the Moon appears to release much less energy in such quakes

annually than Earth does. Total energy released in terres-

trial quakes each year is thought to be more than 1 billion

times greater than on the Moon. Some evidence of moon-

quakes is visible to the unaided eye, as where seismic events

have dislodged boulders from positions on slopes and caused

the rocks to roll downhill. Lunar quakes also are believed to

occur at greater depths than similar events on Earth. These

differences reflect the fact that Earth is more active geologi-

cally than the Moon, both on the surface and at depth. The

ever-changing pattern of plate tectonic activity, with its

characteristic distribution of earthquake foci and volcanic

eruptions, is absent from the Moon. The character of lunar

rock also reflects the difference between processes at work

on the moon and on Earth. Lunar rock is igneous (basalts

in the maria, or “seas,” and a gabbro-like igneous rock

called anorthosite in the highlands), whereas terrestrial rock

may be igneous, sedimentary, or metamorphic, as a result

of erosion, sedimentation, and other processes related to

the action of wind and water on Earth. impact structures

are prominent on the lunar surface. The maria are believed

to have formed when large meteorites struck the Moon and

caused great flows of molten rock. Some of the rills, chan-

nel-like features on the lunar landscape, are thought to have

formed through flows of lava.