CI 5995 Engineering Seismology

1
CI 5995 Engineering Seismology

• Lecture 2 Plate Tectonics

2
Class websitehttp//richter.uprm.edu/jclinton/
CI5995.html
3
Geologic Time
4
Introduction

• Adapted from http//www.ucmp.berkeley.edu/geology/
  http//pubs.usgs.gov/publications/text/dynamic.ht
  ml
• Plate tectonics is the theory that Earth's outer
  layer is made up of plates, which have moved
  throughout Earth's history.
• The theory explains the how and why behind
  mountains, volcanoes, and earthquakes, as well as
  how, long ago, similar animals could have lived
  at the same time on what are now widely separated
  continents.
• 225 million years ago, all the major continents
  formed one giant supercontinent, called Pangaea.
• Perhaps initiated by heat building up underneath
  the vast continent, Pangaea began to rift, or
  split apart, around 200 million years ago. Oceans
  filled the areas between these new
  sub-continents. The land masses continued to move
  apart, riding on separate plates, until they
  reached the positions they currently occupy.
• These continents are still on the move today.
• Exactly what drives plate tectonics is not
  known.
• One theory is that convection within the Earth's
  mantle pushes the plates, in much the same way
  that air heated by your body rises upward and is
  deflected sideways when it reaches the ceiling.
• Another theory is that gravity is pulling the
  older, colder, and thus heavier ocean floor with
  more force than the newer, lighter seafloor
• .Whatever drives the movement, plate tectonic
  activity takes place at four types of boundaries
  
• divergent boundaries, where new crust is formed
  
• convergent boundaries, where crust is consumed
• collisional boundaries, where two land masses
  collide and
• transform boundaries, where two plates slide
  against each other.
• The main features of plate tectonics are
• The Earth's surface is covered by a series of
  crustal plates.
• The ocean floors are continually, moving,
  spreading from the center, sinking at the edges,
  and being regenerated.

5
Historical Perspective (1)

• Close examination of a globe often results in the
  observation that most of the continents seem to
  fit together like a puzzle the west African
  coastline seems to snuggle nicely into the east
  coast of South America and the Caribbean sea and
  a similar fit appears across the Pacific.  The
  fit is even more striking when the submerged
  continental shelves are compared rather than the
  coastlines.  
• In 1912 Alfred Wegener (1880-1930) noticed the
  same thing and proposed that the continents were
  once compressed into a single protocontinent
  which he called Pangaea (meaning "all lands"),
  and over time they have drifted apart into their
  current distribution. He believed that Pangaea
  was intact until the late  Carboniferous period,
  about 300 million years ago, when it began to
  break up and drift apart.
• However, Wegener's hypothesis lacked a geological
  mechanism to explain how the continents could
  drift across the earths surface as he
  proposed.Searching for evidence to further
  develop his theory of continental drift, Wegener
  came across a paleontological paper suggesting
  that a land bridge had once connected Africa with
  Brazil. This proposed land bridge explained
  paleontological observation that the same
  fossilized plants and animals from the same time
  period were found in South America and Africa. 
  The same was true for fossils Europe / North
  America, Madagascar / India.  Many of these
  organisms could not have traveled across the vast
  oceans that currently exist. Wegener's drift
  theory seemed more plausible than land bridges
  connecting all of the continents. But that in
  itself was not enough to support his idea.
• Another observation favoring continental drift
  was the presence of evidence for continental
  glaciation in the Pensylvanian period. Striae
  left by the scraping of glaciers over the land
  surface indicated that Africa and South America
  had been close together at the time of this
  ancient ice age. The same scraping patterns can
  be found along the coasts of South America and
  South Africa.Wegener's drift hypothesis also
  provided an alternate explanation for the
  formation of mountains (orogenesis).
• The theory being discussed during his time was
  the "Contraction theory" which suggested that the
  planet was once a molten ball and in the process
  of cooling the surface cracked and folded up on
  itself.  The big problem with this idea was that
  all mountain ranges should be approximately the
  same age, and this was known not to be true. 
  Wegener's explanation was that as the continents
  moved, the leading edge of the continent would
  encounter resistance and thus compress and fold
  upwards forming mountains near the leading edges
  of the drifting continents.  The Sierra Nevada
  mountains on the Pacific coast of North America
  and the Andes on the coast of South America were
  cited.  Wegener also suggested that India drifted
  northward into the asian continent thus forming
  the Himalayas.Wegener eventually proposed a
  mechanism for continental drift that focused on
  his assertion that the rotation of the earth
  created a centrifugal force towards the equator. 
  He believed that Pangaea originated near the
  south pole and that the centrifugal force of the
  planet caused the protocontinent to break apart
  and the resultant continents to drift towards the
  equator.  He called this the "pole-fleeing
  force".  This idea was quickly rejected by the
  scientific community primarily because the actual
  forces generated by the rotation of the earth
  were calculated to be insufficient to move
  continents.  Wegener also tried to explain the
  westward drift of the Americas by invoking the
  gravitational forces of the sun and the moon,
  this idea was also quickly rejected.  Wegener's
  inability to provide an adequate explanation of
  the forces responsible for continental drift and
  the prevailing belief that the earth was solid
  and immovable resulted in the scientific
  dismissal of his theories

6
Historical Perspective (1)

• Close examination of a globe often results in the
  observation that most of the continents seem to
  fit together like a puzzle the west African
  coastline seems to snuggle nicely into the east
  coast of South America and the Caribbean sea and
  a similar fit appears across the Pacific.  The
  fit is even more striking when the submerged
  continental shelves are compared rather than the
  coastlines.  
• In 1912 Alfred Wegener (1880-1930) noticed the
  same thing and proposed that the continents were
  once compressed into a single protocontinent
  which he called Pangaea (meaning "all lands"),
  and over time they have drifted apart into their
  current distribution. He believed that Pangaea
  was intact until the late  Carboniferous period,
  about 300 million years ago, when it began to
  break up and drift apart.
• However, Wegener's hypothesis lacked a geological
  mechanism to explain how the continents could
  drift across the earths surface as he
  proposed.Searching for evidence to further
  develop his theory of continental drift, Wegener
  came across a paleontological paper suggesting
  that a land bridge had once connected Africa with
  Brazil. This proposed land bridge explained
  paleontological observation that the same
  fossilized plants and animals from the same time
  period were found in South America and Africa. 
  The same was true for fossils Europe / North
  America, Madagascar / India.  Many of these
  organisms could not have traveled across the vast
  oceans that currently exist. Wegener's drift
  theory seemed more plausible than land bridges
  connecting all of the continents. But that in
  itself was not enough to support his idea.
• Another observation favoring continental drift
  was the presence of evidence for continental
  glaciation in the Pensylvanian period. Striae
  left by the scraping of glaciers over the land
  surface indicated that Africa and South America
  had been close together at the time of this
  ancient ice age. The same scraping patterns can
  be found along the coasts of South America and
  South Africa.Wegener's drift hypothesis also
  provided an alternate explanation for the
  formation of mountains (orogenesis).
• The theory being discussed during his time was
  the "Contraction theory" which suggested that the
  planet was once a molten ball and in the process
  of cooling the surface cracked and folded up on
  itself.  The big problem with this idea was that
  all mountain ranges should be approximately the
  same age, and this was known not to be true. 
  Wegener's explanation was that as the continents
  moved, the leading edge of the continent would
  encounter resistance and thus compress and fold
  upwards forming mountains near the leading edges
  of the drifting continents.  The Sierra Nevada
  mountains on the Pacific coast of North America
  and the Andes on the coast of South America were
  cited.  Wegener also suggested that India drifted
  northward into the asian continent thus forming
  the Himalayas.Wegener eventually proposed a
  mechanism for continental drift that focused on
  his assertion that the rotation of the earth
  created a centrifugal force towards the equator. 
  He believed that Pangaea originated near the
  south pole and that the centrifugal force of the
  planet caused the protocontinent to break apart
  and the resultant continents to drift towards the
  equator.  He called this the "pole-fleeing
  force".  This idea was quickly rejected by the
  scientific community primarily because the actual
  forces generated by the rotation of the earth
  were calculated to be insufficient to move
  continents.  Wegener also tried to explain the
  westward drift of the Americas by invoking the
  gravitational forces of the sun and the moon,
  this idea was also quickly rejected.  Wegener's
  inability to provide an adequate explanation of
  the forces responsible for continental drift and
  the prevailing belief that the earth was solid
  and immovable resulted in the scientific
  dismissal of his theories

7
Historical Perspective (2)

• In 1929, about the time Wegener's ideas began to
  be dismissed, Arthur Holmes elaborated on one of
  Wegener's many hypotheses the idea that the
  mantle undergoes thermal convection.  This idea
  is based on the fact that as a substance is
  heated its density decreases and rises to the
  surface until it is cooled and sinks again. This
  repeated heating and cooling results in a current
  which may be enough to cause continents to move. 
  Arthur Holmes suggested that this thermal
  convection was like a conveyor belt and that the
  upwelling pressure could break apart a continent
  and then force the broken continent in opposite
  directions carried by the convection currents. 
  This idea received very little attention at the
  time.Not until the 1960's did Holmes' idea
  receive any attention. Greater understanding of
  the ocean floor and the discoveries of features
  like mid-oceanic ridges, geomagnetic anomalies
  parallel to the mid-oceanic ridges,  and the
  association of island arcs and oceanic trenches
  occurring together and near the continental
  margins, suggested convection might indeed be at
  work. These discoveries and more led Harry Hess
  (1962) and R.Deitz (1961) to publish similar
  hypotheses based on mantle convection currents,
  now known as "sea floor spreading".  This idea
  was basically the same as that proposed by Holmes
  over 30 years earlier, but now there was much
  more evidence to further develop and support the
  idea.
• Advances in sonic depth recording during World
  War II and the subsequent development of the
  nuclear resonance type magnometer
  (proton-precession magnometer) led to detailed
  mapping of the ocean floor and with it came many
  observation that led scientists like Howard Hess
  and R. Deitz to revive Holmes' convection
  theory.
• Hess and Deitz modified the theory considerably
  and called the new theory "Sea-floor Spreading".
  Among the seafloor features that supported the
  sea-floor spreading hypothesis were mid-oceanic
  ridges, deep sea trenches, island arcs,
  geomagnetic patterns, and fault patterns.
• Mid-Oceanic Ridges The mid-oceanic ridges rise
  3000 meters from the ocean floor and are more
  than 2000 kilometers wide surpassing the
  Himalayas in size. The mapping of the seafloor
  also revealed that these huge underwater mountain
  ranges have a deep trench which bisects the
  length of the ridges and in places is more than
  2000 meters deep. Research into the heat flow
  from the ocean floor during the early 1960s
  revealed that the greatest heat flow was centered
  at the crests of these mid-oceanic ridges.
  Seismic studies show that the mid-oceanic ridges
  experience an elevated number of earthquakes. All
  these observations indicate intense geological
  activity at the mid-oceanic ridges.

8
Historical Perspective (3)

• Geomagnetic Anomalies Peridically, the Earth's
  magnetic field reverses. New rock formed from
  magma records the orientation of Earth's magnetic
  field at the time the magma cools. Study of the
  sea floor with magnometers revealed "stripes" of
  alternating magnetization parallel to the
  mid-oceanic ridges. This is evidence for
  continuous formation of new rock at the ridges.
  As more rock forms, older rock is pushed farther
  away from the ridge, producing symmetrical
  stripes to either side of the ridge. In the
  diagram to the right, the dark stripes represent
  ocean floor generated during "reversed" polar
  orientation and the lighter stripes represent the
  polar orientation we have today. Notice that the
  patterns on either side of the line

• representing the mid-oceanic ridge are mirror
  images of one another. The shaded stripes also
  represent older and older rock as they move away
  from the mid-oceanic ridge. Geologists have
  determined that rocks found in different parts of
  the planet with similar ages have the same
  magnetic characteristics.
• Deep Sea Trenches The deepest waters are found
  in oceanic trenches, which plunge as deep as
  35,000 feet below the ocean surface. These
  trenches are usually long and narrow, and run
  parallel to and near the oceans margins. They are
  often associated with and parallel to large
  continental mountain ranges. There is also an
  observed parallel association of trenches and
  island arcs. Like the mid-oceanic ridges, the
  trenches are seismically active, but unlike the
  ridges they have low levels of heat flow.
  Scientists also began to realize that the
  youngest regions of the ocean floor were along
  the mid-oceanic ridges, and that the age of the
  ocean floor increased as the distance from the
  ridges increased. In addition, it has been
  determined that the oldest seafloor often ends in
  the deep-sea trenches.
• Island Arcs Chains of islands are found
  throughout the oceans and especially in the
  western Pacific margins the Aleutians, Kuriles,
  Japan, Ryukus, Philippines, Marianas, Indonesia,
  Solomons, New Hebrides, and the Tongas, are some
  examples.. These "Island arcs" are usually
  situated along deep sea trenches and are situated
  on the continental side of the trench.These
  observations, along with many other studies of
  our planet, support the theory that underneath
  the Earth's crust (the lithosphere a solid array
  of plates) is a malleable layer of heated rock
  known as the asthenosphere which is heated by
  radioactive decay of elements such as Uranium.
  Because the radioactive source of heat is deep
  within the mantle, the fluid asthenosphere
  circulates as convection currents underneath the
  solid lithosphere. This heated layer is the
  source of 1. lava we see in volcanos, 2. heat
  that drives hot springs and geysers, and 3. raw
  material which pushes up the mid-oceanic ridges
  and forms new ocean floor. Magma continuously
  wells upwards at the mid-oceanic ridges producing
  currents of magma flowing in opposite directions
  and thus generating the forces that pull the sea
  floor apart at the mid-oceanic ridges. As the
  ocean floor is spread apart cracks appear in the
  middle of the ridges allowing molten magma to
  surface through the cracks to form the newest
  ocean floor. As the ocean floor moves away from
  the mid-oceanic ridge it will eventually come
  into contact with a continental plate and will be
  subducted underneath the continent. Finally, the
  lithosphere will be driven back into the
  asthenosphere where it returns to a heated state.

9
Historical Perspective (3)

• Geomagnetic Anomalies

10
A Section Through the Earth
The Earth is composed of 3 distinct layers
crust, mantle, and core. Each layer has its
own unique properties and chemical composition.
Crust The crust is the thin, solid, outermost
layer of the Earth. The crust is thinnest
beneath the oceans, averaging only 5 km thick,
and thickest beneath large mountain
ranges.Continental crust (the crust that makes
up the continents) is much more variable in
thickness but averages about 30-35km. Beneath
large mountain ranges, such as the Himalayas or
the Sierra Nevada, the crust reaches a thickness
of up to 100 km. Mantle The layer below the
crust is the mantle. The mantle has more iron and
magnesium than the crust, making it more dense.
The uppermost part of the mantle is solid and,
along with the crust, forms the lithosphere. The
rocky lithosphere is brittle and can fracture.
This is the zone where earthquakes occur.
Its the lithosphere that breaks into the thick,
moving slabs of rock, tectonic plates. As we
descend into the Earth temperature rises and we
reach part of the mantle that is partially
molten, the asthenosphere. As rock heats up, it
becomes pliable or plastic. Rock here is hot
enough to fold, stretch, compress, and flow very
slowly without fracturing (like plasticine/silly
putty). The plates, made up of the relatively
light, rigid rock of the lithosphere float on
the more dense, flowing asthenosphere. Core At
the center of the Earth lies the super-dense
core. With a diameter of 3486 kilometers, the
core is larger than the planet Mars. The core of
the Earth is made up of two distinct layers a
liquid outer layer and a solid inner core.
Unlike the Earths outer layers with rocky
compositions, the core is made up of metallic
iron nickel alloy. The core is about 5 times as
dense as surface rock.
11
Plates
The Main Tectonic Plates
12
Tectonic Evolution Movies1. Pangea to today
Tanya Atwater, UCSB http//emvc.geol.ucsb.edu/do
wnloads.php
13
Western USA
Tanya Atwater, UCSB http//emvc.geol.ucsb.edu/do
wnloads.php
14
Subduction Zone
Tanya Atwater, UCSB http//emvc.geol.ucsb.edu/do
wnloads.php
15
S. Atlantic Spreading Section
Tanya Atwater, UCSB http//emvc.geol.ucsb.edu/do
wnloads.php
16
Another View
Movie of above clip 400MA to now
17
More Movies
Pangea Breakup http//www.scotese.com/sfsanim.htm
Caribbean Tectonic Evolution
http//www.scotese.com/caribanim.htm Future
Tectonics??? http//www.scotese.com/futanima.htm

18
Plate Boundaries

• There are four types of plate boundaries
• 1. Divergent boundaries -- where new crust is
  generated as the plates pull away
• from each other.
• 2. Convergent boundaries -- where crust is
  destroyed as one plate dives under another.
• 3. Transform boundaries -- where crust is
  neither produced nor destroyed
• as the plates slide horizontally past each other.
• 4. Plate boundary zones -- broad belts in
  which boundaries are not well defined
• and the effects of plate interaction are unclear.

19
Divergent boundaries
Divergent boundaries occur along spreading
centers where plates are moving apart and new
crust is created by magma pushing up from the
mantle. (like two giant conveyor belts, facing
each other but slowly moving in opposite
directions as they transport newly formed oceanic
crust away from the ridge crest). Perhaps the
best known of the divergent boundaries is the
Mid-Atlantic Ridge. This submerged mountain
range, which extends from the Arctic Ocean to
beyond the southern tip of Africa, is but one
segment of the global mid-ocean ridge system that
encircles the Earth. The rate of spreading along
the Mid-Atlantic Ridge averages about 2.5
centimeters per year (cm/yr), or 25 km in a
million years. (This rate, similar to fingernail
growth, may seem slow by human standards, but
because this process has been going on for
millions of years, it has resulted in plate
movement of thousands of kilometers). Seafloor
spreading over the past 100 to 200 million years
has caused the Atlantic Ocean to grow from a tiny
inlet of water between the continents of Europe,
Africa, and the Americas into the vast ocean that
exists today. The volcanic country of Iceland,
which straddles the Mid-Atlantic Ridge, is a
natural laboratory for studying on land the
processes also occurring along the submerged
parts of a spreading ridge. Iceland is splitting
along the spreading center between the North
American and Eurasian Plates, as North America
moves westward relative to Eurasia. The
consequences of plate movement are easy to see
around Krafla Volcano, where existing ground
cracks have widened and new ones appear every few
months. From 1975 to 1984, numerous episodes of
rifting (surface cracking) took place along the
fissure zone. Some of these rifting events were
accompanied by volcanic activity the ground
would gradually rise 1-2 m before abruptly
dropping, signalling an impending eruption.
Between 1975 and 1984, the displacements caused
by rifting totalled about 7 m. In East Africa,
spreading processes have already torn Saudi
Arabia away from the rest of the African
continent, forming the Red Sea. The actively
splitting African Plate and the Arabian Plate
meet in what geologists call a triple junction,
where the Red Sea meets the Gulf of Aden. A new
spreading center may be developing under Africa
along the East African Rift Zone. When the
continental crust stretches beyond its limits,
tension cracks begin to appear on the Earth's
surface. Magma rises and squeezes through the
widening cracks, sometimes to erupt and form
volcanoes. Whether or not it erupts, it puts more
pressure on the crust, producing additional
fractures and, ultimately, the rift zone. East
Africa may be the site of the Earth's next major
ocean. Plate interactions in the region provide
scientists an opportunity to study first hand how
the Atlantic may have begun to form about 200
million years ago. If spreading continues, the 3
plates at the triple junction will separate
completely, allowing the Indian Ocean to flood
the area and making the easternmost Africa a
large island.
20
Divergent boundaries
Divergent boundaries occur along spreading
centers where plates are moving apart and new
crust is created by magma pushing up from the
mantle. (like two giant conveyor belts, facing
each other but slowly moving in opposite
directions as they transport newly formed oceanic
crust away from the ridge crest). Perhaps the
best known of the divergent boundaries is the
Mid-Atlantic Ridge. This submerged mountain
range, which extends from the Arctic Ocean to
beyond the southern tip of Africa, is but one
segment of the global mid-ocean ridge system that
encircles the Earth. The rate of spreading along
the Mid-Atlantic Ridge averages about 2.5
centimeters per year (cm/yr), or 25 km in a
million years. (This rate, similar to fingernail
growth, may seem slow by human standards, but
because this process has been going on for
millions of years, it has resulted in plate
movement of thousands of kilometers). Seafloor
spreading over the past 100 to 200 million years
has caused the Atlantic Ocean to grow from a tiny
inlet of water between the continents of Europe,
Africa, and the Americas into the vast ocean that
exists today. The volcanic country of Iceland,
which straddles the Mid-Atlantic Ridge, is a
natural laboratory for studying on land the
processes also occurring along the submerged
parts of a spreading ridge. Iceland is splitting
along the spreading center between the North
American and Eurasian Plates, as North America
moves westward relative to Eurasia. The
consequences of plate movement are easy to see
around Krafla Volcano, where existing ground
cracks have widened and new ones appear every few
months. From 1975 to 1984, numerous episodes of
rifting (surface cracking) took place along the
fissure zone. Some of these rifting events were
accompanied by volcanic activity the ground
would gradually rise 1-2 m before abruptly
dropping, signalling an impending eruption.
Between 1975 and 1984, the displacements caused
by rifting totalled about 7 m. In East Africa,
spreading processes have already torn Saudi
Arabia away from the rest of the African
continent, forming the Red Sea. The actively
splitting African Plate and the Arabian Plate
meet in what geologists call a triple junction,
where the Red Sea meets the Gulf of Aden. A new
spreading center may be developing under Africa
along the East African Rift Zone. When the
continental crust stretches beyond its limits,
tension cracks begin to appear on the Earth's
surface. Magma rises and squeezes through the
widening cracks, sometimes to erupt and form
volcanoes. Whether or not it erupts, it puts more
pressure on the crust, producing additional
fractures and, ultimately, the rift zone. East
Africa may be the site of the Earth's next major
ocean. Plate interactions in the region provide
scientists an opportunity to study first hand how
the Atlantic may have begun to form about 200
million years ago. If spreading continues, the 3
plates at the triple junction will separate
completely, allowing the Indian Ocean to flood
the area and making the easternmost Africa a
large island.
21
Divergent boundaries
Divergent boundaries occur along spreading
centers where plates are moving apart and new
crust is created by magma pushing up from the
mantle. (like two giant conveyor belts, facing
each other but slowly moving in opposite
directions as they transport newly formed oceanic
crust away from the ridge crest). Perhaps the
best known of the divergent boundaries is the
Mid-Atlantic Ridge. This submerged mountain
range, which extends from the Arctic Ocean to
beyond the southern tip of Africa, is but one
segment of the global mid-ocean ridge system that
encircles the Earth. The rate of spreading along
the Mid-Atlantic Ridge averages about 2.5
centimeters per year (cm/yr), or 25 km in a
million years. (This rate, similar to fingernail
growth, may seem slow by human standards, but
because this process has been going on for
millions of years, it has resulted in plate
movement of thousands of kilometers). Seafloor
spreading over the past 100 to 200 million years
has caused the Atlantic Ocean to grow from a tiny
inlet of water between the continents of Europe,
Africa, and the Americas into the vast ocean that
exists today. The volcanic country of Iceland,
which straddles the Mid-Atlantic Ridge, is a
natural laboratory for studying on land the
processes also occurring along the submerged
parts of a spreading ridge. Iceland is splitting
along the spreading center between the North
American and Eurasian Plates, as North America
moves westward relative to Eurasia. The
consequences of plate movement are easy to see
around Krafla Volcano, where existing ground
cracks have widened and new ones appear every few
months. From 1975 to 1984, numerous episodes of
rifting (surface cracking) took place along the
fissure zone. Some of these rifting events were
accompanied by volcanic activity the ground
would gradually rise 1-2 m before abruptly
dropping, signalling an impending eruption.
Between 1975 and 1984, the displacements caused
by rifting totalled about 7 m. In East Africa,
spreading processes have already torn Saudi
Arabia away from the rest of the African
continent, forming the Red Sea. The actively
splitting African Plate and the Arabian Plate
meet in what geologists call a triple junction,
where the Red Sea meets the Gulf of Aden. A new
spreading center may be developing under Africa
along the East African Rift Zone. When the
continental crust stretches beyond its limits,
tension cracks begin to appear on the Earth's
surface. Magma rises and squeezes through the
widening cracks, sometimes to erupt and form
volcanoes. Whether or not it erupts, it puts more
pressure on the crust, producing additional
fractures and, ultimately, the rift zone. East
Africa may be the site of the Earth's next major
ocean. Plate interactions in the region provide
scientists an opportunity to study first hand how
the Atlantic may have begun to form about 200
million years ago. If spreading continues, the 3
plates at the triple junction will separate
completely, allowing the Indian Ocean to flood
the area and making the easternmost Africa a
large island.
22
Convergent boundaries
The size of the Earth has not changed
significantly during the past 600 million years,
and very likely not since shortly after its
formation 4.6 billion years ago - this implies
that the crust must be destroyed at about the
same rate as it is being created. Such
destruction (recycling) of crust takes place
along convergent boundaries where plates are
moving toward each other, and sometimes one plate
sinks (is subducted) under another. The type of
convergence -- called by some a very slow
"collision" -- that takes place between plates
depends on the kind of lithosphere involved
1.oceanic and a largely continental plate, or 2.
between two largely oceanic plates, or 3. between
two largely continental plates. Oceanic-continenta
l convergence the surface of the Pacific Ocean,
beneath the sea, contains a number of long
narrow, curving trenches thousands of kilometers
long and 8 to 10 km deep cutting into the ocean
floor. Trenches are the deepest parts of the
ocean floor and are created by subduction. Off
the coast of South America along the Peru-Chile
trench, the oceanic Nazca Plate is pushing into
and being subducted under the continental part of
the South American Plate. In turn, the overriding
South American Plate is being lifted up, creating
the towering Andes mountains, the backbone of the
continent. Strong, destructive earthquakes and
the rapid uplift of mountain ranges are common in
this region. Even though the Nazca Plate as a
whole is sinking smoothly and continuously into
the trench, the deepest part of the subducting
plate breaks into smaller pieces that become
locked in place for long periods of time before
suddenly moving to generate large earthquakes.
Such earthquakes are often accompanied by uplift
of the land by as much as a few meters. Can
produce massive, deep events such as 9 June 1994,
Mw8.3 event struck about 320 km northeast of La
Paz, Bolivia, within the subduction zone between
the Nazca Plate and the South American Plate
depth of 636 km. One of deepest and largest
subduction earthquakes recorded in South America.
Oceanic-continental convergence also sustains
many of the Earth's active volcanoes, such as
those in the Andes and the Cascade Range in the
Pacific Northwest. The eruptive activity is
clearly associated with subduction, but
scientists vigorously debate the possible sources
of magma partial melting of the subducted
oceanic slab, or the overlying continental
lithosphere, or both?
23
Convergent boundaries
Oceanic-oceanic convergence As with
oceanic-continental convergence, when two oceanic
plates converge, one is usually subducted under
the other, and in the process a trench is formed.
The Marianas Trench (paralleling the Mariana
Islands) marks where the fast-moving Pacific
Plate converges against the slower moving
Philippine Plate. The Challenger Deep, at the
southern end of the Marianas Trench, plunges
deeper into the Earth's interior (nearly 11,000
m) than Mount Everest, the world's tallest
mountain, rises above sea level (about 8,854 m).
Subduction processes in oceanic-oceanic plate
convergence also result in the formation of
volcanoes. Over millions of years, the erupted
lava and volcanic debris pile up on the ocean
floor until a submarine volcano rises above sea
level to form an island volcano. Such volcanoes
are typically strung out in chains called island
arcs. As the name implies, volcanic island arcs,
which closely parallel the trenches, are
generally curved. The trenches are the key to
understanding how island arcs such as the
Marianas, the Outer Antilles and the Aleutian
Islands have formed and why they experience
numerous strong earthquakes. Magmas that form
island arcs are produced by the partial melting
of the descending plate and/or the overlying
oceanic lithosphere. The descending plate also
provides a source of stress as the two plates
interact, leading to frequent moderate to strong
earthquakes. Continental-continental
convergence The Himalayan mountain range
dramatically demonstrates one of the most visible
and spectacular consequences of plate tectonics.
When two continents meet head-on, neither is
subducted because the continental rocks are
relatively light and, like two colliding
icebergs, resist downward motion. Instead, the
crust tends to buckle and be pushed upward or
sideways. The collision of India into Asia 50
million years ago caused the Eurasian Plate to
crumple up and override the Indian Plate. After
the collision, the slow continuous convergence of
the two plates over millions of years pushed up
the Himalayas and the Tibetan Plateau to their
present heights. Most of this growth occurred
during the past 10 million years. The Himalayas,
towering as high as 8,854 m above sea level, form
the highest continental mountains in the world.
Moreover, the neighboring Tibetan Plateau, at an
average elevation of about 4,600 m, is higher
than all the peaks in the Alps except for Mont
Blanc and Monte Rosa, and is well above the
summits of most mountains in the United States.  
24
Convergent boundaries
Oceanic-oceanic convergence As with
oceanic-continental convergence, when two oceanic
plates converge, one is usually subducted under
the other, and in the process a trench is formed.
The Marianas Trench (paralleling the Mariana
Islands) marks where the fast-moving Pacific
Plate converges against the slower moving
Philippine Plate. The Challenger Deep, at the
southern end of the Marianas Trench, plunges
deeper into the Earth's interior (nearly 11,000
m) than Mount Everest, the world's tallest
mountain, rises above sea level (about 8,854 m).
Subduction processes in oceanic-oceanic plate
convergence also result in the formation of
volcanoes. Over millions of years, the erupted
lava and volcanic debris pile up on the ocean
floor until a submarine volcano rises above sea
level to form an island volcano. Such volcanoes
are typically strung out in chains called island
arcs. As the name implies, volcanic island arcs,
which closely parallel the trenches, are
generally curved. The trenches are the key to
understanding how island arcs such as the
Marianas, the Outer Antilles and the Aleutian
Islands have formed and why they experience
numerous strong earthquakes. Magmas that form
island arcs are produced by the partial melting
of the descending plate and/or the overlying
oceanic lithosphere. The descending plate also
provides a source of stress as the two plates
interact, leading to frequent moderate to strong
earthquakes. Continental-continental
convergence The Himalayan mountain range
dramatically demonstrates one of the most visible
and spectacular consequences of plate tectonics.
When two continents meet head-on, neither is
subducted because the continental rocks are
relatively light and, like two colliding
icebergs, resist downward motion. Instead, the
crust tends to buckle and be pushed upward or
sideways. The collision of India into Asia 50
million years ago caused the Eurasian Plate to
crumple up and override the Indian Plate. After
the collision, the slow continuous convergence of
the two plates over millions of years pushed up
the Himalayas and the Tibetan Plateau to their
present heights. Most of this growth occurred
during the past 10 million years. The Himalayas,
towering as high as 8,854 m above sea level, form
the highest continental mountains in the world.
Moreover, the neighboring Tibetan Plateau, at an
average elevation of about 4,600 m, is higher
than all the peaks in the Alps except for Mont
Blanc and Monte Rosa, and is well above the
summits of most mountains in the United States.  
25
Convergent boundaries - Summary
26
Transform boundaries
The zone between two plates sliding horizontally
past one another is called a transform-fault
boundary, or simply a transform boundary. The
concept of transform faults originated with
Canadian geophysicist J. Tuzo Wilson, who
proposed that these large faults or fracture
zones connect two spreading centers (divergent
plate boundaries) or, less commonly, trenches
(convergent plate boundaries). Most transform
faults are found on the ocean floor. They
commonly offset the active spreading ridges,
producing zig-zag plate margins, and are
generally defined by shallow earthquakes.
However, a few occur on land, for example the
San Andreas fault zone in California. This
transform fault connects the East Pacific Rise, a
divergent boundary to the south, with the South
Gorda -- Juan de Fuca -- Explorer Ridge, another
divergent boundary to the north. The San Andreas
fault zone, which is about 1,300 km long and in
places tens of kilometers wide, slices through
two thirds of the length of California. Along it,
the Pacific Plate has been grinding horizontally
past the North American Plate for 10 million
years, at an average rate of about 5 cm/yr. Land
on the west side of the fault zone (on the
Pacific Plate) is moving in a northwesterly
direction relative to the land on the east side
of the fault zone (on the North American Plate).
Oceanic fracture zones are ocean-floor valleys
that horizontally offset spreading ridges some
of these zones are hundreds to thousands of
kilometers long and as much as 8 km deep.
Examples of these large scars include the
Clarion, Molokai, and Pioneer fracture zones in
the Northeast Pacific off the coast of California
and Mexico. These zones are presently inactive,
but the offsets of the patterns of magnetic
striping provide evidence of their previous
transform-fault activity.
27
Transform boundaries
The zone between two plates sliding horizontally
past one another is called a transform-fault
boundary, or simply a transform boundary. The
concept of transform faults originated with
Canadian geophysicist J. Tuzo Wilson, who
proposed that these large faults or fracture
zones connect two spreading centers (divergent
plate boundaries) or, less commonly, trenches
(convergent plate boundaries). Most transform
faults are found on the ocean floor. They
commonly offset the active spreading ridges,
producing zig-zag plate margins, and are
generally defined by shallow earthquakes.
However, a few occur on land, for example the
San Andreas fault zone in California. This
transform fault connects the East Pacific Rise, a
divergent boundary to the south, with the South
Gorda -- Juan de Fuca -- Explorer Ridge, another
divergent boundary to the north. The San Andreas
fault zone, which is about 1,300 km long and in
places tens of kilometers wide, slices through
two thirds of the length of California. Along it,
the Pacific Plate has been grinding horizontally
past the North American Plate for 10 million
years, at an average rate of about 5 cm/yr. Land
on the west side of the fault zone (on the
Pacific Plate) is moving in a northwesterly
direction relative to the land on the east side
of the fault zone (on the North American Plate).
Oceanic fracture zones are ocean-floor valleys
that horizontally offset spreading ridges some
of these zones are hundreds to thousands of
kilometers long and as much as 8 km deep.
Examples of these large scars include the
Clarion, Molokai, and Pioneer fracture zones in
the Northeast Pacific off the coast of California
and Mexico. These zones are presently inactive,
but the offsets of the patterns of magnetic
striping provide evidence of their previous
transform-fault activity.
28
Plate-boundary zones
Not all plate boundaries are as simple as the
main types discussed above. In some regions, the
boundaries are not well defined because the
plate-movement deformation occurring there
extends over a broad belt (plate-boundary zone).
An example is the Mediterranean-Alpine region
between the Eurasian and African Plates, within
which several smaller fragments of plates
(microplates) have been recognized. Because
plate-boundary zones involve at least two large
plates and one or more microplates caught up
between them, they tend to have complicated
geological structures and earthquake
patterns. Puerto Rico and Hispaniola are often
described as lying on microplates between the
Caribbean and North American Plates
29
Rates of Motion of Plates

• How do you determine the rates of plate movement,
  both now and over geologic time?
• The oceans hold one of the key pieces to the
  puzzle. Because the ocean-floor magnetic striping
  records the flip-flops in the Earth's magnetic
  field, scientists, knowing the approximate
  duration of the reversal, can calculate the
  average rate of plate movement during a given
  time span. These average rates of plate
  separations can range widely. The Arctic Ridge
  has the slowest rate (less than 2.5 cm/yr), and
  the East Pacific Rise near Easter Island, in the
  South Pacific about 3,400 km west of Chile, has
  the fastest rate (more than 15 cm/yr).
• Evidence of past rates of plate movement also
  can be obtained from geologic mapping studies. If
  a rock formation of known age -- with distinctive
  composition, structure, or fossils -- mapped on
  one side of a plate boundary can be matched with
  the same formation on the other side of the
  boundary, then measuring the distance that the
  formation has been offset can give an estimate of
  the average rate of plate motion. This simple but
  effective technique has been used to determine
  the rates of plate motion at divergent
  boundaries, for example the Mid-Atlantic Ridge,
  and transform boundaries, such as the San Andreas
  Fault.
• Current plate movement can be tracked directly
  by means of ground-based or space-based geodetic
  measurements geodesy is the science of the size
  and shape of the Earth. Ground-based measurements
  are taken with conventional but very precise
  ground-surveying techniques, using
  laser-electronic instruments. However, because
  plate motions are global in scale, they are best
  measured by satellite-based methods. The late
  1970s witnessed the rapid growth of space
  geodesy, a term applied to space-based techniques
  for taking precise, repeated measurements of
  carefully chosen points on the Earth's surface
  separated by hundreds to thousands of kilometers.
  3 most common space-geodetic techniques -- very
  long baseline interferometry (VLBI), satellite
  laser ranging (SLR), and the Global Positioning
  System (GPS) -- based on technologies developed
  for military and aerospace research (radio
  astronomy and satellite tracking). To date GPS
  has been the most useful for studying the Earth's
  crustal movements. GPS 21 satellites are
  currently in orbit 20,000 km above the Earth,
  continuously transmiting radio signals back to
  Earth. To determine its precise position on Earth
  (longitude, latitude, elevation), each GPS ground
  site must simultaneously receive signals from at
  least four satellites, recording the exact time
  and location of each satellite when its signal
  was received. By repeatedly measuring distances
  between specific points, geologists can determine
  if there has been active movement along faults or
  between plates. By monitoring the interaction
  between the plates, scientists hope to learn more
  about the events building up to earthquakes and
  volcanic eruptions. Space-geodetic data have
  already confirmed that the rates and direction of
  plate movement, averaged over several years,
  compare well with rates and direction of plate
  movement averaged over millions of years.

30
Rates of Motion of Plates

• How do you determine the rates of plate movement,
  both now and over geologic time?
• The oceans hold one of the key pieces to the
  puzzle. Because the ocean-floor magnetic striping
  records the flip-flops in the Earth's magnetic
  field, scientists, knowing the approximate
  duration of the reversal, can calculate the
  average rate of plate movement during a given
  time span. These average rates of plate
  separations can range widely. The Arctic Ridge
  has the slowest rate (less than 2.5 cm/yr), and
  the East Pacific Rise near Easter Island, in the
  South Pacific about 3,400 km west of Chile, has
  the fastest rate (more than 15 cm/yr).
• Evidence of past rates of plate movement also
  can be obtained from geologic mapping studies. If
  a rock formation of known age -- with distinctive
  composition, structure, or fossils -- mapped on
  one side of a plate boundary can be matched with
  the same formation on the other side of the
  boundary, then measuring the distance that the
  formation has been offset can give an estimate of
  the average rate of plate motion. This simple but
  effective technique has been used to determine
  the rates of plate motion at divergent
  boundaries, for example the Mid-Atlantic Ridge,
  and transform boundaries, such as the San Andreas
  Fault.
• Current plate movement can be tracked directly
  by means of ground-based or space-based geodetic
  measurements geodesy is the science of the size
  and shape of the Earth. Ground-based measurements
  are taken with conventional but very precise
  ground-surveying techniques, using
  laser-electronic instruments. However, because
  plate motions are global in scale, they are best
  measured by satellite-based methods. The late
  1970s witnessed the rapid growth of space
  geodesy, a term applied to space-based techniques
  for taking precise, repeated measurements of
  carefully chosen points on the Earth's surface
  separated by hundreds to thousands of kilometers.
  3 most common space-geodetic techniques -- very
  long baseline interferometry (VLBI), satellite
  laser ranging (SLR), and the Global Positioning
  System (GPS) -- based on technologies developed
  for military and aerospace research (radio
  astronomy and satellite tracking). To date GPS
  has been the most useful for studying the Earth's
  crustal movements. GPS 21 satellites are
  currently in orbit 20,000 km above the Earth,
  continuously transmiting radio signals back to
  Earth. To determine its precise position on Earth
  (longitude, latitude, elevation), each GPS ground
  site must simultaneously receive signals from at
  least four satellites, recording the exact time
  and location of each satellite when its signal
  was received. By repeatedly measuring distances
  between specific points, geologists can determine
  if there has been active movement along faults or
  between plates. By monitoring the interaction
  between the plates, scientists hope to learn more
  about the events building up to earthquakes and
  volcanic eruptions. Space-geodetic data have
  already confirmed that the rates and direction of
  plate movement, averaged over several years,
  compare well with rates and direction of plate
  movement averaged over millions of years.

31
Plates and Spreading Rates
http//www2.umt.edu/Geology/faculty/sheriff/437-Se
ismology_Magnetics/Images/Tectonic_Map_World.jpg
32
Hotspots
The vast majority of earthquakes and volcanic
eruptions occur near plate boundaries, but not
all eg Hawaiian Islands, which are entirely of
volcanic origin, have formed in the middle of the
Pacific Ocean more than 3,200 km from the nearest
plate boundary In 1963, J. Tuzo Wilson "hotspot"
theory. He noted in various locations, volcanism
has been active for very long periods of time.
This could only happen, he reasoned, if
relatively small, long-lasting, and exceptionally
hot regions -- called hotspots -- existed below
the plates that would provide localized sources
of high heat energy (thermal plumes) to sustain
volcanism. Specifically, Wilson hypothesized
that the distinctive linear shape of the Hawaiian
Island-Emperor Seamounts chain resulted from the
Pacific Plate moving over a deep, stationary
hotspot in the mantle, located beneath the
present-day position of the Island of Hawaii.
Heat from this hotspot produced a persistent
source of magma by partly melting the overriding
Pacific Plate. The magma, which is lighter than
the surrounding solid rock, then rises through
the mantle and crust to erupt onto the seafloor,
forming an active seamount. Over time, countless
eruptions cause the seamount to grow until it
finally emerges above sea level to form an island
volcano. Wilson suggested that continuing plate
movement eventually carries the island beyond the
hotspot, cutting it off from the magma source,
and volcanism ceases. As one island volcano
becomes extinct, another develops over the
hotspot, and the cycle is repeated. This process
of volcano growth and death, over many millions
of years, has left a long trail of volcanic
islands and seamounts across the Pacific Ocean
floor .According to Wilson's hotspot theory, the
volcanoes of the Hawaiian chain should get
progressively older and become more eroded the
farther they travel beyond the hotspot. The
oldest volcanic rocks on Kauai, the
northwesternmost inhabited Hawaiian island, are
about 5.5 million years old and are deeply
eroded. By comparison, on the "Big Island" of
Hawaii -- southeasternmost in the chain and
presumably still positioned over the hotspot --
the oldest exposed rocks are less than 0.7
million years old and new volcanic rock is
continually being formed.
33
Hotspots
The vast majority of earthquakes and volcanic
eruptions occur near plate boundaries, but not
all eg Hawaiian Islands, which are entirely of
volcanic origin, have formed in the middle of the
Pacific Ocean more than 3,200 km from the nearest
plate boundary In 1963, J. Tuzo Wilson "hotspot"
theory. He noted in various locations, volcanism
has been active for very long periods of time.
This could only happen, he reasoned, if
relatively small, long-lasting, and exceptionally
hot regions -- called hotspots -- existed below
the plates that would provide localized sources
of high heat energy (thermal plumes) to sustain
volcanism. Specifically, Wilson hypothesized
that the distinctive linear shape of the Hawaiian
Island-Emperor Seamounts chain resulted from the
Pacific Plate moving over a deep, stationary
hotspot in the mantle, located beneath the
present-day position of the Island of Hawaii.
Heat from this hotspot produced a persistent
source of magma by partly melting the overriding
Pacific Plate. The magma, which is lighter than
the surrounding solid rock, then rises through
the mantle and crust to erupt onto the seafloor,
forming an active seamount. Over time, countless
eruptions cause the seamount to grow until it
finally emerges above sea level to form an island
volcano. Wilson suggested that continuing plate
movement eventually carries the island beyond the
hotspot, cutting it off from the magma source,
and volcanism ceases. As one island volcano
becomes extinct, another develops over the
hotspot, and the cycle is repeated. This process
of volcano growth and death, over many millions
of years, has left a long trail of volcanic
islands and seamounts across the Pacific Ocean
floor .According to Wilson's hotspot theory, the
volcanoes of the Hawaiian chain should get
progressively older and become more eroded the
farther they travel beyond the hotspot. The
oldest volcanic rocks on Kauai, the
northwesternmost inhabited Hawaiian island, are
about 5.5 million years old and are deeply
eroded. By comparison, on the "Big Island" of
Hawaii -- southeasternmost in the chain and
presumably still positioned over the hotspot --
the oldest exposed rocks are less than 0.7
million years old and new volcanic rock is
continually being formed.
34
Global Hotspots

35
Largest Earthquakes in last 100 years
36
Caribbean Tectonics
Current Plate Boundaries
Earthquake record
37
Puerto Rican Tectonics and Seismic Hazard
Puerto Rico is located on a microplate,
sandwiched between the obliquely subducting North
American and Caribbean plates (Figure 1), which
accommodates approximately 30mm/yr of
deformation. The main sources of seismic activity
in the region are at the supposed boundaries of
the microplate the subduction zones to the North
(the Puerto Rico Trench) and South (the Muertos
Trough), the Anegada Trough to the East, and the
Mona Canyon region to the West. All regions are
capable of producing events greater than M7.0,
and all have evidence of having done so in the
recorded history of the island (Ascencio, 1980
Moya and McCann, 1992 Macari, 1994). Other
shallow faults with less potential for large
magnitude events are interspersed, mainly with
E-W trends, across the island. On average Puerto
Rico is strongly shaken with Modified Mercalli
Intensity (MMI) gtVII once every hundred years,
and MMI gt VI is experienced on the island once
every 50years.
38
Puerto Rican Tectonics and Seismic Hazard
For Mayagüez, the most important earthquake
sources are 1) the Puerto Rico trench, with
damaging events occurring at moderate depths
about 70km to the North, Mmax8.0 2) the Mona
Canyon, 35km distant, Mmax7.5 3) the local
Mayagüez and Cordillera Faults, about 10km
distant, each with Mmax6.5 (Moya and McCann,
1992). The first 2 sources would produce longer
period, longer duration motions across the entire
city, whereas the local faults could generate
very high intensities at short periods over a
small region. The most recent large earthquake to
hit the island is the M7.5 1918 event, located in
the Mona Canyon to the west of Aguadilla. 114
people were killed from the strong shaking and
tsunami triggered by the event (the tsunami was
2m in height at Mayagüez, a maximum height of 6m
was reached at Aguadilla). Structural damage was
widespread throughout Western Puerto Rico, with
Mayagüez being severely affected. The USGS (USGS,
2001) suggest the seismic hazard at Mayagüez is
similar to that at Seattle, Washington State. The
overall seismic hazard of the island is similar
to that found in the Basin and Range province of
the Western United States (http//eqhazmaps.usgs.g
ov/html/prvi2003doc.html). The local geology at
Mayagüez creates the potential for ground motion
amplifications, which increases the seismic risk
for the town. Quaternary alluvial deposits are
widespread across the low-lying valley where
population is most concentrated. This is
especially true of the test-bed site, which is
only tens of meters from the banks of the Yagüez
River, the main river running through the city.
These unconsolidated sediments are typically of
NEHRP site class E, and are of unknown depth
boreholes in excess of 45m do not reach competent
bedrock. These sediments can produce
amplification of the seismic waves as they pass
from the bedrock to the surface, and also
increased duration of strong shaking as trapped
waves can resonate through the soil column.
Capacete et al (1972) note extensive structural
damage occurred right at the test-bed site during
the 1918 event, Moya and McCann (1992) suggest
that these damages could be caused by the
amplification of the unconsolidated sediments.
39
Plate Kinematics
Euler discovered relative motion of irregular
plates on a spherical surface can be simplified
as rotation around a pole, now called a Euler
Pole.  Euler Poles - movement of a plate on a
sphere can be compactly described as rotation
about a pole. Euler poles (latitude, longitude,
and rotation rate) have been compiled for all of
the plates. The latitude and longitude of a point
can be used to determine the distance between the
point and the Euler pole. The rotation rate can
be converted into a linear velocity using this
distance from the Euler pole. This center of
plate rotation (Euler pole) is like an axle,
running through the plate, down to the centre of
the earth.  North and South Poles, around
which the whole earth rotates, give an idea of
how a Euler pole works .  Close to the poles,
rotation is slow, but as you move from the pole,
rotation speeds up. The same as a bicycle wheel,
where the rim rotates faster than the hub. As
earth rotates on its axis, the equator moves
faster than anywhere else, and is 90 degrees of
latitude from the North and South Poles. Plate
motion is also fastest at 90 degrees from the
Euler Pole, then slows again as you go even
further from the rotation axis.
40
Plate Kinematics cont

• all plates are moving about on a sphere
• Euler's theorem the relative motion between two
  plates across the surface of the sphere is
  uniquely defined by a singular angular rotation
  about a pole of rotation (the Euler Pole)
• the Euler pole tends to remain fixed for long
  periods of time
• transform faults are geometrically linked to the
  Euler poles
• for the transform fault to act with true
  tangential motion, it must lie on a small circle,
  the centre of which is the Euler pole.

41
vij?ij x r vij linear velocity of plate i with
respect to plate j ?ij angular velocity of
plate i with repect to plate j r distance from
Euler pole to point on boudary
42
Homework due Wed 24 Aug 05

• Briefly discuss the hazard and risk Natural
  Hazards pose to Puerto Rico, with focus on
  hazards from earthquakes.
• Explain why the Australian continent has few
  earthquakes.  However, note that there are very
  active earthquake zones near Australia.
• Compare the earthquake activity, volcanic
  activity and topography of the west and east
  coasts of South America.  Why are these
  continental margins so different?
• Stein p367 Problems1,2,3,5 (handout in class)
• Note Seismic moment M0 ?AD (dyne cm)
• rigidity of rock (assume 3x1011dyne/cm2)
• A area of fault slip
• D average displacement/slip over the area of
  fault slip
• Moment Magnitude Mw (log M0 / 1.5)
  - 10.73