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Chapter 4
Plate Tectonics and
Earthquakes
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Plate Tectonics and Earthquakes
China, May 12, 2008:
• Major earthquake  great natural disaster
– Many poorly constructed buildings collapsed
• Nearly 90,000 people killed
Insert Figure 4.1 here
Figure 4.1
Plate Tectonics and Earthquakes
Most earthquakes can be explained by plate tectonics:
• Divergent plate boundaries
– Divergent motion and high temperatures cause rocks to fail
easily in tension
– Earthquakes are small and generally non-threatening
• Transform plate boundaries
– Plates slide past each other in horizontal movement, retarded at
irregularities in plate boundaries
– Energy required to move plates is released as large earthquakes
• Convergent plate boundaries
– Great amounts of energy are required to pull a plate back into the
mantle or push continents together
– Largest earthquakes are generated at convergent boundaries
Plate Tectonics and Earthquakes
Figure 4.2
Plate Tectonics and Earthquakes
Examine example of Pacific plate:
• Created at spreading centers on eastern and southern edges,
producing small earthquakes
• Slides past other plates on transform faults (Queen Charlotte fault,
Canada; San Andreas fault, California; Alpine fault, New Zealand),
generating large earthquakes
• Subducts along northern and
western edges, generating
enormous earthquakes
Figure 4.3
Spreading Center Earthquakes
Iceland
• Volcanic island fed by hot spot along the mid-Atlantic ridge
• Swarms of moderate earthquakes too small to destroy
buildings or kill people
Figure 4.4
Figure 4.5
Spreading Center Earthquakes
Red Sea and Gulf of Aden
• Young spreading center, new ocean basin
• Hot upper mantle under Africa melts and
uplifts crust, which gravity then pulls
apart and downward, creating pull-apart
basins (or rift valleys)
• As down-dropped pull-apart basins widen,
are flooded by ocean to form new arm
• At south end of Red Sea, three pull-apart
basins meet at triple junction
• Spreading has split Arabian plate from
Africa
• East African Rift Valley may someday
split ‘Somali’ plate from African plate
Figure 4.6
Spreading Centers and Earthquakes
Figure 4.7
Spreading-Center Earthquakes
• Spreading center underlies Gulf of
California, extends north into U.S.,
including Salton Sea, Coachella and
Imperial Valleys
• Salton Trough:
– High heat flow, glassy volcanic domes,
boiling mud pots, geothermal reservoirs,
earthquake swarms from moving magma
– One of most earthquake-active U.S. areas
– Brawley seismic zone – often hundreds
of small earthquakes over few days
(swarms), continually releasing stress
rather than building for big earthquake
– Other nearby faults generate larger
earthquakes, up to magnitude 7
Figure 4.9
Convergent Zones and Earthquakes
• Largest earthquakes
• Three types of convergence:
– Ocean-ocean: older, denser oceanic plate is subducted
– Ocean-continent: oceanic plate is subducted
– Continent-continent: both plates are too buoyant to be
subducted; continental upheaval results
Subduction Zone Earthquakes
Subduction Zones:
• Sites of great earthquakes
• Shallow earthquakes:
– Compressive movements of overriding plate and subducting
plate
– Pull-apart movements where subducting plate bends downward
– Most damaging earthquakes
• Inclined plane of deep earthquakes, defining descending
slab of oceanic lithosphere
– Rigid interior of slab can stay cold enough to generate
earthquakes down to depths of 700 km
– Most seismic energy is dissipated before reaching surface
Subduction Zone Earthquakes
Seismic-Gap Method:
• If some segments of a fault have moved recently, it is reasonable
to expect that unmoved portions will move next, to fill the gaps
• Yields expectations, not guarantees; segments may move in two or
more earthquakes before adjacent unmoved segments move once
Figure 4.11
Figure 4.10
Subduction Zone Earthquakes
Indonesia, 2004 to 2009
• Indian-Australian plate collides with Indonesia at
5.3-5.9 cm/yr  frequent, huge subduction-caused
earthquakes, frequent tsunami
• December 26, 2004: 1,200 km long fault ruptured
over 11 minutes
• March 28, 2005: 400 km rupture south of earlier
rupture
• Additional quakes in 2007, 2008 and 2009
• Earthquake cluster is under way
• More seismic gaps to fill
Subduction Zone Earthquakes
Tokyo, Japan, 1923
• One of world’s most deadly disasters (probably about
144,000 people killed)
• Series of earthquakes, with principal one worst of year
globally
• Tsunami 11 m high hit city
• Fires raced through city for 2½ days, destroying 71% of
Tokyo and all of Yokohama
– 38,000 people were killed by fire, crowded into a park that was
consumed by fire from three sides
• Region SW of Tokyo has had five very strong
earthquakes in last 400 years – about every 73 years,
most recent in 1923
Subduction Zone Earthquakes
Insert Table 4.1
Subduction Zone Earthquakes
• Subduction of Pacific plate under Alaska creates truly
large earthquakes (not San Andreas fault)
• The Good Friday Earthquake, Alaska, 1964
– Major subduction movement created magnitude 9.2
earthquake, as Pacific slab shoved under Alaska in seven
lurches
– Seven minutes of shaking induced avalanches, landslides,
ground settling and tsunami, killing 131 people
– Relatively low loss of life because area is sparsely inhabited,
few people downtown, low tide and warm weather
Subduction Zone Earthquakes
Mexico City, 1985
• Subduction of Cocos plate under North America created magnitude
8.1 earthquake, with magnitude 7.5 and 7.3 aftershocks
• Earthquake was 350 km from Mexico City, but intense damage
• 8,000 people were killed by building collapses
• Earthquake was
expected 
Michoacan
seismic gap
• Guerrero seismic
gap still exists
Figure 4.13
Subduction Zone Earthquakes
Mexico City, 1985
• Earthquakes Don’t Kill, Buildings Do
• 5,700 buildings in Mexico City were damaged, while small towns
closer to the epicenter had much less damage
Figure 4.12
Figure 4.14
Subduction Zone Earthquakes
Mexico City, 1985
• Earthquakes Don’t Kill, Buildings Do
• Resonance between seismic waves, soft lake-sediment foundations
and improperly designed buildings, all vibrating most intensely at 1
to 2-second periods, destroyed many buildings
Figure 4.16
Figure 4.15
Subduction Zone Earthquakes
Pacific Northwest, The
Upcoming Earthquake
• Cascadia subduction zone –
small plates subducting under
North America
• Why no major earthquakes
in last 200 years in Pacific
Northwest?
– Has subduction stopped?
• No, because volcanism still occurs
Figure 4.17
– Are plates subducting so smoothly that earthquakes don’t occur?
• Probably not, since subducting lithosphere is so young
Subduction Zone Earthquakes
Pacific Northwest, The Upcoming Earthquake
• Similar subduction setting to Japan and Chile
– Japan’s subduction zone relieved stress with two magnitude 8.1
earthquakes in 1944 and 1946
– Chilean subduction zone generated world’s largest measured
earthquake in 1960, magnitude 9.2
– 1960 Chilean events would be comparable to earthquake series
that shifts entire Cascadian subduction zone
• Could Cascadia have a magnitude 9 earthquake?
– Yes, it has and will again
Subduction Zone Earthquakes
Earthquake in 1700
• Last major earthquake in Pacific Northwest was about
magnitude 9, in 1700
– Trees in forests drowned, died by salt-water flooding when
ground dropped in 1700
– Tsunami hit Japan in 1700 – might have been caused by
magnitude 9 earthquake along Washington-Oregon coast
• Effects of magnitude 9
earthquake:
Figure 4.18
– Three to five minutes of
intense shaking
– Tsunami 10 m high, 15 to 40
minutes after earthquake
Convergent Zones and Earthquakes
Continent-continent collisions
• India has moved 2,000 km
north into Asia since initial
contact
– Pre-collision, Indian and Asian
crusts were 35 km thick
– Crust under Tibetan plateau is
70 km thick, highest continental
area on Earth
Figure 4.19
Convergent Zones and Earthquakes
Continent-continent collisions
• India continues to move 5 cm/year into Asia,
along 2,000 km front, affecting India, Pakistan,
Afghanistan, Tibetan Plateau, eastern Russia,
Mongolia and
China with great
earthquakes, and
pushing parts of
China to east and
southeastern Asia
farther to
southeast
Figure 4.20
Figure 4.21
Convergent Zones and Earthquakes
• China, Pakistan and India, 2008, 2005 and 2001
– Caused by India’s collision into Asia
– May 2008 earthquake in Sichuan, China killed 87,500
people
– October 2005 earthquake in Kashmir, Pakistan killed
88,000 people
– January 2001 earthquake in Gujarat, India killed more
than 20,000 people
– Seismic gaps (such as 600 km long section of central
Himalayas) throughout region
– Rapid population growth in region, sporadic seismicsafety codes, poor construction practices
– None of recent earthquakes were direct hit on major cities
Convergent Zones and Earthquakes
• Shaanxi province, China, 1556
– Deadliest earthquake in history: about 830,000 people killed
– Soft soil (with little cohesion) made caves practical homes, but
shaking caused ground and caves to collapse
• Tangshan, China, 1976
– Deadliest earthquake in recent history: more than 240,000
people killed
– Dense mining city of 2 million, with most buildings of mudbrick built under lenient building codes  93% of residential
buildings collapsed
– Earthquake weather:
• No connection between fault movements many kilometers underground
and weather above the surface
• Shaanxi and Tangshan earthquakes during opposite kinds of weather
Transform Faults and Earthquakes
Arabian Plate:
• Spreading in Red Sea and Gulf of Aden pushes Arabian
plate into Eurasia, uplifting mountains and creating
large earthquakes
Figure 4.22
The Arabian Plate
Continent-continent collision earthquakes:
• Iran, 1962-2009
– In last 48 years, 10 earthquakes killed over150,000 people in
Iran
– December 2003: Construction methods of mud-brick walls
with heavy roofs caused collapsing homes to kill 41,000
people
– “Earthquakes don’t kill people, buildings do”
The Arabian Plate
Transform fault earthquakes:
• On western side, Arabian plate
slides past African plate at
edge of Mediterranean Sea
along Dead Sea fault zone
– Runs through Holy Land
– Steps in fault zone have created
pull-apart basins that hold Dead
Sea, Sea of Galilee
– Dead Sea fault zone generates
magnitude 6-7 earthquakes
about every 100-200 years
Figure 4.23
The Arabian Plate
Transform fault earthquakes:
• On western side, Arabian plate slides past African plate at
edge of Mediterranean Sea along Dead Sea fault zone
Figure 4.24
Transform Faults and Earthquakes
• Horizontal movements cause major earthquakes
• Turkey, 1999:
– Segment of North Anatolian fault ruptured for 120 km in
magnitude 7.4 earthquake near Izmit, followed weeks
later by rupture to the east in magnitude 7.1 earthquake
– Residential buildings on soft ground, adding sand to
concrete resulted in buildings collapsing during shaking
– 19,000 people killed, $20 billion in damages
Transform Faults and Earthquakes
• Turkey, 1999:
– As Arabian plate pushed in Eurasia, Turkey is pushed
westward, rotated counter-clockwise in escape tectonics
along the North Anatolian fault, which runs for 1,400 km
along the Black Sea
– Since 1939, North Anatolian fault has ruptured in 11
earthquakes greater than 6.7, from east end of fault to west
• Unique, semi-regular pattern
• Next event? Probably to west of Izmit, closer to Istanbul
• Probably within next 30 years
Transform Faults and Earthquakes
• Turkey, 1999
Figure 4.26
San Andreas Fault Tectonics
• Opening of Atlantic
Ocean basin pushes
North and South
America west
• Farallon plate
subducts beneath North
America
• By 28 million years
ago, enough of the
Farallon plate has
subducted under North
America for the Pacific
spreading center to
make contact with
North America
Figure 4.27
San Andreas Fault Tectonics
• Remaining spreading
centers to the north and
south are connected by
a transform fault 
ancestor of San
Andreas fault
• In last 5.5 million years,
Gulf of California has
opened 300 km, tearing
peninsular California
(Baja California and
California west of the
San Andreas fault)
away from the North
American plate and on
the Pacific plate
Figure 4.27
Side Note: San Francisco , 1906
Home to about 400,000 people
• Intense ground shaking for about one minute in early
morning hours
• Damage much worse in areas constructed on artificial
fill rather than rock or consolidated sediment
• Many fires broke out and water lines were destroyed,
making fire-fighting impossible – ten times more
damage than shaking
San Andreas Fault Tectonics
• Different sections of the fault behave differently:
– South of 1906 section, between San Juan Bautista and
Cholame:
• Frequent small to moderate earthquakes and creep
(millimeters per year of ongoing offset)
– South of creeping section, between Cholame and San
Bernardino:
• Locked section with recent deficit of earthquake activity
• Most recent large rupture in 1857 Fort Tejon earthquake of
magnitude 7.9
– South of locked Fort Tejon section:
• Complex zone with locked sections
San Andreas Fault Tectonics
World Series (Loma Prieta) Earthquake, 1989
• Just prior to scheduled start of Game 3 of the World
Series, at Candlestick Park of San Francisco
• Rupture of southernmost 42 km of 1906 section
• Fault rupture was deep, did
not offset ground surface
• Fault movement was
vertical and horizontal,
not just horizontal
• Rupture lasted only 7
seconds (short for
Figure 4.33
magnitude 7.0 earthquake)
San Andreas Fault Tectonics
World Series (Loma Prieta)
Earthquake, 1989
• Occurred at bend in San
Andreas fault near Santa Cruz
Mountains’ highest peak, Loma
Prieta
– Bend in fault uplifted Santa Cruz
Mountains
• At bend, fault plane is inclined
at 70 degrees (not vertical), so
fault movement shifted Pacific
plate 1.9 m to north, and uplifted
Santa Cruz Mountains 36 cm
Figure 4.32
San Andreas Fault Tectonics
World Series (Loma Prieta) Earthquake, 1989
• Mainshock was magnitude 7.0, numerous aftershocks
• Loma Prieta region was seismic gap before earthquake
– Mainshock and aftershock filled in seismic gap
• Destruction was much less than could have been, as
shaking lasted only 11 seconds
– 67 people killed, 3,757 people injured, 12,000 people homeless
– $6 billion in damages
Figure 4.34
San Andreas Fault Tectonics
World Series (Loma Prieta) Earthquake, 1989
• Earthquakes don’t kill, buildings do
• Near epicenter, damage was mostly to older and
poorly designed buildings
• Seismic waves were amplified by soft muds and
artificial-fill foundations around San Francisco Bay
• Ground motion on soft-sediment sites was 10 times
stronger than at nearby rock sites
• Farther from epicenter, considerable damage to
poorly sited or poorly designed structures
– Marina District
– Interstate 880
San Andreas Fault Tectonics
World Series (Loma Prieta)
Earthquake, 1989
• Marina District
Figure 4.35
San Andreas Fault Tectonics
World Series (Loma Prieta) Earthquake, 1989
• Marina District
– Built on fill from debris from 1906 earthquake
• Amplified shaking
• Deformation and liquefaction of artificial-fill
foundations
• Soft first-stories – ground floors of buildings cleared
out to make room for parking, leaving building
without adequate support so rest of building pancakes
onto first floor
San Andreas Fault Tectonics
World Series (Loma Prieta) Earthquake, 1989
• Interstate 880
– 42 people killed when upper deck collapsed onto lower deck
Figure 4.36
San Andreas Fault Tectonics
World Series (Loma Prieta) Earthquake, 1989
• Interstate 880
– Section where collapse occurred was built on soft muds (other
sections on sand and gravel survived)
– Freeway’s natural resonance was amplified by shaking of muds
– Could have been predicted by collapses of freeways in 1971 San
Fernando earthquake
Figure 4.37
San Andreas Fault Tectonics
Bay Area Earthquakes – Past and Future
• Historic records for last 225 years (good records only last
160 years)
– Major earthquake recurrence
interval is more than 160 years
• 19th century earthquake history
quite different than 20th century
– 19th century: Seven greater than
6 magnitude earthquakes before
1906 earthquake
– 20th century: No large
earthquakes until 1970s
Figure 4.39
San Andreas Fault Tectonics
Bay Area Earthquakes – Past and Future
Pattern 1: common large earthquakes vs. rare giant earthquakes
• Plate tectonic movement is satisfied by
either one magnitude 8 earthquake in a
century or one magnitude 6-7 earthquake
every decade
Pattern 2: East-west pairings of
earthquakes
• In 1836, Monterey Bay area had
earthquake about magnitude 6.5; in 1838,
San Francisco peninsula had earthquake
of magnitude 6.8
• In 1865, Santa Cruz had a large
earthquake; in 1868, Hayward had a
Figure 4.39
magnitude 6.9 earthquake
San Andreas Fault Tectonics
Bay Area Earthquakes – Past and Future
Pattern 3: northward progression of earthquakes
• Earthquakes of 1865 and 1868 on the Hayward fault were
preceded by five moderate earthquakes that moved
northward up the Calaveras fault
– Area of Hayward fault today is populated by 2 million
people, with schools, hospitals, city halls, houses and
the University of California built on the fault itself
San Andreas Fault Tectonics
Kobe, Japan, 1995 Versus
Oakland, California, 20??
• Most expensive earthquake in
history
• Magnitude 6.9 earthquake with
50 km long rupture of Nojima
fault and 100 seconds of shaking
• Tile roofs and little lateral
support caused collapse of
buildings causing 6,308 fatalities
• More than 140 fires resulted
Figure 4.41
San Andreas Fault Tectonics
Kobe, Japan, 1995 Versus
Oakland, California, 20??
• Hayward fault has 27% probability of
causing magnitude 6.7 or greater
earthquake before 2032
• San Francisco Bay region, 62%
probability of 6.7 magnitude
earthquake on a fault before 2032
• Hayward fault and Nojima fault:
– About 50 km long, densely
populated areas, with large areas
of weak ground materials
– Both capable of generating
magnitude 7 earthquakes
Figure 4.43
End of Chapter 4