Plan for this lecture You are herecourses.washington.edu/ess202/L08_wc_faults.pdf · Earthquakes...
Transcript of Plan for this lecture You are herecourses.washington.edu/ess202/L08_wc_faults.pdf · Earthquakes...
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ESS 202 -
Earthquakes
Profs. John & Ken
TA Josh
Bolt, 5-17
Overall outline
! Plate tectonics
! Earthquakes
! Structure of the the Earth
! Measuring intensity
! Seismometers, magnitude
! Where are west coast faults?
! Some famous quakes
! More Tsunamis
! Rest of quarter - Hazards of quakes
Light at the
end of the
tunnel
Plan for this lecture
! Faults on the
west coast
You are here
Press, 20-12
Pacific and North American plates
are main players
Where are faults, exactly?
! We’ll swing down the coast
! Alaska
! British Columbia, Oregon,Washington
! California
! Wasatch Fault Zone
" Utah, Idaho, Montana, Wyoming
(Yanev,
Appendix A)
USGS Prof. Paper, 1-2
North America map
Show
Movie #01
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Subduction under Alaska &
Aleutian Is.
! 8 cm/yr of plate convergence
" almost strike-slip in Aleutian Is.
! Site of large underthrusting earthquakes
" 1964 Alaska Earthquake MW=9.2
! Volcanic arc & eruptions
Not subduction!
Embarrassing.
(UT Austin)
Strike-slip
Alaska - Pacific boundary
Queen Charlotte Fault SystemAlaska
! Most dangerous faults in US
" 8 quakes over M=8 in last 100 years
! Sparsely populated
! Main fault is subduction thrust
" Only surfaces on ocean floor
" Many secondary faults
! Also has volcanoes, tsunamis
1899-1979 Alaska quakes
Stars - M > 7.5
Triangles - 6.5 to 7.5
Yanev, p. 175
6.7 & 8 quakes in 2002
Tectonic Setting
of N. America
! Old stable interior
! East coast “passive margin”
! West coast “active margin”
" Pacific - N. Am. plate interactions
# Transform motion at SAF
# Subduction under Alaska & Aleutian Is.
" Juan de Fuca - N. Am. interactions
# Subduction under Northern California,Oregon, Washington, and British Columbia
”
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US
Geology
US Topography
Topographic action mark tectonic activity
Press, 20-6c
Continental collision is
cause of Appalachian Mts
Now
Passive marginUSEurope
Rifting
USA seismicity
! Notice that all the action is in the West
" most dramatic topography in the West
# Yosemite, Cascade Mts., Big Sur coast, etc.
" few earthquakes in the East
! Some faults as far east as Yellowstone
! No action to west in Pacific plate either
" Hawaii is special case - hot spot volcano
US Seismicity
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Geology
of western US -
Fabric related to
deformation
Western boundary ofNorth American Plate
! Type of boundary depends on orientation
! Plate boundaries move and change in time
! Past 30 My saw major changes &development of San Andreas Fault
! From 80 to 30 My Farallon plate subductedunder west coast.
" Juan de Fuca & Cocos plates are remnants ofFarallon plate
Cartoon of
Western
N. America
tectonics
Vidale
Details: US and Mexico coast
! Three little plates subducting offshore
Oregon, Washington, and B. Columbia
" Juan de Fuca Plate
" Gorda Plate
" Explorer Plate
! Spreading ridge splitting Gulf of California
" Separating Baja from N. America
" Oblique because ridges are combined with
transform faults
! Cocos Plate subducting to the south
Exotic
terrain
placement
Accretion of fragments
to continent
Exotic terrains
Western N. Am (Cordillera)
assembled from small
pieces over past 200 My
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Changes in west coast
! A mid-ocean ridge subducted
" Before that, just subduction on coast
" First hit near LA 25 Mya
" San Andreas fault system started then
! As ridge is subducting, two triplejunctions are moving apart on coast
" Mendocino Triple Junction moving north
# NA-Pacific-Juan de Fuca junction
" NA-Pacific-Rivera junction off BajaCalifornia
Note present San Andreas fault
Show movie #02
Oregon-Washington
! M 9 every 1000 years, last in 1700
! Recent quakes
" M 7.0 in 2000, 6.5 in 1965 in Seattle
" M 7.1 in 1949 in Olympia
! Main fault is subduction zone
! Also volcanoes (like Mt. St. Helens)
! Not adequately prepared
! We’ve looked at these faults before
" But biggest surface offshore
! Cascadia
subduction zone
! 4 cm/yr
convergence rate
Pacific - Juan deFuca spreading
Subduction of J. deF. under N. Am. Iacopi, 27
Note:
Plate motions
Triple Junctions
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Cartoon view
Show movie #09
Also smaller
faults in
Seattle
Oregon-
Wash.
Close-up
Yeats, 5-2
Note:
Volcanoes
San Andreas
Triple Junctions
Mt. Hood
From Portland
Mt Rainier and Seattle
Subduction under Pacific
Northwest! 4 cm/yr of plate convergence
" like small convection cell - up at ridge down at
subduction zone
! Site of great underthrusting earthquakes
" None in historic record
" But evidence for magnitude 9 quake in 1700
# Tsunami sands in buried marsh
# Tsunami in Japan in 1700
! Volcanic arc & eruptions
" Mt. St. Helens, Mt. Rainier, Mt. Shasta
Oregon-Washington X-sectOregon-Washington X-sectCascadia cross-section
Keller, 7-5
Ridge
Olympic
Mts
Cascade
Mts
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Tsunami sand associated with the 1700 earthquake covering a Native fire pit on the
banks of the Salmon River in Oregon. Fire-cracked rocks, charcoal and ashes delineate
the fire pit that was dug into the sandy beach.
Last 30 Mya of west coast tectonics
Cape
Mend.
Quakes in California
! The highest concentration is near the big
faults; San Andreas, Cape Mendocino
! Other areas also have many little quakes
" Mammoth Mts. - 1000’s of quakes per year
" Coso Hot Springs - geothermal activity
! Probably no area in California is safe
from occasional little quakes
California
! Last 150 years, 2-3 quakes of M8
! Main fault is strike-slip, on land
" San Andreas fault zone
" Easy to map in detail
! Many secondary faults
" Some offshore, most farther East
Quakes
and
San
Andreas
fault
Mammoth
Coso
San Andreas Fault
! Right-lateral slip - LA moving northwest
" 3.5 cm/yr
! Segments
" 1906 San Francisco quake
" Creeping section (gradual aseismic slip)
" Parkfield segment
" 1857 Fort Tejon quake
" Southern segment
! Big Bend
" causes N-S compression in So. Cal.
“Big ones”
M ~ 7.9
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Fault
segments
Big bend
Active
faults and
segments
Official
faults
San Andreas,
Garlock,
SoCal and Bay
Area branchesYanev, p. 42
Special Studies Zone
Act of 1972
(Alquist-Priolo Act)
Alquist-Priolo Map Detail
Fault zones
and Faults
! The ideal fault:
San Gabriel Fault
Photo Matt D’Allessio
Complication #1
! Faults can splay (splitnear surface)
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Complication #2: Gouge zonesNorthern California
! San Andreas
" Runs through Point Arena, Point
Reyes, San Francisco, San Jose,
Watsonville
! East Bay Faults
" Calaveras-Hayward-Rodger’s Creek
" Some farther east
! Offshore faults?
! Bay Areafaults
" San Andreas
" Hayward
" Calaveras
GPS view
of Bay Area
Location of Loma Prieta break Bay Area faultsNotice relation of faults
and topography
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Marin
County
San
Andreas
Iacopi, 42
Point
Reyes
More Point Reyes
San
Fran Oakland:
Problems
with the
Hayward
Fault
Yanev, 32
San Andreas
Yanev, 180
SF to Salinas Hollister
From AAA map
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Hollister - city on a creeping fault
Hollister
problems
with fault
creep
1
3
Sierra Nevada Mts.
! From 80 to 30 My Farallon plate was
subducting under west coast.
! Produced great range of volcanoes,
like present-day Andes Mt.
! Sierras are the cooled, solidified,
uplifted magma chambers of the
volcanic arc (Yosemite granite)
Present Sierra mountains:cooled, solidified, uplifted magma chambers, which
formed during Farallon subduction
! Subduction slowsthen ceases
! Erosion occurs
! Regional upliftand tilt
" exposes solidifiedmagma chambers
Subduction of
Farallon plate
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Garlock Fault
San Andreas Fault
Grapevine
Great Valley
Mojave Desert
San Andreas Fault in the Carrizo Plain
Right-lateral slip
LA moving northwest
3.5 cm/yr
Mojave Fault - the Garlock
Iacopi, 31
Garlock Fault
San Andreas Fault
Grapevine
Great Valley
Mojave Desert
Southern California Faults
! Complex system driven by Pacific-North
America interaction and Big Bend
! Some faults don’t reach surface
! NW-SE trending faults mostly right-lateral
strike-slip
! E-W trending faults mostly thrust
" usually thrust faults, “blind thrusts”
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Big
Bend
builds
mountains
Show Movie #03
Southern California
! San Andreas
" Runs 30 km north of LA, thru San Bernadino, Palm
Springs, Salton Sea, into Mexico
! LA to San Diego - system of faults
" SAF, San Jacinto, Elsinore faults
! Faults under LA are hard to find
" Mostly not strike-slip
" Surface reworked by civilization
Southern Cal. Faults
Show movie #14
Yanev, 195
LA regional faults
Diblee Maps
! Tom Diblee single-handedly mapped largesections of Californiageology … mainly onfoot
! Tom often mapped in remote areas,camping out with enough food andwater for a week, sleeping each nightsheltered from the wind on the car seatwith one door open and a boardextending outward on which to rest hislegs. This enabled him to cover a lot ofground at little expense.
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Example of
Geological
MapYanev, 197
Southern LA faults
Newport-Inglewoodschools and hospitals
Yanev, 33
Yanev, 187San Andreas near LA
San Andreas meets I5at Tejon Pass
Iacopi, 48
Northern LA faultsYanev, 193
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Yanev, 191
Faults in San Berdoo
Faults
south
of LA
Yanev, 187
San Andreas: Indio Hills
Iacopi, 53
Near Mexican
border
Iacopi, 59
Imperial Valley 1940
Earthquakes in SoCalSCEC webpage Earthquake depth on San Jacinto
LA
SD
SB
USGS, 5.10
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Seismicity cross-section
USGS, 5-10
Continuously deforming
surface
Salton Sea
Locked?
Depth of California quakes
! Definite lower limit to seismicity that variesfrom 5-25 km
" Set by temperature and composition of rock# Mostly temperature
" Shallowest under the Salton Sea# Where it is the hottest
! Spots surrounded by seismicity with fewerquakes may be either creeping or locked
! Not so many earthquakes in top few km
" Rock is not so strong there?
Gulf of
California
! Spreading center
Rest of US
! Wasatch fault zone
" Utah, Idaho, Montana, Wyoming
" About 10-25% as active as San Andreas
" Mainly normal faults
! New Madrid
" Had some big quakes
" We don’t know how often they strike# Every 5000 years? Every 500?
! Charleston, plus a few others
! We’ll talk about because of old quakes
" Next one of my lectures
Wasatch fault zone Yanev, 203 Basin and Range
Topography
http://geography.sierra.cc.ca.us/booth/California/1_lithosphere/west_relief_map.jpg
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US SeismicityWhere are the faults?
! Alaska" Subduction and Queen Charlotte fault
zone
! British Columbia, Oregon, Washington" Subduction, Juan de Fuca,
! California" Mendocino, San Andreas, Big bend
! Utah, Idaho, Montana, Wyoming" Wasatch Fault Zone