ESS 202 - Earthquakes

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ESS 202 - Earthquakes

• Profs. John Ken
• TA Josh

Bolt, 5-17
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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
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Plan for this lecture

• Faults on the west coast

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You are here
Pacific and North American plates are main players
Press, 20-12
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Where are faults, exactly?

• Well swing down the coast
• Alaska
• British Columbia, Oregon, Washington
• California
• Wasatch Fault Zone
• Utah, Idaho, Montana, Wyoming

(Yanev,Appendix A)
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North America map
Show Movie 01
USGS Prof. Paper, 1-2
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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 MW9.2
• Volcanic arc eruptions

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Alaska - Pacific boundary
Not subduction! Embarrassing. (UT Austin)
Strike-slip
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Queen Charlotte Fault System
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Alaska

• Most dangerous faults in US
• 8 quakes over M8 in last 100 years
• Sparsely populated
• Main fault is subduction thrust
• Only surfaces on ocean floor
• Many secondary faults
• Also has volcanoes, tsunamis

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1899-1979 Alaska quakes
6.7 8 quakes in 2002
Stars - M gt 7.5 Triangles - 6.5 to 7.5
Yanev, p. 175
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Tectonic Settingof 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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USGeology
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US Topography
Topographic action mark tectonic activity
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Continental collision iscause of Appalachian Mts
Press, 20-6c
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Now
Rifting
Europe
US
Passive margin
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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

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US Seismicity
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Geology of western US - Fabric related
to deformation
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Western boundary of North 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 subducted under
  west coast.
• Juan de Fuca Cocos plates are remnants of
  Farallon plate

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Cartoon ofWesternN. Americatectonics
Vidale
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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

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Exoticterrainplacement
Accretion of fragments to continent
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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 triple junctions are
  moving apart on coast
• Mendocino Triple Junction moving north
• NA-Pacific-Juan de Fuca junction
• NA-Pacific-Rivera junction off Baja California

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(No Transcript)
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Note present San Andreas fault
Show movie 02
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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
• Weve looked at these faults before
• But biggest surface offshore

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• Cascadia subduction zone
• 4 cm/yr convergence rate

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Pacific - Juan de Fuca spreadingSubduction of
J. de F. under N. Am.
Iacopi, 27
Note Plate motions Triple Junctions
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Cartoon view
Show movie 09
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Also smaller faults in Seattle
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Oregon-Wash.Close-up
Note Volcanoes San Andreas Triple Junctions
Mt. Hood From Portland
Yeats, 5-2
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Mt Rainier and Seattle
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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

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Oregon-Washington X-sect
Cascadia cross-section
Cascade Mts
Olympic Mts
Ridge
Keller, 7-5
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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.
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Last 30 Mya of west coast tectonics
Cape Mend.
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Quakes in California

• The highest concentration is near the big faults
  San Andreas, Cape Mendocino
• Other areas also have many little quakes
• Mammoth Mts. - 1000s of quakes per year
• Coso Hot Springs - geothermal activity
• Probably no area in California is safe from
  occasional little quakes

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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

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QuakesandSan Andreasfault
Mammoth
Coso
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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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Faultsegments
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Activefaults andsegments
Big bend
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Officialfaults
Special Studies Zone Act of 1972 (Alquist-Priolo
Act)
San Andreas, Garlock, SoCal and Bay Area branches
Yanev, p. 42
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Alquist-Priolo Map Detail
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Fault zones and Faults

• The ideal fault

San Gabriel Fault Photo Matt DAllessio
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Complication 1

• Faults can splay (split near surface)

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Complication 2 Gouge zones
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Northern California

• San Andreas
• Runs through Point Arena, Point Reyes, San
  Francisco, San Jose, Watsonville
• East Bay Faults
• Calaveras-Hayward-Rodgers Creek
• Some farther east
• Offshore faults?

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• Bay Area faults
• San Andreas
• Hayward
• Calaveras

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GPS viewof Bay Area
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Location of Loma Prieta break
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Bay Area faults
Notice relation of faults and topography
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MarinCountySanAndreas
Iacopi, 42
Point Reyes
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More Point Reyes
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SanFran
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OaklandProblemswith theHaywardFault
Yanev, 32
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SF to Salinas
San Andreas
Hollister
Yanev, 180
From AAA map
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(No Transcript)
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Hollister - city on a creeping fault
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Hollisterproblemswith faultcreep
1
3
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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)

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Present Sierra mountainscooled, solidified,
uplifted magma chambers, whichformed during
Farallon subduction
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Subduction of Farallon plate

• Subduction slows then ceases
• Erosion occurs
• Regional uplift and tilt
• exposes solidified magma chambers

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Great Valley
San Andreas Fault
Grapevine
Garlock Fault
Mojave Desert
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San Andreas Fault in the Carrizo Plain
Right-lateral slip LA moving northwest 3.5 cm/yr
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Mojave Fault - the Garlock
Iacopi, 31
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San Andreas Fault
Mojave Desert
Garlock Fault
Grapevine
Great Valley
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Southern California Faults

• Complex system driven by Pacific-North America
  interaction and Big Bend
• Some faults dont 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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BigBendbuildsmountains
Show Movie 03
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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

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Southern Cal. Faults
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Show movie 14
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LA regional faults
Yanev, 195
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Diblee Maps

• Tom Diblee single-handedly mapped large sections
  of California geology mainly on foot
• Tom often mapped in remote areas, camping out
  with enough food and water for a week, sleeping
  each night sheltered from the wind on the car
  seat with one door open and a board extending
  outward on which to rest his legs. This enabled
  him to cover a lot of ground at little expense.

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Example of Geological Map
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Southern LA faults
Yanev, 197
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Newport-Inglewoodschools and hospitals
Yanev, 33
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San Andreas near LA
Yanev, 187
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San Andreas meets I5at Tejon Pass
Iacopi, 48
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Northern LA faults
Yanev, 193
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Faults in San Berdoo
Yanev, 191
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Faultssouthof LA
Yanev, 187
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San Andreas Indio Hills
Iacopi, 53
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Imperial Valley 1940
Near Mexican border
Iacopi, 59
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Earthquakes in SoCal
SCEC webpage
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Earthquake depth on San Jacinto
USGS, 5.10
SB
LA
SD
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Seismicity cross-section
Locked?
Salton Sea
surface
Continuously deforming
USGS, 5-10
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Depth of California quakes

• Definite lower limit to seismicity that varies
  from 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 fewer quakes
  may be either creeping or locked
• Not so many earthquakes in top few km
• Rock is not so strong there?

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Gulf of California

• Spreading center

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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 dont know how often they strike
• Every 5000 years? Every 500?
• Charleston, plus a few others
• Well talk about because of old quakes
• Next one of my lectures

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Wasatch fault zone
Yanev, 203
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Basin and Range Topography
http//geography.sierra.cc.ca.us/booth/California/
1_lithosphere/west_relief_map.jpg
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US Seismicity
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Where 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