San Andreas fault

Geological Sciences 107

Our Dynamic Planet

PLANET EARTH IS A DYNAMIC EVOLVING SYSTEM - FROM 4.6 BILLION YEARS AGO TO NOW

STRUCTURE & COMPOSITION REFLECT ONGOING EVOLUTION

EVOLUTION DEPENDS ON HOW HEAT- "THE GEOLOGICAL LIFEBLOOD OF PLANETS" - TRANSFERRED OUT OF COOLING EARTH BY THERMAL

CONVECTION (Hot stuff rises)

THERMAL CONVECTION CAUSES PLATE TECTONICS: PLATES OF EARTH'S SURFACE MOVE RELATIVE TO EACH OTHER AT A FEW mm/yr

(about speed fingernails grow)

PLATE MOTIONS CAUSE EARTHQUAKES, VOLCANOS, MOUNTAIN BUILDING AT PLATE BOUNDARIES

PLATE TECTONICS MAKES EARTH WHAT IT IS - DIFFERENT FROM NEIGHBOR PLANETS (Mars, Venus, & Earth: 3 bears principle)

PLATE MOTIONS ARE CRUCIAL FOR THE ORIGIN OF LIFE, ITS SURVIVAL, AND OUR CLIMATE

PLATE MOTIONS PROVIDE RESOURCES AS WELL AS HAZARDS TO SOCIETY

"CIVILIZATION EXISTS BY GEOLOGICAL CONSENT"

EARTH- A LAYERED PLANET DUE TO ITS EVOLUTION

THE PLATES (= LITHOSPHERE) ARE THE COLD, STRONG OUTER

BOUNDARY LAYER ("thin scum": 100 /

6371 km)MOVING OVER

WEAKER ASTHENOSPHERE

Earth’s surface divided up into about a dozen major lithospheric plates

Three Types of Plate Boundaries (Margins)

NORMAL FAULTING

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Wasatch Fault

Salt Lake City, Utah

Region under extension

REVERSE FAULTING

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STRIKE SLIP FAULTING

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Offset fence showing 3.5 m of left-lateral strike-slip motion on San

Andreas fault in 1906 San Francisco earthquake

Western North America: Three Major Plate Boundaries

CONVERGENT (SUBDUCTION)-

Kirby et al., 1996

2001 NISQUALLY, WASHINGTON EARTHQUAKE (M 6.8, $2B damage)

Within subducting Juan de Fuca plate

Intraslab earthquakes now recognized hazard

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CASCADE VOLCANOES INDICATE JUAN DE FUCA

PLATE SUBDUCTION BENEATH NORTH

AMERICA

Mt Saint Helens1980 eruption

USGS

Mt. St. Helens: May 18, 1980

www.fs.fed.us/gpnf/volcanocams/msh

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Mount St. Helens as part of the new dome collapses. USDA Forest Service photograph by taken moments after a 3.2 magnitude earthquake triggered the event

on July 18, 2005

TRANSFORM- San Andreas Fault System: Motion Between North American & Pacific Plates

San Andreas Lake- San Francisco Bay

Area

1906 SAN FRANCISCO EARTHQUAKE (Magnitude

7.8)

~ 4 m of slip on 450 km of San Andreas Fault

~2500 deaths, ~28,000 buildings destroyed (most

by fire)

San Andreas Fault- Carrizo Plane, California

San Andreas Fault- Palmdale, California

DIVERGENT: Seafloor Spreading in the Gulf of California

Plate Boundaries in Western North America Evolve Over Time

Western North America Tectonics- The Movie

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ICELAND - Part of the Mid-Atlantic Ridge

- Made of Recently Erupted Basalt

- Formed in past 15 million years

Thingvellir

Eurasian plate

North American plate

20 mm/yr

1973 Eruption on the island ofHeimaey, Iceland

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East Africa Rift - New Spreading Center Forms by Rifting Continental Crust

Africa is splitting up into Nubia (West Africa) and Somalia (East Africa)

2001 Eruption near Goma, Congo

Along the East African Rift

DECEMBER 2004 INDIAN OCEAN EARTHQUAKE AND TSUNAMI

INDIAN PLATE MOVES NORTH COLLIDING WITH EURASIA

COMPLEX PLATE BOUNDARY ZONE

IN SOUTHEAST ASIA

Northward motion of India deforms large region

Many small plates (microplates) and blocks

Molnar & Tapponier, 1977

India subducts beneath Burma

microplate

Earthquakes occur at plate interface

along the Sumatra arc (Sunda trench)

Largest & most destructive

earthquakes results from many years of accumulated motion

BETWEEN EARTHQUAKES:

India subducts beneath Burma at about 20 mm/yr

Fault interface is locked

EARTHQUAKE:

Fault interface slips, overriding plate rebounds, releasing accumulated motion and generating tsunami

HOW OFTEN:

Fault slipped ~ 10 m --> 10000 mm / 20 mm/yr = 500 yr

Faults aren’t exactly periodic - intervals vary

Crucial for assessing hazards

Stein & Wysession, 2003

INDIA BURMA

Tsunami generated

SUMATRA TRENCH

Earthquakes rupture a patch along fault's surface. Generally speaking, the larger the rupture patch, the larger the earthquake magnitude. Initial estimates from the aftershock distribution show the magnitude 9.0 Sumatra-Andaman Islands Earthquake ruptured a patch of fault roughly the size of California, and modeling seismic waves show that most of the slip occurred in the southern 400 km of the patch. For comparison, a magnitude 5 earthquake would rupture a patch roughly the size of New York City's Central Park.

MAGNITUDE 9 EARTHQUAKES ARE INFREQUENT

Stein & Wysession after IRIS

TSUNAMI - water wave generated by earthquakes, volcanic eruptions & landslides

NY Times

TSUNAMI GENERATED ALONG FAULT, WHERE SEA FLOOR DISPLACED, AND SPREADS OUTWARD

http://staff.aist.go.jp/kenji.satake/animation.gif

Red - up motion, blue downHyndeman and Wang, 1993

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TSUNAMI SPEED IN DEEP WATER of

depth d

c = (gd)1/2

g = 9.8 m/s2 d = 4000 m

c = 200 m/s = 720 km/hr = 450 m/hr

Tsunami generated along fault, where sea floor displaced, and

spreads outward

Reached Sri Lanka in 2 hrs, India in 2-3

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Because seismic waves travel much faster (km/s) than tsunamis, rapid analysis of seismograms can identify earthquakes likely to cause major tsunamis and predict when waves will arrive

TSUNAMI WARNING

Deep ocean buoys can measure wave heights, verify tsunami and reduce false alarms

HOWEVER, HARD TO PREDICT EARTHQUAKES recurrence is highly variable

M>7 mean 132 yr 105 yr Estimated probability in 30 yrs 7-51%

Sieh et al., 1989

Extend earthquake history with geologic records -paleoseismology

Plate boundary volcanism produces atmospheric gases (carbon dioxide CO2 ; water H2O) needed to support life and keep planet warm enough for life ("greenhouse" )

May explain how life evolved on earth (at midocean ridge hot springs)

Plate tectonics raises continents above sea level

Plate tectonics produces mineral resources including fossil fuels

PLATE TECTONICS IS DESTRUCTIVE TO HUMAN

SOCIETY BUT CRUCIAL FOR HUMAN LIFE

Press & Siever

“CIVILIZATION EXISTS BY GEOLOGICAL CONSENT

- SUBJECT TO CHANGE WITHOUT NOTICE”

The same geologic processes that make our planet

habitable also make it dangerous