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6. Earthquakes that shouldn’t happen. DIFFUSE BOUNDARY ZONES. INTRAPLATE. NARROW BOUNDARIES. Most earthquakes are on plate boundaries, but some are within plates - intraplate. 1900 -. Almost all large earthquakes in North America are in the western plate boundary zone.

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slide1

6. Earthquakes that shouldn’t happen

DIFFUSE BOUNDARY

ZONES

INTRAPLATE

NARROW BOUNDARIES

Most earthquakes are on plate boundaries, but some are within plates - intraplate.

slide2

1900 -

Almost all large earthquakes in North America are in the western plate boundary zone.

To see New Madrid, we plot down to magnitude 5, which aren’t very big & do little harm.

To resolve details, plot eastern North America down to magnitude 3.

See New Madrid and other similar eastern earthquake zones.

M>3

DD 10.1

slide3

Charleston, South Carolina 1886 M ~ 7

Similar to New Madrid 1811-12

Damaged buildings - especially brick & caused about 60 fatalities

Similar sand blows

Recovery took about five years

DD 10.2

slide4

Grand Banks, Newfoundland 1933 M 7.3

Caused landslide & tsunami

Broke transatlantic cables

Intensity pattern like 1811-12

GSC

slide5

Earthquakes require

  • Faults to slip on
  • Force (stress) to cause motion of faults

At plate boundaries, faults are the boundaries and plate motions provide stresses that cause motion

What about within continental plates?

slide6

Granite continents vs basalt oceans

Continental crust is less dense, so continents are higher than ocean basins

Continents float above denser mantle

Continents don’t subduct, unlike oceans

Continents formed earlier in Earth history and can reach ages of billions of years compared to 200 Ma oceans

DD 10.3

slide7

WILSON CYCLE

Oceanic lithosphere is formed at midocean ridges & destroyed at subduction zones

Continental plates are rifted apart and then collide, but survive

East African Rift

Gulf of Aden, Gulf of California

Andes

Southern Europe

Himalaya Zagros

Continental rifting

Young ocean

Ocean-continent convergence

Closing ocean

Stein & Wysession, 2003

Continental collision

DD 10.4

slide8

Global Plate TectonicsJurassic to Present Day

By

L.A. Lawver, M.F. Coffin, I.W.D. Dalziel

L.M. Gahagan, D.A. Campbell, and R.M. Schmitz

2001, University of Texas Institute for Geophysics

February 9, 2001

activity 6 1 wilson cycle

Activity 6.1: Wilson cycle

When does the North Atlantic rift open?

When does the South Atlantic rift open?

When does India collide with Eurasia?

When does Arabia rift away from Africa?

slide32

Some continental rifts fail to split a continent

Leave “failed rift” - long valley of stretched & faulted rock

that gets filled up & buried by sediments

MCRS

Midcontinent Rift system

Failed to split North America ~1.1 Ga (billion years ago)

slide33

Activity 6.2

Why does the MCRS show up so well in gravity & magnetic maps?

USGS

slide35

Over billions of years, continents retain structures formed by rifting, collisions, failed rifts, basin formation, faulting, etc

Stresses within the plate - from various sources - can reactivate these & cause intraplate earthquakes

S. Marshak

slide36

A set of failed rifts, the Reelfoot Rift and Rough Creek Graben, are associated with the New Madrid & Wabash Valley seismic zones

S. Marshak

slide37

Activity 6.3: Spot formation of the MCRS and Reeflfoot/Rough creek rifts

Whitmeyer & Karlstrom 2007

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