Tectonics_Lecture-1

Why, Where and How do Earthquakes Occur?

Supriyo Mitra Dept. of Geology and Geophysics Indian Institute of Technology Kharagpur Seismology Group IIT Kharagpur

FROM CONTINENTAL DRIFT TO PLATE TECTONICS The Story of our Dynamic Planet…

Because tectonic motions cause earthquakes, it is critical to understand plate tectonics when trying to reduce the societal hazard associated with faulting…

Lay and Wallace.

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The Theory of Plate Tectonics: The Earth’s outermost layer is fragmented into a dozen or more large and small plates that are moving relative to one another as they ride atop hotter, more mobile material.

Compositional Structure of the Earth Average 35 km (continent) 7-8 km (ocean)

John Milne Lord Rayleigh Lord Rutherford

Andrya Mohorovicic (Moho) 1909 Mantel or ‘coat’ by Emil Wiechert (in German) -Discovered by R.D. Oldham 1906.

- Correctly delineated by Beno Gutenberg in 1912 from earthquake data.

-Fluid Outer Core (1926) From works on tides by Sir Harold Jeffreys.

Crust (rich in silica)

Mantle

Derived from the mantle over the aeons by series of melting & reworking

Outer Core

Inner Core 1929 Buller earthquake south Island of New Zealand.

Inge Lehmann 1936 – solid Inner Core within the liquid Outer Core.

Mechanical Layering of the Earth Continental crust Oceanic crust

D’’ Layer

Mechanical Layering of the Earth

Combined View (A comparison)

Purely Mechanical terms

Earth as a Heat Engine

Strong Lithosphere floats on a weak Asthenosphere.

Lithosphere - an outer skin; conductive heat loss.

The high temperature and pressure which exist at the depth of the asthenosphere causes its viscosity to be low enough to allow viscous flow, on a geological time scale.

Asthenosphere - interior shell; convective heat transfer.

Distribution of seismic velocities and density within the Earth

No S-wave

Plate Tectonics • • •

•

A relatively new scientific concept, introduced in the late 1960s.

Revolutionized our understanding of the dynamic Earth.

Unified the study of the Earth: drawing together many branches of Earth Sciences, from Paleontology (study of fossils) to Seismology (the study of earthquakes).

Provided explanations to questions that scientists speculated upon for centuries:1. Why earthquakes and Volcanic eruptions occur in very specific areas around the world?

2. How and why great mountain ranges like Himalayas and Alps formed?

Catastrophism 1700s: all earth changes were sudden and caused by a series of catastrophes.

Uniformitarianism 1785 (James Hutton): The present is the key to the past. (geologic forces and process – gradual and catastrophic – acting on the Earth today are same as those that have acted in the geologic past.

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Development of the Theory: A Historical Perspective 1596 – Dutch mapmaker – Abraham Ortelius in his work Thesaurus Geographicus suggested that America were “torn away from Europe and Africa ….by earthquakes and floods.”

“The vestiges of the rupture reveal themselves, if someone brings forward a map of the world and considers carefully the coasts of the three (continents).”

1915 – Alfred Lothar Wegener – German Meteorologist – observed the same and in his book The Origin of Continents and Oceans developed his ideas into the Theory of Continental Drift, which he suggested as an explanation of several observations about the continents.

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Evidences for the Theory of Continental Drift: 1. Remarkable fit of South American and African continents.

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2. Occurrences of unusual geological structures and plant and animal fossils found on matching coastlines, now widely separated by the Oceans.

3. Dramatic climate changes on some continents:

* Discovery of coal deposits (fossils of tropical plants) in Antarctica (must have been situated closer to the equator in temperate climate conducive for lush, swampy vegetation). * Fossil fern (Glossopteris) discovered in now polar regions. * Occurrence of glacial deposits in present-day arid Africa, eg. Vaal River valley of South Africa.

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Wegner stated: Continents simply plowed through the ocean floor and reached their present configuration.

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Wegner stated: Continents simply plowed through the ocean floor and reached their present configuration.

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Criticism of Wegner’s Theory: • Physically impossible for a large mass of solid rock to plow through the ocean floor without breaking up.

• What kind of forces could be strong enough to move such land masses of solid rock over such great distances?

However beginning in the 1950s a wealth of new scientific evidences emerged to revive the debate about Wegner’s provocative ideas and their implications

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Major scientific developments towards the formulation of Plate Tectonics Theory Æ Demonstration of the ruggedness and youth of the ocean floor Æ Confirmation of repeated reversals of the Earth magnetic field in the geologic past Æ Emergence of the seafloor-spreading hypothesis and recycling of oceanic crust Æ Concentration of earthquakes along narrow belts.

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Ocean floor mapping - Discovery of underwater mountains chains encircling the earth (> 50,000 kms) - Sediment layers on seafloors: much thinner than previously thought.

Most prominent topographic feature on the surface of the earth (called MOR)

Magnetic striping and polar reversals Vine and Matthews (1963) - Alternating magnetic stripes of Normal and reversed polarity on the seafloor.

- Symmetric around the underwater chain of mountains that was discovered.

Magnetic stripes prompted more questions: 1 How does the magnetic striping pattern form?

2 Why are these stripes symmetrical around the crest of MOR? These could not be answered without knowing the significance of the ridges

Seafloor spreading and recycling of Oceanic crust 1961- scientist began to theorize 1. MOR marked structurally weak zones.

2. Ocean floor was being ripped apart lengthwise along the ridge crest.

3. New magma from deep within the earth rises through these weak zones 4. Eventually erupts along crest of the ridge to create new oceanic crust.

Process was called:

SEAFLOOR SPREADING.

Evidences for Seafloor Spreading: 1. At or near the crest of the ridge, the rocks are very young, becomes progressively older away from the ridge crest.

2. Younger rocks at the ridge crest always have present-day normal polarity.

3. Stripes of rocks parallel to the ridge crest alternated in magnetic polarity, suggesting that the earth’s magnetic polarity has flip-flopped many times.

1968: Year Long Scientific expedition to collect drill cores from Mid-Atlantic Ridge.

Age determined using paleontology and isotopic dating studies.

Consequence of Seafloor spreading : Expanding

Earth Hypothesis..

i.e. New crust was, and is now, being continually created along ocean ridges

Harry H. Hess and Robert S. Deitz: coined the term Seafloor Spreading.

Earth’s crust must be shrinking elsewhere.

- New Oceanic crust continuously spread away from the ridge in a conveyor belt-like motion.

- Many million yrs later, the oceanic crust eventually descends into the Oceanic Trenches – deep narrow canyons along the rim of the Pacific Ocean basin - So the Atlantic Ocean was expanding while the Pacific Ocean was shrinking.

- In effect, the ocean basins were perpetually being “recycled” – with creation of new crust and the destruction of old oceanic lithosphere.

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Hess’ idea neatly explained: 1. Why the Earth does not get bigger with sea floor spreading.

2. Why there is little sediment accumulation on the ocean floor.

3. Why oceanic rocks are much younger than continental rocks.

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Distribution of earthquakes 1960s WWSSN stations recorded earthquake data delineating ridges and trenches. Reconfirmed Hess’ prediction.

The unifying theory: Plate Tectonics Jason Morgan (1968)

The Basic concepts: • The outermost layer of the Earth called the lithosphere is divided into small number of nearly rigid plates (like curved caps on a sphere) moving over the asthenosphere. • Most of the deformation which results from the plate motions (earthquakes) takes place along the edge, or boundary, of a plates. Deformation inside the boundary is not significant.

This is known as the kinematic theory, because it is not concerned with the forces that maintain the motions.

Important assumptions: • The generation of new plate material occurs by seafloor spreading: that is, new oceanic lithosphere is generated along the active Mid Ocean Ridges.

• The new oceanic lithosphere, once created, forms part of a rigid plate, this plate may or may not include continental material.

• The earth’s surface area remains constant; therefore seafloor spreading must be balanced by consumption of plate elsewhere.

• The lithospheric plates are capable of transmitting stresses over great horizontal distances without buckling; in other words, the relative motion between plates is taken up only along plate boundaries.

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Earthquakes and Plate Boundaries Most Earthquakes result from motion of plates and are distributed along the plate margins.

Analysing Plate Boundaries Seven Major Plates (107 to over 108 sq km area) Several intermediate and smaller size plates (105 to 107 sq km area)

Divergent Plate Boundary - Also called accreting or constructive plate boundary, - Plates move away from each other.

- New plate material derived from mantle, is added to the lithosphere.

- Represented by MOR. System - Normal fault mechanism.

An example of Divergent Plate Boundary East African Rift Zone: Red Sea

Two African Plates Nubian Plate Somalian Plate Splitting along the East African Rift Zone.

Convergent Plate Boundary - Also called consuming or destructive plate boundary - Plates approach each other.

- Represented by the oceanic trench, island arc system of subduction zone, where one -of the colliding plates descend into the mantle and is destroyed.

-The downgoing plate often penetrates the mantle to depths of about 700 km.

An example of Convergent Plate Boundary Peru-Chile trench: Off the coast of South America Oceanic Nazca Plate is being subducted under the continental part of the South American Plate.

The overriding South American Plate is being lifted up, creating the Andes mountains.

Limestone strata pushed up to form towering peaks of the Andes.

Collisional Boundary Some convergent boundaries occur on land due to the collision of two continental plates. Himalaya is a grand example of collisional boundary.

Both these types have thrust fault mechanism

An example of Collisional Plate Boundary

India-Eurasia Collision: Himalayan Mountains

Conservative Plate Boundary -Lithosphere is neither created nor destroyed.

- The plates move laterally relative to each other.

- Represented by Transform faults.

- Six basic classes: most common is the ridge-ridge fault.

- Strike-slip fault mechanism.

Note: Although the plates are made up of both oceanic and continental material, usually only oceanic part of any plate is created or destroyed.

An example of Conservative Plate Boundary The San Andreas Fault -The San Andreas Fault zone, which is about 1,300 km long and in places tens of kilometers wide, slices through two thirds of the length of California.

- Along it the pacific plate has been grinding horizontally past the North American Plate for 10 million years.

-Dextral sense of motion at an average rate of about 5 cm/yr.

Field Photograph

The Global Picture

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