Plate Tectonicsand earthquakes

• Alfred Wegener• Proposed hypothesis in 1915

• Continental drift hypothesis • Supercontinent Pangaea began breaking

apart about 200 million years ago

Continental drift: An idea before its time

Pangaea approximately 200 million years ago

• Continental drift hypothesis• Continents "drifted" to present positions

• Evidence used in support of continental drift hypothesis

• Fit of continents • Fossil evidence• Rock type and mountain belts • Paleoclimatic evidence

Match: - shape - fossil types - rock types - mountain belts

The great debate

• Objections to drift hypothesis • Inability to provide a mechanism capable

of moving continents across globe • Wegner suggested that continents broke

through the ocean crust, much like ice breakers cut through ice

Plate tectonics

• Earth’s major plates • Associated with Earth's strong, rigid

outer layer– Known as the lithosphere

– Consists of uppermost mantle and overlying crust

– Overlies a weaker region in the mantle called the asthenosphere

Basal tractions drive plate motions

• Earth’s major plates• Seven major lithospheric plates• Plates are in motion and change in shape

and size• Largest plate is the Pacific plate• Several plates include an entire continent

plus a large area of seafloor

• Earth’s major plates• Plates move relative to each other at a

very slow but continuous rate

– Average about 5 centimeters (2 inches) per year

– Cooler, denser slabs of oceanic lithosphere descend into the mantle

• Plate boundaries• Interactions among individual plates

occur along their boundaries • Types of plate boundaries

– Divergent plate boundaries

– Convergent plate boundaries

– Transform fault boundaries

Types of Plate Margins

• Plate boundaries• Each plate bounded by combination of all

three boundary types

Divergent plate boundaries

• Most are located along the crests of oceanic ridges

• Oceanic ridges and seafloor spreading • seafloor is elevated forming oceanic ridges

• Oceanic ridges and seafloor spreading• Seafloor spreading occurs along the oceanic ridge

system

• Spreading rates and ridge topography • Ridge systems exhibit topographic differences

• Topographic differences are controlled by spreading rates

Divergent boundaries are located mainly along oceanic ridges

Development of Continental Rift into Ocean Basin

Convergent plate boundaries

• Old portions of oceanic plates are returned to the mantle

• Surface expression of descending plate is an ocean trench

• Called subduction zones • Average angle at which oceanic

lithosphere descends into the mantle is about 45

• Types of convergent boundaries • Oceanic-continental convergence

– Denser oceanic slab sinks into the asthenosphere

– As plate descends, partial melting of mantle rock makes basaltic or andesitic magmas

– Volcanic mountains associated with subduction of oceanic lithosphere are called continental volcanic arcs (Andes and Cascades)

Types of Arcs

• Types of convergent boundaries• Oceanic-oceanic convergence

– When two oceanic slabs converge, one descends beneath the other

– Often forms volcanoes on the ocean floor

– If the volcanoes emerge as islands, a volcanic island arc is formed (Japan, Aleutian islands, Tonga islands)

Types of Arcs

• Types of convergent boundaries• Continental-continental convergence

– Result is a collision between two continental blocks

– Process produces mountains (Himalayas, Alps, Appalachians)

The collision of India and Asia produced the Himalayas

Transform fault boundaries

• Third type of plate boundary • Plates slide past one another and no new

lithosphere is created or destroyed

Transform plate boundaries

Tectonic plates slide horizontally past each other

Transform plate boundaries

The San Andreas fault

Testing the plate tectonics model

• Plate tectonics and earthquakes • Plate tectonics model accounts for the

global distribution of earthquakes – Absence of deep-focus earthquakes along the

oceanic ridge is consistent with tectonic theory

– Deep-focus earthquakes associated with subduction zones

– Deep-focus earthquakes occur along convergent boundaries

Deep-focus earthquakes occur along convergent boundaries

Earthquakes near Japan trench

• Hot spots • Caused by rising plumes of mantle

material • Volcanoes form over them (Hawaiian

Island chain)

The Hawaiian Islands form over stationary hot spot

Importance of plate tectonics

• Provides a unified explanation of Earth’s major surface processes, especially oceans

• Within framework of plate tectonics, we find explanations for the distribution of earthquakes, volcanoes, and mountains

• Plate tectonics provides explanations for distribution/evolution of plants and animals and climate record

Earthquakes

• Why study earthquakes?

• What is an earthquake?

• Seismographs• Locating

earthquakes• Recent earthquakes• CA and AZ

earthquakesDamage in Golcuk, Turkey after a magnitude 7.4 earthquake, August 17, 1999

Why study earthquakes?• Understand the hazard to minimize the risk.

Earthquakes

• Earthquakes release accumulated strain by the sudden displacement of faults.

• Elastic Rebound Theory – energy can be stored in elastically deformed rock, earthquakes release that stored energy and the rock returns to its undeformed state.

Elastic rebound. As rock isdeformed it bends, storingelastic energy. Once therock is strained beyond itsbreaking point it rupture,releasing the store-up energy in the form of earthwaves.

An example of elastic rebound

• Slip of nearly 3 m as a result of the 1906 San Francisco earthquake.

Seismology and Seismographs

• Seismology – the study of earthquakes

• Seismographs are used to observe and record earthquake motion and vibrations.

Location of an Earthquake• Earthquake focus – location where seismic energy

is first released.• Epicenter – the point on the surface directly above

the focus.

• Types of seismic waves• Body waves

– Travel through Earth’s interior – Two types based on mode of travel – Primary (P) waves

» Primary, Push-pull motion» Travel through solids, liquids & gases» Travel fastest of the seismic waves

– Secondary (S) waves» Secondary, shaking, shear, side-to-side» Moves at right angles to their direction of

travel » Travels only through solids

Body waves travel through the interior of Earth

Body waves

• Types of seismic waves • Surface waves

– Complex motion (up-and-down and side-to-side)

– Causes damage to structures during an earthquake

– High amplitude and low velocity

– Longest periods (interval between crests)

– Termed long, or L waves

– Slowest

• Types of seismic waves • Surface waves

– Complex motion (up-and-down and side-to-side)

– Causes damage to structures during an earthquake

– High amplitude and low velocity

– Longest periods (interval between crests)

– Termed long, or L waves

– Slowest

Surface waves travel along the outer part of Earth

Surface waves

Seismic waves within the Earth• Seismic waves (P and S waves) bend due to

changing density with depth

• Knowledge of the behavior of seismic waves is used to estimate the density, temperature, and pressure within the Earth.

Travel times of seismic waves

• Travel times of p- and s-waves differ.

• The difference in the velocity of p- and s-waves used to calculate the distance to an earthquake focus.

Locating the earthquake

• The distance from each station to the earthquake is plotted.

• The intersection of three or more circles shows the location of the epicenter.

• Much of the calculation is now automated to give preliminary estimates of magnitude and location.

Measuring the size of earthquakes

• Two measurements describe the size of an earthquake

• Intensity – a measure of earthquake shaking at a given location based on amount of damage

• Magnitude – estimates the amount of energy released by the earthquake

• Magnitude scales • Richter magnitude - concept introduced

by Charles Richter in 1935 • Richter scale

– Based on amplitude of largest seismic wave recorded

– Each unit of Richter magnitude corresponds to 10X increase in wave amplitude and 32X increase in energy

The Richter scale

The Richter scale is based on the relationship between distance from the epicenter (or the difference in arrival time between P and S waves), and the maximum amplitude

The amplitude at a given distance is a function of the energy released by the earthquake

World Seismicity

Earthquake damage

• Ground motion due to seismic waves

• Fault rupture of the ground surface

• Fire

• Liquefaction

• Tsunami