The Big Picture:

Earthquakes and Plate Tectonics

World Seismicity Chart (

Earthquake Belts at Plate BoundariesBelts- a geographic zone on the Earth’s surface along which most earthquake activity occurs

Ring of Fire- one of the best known belts surrounds the Pacific

Ocean

Shallow-Focus Earthquakes at Divergent BoundariesDeep-Focus Earthquakes at Convergent BoumdariesShallow Focus Earthquakes within Plates

Shallow-Focus Earthquakes at

Divergent Boundaries

These earthquakes are associated with two components -one is the spreading centre or mid-oceanic ridge, and the other is the set of small transform offsets that link short segments of oceanic ridge

Deep-Focus Earthquakes at

Convergent Boundaries

These are earthquakes that originate at depths greater than about 100 km

Found to coincide with continental margins or island chains that are adjacent to ocean trenches and young volcanic mountains

Examples of such areas are West coast of South America and the chains of islands that make up Japan and Philippines

Shallow-Focus Earthquakes within Plates

Earthquake Destructiveness Each year 800 000 little tremors that

are not felt by humans are not recorded by instruments around the world

100 earthquakes occur each year with Richter magnitude between 6 and 7

Once every 5 to 10 years great earthquakes occur with a Richter magnitude exceeding 8

Destructive earthquakes are even more frequent in Japan than in California

How Earthquakes Cause Their Damage?

Ground vibrations

Fires ignited by ruptured gas lines or downed electrical power lines

Power seismic waves also take their toll on land forms and underlying soils

Avalanches Liquefaction

Tsunami

• sea wave that is triggered by undersea event such as an earthquake or landslide or eruption of an oceanic volcano

• Popularly known as tidal waves

• It travels across the ocean at speeds of up to 800 km per hour and form walls of water that can be higher than 20 m when they reach the coast

Generation of a tsunami by fault movements caused by an earthquake on the seafloor.

Tsunamis in Japan

Mitigating the Destructiveness of

Earthquake

*Earthquake Protection Program

provides a basis for organizing local earthquake protection programs consonant with the degree of danger

Seismic-risk Map

Seismic Hazard Map for North and Central America

1. Izmit, Turkey(1999)2. Papua New Guinea3. Northern Iran4. Windward Islands5. Sakhalin Island6. Kobe, Japan7. Kuril Islands8. Northern Bolivia9. Northridge CA10.Southern India11.Republic of South12.Flores region13.Switzerland14.Northern Colombia15.Landers, CA16.Northern India17.Luzon, Philippines18.Western Iran19. Loma Prieta, CA

Housing tracts constructed in recent years within the San Andreas fault zone, San Francisco Peninsula

*These are the possible premonitory indicators being examined by some scientists:• A rapid tilting of the ground or other forms of surface deformation• An unusual aseismic slip on a fault• An episode of stretching of the crust across fault• Changes in the physical properties of rock in the vicinity of a fault• Changes in the level of water in wells• An usual increase in the frequency of smaller earthquakes in a

region before a main shock

Early-Warning Indicators

Seismic gap

a segment of an active fault known to produce significant earthquakes, that has not slipped in an unusually long time when compared with other segments along the same structure

Seismic gap hypothesis/theory

states that, over long periods of time, the displacement on any segment must be equal to that experienced by all the other parts of the fault.[1] Any large and longstanding gap is therefore considered to be the fault segment most likely to suffer future earthquakes.

*Worrisome Developments

Exploring Earth’s Interior

Exploring the Interior with Seismic Waves

• Any differences detected for different paths can be used to infer the properties of materials the waves have been encountered along these paths

• A boundary between two materials, some of the waves bounce off and others are transmitted into the second material

• There are waves bend or refract when cross the boundary between two materials

Two beams of light enter the bowl of water from the top

Paths of Seismic Waves in the Earth

• Shadow Zone A zone where no P wave reaches the surface between 105° and 142°. Discovery of it led geologists to surmise that the Earth has a core made up of a material different from than that of overlying mantle.

*Waves traveling through the Earth

The pattern of P-wave paths through earth’s interior

The larger S-wave shadow zone

*Waves traveling through the Earth

• PcP wave- bounces off the core; used to determine the depth of the core

• SS wave- S waves that reflect back into the Earth

• PKP wave- P waves that penetrate the outer core

• PKIP wave- P wave that penetrate the inner core

*Waves reflected in the Earth

Composition and Structure of the Interior

Changes in P- and S- wave velocities with depth in the Earth reveal the sequence of layers that make up Earth’s interior

*The Crust

• It varies in thickness- thin under oceans (about 5 km), thicker under continents ( about 40 km) and thickest under high mountains ( ranging up to 65 km).

• P waves move through crustal rocks at about 6-7 km per second

Mohorovicic discontinuity (Moho)

• Discovered by seismologist Yugoslav in 1909 .The boundary between the crust and mantle

Principle of Isostasy

• The idea that continents are less dense than the mantle and float on it

*The Mantle

It is mainly made up of olivine and pyroxene, the two silicates of magnesium and iron

• lithosphere- outermost zone, is a slab up to 100 km thick in which the continents are embedded tectonic plates are large fragments of the lithosphere . S waves pas through easily without being absorbed

• asthenosphere- zone of weakness; a partially fluid solid .It rises close to the surface at mid-ocean ridges where plates separate and is found at depths up to about 100 km and ends at a about 200 km where the velocity of S waves increases to a value that fits that of solid peridotite.

• From about 200 to 400 km, the velocity of S waves increases gradually with depth

• About 400 km below the surface waves increases rapidly• In the region from 450 to 650 km, properties change little as

depth increases• Near 670 km, the velocity increases again• From 700 km to the core at a depth of 2900 km, changes little

in composition and crystal structure with depth

• *The Core• It is 2900 km from the surface• P waves that penetrate to depths of 5100 km suddenly speed up, a discovery

made by a Danish seismologist, Inge Lehman• Composition of the core derived from astronomical data, and seismological data • To be consistent with the that the core is made up material that sank during the

initial formation and differentiation of the Earth, geologists searched for the substances that were dense

• Because the core contains one- third of Earth’s mass, geologists considered substances that were abundant in the universe

• Geologists concluded that Earth’s core is composed mostly of iron, molten in the inner core and solid in the inner core