Dynamic Earth Class 8 2 February 2006

Any Questions?

Note: Exam #1 Date shown on hardcopy syllabus as February 14 th, but said February 15 th. The correct date is February 14 th (On-line version is correct)

What Plate Tectonics Theory explains Age distribution of oceanic crust (and why the oceans are MUCH younger than the continents) Age distribution of oceanic crust (and why the oceans are MUCH younger than the continents) Distribution of earthquakes and volcanoes Distribution of earthquakes and volcanoes

Seismology Study of the propagation of mechanical energy released by earthquakes. Study of the propagation of mechanical energy released by earthquakes. When energy is released in this fashion, waves of motion (like the effect of a pebble tossed into a pond) are set up in the Earth. When energy is released in this fashion, waves of motion (like the effect of a pebble tossed into a pond) are set up in the Earth.

Earthquakes earthquake: movement of rock bodies past other earthquake: movement of rock bodies past other fault: locus of the earthquake movement fault: locus of the earthquake movement faults come at all scales, mm to separation of lithospheric plates (e.g., San Andreas). faults come at all scales, mm to separation of lithospheric plates (e.g., San Andreas).

Earthquake terms focus: site of initial rupture

Focus = point where an earthquake originates

Earthquake terms focus: site of initial rupture epicenter: point on surface above the focus

Seismic Waves Radiate from the Focus of an Earthquake

Eric Marti/AP PhotoEarthquakes Most damage from ground shaking

Chile, May 22, 1960 Magnitude 9.6 earthquake, the largest earthquake ever recorded Magnitude 9.6 earthquake, the largest earthquake ever recorded The city of Valdivia suffered catastrophic damage The city of Valdivia suffered catastrophic damage Severe shaking lasted for over 15 minutes Severe shaking lasted for over 15 minutes Coastal areas subsided Coastal areas subsided A 10-meter tsunami was generated A 10-meter tsunami was generated

The earth ruptured for 450 miles along the Chile coast The earth ruptured for 450 miles along the Chile coast The area of rupture was the size of California The area of rupture was the size of California Chile, May 22, 1960

Elastic Rebound Theory

Fig San Francisco Earthquake

Fault Trace Fault Offset (~2.5m) 1906 San Francisco Earthquake

Seismic waves Waves are started because of initial tension or compression in the rock. Waves are started because of initial tension or compression in the rock. Instruments used to measure these waves are called seismographs. Instruments used to measure these waves are called seismographs.

Seismographs Record Vertical or Horizontal Ground Motion VerticalHorizontal

Kinematics Modern Seismograph

Seismograph Record and Pathway of Three Types of Seismic Waves

Two kinds of waves from earthquakes P waves (compressional) 6–8 km/s. Parallel to direction of movement (slinky), also called primary waves. Similar to sound waves. P waves (compressional) 6–8 km/s. Parallel to direction of movement (slinky), also called primary waves. Similar to sound waves. S waves (shear) 4–5 km/s. Perpendicular to direction of movement (rope); also called secondary waves. Result from the shear strength of materials. Do not pass through liquids. S waves (shear) 4–5 km/s. Perpendicular to direction of movement (rope); also called secondary waves. Result from the shear strength of materials. Do not pass through liquids.

Comparison of P-wave and S-wave Motion

Two Types of Surface Waves

Time Lag Between S and P waves with Distance from Epicenter

Seismic Travel-time Curve

Locating the Epicenter

Locating an epicenter The difference between the arrival times of the P and S waves at a recording station is a function of the distance from the epicenter. The difference between the arrival times of the P and S waves at a recording station is a function of the distance from the epicenter. Therefore, you need three stations to determine the location of an epicenter. Therefore, you need three stations to determine the location of an epicenter.

Measuring the force of earthquakes 1. Surface displacement 1964 Alaska earthquake displaced some parts of the seafloor by ~ 50 ft Alaska earthquake displaced some parts of the seafloor by ~ 50 ft San Francisco earthquake moved the ground ~8.5 ft San Francisco earthquake moved the ground ~8.5 ft. 2. Size of area displaced Alaska — 70,000 sq. miles Alaska — 70,000 sq. miles

Measuring the force of earthquakes 3. Duration of shaking Up to tens of seconds Up to tens of seconds 4. Intensity scales Based on damage and human perception Based on damage and human perception 5. Magnitude scales Based on amount of energy released Based on amount of energy released

Modified Mercalli Intensity Scale INot felt IIFelt only by persons at rest III–IVFelt by persons indoors only V–VIFelt by all; some damage to plaster, chimneys VIIPeople run outdoors, damage to poorly built structures VIIIWell-built structures slightly damaged; poorly built structures suffer major damage IXBuildings shifted off foundations XSome well-built structures destroyed XIFew masonry structures remain standing; bridges destroyed XIIDamage total; waves seen on ground; objects thrown into air

Richter scale Richter scale: amount of energy received 100 km from epicenter Richter scale: amount of energy received 100 km from epicenter Largest quake ever recorded = 9.6 (rocks not strong enough for more). Largest quake ever recorded = 9.6 (rocks not strong enough for more). Earthquakes less than M = 2 are not felt by people. Earthquakes less than M = 2 are not felt by people. Scale is logarithmic: Scale is logarithmic: Increase 1 unit = 10 times greater shaking Increase 1 unit = 10 times greater shaking Increase 1 unit = 30 times greater energy Increase 1 unit = 30 times greater energy

Maximum Amplitude of Ground Shaking Determines Richter Magnitude

Richter Magnitude Versus Energy

Earthquake prediction Long term—imprecise (can be done) Short term—precise (very difficult) We can't stop earthquakes, so we have to be prepared for them.

New Housing Built Along the 1906 Trace of the San Andreas Fault R.E. Wallace, USGS

Seismic Hazard Map

Distribution of earthquakes Not random Not random Focused in linear zones Focused in linear zones

World Seismicity, 1963–2000

Earthquake distribution defines plate boundaries

The Earth has a number of layers, just like an egg or an apple Yoke Core Mantle Crust Core ?

Earth’s layers

Lithosphere Asthenosphere

Plates Rigid Lithosphere with definite boundaries Rigid Lithosphere with definite boundaries Can have both oceanic and continental crust or just one kind. Can have both oceanic and continental crust or just one kind.

Note that crust under continents is thicker (~45 km) than under oceans (~8 km).

The Earth’s Major Plates

Types of plate boundaries Divergent: mid-ocean ridges Divergent: mid-ocean ridges Convergent: collision zones volcanic arcs Convergent: collision zones volcanic arcs Strike-slip: San Andreas Fault (California) Anatolian Fault (Turkey) Strike-slip: San Andreas Fault (California) Anatolian Fault (Turkey)

Three Types of Plate Boundaries Divergent (Spreading) Convergent (Subduction Zone) Transform

Earthquakes Associated with Divergent and Transform Margins

Strike-slip Faults

Subduction Zones

Benioff- Wadati Zone Earth- quakes in subduction zones

Lithosphere is created at spreading centers and destroyed at Trenches (Subduction Zones) Convection within the Earth

Subduction zones Ocean-continent convergence Ocean-ocean convergence Continent-continent collision

Growth of Continents Addition of volcanic arc material to continent (e.g. Andes). Addition of volcanic arc material to continent (e.g. Andes). Collisions: Continents Collisions: Continents Collisions: Exotic terranes Collisions: Exotic terranes

Exotic terranes: approaching arc or microcontinent

Collision

Accreted Terrane

The West Coast of North America

Exotic Terranes: plastered to the continent at subduction zones

Any Questions?

Tuesday Finish reading Chapter 3 in the text Finish reading Chapter 3 in the text Thursday Homework # 3 due Homework # 3 due