Earthquakes Earthquakes are caused by a sudden release of energy The amount of energy released determines the magnitude of the earthquake Seismic waves carry the energy away from its origin
Fig. 18.1 Origin of Earthquakes by elastic rebound
Seismic Waves P-waves Primary waves, arrive first Alternating pulses of compression and dilation (expansion) parallel to wave path P waves may pass through solids, liquids, and gases Compression produces temporary changes in volume & density of material
Seismic Waves S-waves Secondary waves, arrive second S waves cause a shearing effect Waves are perpendicular to the direction of travel Elastically change the shape of materials Liquids and gases do not behave elastically
Seismic Waves Surface waves Restricted to traveling along the Earth s surface Travel more slowly than P or S waves Similar to ocean waves travelling through rock Orbital motion
Fig. 18.3. Motion of seismic waves
Earthquake Locations The arrival time method determines the location on the map below which the earthquake occurred Epicenter The exact point is at some depth below the surface Focus
Earthquake epicenter and focus
Earthquake Magnitude The Richter scale measures the amplitude of seismic waves The Richter scale is logarithmic - Each unit on the scale relates a 10 fold increase in the amplitude of the seismic wave
Examples of earthquake damage
Biggest in US history Most frequent Seismic risk map of the U.S.
Fig. 18.14. Earth s seismicity
Seismic waves in a homogeneous planet
Seismic waves in a differentiated planet
S wave shadow zone
Fig. 18.18. P wave shadow zone
The Earth s Magnetic Field
Seismic Structure of the Earth Seismic wave velocities vary with depth Variation with depth is not regular Discontinuities exist at certain depths Represent discrete changes in the layered Earth structure
Seismic Structure of the Earth Mohorovicic Discontinuity (Moho) First discovered by Andrija Mohorovicic Occurs between 5 and 70 km deep Represents the base of the crust Compositional change from feldspar rich to olivine rich causes change in seismic velocities
Seismic Structure of the Earth Low-velocity zone Layer from ~100 to 250 km deep Seismic velocities usually increase with depth Decrease by ~ 6% in low velocity zone Caused by partially molten mantle that slows seismic waves
Internal structure of the Earth
Convection in the Earth In the core Seismic waves show composition & structure of core 3-D models may show flowing molten iron In the mantle Investigations show a complex convection system occurring in the entire mantle system
Convection in the Earth
Midoceanic Ridges Divergent margin in an ocean basin Longest mountain chain on Earth New oceanic crust created New crust pushes old crust aside Spreading a few centimeters per year Seawater interacts with new crust
Fig. 19.1. Midocean Ridge System
Ridge Topography Most pronounced tectonic feature 1500 km wide with peaks up to 3 km Total length of 70,000 km Structure dominated by normal faults and basaltic volcanism Broken into segments by transform faults
Ridge Topography Ridge consists of broad ridge with central rift valley Detailed topography depends on spreading rate Prominence of rift valley Associated volcanoes Sedimentary cover
Ridge Topography Cooling & subsidence Elevation is influenced by temperature New crust cools as it spreads away from ridge Cooling crust becomes more dense Increase in density causes subsidence Age correlates with water depth
Mid-Atlantic Ridge slow spreading
East Pacific Rise Fast spreading
Ridges subside and are covered with sediment
Juan de Fuca Plate
A Close Up View Surface of ridge covered with fresh lava flows and pillow basalts Fissures in the crust are apparent & parallel the ridge axis Hydrothermal vents form chimney structure Unique ecological community
Pillow basalt Fissures
Pillow lav a.mov Smoker.mov Tube3.mo v
Model of Oceanic Ridge
Seafloor Metamorphism Interaction of seawater with hot basalt Water T of 400-450 o C Hydrothermal fluids react with basalt to form new mineral suite Chlorite, epidote, serpentine Hydrothermal fluids leach minerals Form black & white smokers Precipitate Cu, Zn, Pb sulfides
Ophiolites Fragments of oceanic crust with 5 layers Accreted onto continental crust Consist of 5 layers Marine Sediments Pillow Basalts Sheeted Dike Complex Gabbro Tectonites (upper mantle peridotite)
Major units in an ophiolite sequence
Evolution of oceanic crust
Marine sediments
Pillow Basalts
Sheeted dikes
Layered Gabbro
Iceland Exposed portion of Mid-Atlantic ridge Flood basalts dominate with fissures and rifts parallel to Mid-Atlantic ridge Fissures are bounded by normal faults Fissure eruptions emit large volumes of magma
Shallow earthquakes and magma chambers Groundwater heated to produce geothermal energy Iceland
Line of fissures and volcanoes in Iceland
Continental Rifting Elongate depression bounded by normal faults Lithosphere is deformed Crust is arched, extended, & pulled apart Normal faults produce down-dropped grabens (rift valley)
Continental Rifting Volcanism and sedimentation Basaltic volcanism similar to oceanic ridges Rhyolitic magma formed by partial melting of crust Old rift valleys contain thick layers of sediments Oil fields
Basin & Range Complex rift system that extends from Mexico to Canada Basins (valleys) and ranges formed by tilted fault blocks Heat flow is 3X normal Crust has thinned to 25 km from ~ 50 km Recent and active volcanism Slower spreading rate than mid-ocean ridges
Fig. 19.29. Basin & Range Province
East Africa East African system extends 3000 km from Ethiopia to Mozambique Red Sea rift extends from Ethiopia to the Sinai and Dead Sea Complex volcanism throughout Oceanic crust present in SE Red Sea Lakes form in isolated down-dropped blocks Several areas below sea level
Fig. 19.33. The Red Sea
North American Mid-Continent Ancient rift system ~ 1 bya Stretches 1400 km from Minnesota to Kansas Buried beneath younger marine sediments and glacial till No topographic evidence Revealed by gravity anomalies Dense rocks (basalt or gabbro) produce gravity high
Characteristics Transform boundaries are strike-slip faults Faults are nearly vertical & parallel to movement Plates move laterally past one another No lithosphere is created or consumed Most associated with divergent margins
Oceanic Transform Boundaries Active displacement only occurs between ridge crests Only region of fracture zone with opposite plate motion Remainder of fracture zone is inactive Vertical relief, ridge & trough, due to age of crust on opposite sides of boundary
Fracture Zones Oceanic transform boundaries are part of fracture zones Large scale features up to 10,000 km long Generally very narrow, 10 s of km at most, but contain numerous faults Appear as faults offsetting oceanic ridges Transform boundary is a small portion of fracture zone
Continental Transform Faults Not as common as oceanic transform faults Similar in structure Seismically active Penetrate entire lithosphere Distinct linear features San Andreas, Dead Sea systems
San Andreas System Ridge-ridge system extending ~ 3000 km System is composed of numerous faults Accommodate motions of Pacific and N. American plates Earthquakes are shallow 30 my old with ~ 300 km of offset
Fig. 20.13. The San Andreas transform fault system
Fig. 20.12b. Landforms along the San Andreas
Earthquakes Especially abundant along transforms More abundant in oceanic systems than ridge related quakes Results of colder, more brittle crust Typically shallow in thinner crust Relatively low intensity Quakes are more intense along continental transforms