Geos 218: Geological Disasters & Society - Subduction & Earthquakes - PDF

Geos 218: Geological Disasters & Society Unit 4a: Subduction & Wilson Cycle Earthquakes EQ basics & terms 3 Kinds of Faults 2021 Haiti Earthquake damage USGS http://gallery.usgs.gov/photos 1 Main Concept in Pl...

Geos 218: Geological Disasters & Society Unit 4a: Subduction & Wilson Cycle Earthquakes EQ basics & terms 3 Kinds of Faults 2021 Haiti Earthquake damage USGS http://gallery.usgs.gov/photos 1 Main Concept in Plate Tectonics Last time: Creation of lithospheric plates creation creation i on uct s tr de 2 1  Plate Tectonics (Last time) Tectonic cycle: Melted asthenosphere flows upward as magma Cools to form new ocean crust (lithosphere) New oceanic lithosphere (slab) diverges from zone of formation atop asthenosphere (seafloor spreading) When slab of oceanic lithosphere collides with another slab, older, colder, denser slab subducts under younger, hotter, less dense slab Subducted slab is reabsorbed into the mantle Cycle (Wilson Cycle) takes on order of 400 million years https://www.youtube.com/watch?v=I_q3sAcuzIY 3 Main Concept in Plate Tectonics Today: Destruction of lithospheric plates creation n creation tio uc str de 4 2  Plate Tectonics Tectonic cycle: Melted asthenosphere flows upward as magma Cools to form new ocean crust (lithosphere) New oceanic lithosphere (slab) diverges from zone of formation atop asthenosphere (seafloor spreading) When slab of oceanic lithosphere collides with another slab, the older, colder, denser slab subducts under younger, hotter, less dense slab Subducted slab is reabsorbed into the mantle Cycle (Wilson Cycle) takes on order of 400 million years https://www.youtube.com/watch?v=I_q3sAcuzIY 5 Animation: Subduction https://www.iris.edu/hq/inclass/animation/plate_boundary_convergent_margin 6 3  Plate Tectonics Tectonic cycle: Melted asthenosphere flows upward as magma Cools to form new ocean crust (lithosphere) New oceanic lithosphere (slab) diverges from zone of formation atop asthenosphere (seafloor spreading) When slab of oceanic lithosphere collides with another slab, older, colder, denser slab subducts under younger, hotter, less dense slab Subducted slab is reabsorbed into the mantle Cycle (Wilson Cycle) takes on order of 400 million years https://www.youtube.com/watch?v=I_q3sAcuzIY (next slide) 7 Wilson Cycle https://www.youtube.com/watch?v=I_q3sAcuzIY 8 4  The Grand Unifying Theory Tectonic cycle: When two plates collide, denser (colder, older) plate goes beneath less-dense (warmer, younger) plate in subduction – Oceanic plate beneath oceanic plate: Volcanic island arc next to trench (Aleutian Islands of Alaska) – Oceanic plate beneath continental plate: Volcanic arc on continent edge next to trench (Cascade Range) Plate tectonics requires time perspective of millions and billions of years – Plate movement may be 1-10 cm/year 5cm = 4-5m during your life, = 40-50 km in a million years = 4000-5000 km in 100 million yrs – Uniformitarianism: small events add up to big results 9 What is a Plate? Geochemical/Density Layers Physical Layers Temperature Surface Crust = low density minerals (Si + O) Lithosphere = crust + uppermost mantle à RIGID layer = PLATE! Cool ~ 100km thick EQ happen here! Asthenosphere = within the mantle MUSHY (like silly putty), can flow! ~ 100km thick Depth Mantle = higher density minerals Si + O + Fe + Mg Lower mantle = RIGID ~2500 km thick Core = metals (iron/nickel) Outer Core = LIQUID HOT Inner Core = SOLID Layer thickness NOT to scale! Layer thickness NOT to scale! 10 5  What is an Earthquake? n An event in which the Earth quakes, a section of lithosphere cracks and vibrations (known as seismic waves) are felt or recorded n Most commonly caused by movement of the Earth across a fault n Also caused by volcanic activity, meteorite impacts, undersea landslides, explosions of nuclear bombs n A Fault is a fracture in the Earth across which the two sides move relative to each other n Stresses build up until enough to cause rocks to fracture and shift, sending off waves of seismic energy, felt as earthquake Figure 3.2 11 Elastic Rebound Theory Stress builds; releases in earthquake; builds again Elastic energy stored in bent rock as stress builds. Stored elastic energy released in earthquake rupture. 12 6  uake_machine__graphing_time_vs_distance http://www.iris.edu/hq/inclass/animation/earthq Elastic Rebound Theory Time vs Distance (the block slides) Stress builds; releases in EQ; uake_machine_graphing_time_vs_strain https://www.iris.edu/hq/inclass/animation/earthq builds again Time vs Strain (tension in rubber band) 13 Deforming Earth’s Crust 3 Types of stress: Extensional Pulling apart Compressional Pushing together Shear Opposite directions Results in 3 kinds of faults: Normal Thrust or Reverse Strike slip Earthquakes occur on faults 14 7  Terminology Epicenter: point on surface directly above focus Focus or hypocenter: point of rupture within Earth EQ generates seismic waves that travel through the Earth 15 The Rupture Area Faults are complex zones of breakage with irregular surfaces, many miles wide and long n Stress builds up until rupture occurs at weak point and propagates along fault surface n Point where rupture first occurs is hypocenter or focus n Point directly above hypocenter on surface is epicenter Fig. 3.12 16 8  Extensional Stress = Normal Fault Normal fault: block above the fault moves down; e.g., Wasatch Fault in Salt Lake City Example: Rocky Mtns. / Northridge earthquake 17 Ariel view over BYU campus showing the Wasatch Mountains to the east. The last time this fault ruptured, 400 years ago, there were probably few inhabitants in the area. There are now approximately 1.5 million people living on top of the fault as it dips beneath the valley. 18 9  (Photo by Bill Harris) Photo of exposure of the Wasatch Fault scarp. This is one of the best exposures anywhere along the 240 mile fault. A history of movement of the fault is seen in the striations or tool marks on the face of the scarp. Large earthquakes occur along the fault on average about every 350 years. This Provo segment of the fault last ruptured about the year 1400. 19 Compressional Stress = Thrust Fault Thrust or Reverse fault (same thing): Block above the fault moves up. Example: Great Sumatra earthquake 20 10  Thrust Fault, Ketobe Knob, Utah 21 Shear Stress = Strike-Slip Fault Strike-slip fault: Horizontal motions on a vertical fault; e.g., The San Andreas Fault 22 11  A stream channel offset by the San Andreas fault, Carrizo Plain, central California ‘ (photo by Robert E. Wallace) 23 Identifying Faults: Right or Left? Strike-slip faults: n Dominated by horizontal movement n When straddling a fault, if the right-hand side moved towards you, it is a right-lateral fault n When straddling a fault, if the left-hand side moved towards you, it is a left-lateral fault n Convention works in either direction n This is the “older” way to describe it (and harder to remember!) Figure 3.11 24 12  Identifying Faults: Right or Left? Strike-slip faults: n Dominated by horizontal movement n When looking across a fault, if the far side moved towards the right, it is a right-lateral fault n When looking across a fault, if the far side moved towards the left, it is a left-lateral fault n Convention works in either direction n Which is the San Andreas? n Right Lateral! 25 Identifying Faults Transform Faults (somewhat like strike-slip): As oceanic plates spread apart at mid-ocean ridges, they must slide past other plates, along transform faults Transform faults link spreading centers, or connect spreading center to subduction zone Between two spreading centers, transform fault motion is same as strike-slip fault Outside two spreading centers, plates are moving at same rate so there is no offset – fracture zone Figure 3.15 26 13  Transform Boundary – “square cut” Observe how these two “plates” are moving past each other D Plates move horizontally Sp IVE re RG past one another ad EN in T g ce bou nt nd 2 er a s r ie s 1 27 Quick Review When plates come together, you get a: A. Normal Fault B. Thrust fault C. Reverse fault D. B or C (they are the same!) 28 14  Next Time n Seismology 29 15