Seismic Waves and Earthquakes

Seismic Waves

• Seismic waves are energy radiating from the focus of a tectonic earthquake, volcanic eruption, landslide, or explosion
• Seismometers detect and seismographs record seismic waves
• Earthquakes are vibrations of the Earth caused by rapid energy release, often from slippage along a fault

Seismology and Seismologists

• Seismology studies earthquakes and seismic waves that move through the earth
• Seismologists study earthquakes and seismic waves

Types of Seismic Waves

• Body waves travel through Earth's inner layers and are studied to understand Earth's interior
• P-waves travel through solids, liquids, and gases as a pulse of energy that compresses and expands the ground
• S-waves travel only through solids and move as shear or transverse waves
• Surface waves travel through Earth's surface and arrive after P and S waves
• Love waves cause the most structural damage during an earthquake, moving the ground side-to-side or horizontally, and are named after Augustus Edward Hough Love
• Rayleigh waves cause the most shaking, move the ground up and down or in a circular motion, and are named after John William Strutt, Lord Rayleigh

Wave Speed

• P-waves arrive first, followed by S-waves, Love waves, and Rayleigh waves

Sources of Earth's Interior Information

• Indirect sources include:
  • Temperature and pressure
  • Meteors
  • Gravity anomaly
  • Seismic waves
  • Gravitation
  • Magnetic Sources

Body Waves and Internal Structure

• P-waves determine Earth's internal structure
• The refraction of seismic waves provides information about Earth
• Discontinuity marks the boundary between two layers of different densities

Shadow Zone

• Shadow zones lack seismic waves from earthquakes because P-waves refract and S-waves are absorbed by Earth's liquid outer core
• S-waves cannot travel through the liquid outer core
• P-waves refract, and S-waves are absorbed by the Earth's outer core

Earth's Discontinuities

• Andrija Mohorovicic (1909) found seismic wave velocity changes at around 50 km below the surface and that there is a density difference between the crust and mantle
  • This boundary is the Mohorovicic Discontinuity (Moho), separating the crust and mantle
• Beno Gutenberg (1914) explained the shadow zone by a core of material different from that of the mantle, causing P-wave bending
  • This boundary is the Gutenberg Discontinuity, separating the mantle and core
• Inge Lehmann (1966) predicted the solid inner core within the liquid outer core
  • This boundary is the Lehmann Discontinuity, separating the outer core and inner core
• Conrad Discontinuity separates the crust and Repetti Discontinuity separates the mantle

Earth's Interior Structure

• Crust: The thinnest, outermost solid part of Earth, 8-40 km thick, 1% of Earth's volume, 0.5% of Earth's mass
  • Composed of continental and oceanic crust where oceanic crust is mostly basalt, >200 million years old, while continental crust is mostly granite, ~4 billion years old
  • Major elements: Silicon and Aluminum (SIAL)
• Mantle: The interior portion beyond the crust, ~2900 km thick, 84% of Earth's volume, 67% of Earth's mass
  • Major elements: Silicon and Magnesium (SIMA)
• Core: Innermost layer, two sub-layers of inner and outer core, 15% of Earth's volume, 32.5% of Earth's mass
  • Outer Core is liquid, location of gravitational force of earth
  • Inner Core is solid under extreme pressure
  • Major elements: Nickel and Iron (NIFE)
• Asthenosphere (Mantle): Layer over which continental plates move; highly viscous
• Lithosphere (Crust): Uppermost solid part of mantle and entire crust
• Mesosphere (Mantle): Portion of mantle just below asthenosphere and above core
• Barysphere (Core): Core of Earth or entire interior
• Temperature, pressure, and density increase from the surface to the Earth's center

Continental Drift Theory

• Abraham Ortelius (1500): A Dutch cartographer who noticed the fit of continents across the Atlantic and proposed separation due to earthquakes and floods
  • In 1596, he stated that the Earth's continents were not always in their current positions
• Antonio Snider Pellegrini (1858): Stated that the continents were once connected during the Pennsylvanian period
  • Illustrated two maps of how Africa and South America may have fit together, later separated
• Alfred Wegener, father of plate tectonics, proposed the Continental Drift Theory
  • The continents that had once been joined as a single landmass which then broke apart and sent the continents adrift; Earth's continents had once been joined as a single landmass that broke apart and sent those drifting continents
  • The theory had first been rejected, because it could not explain why the continents were moving
  • Advances in seafloor mapping and Earth's magnetic field provided the necessary evidence to show how continents move and the source of the forces involved

Pangaea, Laurasia, and Gondwanaland

• Pangaea: Greek for "all Earth"
• Laurasia (North): Contained North America, Europe, and Asia
• Gondwanaland (South): Contained Africa, Antarctica, Australia, India, and South America

Evidence of Continental Drift

• Rock Formations: Continental coastlines fit together; rock layers in the Appalachian Mountains match layers in mountains of Greenland and Europe
• Climatic Evidence:
  • Glossopteris fossils in temperate climates indicate closer proximity to the equator
  • Coal beds found in Antarctica suggest a tropical climate at one time
  • Glacial deposits found in parts of Africa, India, Australia, and South America
• Fossils: Similar fossils of different species found on widely separated continents; example animals include:
  • Cynognathus: Mammal-like reptile found in South America and South Africa
  • Mesosaurus: Freshwater reptile found in South Africa and Eastern South America
  • Glossopteris: Found in Australia, Antarctica, India, South Africa, and South America. Its seed is known to be large and bulky and therefore could not have drifted or flown across the oceans to a separate continent.
  • Lystrosaurus fossils found in Antarctica, India, and South Africa, lacked the swimming capability to traverse any ocean

Seafloor Spreading

• Harry Hess proposed the seafloor spreading process in 1960 where new oceanic crust forms at ocean ridges, moves away, and is subducted and recycled at deep-sea trenches
• Isochron: Imaginary line on a map shows points of the same age
• The seafloor's topography is rough and irregular with mountains and depressions
• Bathymetry: Study of the shape of the seafloor
• Sounding Line Technique measures depth using a plumb bob
• Sound Navigation and Ranging (SONAR) is used to measure the depth and maps the topography of the ocean floor using sound waves
• Magnetometer detects small changes in magnetic fields

Features of the Seafloor

• Continental Margin: The area where continent edges meet the ocean
  • Continental shelf: The shallowest part of a continental margin extending from the shore with an average depth of 130 m
  • Continental slope: True edge of a continent; a gently sloping accumulation of deposits at the base of the continental slope from turbidity currents
  • Continental rise consists mainly of silts, mud, and sand, deposited by turbidity flows
• Deep-Ocean Basins: Occupy 70% of Earth's surface
• Abyssal Plains: Smooth, flat areas covered with fine sediments
• Deep-sea trenches: Narrow, elongated depressions
• Mid-ocean ridges: The longest continuous mountain range
  • Crests of ridges have rifts
  • Hydrothermal vents: Holes in the seafloor where fluid heated erupts; commonly located along the bottom of the rifts
  • Fracture Zones: Broken into shorter stepped sections
  • Site of volcanic eruptions and earthquake activity
• Seamounts: Submerged volcanoes >1 km high
• Guyots: Large, extinct volcanoes with flat, submerged tops

Ocean Rock Discoveries

• Ocean Rocks: The age of the oceanic crust consistently increases with distance from a ridge so the oldest seafloor is about 340 million years old
• Ocean floor sediments: Thickness increases with distance from an ocean ridge, and sediments are typically a few hundred meters thick
• Magnetic Reversal: Where the study of the history of earth's magnetic field is called Paleomagnetism; Earth's magnetic field is generated by the flow of molten iron in the outer core
  • A magnetic reversal happens when the flow in the outer core changes and Earth's magnetic field changes direction
  • Rocks of the ocean floor lie in magnetized stripes, holding records of reversals in Earth's magnetic field

Tectonic Plates

• Tectonic plates are huge pieces of crust and rigid upper mantle
• There are 7 major tectonic plates worldwide
• Plate tectonics theory describes how tectonic plates move and shape Earth's surface
• Plates move in different directions, at different rates, and interact at their boundaries

Types of Plate Boundaries

• Convergent Boundaries lead to the destructions of plates, causing tectonic plates to move towards each other
  • Denser plates eventually descend below less-dense ones in subduction
• Oceanic-Continental:
  • Oceanic plate descends under the continental plate because it's denser, called subduction
  • Produces trenches, a mountain range, and many volcanoes (e.g., Andes)
• Oceanic-Oceanic:
  • Both plates descend by age and temperature
  • Produces deep-sea trenches and volcanic island arcs (e.g., Philippines)
• Continental-Continental:
  • Plates collide upward
  • A vast mountain range is formed
• Divergent Boundaries form due to constructive forces, where tectonic plates move away from each other
  • Are mostly found along the seafloor and specifically the mid-ocean ridge
  • Seafloor spreading begins as tectonic plates move away, and a new ocean crust beings to form
  • Characterized by high heat flow, volcanism, and earthquakes
  • Example of this: East African Rift
• Transform Boundaries are considered conservative boundaries, where tectonic plates slide horizontally past the other
  • Characterized by shallow Earthquakes
  • Results in deformation
  • Example of a transform boundary is the San Andreas fault where the Pacific and North American Plates slide horizontally past each other, causing earthquakes

Crust

• New Crust comes from Divergent Boundaries and destroying crust is caused through Convergent Boundaries, while Crust transformation is caused through transform boundaries

Plate Motion Mechanism

• Convection is the transfer of thermal energy by the movement of heated material
  • Drives plate movement, causing continental drift and convection in the mantle
  • Converging currents drive plates into each other, and diverging currents pull them apart
• Ridge Push: Uplifted ridges push oceanic plates towards subduction zone trenches
• Slab Pull: Subducting plates pull the trailing slab

Earthquakes and Volcanoes

• Convergent plate boundary volcanoes align into the Circum-Pacific Belt ("Pacific Ring of Fire") and the Mediterranean Belt
• Convergent Volcanism: Characterized by explosive eruptions with volcanoes located from oceanic-continental subduction
• Divergent Volcanism: Tends to be non-explosive, with the effusion of large amounts of lava
• Hot spot volcanism: Occurs at hot spots, which are Earth's unusually hot regions of Earth’s mantle where high-temperature plumes of magma rise to the surface
  • Examples include the Yellowstone Caldera and the Mauna Loa in Hawaii