earthquakes week 1

seismic waves

P waves-result in back-and-forth motion of the ground in the direction of propagation of the wave. faster than s or surface (p for primary)

S waves-the motion of the ground that is perpendicular to the direction of the wave. faster than surface waves (s for secondary)

surface waves- when there is a boundary in the earth, such as the earth's surface, two kinds of surface waves

-reyliegh waves

-love waves

reyleigh waves- results in a rolling motion of the ground's surface, very much reminiscent of a water wave on a beach

love waves- back and forth ground motion perpendicular to the direction of propagation of the wave, like S waves, but they are strictly oriented parallel to the earth's surface

Surface waves are slower than P and S waves. When we measure the time for seismic waves to travel a certain distance, we create a travel time curve.

• If the Earth was flat and wave speeds constant, the travel time curve for P waves would be a straight line, meaning the farther from the source, the longer the waves take to arrive.

• The slope of the P wave curve helps measure its speed. The S wave speed can also be measured, but is slower than P waves, which is why S waves arrive later.

• The time delay between P and S waves increases with distance from the source. This delay helps locate earthquakes.

• In reality, travel time curves are complex and curved instead of straight due to waves traveling through the Earth's interior and varying speeds with depth.

• P and S waves can reflect off different layers in the Earth and some may not travel through certain regions.

• P waves can move through all Earth regions, whereas S waves cannot go through the innermost parts, which helps scientists understand what those areas are made of.

• By analyzing the slopes of travel time curves, scientists can trace the paths of seismic waves and map the Earth's interior, revealing shells with different seismic properties.

Seismic Wave Observations and Earth's Layers

• The Earth consists of a series of spherical layered shells.

• Layers are distinguishable by seismic wave speed and reflection off boundaries.

• Primary Layers from Top to Bottom:

  • Crust: The uppermost layer, can be oceanic or continental.

  • Mantle: The layer beneath the crust, making up bulk of the Earth.

  • Core: The innermost layer.

• Mohorovičić Discontinuity: Boundary between the crust and mantle, recognized by Andrija Mohorovičić in 1915 through seismic P waves from earthquakes in Eastern Europe.

• Boundary between the mantle and core measured by Beno Gutenberg in 1913 using reflected seismic waves.

Composition and Properties of Layers

• Crust and Mantle: Composed mainly of silicate minerals, structure influenced by depth pressure.

• Core Composition: Predominantly metallic, mainly iron and nickel.

• Core consists of:

  • Outer Core: Behaves like a fluid.

  • Inner Core: Believed to be solid, recognized by Inge Lehmann.

• P and S Waves: Generally, their velocities increase with depth, but:

  • At core-mantle boundary, P wave velocity drops ~50%, and S wave velocity drops to 0 (no shear waves can traverse).

Earth’s Magnetic Field

• Fluid Motion in Outer Core: Source of Earth's magnetic field.

• Magnetic field polarity varies with time, aiding in age estimation of magnetic rocks.

Advancements in Understanding Earth's Interior

• New techniques and data from seismic travel time observations allow mapping of details in the Earth's interior.

• Seismic Tomographic Technique: Reveals complex patterns, such as fast material slabs dipping through outer layers, enhancing understanding of earthquakes and volcanic activity.

Earth's Interior and Plate Tectonics

• The earth's interior is dynamic; characterized by earthquakes, volcanoes, mountains, and ocean basins.

• Theory of Plate Tectonics: Developed from the study of natural hazards and the shapes of continents.

• Continental Drift: The idea that continents were once joined (e.g., South America and Africa) and have drifted apart over time.

  • Supported by:

    • Matching coastlines

    • Similar geology and fossils across continents

• Key Proponents:

  • Alfred Wegener (German explorer/climatologist)

  • Arthur Holmes (British geologist)

• Early objections included doubts about the strength of continental crust to push through oceanic crust like an icebreaker.

Global Seismicity and Volcanism

• New geophysical observations in the mid-20th century helped evolve the theory of continental motion, overcoming previous objections.

• Earthquakes are concentrated along

  • Well-defined globally interconnected belts, indicating active tectonic processes.

• Volcanic activity also aligns with earthquake patterns, largely in similar bands circling the earth.

• These belts are not confined to continental margins but also transect oceans, indicating:

  • A mosaic of tectonic plates.

Tectonics

• Definition: Processes that shape the solid earth on a large scale.

• Tectonic plates are defined by regions of earthquake and volcanic activity, indicating their dynamic nature and behavior.

Earthquakes, Volcanoes, and Plate Boundaries

• Earthquakes and volcanoes at plate boundaries provide clues about the origin and fate of tectonic plates.

• The Atlantic Ocean features a band of shallow earthquakes and subring volcanism along the Mid-Atlantic Ridge.

• Geophysical measurements reveal that the ocean floor has magnetically defined stripes, which are parallel to mid-ocean ridges.

Key Discoveries in Geophysics

• Fred Vine and Drum Matthews discovered that these stripes are caused by changes in the polarity of the Earth's magnetic field and how lava is magnetized at mid-ocean ridges.

• This finding provides a method to date oceanic crust.

• Harry Hess proposed that magma from the mantle forms new ocean crust that spreads away from the ridge, confirming sea-floor spreading as a means of oceanic crust formation.

Sea-Floor Spreading

• Magma solidifies to form new ocean crust, pushing aside older material.

• Basalt, the rock formed during this process, becomes magnetized upon cooling below the Curie temperature (~500°C).

Lithosphere vs. Crust

• Lithosphere: Consists of crust and some underlying upper mantle, mechanically defined.

• Crust: Defined by its composition, contrasting with the mantle.

• The lithosphere floats on the asthenosphere, which is weaker and capable of ductile flow.

Plate Tectonics Dynamics

• As new oceanic crust forms at divergent plate boundaries, it pushes older crust away.

• This process overcomes the objection that continents cannot push through oceanic crust; instead, they are rafted away with the lithospheric plate.