Earthquakes and Earth's Interior

Earthquakes and Earth's Interior

What are Earthquakes?

  • Definition: Shaking or trembling caused by the sudden release of energy.

  • Associated Processes:

    • faulting or breaking of rocks.

    • Continuing adjustments in position after initial quake often result in aftershocks.

What is the Elastic Rebound Theory?

  • Description: This theory explains the process of energy storage and release in rocks.

    • Mechanism:

    • Rocks bend under stress until the rock's strength is exceeded.

    • A rupture occurs, causing the rocks to quickly rebound to their undeformed shape.

    • The energy that was stored due to accumulated pressure is released in waves that radiate outward from the fault as the rock snaps back to its original position.

What is Seismology?

  • Definition: The study of earthquakes.

  • Key Terms:

    • Focus (Hypocenter): The actual point within Earth where an earthquake's energy is released.

    • Epicenter: The point directly above the focus on the Earth's surface.

Seismological Equipment

Seismic Devices:

  • A seismograph records earthquake waves electronically into a seismogram

Where Do Earthquakes Occur and How Often?

  • Distribution Patterns:

    • Most = circum-Pacific belt, primarily from convergent margin activity. Ring of Fire

    • Then Mediterranean-Asian belt.

    • Then interiors of plates and at spreading ridge centers.

Earthquake Depths

Classification of Earthquake Foci:

  • Shallow Focus: <70km - oceanic ridges from divergent margin

  • Intermediate Focus: 70 km - 300 km.

  • Deep Focus: Over 300 km. Circum-pacific belt.

What are Seismic Waves?

  • Definition: These waves are the response of material to the arrival of energy fronts released during an earthquake rupture.

  • Types:

    1. Body Waves:

    • P Waves (Primary Waves):

      • Fastest seismic waves, traveling through solids, liquids, and gases.

    • Type of compressional wave where material movement aligns with wave movement.

      • S Waves (Secondary Waves):

        • Slower than P waves and can only travel through solids.

      • Shear waves that move material perpendicular to wave motion

    1. Surface Waves:

    • Generated when body waves arrive at the surface.

    • Travel just beneath or along the surface, causing significant damage.

    • Types:

      • Rayleigh Waves (R-Waves): Move like water waves; particles move in an elliptical path. Up and down

      • Love Waves (L-Waves): Faster than Rayleigh waves; particles move back and forth in a horizontal plane. Stay flat side to side

Locating Earthquake Epicenters

  • Seismic Wave Behavior:

    • Sequence: P waves arrive first, followed by S waves, then surface waves (L and R).

    • The difference in arrival times allows for the calculation of distance from the seismograph to the epicenter..

How is the Size and Strength of an Earthquake Measured?

  • Magnitude:

    • Measures total energy released; expressed on the Richter magnitude scale, a logarithmic scale.

  • Intensity:

    • subjective measure of damage and people's reactions.

  • Identified areas of equal intensity using isoseismal lines and expressed on the Modified Mercalli Intensity Scale (1-12)..

Earthquake Disasters

  • The deadliest earthquake on record happened in 1556 in Shaanxi province, China, with approximately 830,000 fatalities.

Destructive Effects of Earthquakes

  • Primary Hazards:

    • Ground Shaking & Ground Rupture: Responsible for most destruction. Increased amplitude and duration amplify damage in poorly consolidated materials.

    • Building Collapse, Fire, Tsunami, Ground Failure, and Landslides

Looking at Ground Shaking and Rupture

  • Consequences:

    • Ground shaking can lead to ruptured gas lines, collapsing structures, and injuries to individuals.

    • Ground rupture can cause horizontal, vertical, or oblique displacements.

    • Regional Deformation: Earth can undergo subsidence or uplift (e.g., 1964 Alaska).

    • Fires: Result from ruptured lines and access issues to firefighting resources.

    • Liquefaction: Occurs when saturated materials transform into a liquid state under shock (e.g., Alaska 1964).

    • Tsunamis: Seismic sea waves can increase in height from 1 m in open water to 20-40 m in coastal regions with disastrous effects.

Global Earthquake Statistics

  • Annual Statistics:

    • Approximately 1 million earthquakes occur yearly.

    • 200 earthquakes typically exceed magnitude 6.0.

    • Average of 20 earthquakes reach magnitude 7.0 or larger.

    • Each year, on average, there is one "great" earthquake with a magnitude of 8.0 or larger.

Canadian Earthquakes

  • Locational Causes:

    • West Coast: Active continental margin (subduction, fault movement) leads to frequent earthquakes.

    • East Coast: Passive continental margin; fault mechanisms are not fully understood and quakes are less common.

    • Prairies: Intraplate areas rarely experience earthquakes but have notable historical events (e.g., 1919).

Can Earthquakes be Predicted?

  • Prediction Approaches:

    • Short-Range Predictions: Aimed at predicting the location and magnitude of imminent earthquakes within a narrow timeframe (currently no reliable methods exist).

    • Long-Range Forecasts: Indicate probabilities of major earthquakes occurring over years based on cyclic behavior using historical seismic records.

Techniques for Earthquake Prediction

  • Precursor Indicators:

    • Includes elevation changes, groundwater fluctuations, shifts in magnetic fields, and anomalies in animal behavior.

Can Earthquakes be Controlled?

  • A speculative approach includes injecting fluids into seismic gaps to encourage smaller quakes and prevent larger ones.

    Seismic gap: area of earthquakes that is now inactive

Earth's Interior

  • Understanding Earth's Interior:

    • By 1860, significant data was accumulated regarding the mass, volume, and density of Earth, alongside understanding pressure and temperature variances increasing with depth.

    • The average geothermal gradient is about 25°C/km.

    • Direct observation of Earth’s interior is impossible, necessitating reliance on various forms of evidence, such as density variations, gravitational fields, and mineral composition.

How do We Know about Earth’s Interior?

  • Direct Evidence:

    1. Mining: Extensive studies from deep mining operations, such as the TauTona Mine in South Africa, which is the world's deepest mining operation.

    2. Drilling: Notable drilling expeditions, such as the Russian Kola Superdeep Borehole, which aimed to reach 15 km but only achieved 12.3 km due to technological and geological challenges.

    3. Volcanic Materials: Samples such as xenoliths from mantle rocks brought to the surface during volcanic eruptions.

Analyzing Seismic Waves and Earth’s Internal Structure

  • Seismic Indications:

  • Body waves (P-waves and S-waves) help determine Earth's internal structure through changes in velocity contingent on density and elasticity changes.

Earth’s Interior Structure

  • Composition and Layers:

    • The interior comprises the crust, mantle, and core. The nature of seismic waves through each layer aids in understanding the layering and material properties.

    • Example: Core findings indicate a liquid outer core and solid inner core based on seismic wave behaviors and velocities (P-waves show reduction and S-waves are non-existent in certain zones).

The Mantle

  • Characteristics:

    • The mantle is over 80% of Earth's volume, composed mainly of ultramafic rock, with density increasing with depth.

    • Discontinuities, such as the Moho, define the boundary between the crust and mantle.

Earth’s Internal Heat

  • Geothermal Gradient:

    • Heat increases with depth, averaging 25°C per km, impacted by volcanic activity.

    • Estimated temperatures at different depths, with the core reaching up to 6,500°C.