Earthquakes and Earth's Interior

Earthquakes and Earth's Interior

8.1 What Is an Earthquake?

  • Definition of Earthquake:

    • An earthquake is defined as the vibration of Earth, produced by the rapid release of energy.

  • Focus and Epicenter:

    • Focus: The point within the Earth where the earthquake starts.

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

  • Faults:

    • Faults are defined as fractures in the Earth where movement has occurred, which can lead to earthquakes.

8.1 What Is an Earthquake? (Continued)

Cause of Earthquakes

  • Elastic Rebound Hypothesis:

    • Most earthquakes originate from the rapid release of elastic energy stored in rocks subjected to great forces. Once the strength of the rock is exceeded, it breaks, leading to the vibrations associated with earthquakes.

Visual Representation of Elastic Rebound Hypothesis

  • Energy buildup occurs as rocks are deformed (similar to bending a limber stick).

  • Key Stages:

    • Original position

    • Buildup of energy

    • Rupture (slippage leading to earthquake)

    • Energy is released, causing vibrations.

Aftershocks and Foreshocks

  • Aftershock: A smaller earthquake that follows the main earthquake.

  • Foreshock: A small earthquake that often proceds a major earthquake.

8.2 Measuring Earthquakes

Earthquake Waves

  • Seismographs: Instruments that record earthquake waves.

  • Seismograms: Traces of amplified electronically recorded ground motion made by seismographs.

  • Surface Waves: Seismic waves that travel along the Earth's outer layer.

Types of Body Waves

  • P Waves (Primary Waves):

    • Definition: Push-pull waves that compress and expand in the direction of travel.

    • Characteristics:

    • Travel through solids, liquids, and gases.

    • Have the greatest velocity of all earthquake waves.

  • S Waves (Secondary Waves):

    • Definition: Seismic waves that travel along Earth's surface and shake particles at right angles to the direction of travel.

    • Characteristics:

    • Travel only through solids.

    • Slower than P waves.

Behavior of Seismic Waves

  • P Waves: Cause compression and expansion of materials; can buckle and fracture the ground.

  • S Waves: Cause ground to shake both up-and-down and sideways.

  • Surface Waves: Move the ground side-to-side or in an elliptical pattern, potentially damaging structures.

Locating an Earthquake

  • Epicenter Calculation:

    • Determined using the difference in arrival times between P and S wave recordings, which relate to distance.

    • Travel-Time Graphs from multiple seismographs can pinpoint the epicenter of an earthquake.

  • Earthquake Zones: Approximately 95% of major earthquakes occur in a few narrow zones.

8.2 Measuring Earthquakes (Continued)

Measuring Earthquake Size

  • Historical measurements classified earthquakes by two indicators: intensity and magnitude.

Richter Scale

  • Based on the amplitude of the largest seismic wave.

  • Each unit of Richter magnitude corresponds to an approximate 32-fold increase in energy release.

  • Does not adequately estimate the size of very large earthquakes.

Moment Magnitude

  • Derived from the displacement occurring along a fault zone.

  • Most widely used for measuring earthquakes; estimates the energy released during the event.

  • Effective for measuring very large earthquakes.

Earthquake Magnitudes

  • Magnitude Classification:

    • < 2.0: Generally not felt, but can be recorded (estimated > 600,000 annually).

    • 2.0 - 2.9: Potentially perceptible (estimated > 300,000).

    • 3.0 - 3.9: Rarely felt (estimated > 100,000).

    • 4.0 - 4.9: Strongly felt (estimated 13,500).

    • 5.0 - 5.9: Can cause damage (estimated 1,400).

    • 6.0 - 6.9: Destructive in populous areas (estimated 110).

    • 7.0 - 7.9: Major earthquakes inflicting serious damage (estimated 12).

    • 8.0 and above: Great earthquakes capable of destruction (0-1 annually).

Notable Earthquakes

  • Sample table of significant earthquakes and their impact:

    • 1886: Charleston, SC; 60 deaths; noted as the greatest historical earthquake in the eastern U.S.

    • 1906: San Francisco, CA; 1,500 deaths; magnitude 7.8; extensive damage due to fires.

    • 1923: Tokyo, Japan; 143,000 deaths; magnitude 7.9; severe destruction caused by fire.

    • 1960: Southern Chile; 5,700 deaths; magnitude 9.6; noted as possibly the largest ever recorded.

    • 1995: Kobe, Japan; 5,472 deaths; magnitude 6.9.

8.3 Destruction from Earthquakes

Seismic Vibrations

  • Damage from earthquake waves relies on several factors:

    • Intensity and duration of vibrations.

    • Foundation material where the structure is built.

    • Structural design and resilience.

Building Damage Factors

  • Unreinforced stone or brick buildings significantly increase safety risks.

  • The underlying material and structure design critically affect damage levels.

Liquefaction Effects

  • Occurs when saturated materials transform into fluid-like states.

  • Underground structures may surface due to liquefaction.

Tsunamis

  • Cause: Triggered when an earthquake causes a slab of the ocean floor to displace vertically along a fault. Underwater landslides may also initiate a tsunami, which is a Japanese term for "seismic sea wave."

Tsunami Movement and Speed

  • Tsunami speed varies depending on water depth:

    • 835 km/h at 5,500 meters depth

    • 340 km/h at 900 meters depth

    • 50 km/h at 20 meters depth

Tsunami Warning Systems

  • Reports of large earthquakes transmitted to Hawaii provide adequate evacuation time, except for areas closest to the epicenter.

Other Dangers Associated with Earthquakes

  • Landslides: Significant damage often stems from landslides and ground subsidence triggered by earthquake vibrations.

  • Fires: Historical events, such as the 1906 San Francisco earthquake, exemplify the destructive power of fires ignited by ruptured gas and electrical lines.

Predicting Earthquakes

  • Short-Range Predictions: Current methods have been unsuccessful for short-term earthquake predictions.

  • Long-Range Forecasts: Insufficient understanding regarding the occurrence of earthquakes prevents accurate long-term predictions.

  • Seismic Gaps: Areas along a fault that have experienced no seismic activity for extended periods are termed seismic gaps.

8.4 Earth's Layered Structure

Layers Defined by Composition

  • The Earth's interior is comprised of three primary zones based on chemical composition:

    • Crust: Thin, rocky outer layer.

    • Thickness: Roughly 7 km in oceanic regions; averages 8-40 km in continental regions; exceeds 70 km in mountainous areas.

    • Mantle: Extends below the crust to a depth of 2900 kilometers; upper mantle consists primarily of the igneous rock peridotite.

    • Core: Below the mantle; spherical with a radius of 3486 kilometers; primarily composed of an iron-nickel alloy with an average density near 11 g/cm³.

Layers Defined by Physical Properties

  • Lithosphere: Comprises the crust and uppermost mantle (approximately 100 km thick); characterized as cool, rigid, and solid.

  • Asthenosphere: Below the lithosphere; upper mantle to about 660 kilometers depth; soft and easily deformed.

  • Lower Mantle: From 660 to 2900 km, a more rigid layer where rocks are hot and capable of gradual flow.

  • Inner Core: Solid sphere with a radius of 1216 km.

  • Outer Core: Liquid layer approximately 2270 km thick; convective flows of metallic iron generate Earth's magnetic field.

Discovering Earth's Layers

  • Moho: Boundary signifying an abrupt increase in seismic wave velocity below a depth of 50 km, separating the crust from the mantle.

  • Shadow Zone: The absence of P waves from about 105 to 140 degrees around the globe represents a liquid core that differs from the overlying mantle.

Discovering Earth's Composition

  • Crust: Studies indicate that the continental crust predominantly consists of lighter, granitic rocks.

  • Mantle: Largely speculative composition; some surface lava originates from the mantle.

  • Core: Believed primarily to consist of dense iron and nickel, akin to metallic meteorites, while the surrounding mantle is akin to stony meteorites.