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Earth Science
Table of Contents
Unit 8 Overview: Natural Hazards and Earth's Processes
8.1 Earthquakes and Seismic Waves
8.2 Volcanoes and Volcanic Eruptions
8.3 Tsunamis and Coastal Hazards
8.4 Landslides and Mass Wasting
8.5 Floods and Droughts
8.6 Tornadoes and Hurricanes

Earthquakes shake up our world, literally! These powerful events release pent-up energy in Earth's crust, causing seismic waves to ripple through the ground. Understanding how they work is key to staying safe and prepared.

From the deep rumble of P-waves to the destructive force of surface waves, earthquakes pack a punch. We'll explore their causes, how they're measured, and their impacts on our cities and lives. Get ready to dive into the shaky science of quakes!

Causes of Earthquakes

Elastic Strain Energy and Fault Rupture

  • Earthquakes caused by sudden release of stored elastic strain energy in Earth's lithosphere resulting in seismic waves
  • Elastic strain energy builds up from stress of tectonic forces acting on rock
  • Released when stress exceeds rock's strength causing rock to break or slip along a fault
  • Elastic rebound theory: crust suddenly snaps back to original unstressed shape when strain exceeds rock strength
    • Releases energy in form of seismic waves causing an earthquake

Earthquake Focus and Epicenter

  • Focus (hypocenter): point within earth where seismic waves originate during an earthquake
  • Epicenter: point on Earth's surface directly above focus
  • Foci usually concentrated in crust and upper mantle particularly along plate boundaries
  • Majority of foci are shallow originating within a few tens of kilometers of surface
    • Deep focus earthquakes can occur up to depths of about 700 km but are more rare

Seismic Wave Types

Body Waves

  • Can travel through earth's inner layers
  • Two types: P-waves and S-waves
    • P-waves (primary waves): fastest seismic waves and first to arrive
      • Travel through solids, liquids, and gases and can pass through earth's core
      • Push and pull ground in direction wave is traveling
    • S-waves (secondary waves): second waves to arrive
      • Only travel through solids and cannot pass through earth's core
      • Move rock particles up and down or side-to-side perpendicular to direction wave is traveling

Surface Waves

  • Can only move along surface of planet like ripples on water
  • Two types: Love waves and Rayleigh waves
    • Love waves: fastest surface waves
      • Cause horizontal shearing of ground side-to-side perpendicular to direction of wave propagation
    • Rayleigh waves: slower than Love waves but tend to be larger and most destructive
      • Cause rock particles to move in elliptical motion with horizontal and vertical ground motion

Analyzing Seismograms

Seismology and Seismographs

  • Seismology: study of earthquakes and seismic waves
  • Seismologists use seismographs to record seismic waves and produce seismograms
  • Seismograms: zig-zag trace recordings of ground motion detected during an earthquake
    • Seismographs record motion of ground as seismic waves pass a certain point

Determining Earthquake Location and Magnitude

  • Time of arrival of different seismic waves (P-wave and S-wave) used to determine distance of seismograph from epicenter
    • Difference in arrival times between P and S waves used to determine distance
    • Longer time between P and S wave arrivals indicates earthquake occurred farther away
  • Amplitude of seismic waves indicates amount of ground motion and energy released
    • Larger amplitudes generally indicate more powerful earthquake
  • Seismologists calculate magnitude using amplitude information
    • Richter scale and moment magnitude scale used to quantify and compare seismic energy released by earthquakes

Earthquake Impacts

Damage to Buildings and Infrastructure

  • Ground shaking: primary cause of earthquake damage to buildings and infrastructure
    • Intensity depends on magnitude, distance from epicenter, and local geology
  • Liquefaction: strong shaking causes water-saturated sediments to temporarily lose strength and act as fluid
    • Can cause buildings to collapse and pipelines to rupture
  • Poorly constructed buildings, particularly those not built to seismic safety codes, most vulnerable to damage and collapse
    • Unreinforced masonry buildings particularly at risk

Secondary Hazards and Consequences

  • Landslides and avalanches: common secondary hazards triggered by ground shaking particularly in mountainous areas
  • Tsunamis: large seismic sea waves triggered by earthquakes occurring under ocean
    • Can cause extensive damage and loss of life in coastal areas
  • Fires: common result as gas lines may be damaged and electrical shorts can spark fires in damaged buildings
  • Large earthquakes can cause substantial damage and loss of life particularly in populated areas with vulnerable infrastructure

Hazard Mitigation Strategies

  • Adopting and enforcing seismic building codes
  • Retrofitting older buildings to improve seismic resistance
  • Preparing emergency response and recovery plans
  • Educating public about earthquake safety and preparedness measures (securing heavy objects, identifying safe spots)

Key Terms to Review (20)

Inge Lehmann: Inge Lehmann was a Danish seismologist known for her groundbreaking work in understanding the Earth's interior structure, particularly the discovery of the Earth's inner core. Her research, based on the analysis of seismic waves generated by earthquakes, revealed that the Earth's core is not a uniform mass but consists of a solid inner core and a liquid outer core, providing crucial insights into the dynamics of the planet.
Building codes: Building codes are sets of regulations that establish the minimum standards for construction and design of buildings, ensuring safety, health, and welfare for occupants. These codes address various factors like structural integrity, fire safety, electrical systems, plumbing, and accessibility. They are crucial for guiding builders and architects to create resilient structures that can withstand challenges such as natural disasters, including earthquakes.
Charles F. Richter: Charles F. Richter was an American seismologist best known for developing the Richter scale, a logarithmic scale used to measure the magnitude of earthquakes. His work significantly advanced the field of seismology by providing a standardized method for quantifying the size of seismic events, making it easier to compare different earthquakes and understand their impact on society.
Earthquake drill: An earthquake drill is a structured practice exercise that prepares individuals and communities for the occurrence of an earthquake by teaching them proper safety protocols and responses. These drills aim to familiarize participants with the actions they should take during an actual earthquake, such as 'Drop, Cover, and Hold On', which can significantly reduce injury and panic in real situations. Regularly conducting these drills ensures that everyone knows what to do and where to go, improving overall preparedness.
Seismograph: A seismograph is an instrument that measures and records the vibrations caused by seismic waves as they travel through the Earth. This device plays a crucial role in detecting earthquakes, helping scientists understand the intensity and location of seismic activity. By analyzing the data collected from seismographs, researchers can gain insights into the Earth's internal processes and improve earthquake preparedness and response efforts.
Epicenter: The epicenter is the point on the Earth's surface that is directly above the focus of an earthquake, where seismic waves originate. It serves as a critical reference point in seismology for determining the location and intensity of an earthquake, as the strength of shaking typically decreases with distance from the epicenter.
Aftershock: An aftershock is a smaller earthquake that follows the main shock of a larger seismic event. Aftershocks occur as the crust adjusts to the changes in stress caused by the initial earthquake, making them an important aspect of the seismic cycle. They can vary in magnitude and frequency, often decreasing in intensity over time, but may still pose risks to structures and communities already affected by the main quake.
Seismology: Seismology is the scientific study of earthquakes and the propagation of seismic waves through the Earth. This field involves understanding how seismic waves are generated by tectonic activities and how they travel through various layers of the Earth's structure. Seismology is crucial for assessing earthquake risk, understanding Earth's internal composition, and developing early warning systems to mitigate damage from seismic events.
Foreshock: A foreshock is a smaller earthquake that occurs in the same area as a larger earthquake, known as the mainshock, and often happens hours to days before it. These preliminary tremors can indicate an impending larger seismic event, although not every earthquake has foreshocks. Understanding foreshocks helps scientists assess potential earthquake risks and improves the ability to predict seismic activity.
Mainshock: The mainshock is the largest shock in a sequence of seismic events, typically associated with the initial rupture of a fault line during an earthquake. It serves as the primary event that triggers subsequent aftershocks, which are smaller tremors that occur in the same general area as the mainshock. Understanding the mainshock is crucial for assessing earthquake intensity and potential damage.
Fault line: A fault line is a crack or fracture in the Earth's crust where tectonic plates meet, leading to potential seismic activity. These lines are critical in understanding how and where earthquakes can occur, as they mark the boundaries between different geological structures. Movement along fault lines can release built-up stress and energy, resulting in earthquakes and other geological phenomena.
Tsunami: A tsunami is a series of ocean waves generated by the displacement of a large volume of water, often caused by underwater earthquakes, volcanic eruptions, or landslides. Tsunamis can travel across entire ocean basins at high speeds, leading to devastating impacts when they reach coastal areas. The immense energy released during these events is a critical factor in understanding the behavior and consequences of tsunamis.
Tectonic plate: A tectonic plate is a large, rigid slab of solid rock that makes up the Earth's lithosphere and moves over the semi-fluid asthenosphere beneath it. These plates interact at their boundaries, leading to various geological phenomena such as earthquakes, volcanic activity, and mountain building. The movement of tectonic plates shapes the Earth's surface and is responsible for many natural events.
Moment magnitude scale: The moment magnitude scale is a logarithmic scale used to measure the total energy released by an earthquake, providing a more accurate assessment of earthquake size than previous scales. This scale considers factors like the area of the fault that slipped and the amount of slip, allowing for a better understanding of the earthquake's potential impact on structures and communities. It's crucial for both seismologists and disaster management teams to assess seismic hazards effectively.
Richter Scale: The Richter Scale is a logarithmic scale used to measure the magnitude of earthquakes, quantifying the amount of energy released during an earthquake. Developed in 1935 by Charles F. Richter, this scale provides a way to compare the sizes of different earthquakes, with each whole number increase on the scale representing a tenfold increase in measured amplitude and approximately 31.6 times more energy release. This scale is essential for understanding seismic activity and its potential impact.
P-waves: P-waves, or primary waves, are the fastest type of seismic waves produced during an earthquake. They travel through solids, liquids, and gases by compressing and expanding the material in the same direction as the wave travels. Their ability to move through different states of matter makes them crucial for understanding the Earth's internal structure and the nature of seismic activity.
S-waves: S-waves, or secondary waves, are a type of seismic wave that move through the Earth during an earthquake. They are shear waves that only travel through solids, making them critical in understanding the Earth's internal structure and the composition of its layers, as they provide insights into the behavior of materials beneath the surface.
Surface Waves: Surface waves are seismic waves that travel along the Earth's surface, causing the ground to move in an up-and-down or side-to-side motion. These waves are responsible for most of the damage during an earthquake due to their larger amplitude and slower speed compared to other seismic waves, such as primary and secondary waves. They occur after the initial shock of an earthquake, making them particularly dangerous as they can create significant ground shaking and displacement.
Subduction zone: A subduction zone is a geological feature that occurs where one tectonic plate moves under another and is forced into the mantle. This process leads to the formation of deep ocean trenches, volcanic arcs, and is a significant driver of seismic activity. The intense pressure and friction at these zones can result in powerful earthquakes and the emergence of volcanoes, making them crucial in understanding the dynamics of Earth's lithosphere.
Fault: A fault is a fracture or zone of fractures in rock along which there has been displacement of the sides relative to each other. Faults are crucial in understanding the rock cycle and are often associated with the movement of tectonic plates, leading to various geological features and phenomena. They can significantly influence the types of rocks formed, as well as play a vital role in the generation of earthquakes and seismic activity.