🌋natural and human disasters review
2.1 Earthquakes: causes, measurement, and effects
Last Updated on August 14, 2024
Earthquakes shake up our world, literally. They're caused by sudden energy releases in Earth's crust, usually along fault lines where tectonic plates meet. Understanding their causes, measurement, and effects is crucial for predicting and mitigating their impact.
Measuring earthquakes involves magnitude scales like Richter and moment magnitude, plus intensity scales like Modified Mercalli. Their impacts range from ground shaking and liquefaction to landslides and tsunamis. Damage depends on factors like magnitude, location, and building quality.
Earthquakes and Plate Tectonics
Causes of Earthquakes
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- Earthquakes are caused by the sudden release of stored elastic strain energy in the Earth's crust, primarily along fault lines where tectonic plates meet and interact
- The elastic rebound theory states that as tectonic plates move, the rocks along the fault lines experience increasing stress and strain until they reach a point of sudden rupture, releasing the accumulated energy as seismic waves
- Intraplate earthquakes can also occur within the interior of tectonic plates, often due to the reactivation of ancient faults or the formation of new faults in response to regional stresses
Relationship Between Earthquakes and Plate Boundaries
- The majority of earthquakes occur along plate boundaries, with the type of boundary influencing the frequency and magnitude of earthquakes
- Divergent boundaries (mid-ocean ridges) experience shallow, low-magnitude earthquakes due to the pulling apart of plates
- Convergent boundaries (subduction zones) experience frequent, high-magnitude earthquakes due to the collision and subduction of plates
- Transform boundaries (San Andreas Fault) experience shallow, moderate to high-magnitude earthquakes due to the sliding of plates past each other
Measuring Earthquake Magnitude and Intensity
Magnitude Scales
- Earthquake magnitude is a measure of the energy released at the source of the earthquake
- The Richter scale, developed by Charles Richter in 1935, is a logarithmic scale that measures the magnitude of earthquakes based on the amplitude of the largest seismic wave recorded by a seismograph
- Each whole number increase on the Richter scale represents a tenfold increase in the amplitude of the seismic waves and a 32-fold increase in the energy released
- The moment magnitude scale (Mw), introduced in 1979, is now more commonly used than the Richter scale
- It measures the total energy released by an earthquake based on the seismic moment, which is the product of the rock rigidity, the area of the fault rupture, and the average amount of slip along the fault
Intensity Scales and Seismographs
- Intensity is a measure of the strength of shaking and damage experienced at a particular location
- The Modified Mercalli Intensity (MMI) scale is used to describe the intensity of an earthquake at a specific location based on observed effects on people, structures, and the natural environment
- The scale ranges from I (not felt) to XII (total destruction)
- Seismographs are instruments used to detect and record seismic waves generated by earthquakes
- They consist of a mass suspended by a spring or a pendulum, which moves relative to the ground motion, and a recording device that tracks the motion of the mass over time
Earthquake Impacts on the Environment
Ground Shaking and Liquefaction
- Ground shaking is the primary cause of damage during an earthquake, as the seismic waves cause structures to vibrate and potentially collapse
- The intensity of ground shaking depends on factors such as the magnitude of the earthquake, the distance from the epicenter, and the local geology
- Liquefaction occurs when water-saturated, loose, and unconsolidated sediments (sand and silt) temporarily lose strength and behave like a liquid due to the intense shaking during an earthquake
- This can cause buildings to sink, tilt, or collapse, and can damage underground utilities and infrastructure
Landslides and Other Effects
- Earthquake-induced landslides can occur in hilly or mountainous regions, where the shaking can destabilize slopes and trigger the downward movement of soil, rock, and debris
- Landslides can cause damage to structures, block roads and waterways, and pose risks to public safety
- Other effects of earthquakes include:
- Ground rupture along fault lines, which can damage buildings and infrastructure that cross the fault
- Tsunamis, which can be generated by large underwater earthquakes and cause extensive damage and loss of life in coastal areas
- Fires, which can be triggered by ruptured gas lines or electrical short circuits
Earthquake Damage and Casualties
Factors Influencing Damage and Casualties
- The magnitude and intensity of the earthquake are primary factors in determining the level of damage and casualties, with higher magnitude earthquakes generally causing more widespread and severe impacts
- The proximity of populated areas to the epicenter of the earthquake is a critical factor, as the intensity of shaking and the potential for damage decrease with distance from the source
- The local geology and soil conditions can amplify or attenuate seismic waves, influencing the intensity of ground shaking
- Soft, loose soils tend to amplify seismic waves and increase the risk of liquefaction, while solid bedrock can reduce the intensity of shaking
Building Construction and Preparedness
- The quality and seismic resistance of building construction play a significant role in the level of damage and casualties
- Buildings designed and constructed to withstand earthquakes, using techniques such as base isolation and reinforced concrete, are more likely to survive with minimal damage compared to older, unreinforced structures
- The level of earthquake preparedness and risk reduction measures in a region, such as building codes, land-use planning, and public education, can influence the resilience of communities and their ability to cope with and recover from earthquakes
- Socioeconomic factors, such as population density, poverty, and access to resources and emergency services, can affect the vulnerability of communities to earthquake impacts and the ability to respond effectively to disasters
Key Terms to Review (18)
Disaster relief: Disaster relief refers to the organized response and assistance provided to individuals and communities affected by natural disasters, aiming to meet their immediate needs and facilitate recovery. This process involves a range of services including medical aid, food distribution, shelter provision, and psychological support, which are critical in the aftermath of events like earthquakes and tsunamis. Effective disaster relief is essential for minimizing the impacts of disasters on society and the environment and can significantly influence patterns of displacement and migration as affected populations seek safety and stability.
Emergency Management: Emergency management is the coordinated effort to plan for, respond to, recover from, and mitigate the effects of disasters and emergencies. This involves assessing risks, preparing response strategies, and ensuring communities are equipped to handle incidents like natural disasters or human-made crises effectively.
Early warning systems: Early warning systems are organized methods that detect potential disasters and provide timely alerts to help communities prepare and respond effectively. These systems integrate data collection, monitoring technologies, and communication networks to anticipate hazards such as earthquakes, floods, and extreme weather conditions, ultimately aiming to reduce the impact of these events on human lives and infrastructure.
Seismology: Seismology is the scientific study of earthquakes and the propagation of seismic waves through the Earth. This field focuses on understanding the causes of earthquakes, how they are measured, and their various effects on the environment and human structures. By studying seismic activity, seismologists can gain insights into Earth's internal processes and improve safety measures in earthquake-prone areas.
San Francisco Earthquake of 1906: The San Francisco Earthquake of 1906 was a massive seismic event that struck the San Francisco Bay Area on April 18, resulting in widespread destruction and loss of life. This earthquake, measuring approximately 7.9 on the moment magnitude scale, is notable for its causes, which include the movement of the San Andreas Fault, and its effects, which included not only ground shaking but also devastating fires that ravaged the city.
Haiti Earthquake of 2010: The Haiti Earthquake of 2010 was a devastating magnitude 7.0 earthquake that struck near Port-au-Prince, Haiti, on January 12, 2010. This seismic event caused widespread destruction, leading to significant loss of life and displacement of populations, highlighting the vulnerabilities in earthquake-prone regions, particularly in developing countries. Its aftermath demonstrated the critical importance of understanding earthquake causes, measuring seismic activity, and assessing the effects on human populations and infrastructure.
Economic disruption: Economic disruption refers to the significant disturbance or interruption of economic activity, which can lead to a decrease in production, trade, and overall economic stability. In the context of natural disasters such as earthquakes, this disruption can result in widespread damage to infrastructure, loss of jobs, and a decline in local and regional economies. Understanding how earthquakes cause economic disruption helps highlight the interconnections between physical damage and economic consequences.
Infrastructure damage: Infrastructure damage refers to the harm or destruction of essential physical structures and facilities that support a society's economy and quality of life, including transportation systems, utilities, communication networks, and public services. The extent of this damage can significantly influence recovery efforts, economic stability, and public safety in the aftermath of disasters.
Evacuation Plans: Evacuation plans are detailed strategies designed to guide individuals and communities in safely exiting an area that may be threatened by natural or man-made disasters. These plans are crucial for ensuring public safety, coordinating rescue efforts, and minimizing chaos during emergencies such as earthquakes, tsunamis, hurricanes, and nuclear incidents.
Liquefaction: Liquefaction is a geological phenomenon that occurs when saturated soil loses its strength and stiffness in response to an applied stress, often due to seismic activity such as an earthquake. This process transforms solid ground into a more liquid state, leading to severe ground failure, which can cause buildings to tilt, sink, or even collapse. Understanding liquefaction is crucial for assessing the risks associated with earthquakes and designing structures that can withstand potential ground movement.
Seismic waves: Seismic waves are energy waves generated by the sudden release of energy in the Earth, typically due to tectonic activity such as earthquakes. These waves travel through the Earth and are responsible for transmitting the energy released during an earthquake, which can result in ground shaking and damage. Understanding seismic waves is crucial for measuring earthquakes and assessing their effects on structures and landscapes.
Moment Magnitude Scale: The Moment Magnitude Scale (Mw) is a logarithmic scale used to measure the energy released by an earthquake, providing a more accurate representation of an earthquake's size compared to previous scales. This scale considers the seismic moment, which incorporates factors such as fault area, average slip, and the rigidity of the rocks involved in the earthquake. By measuring these elements, the Moment Magnitude Scale allows for a better understanding of an earthquake's potential impact on the affected region.
Fault lines: Fault lines are fractures or zones of weakness in the Earth's crust where tectonic plates meet, allowing for the movement of these plates during geological events such as earthquakes. These lines are crucial in understanding the distribution and intensity of earthquakes, as they represent areas where stress accumulates and is eventually released, leading to seismic activity.
Tectonic plate movement: Tectonic plate movement refers to the continuous shifting of the Earth's lithospheric plates over the semi-fluid asthenosphere beneath them. This movement is driven by forces such as mantle convection, slab pull, and ridge push, leading to significant geological events and phenomena like earthquakes and tsunamis. Understanding tectonic plate movement is crucial for analyzing how these events occur, how they can be measured, and their various effects on coastal regions and ecosystems.
Transform fault earthquake: A transform fault earthquake occurs along a transform boundary where two tectonic plates slide past each other horizontally. This movement can lead to significant stress accumulation along faults, which, when released, results in an earthquake. Understanding transform fault earthquakes is crucial for grasping how they affect geological structures and human activities in areas prone to such seismic events.
Richter Scale: The Richter Scale is a numerical scale used to measure the magnitude of earthquakes, quantifying the amount of energy released at the source of the quake. Developed in 1935 by Charles F. Richter, this scale provides a way to compare the size of different earthquakes, helping scientists and engineers assess potential impacts and risks associated with seismic activity.
Subduction Zone Earthquake: A subduction zone earthquake occurs when one tectonic plate is forced beneath another, creating intense stress that can lead to sudden seismic activity. These earthquakes often occur at convergent plate boundaries where an oceanic plate subducts under a continental plate or another oceanic plate, resulting in powerful tremors and significant geological activity. They are a primary cause of both local and distant tsunamis, making them particularly dangerous.
Disaster Resilience: Disaster resilience refers to the ability of individuals, communities, and systems to anticipate, prepare for, respond to, and recover from disasters effectively. This concept encompasses not only the immediate response to a disaster but also the long-term recovery and adaptation processes that reduce vulnerability to future events. By understanding historical patterns of disasters and integrating knowledge about hazards, communities can enhance their resilience and improve preparedness and response strategies.