19.1 - Causes of Earthquakes
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Tectonic Movements
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Let's start by discussing tectonic movements. Earthquakes mainly occur due to the relative motion between tectonic plates, where energy builds up and is suddenly released. Can anyone tell me what types of boundaries exist where these movements happen?
Are there specific types of boundaries, like between plates that move apart or come together?
Exactly! There are three types: divergent, convergent, and transform boundaries. Let's break them down. Divergent boundaries occur where plates move apart, allowing magma to rise. What happens at convergent boundaries?
At convergent boundaries, the plates collide, and one goes under the other, right? Isn’t that subduction?
Correct! This process can produce significant earthquakes. Finally, transform faults, like the San Andreas Fault, allow plates to slide past one another. Now, why do you think understanding these boundaries is critical for earthquake engineering?
Because knowing where earthquakes can occur helps us build safer buildings in those areas?
Absolutely! Great insights! Remember, 'Tectonic tendencies create tremors.'
Volcanic Activity and Induced Seismicity
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Next, let's explore volcanic activity. Can someone explain how volcanic eruptions lead to earthquakes?
I think when a volcano erupts, it can shake the ground. Are these earthquakes different from tectonic ones?
Good question! Yes, while volcanic earthquakes are less frequent, they can be localized and very destructive. Now, let's touch on induced seismicity. What human activities can lead to earthquakes?
Things like reservoir-induced seismicity, where water from a big reservoir can cause pressure in the ground, right?
Exactly! Mining explosions and fracking also contribute. Understanding these induced factors helps us anticipate earthquakes related to human activities. Can anyone think of a consequence of these induced earthquakes?
They could damage buildings or infrastructure that wasn’t designed for earthquakes!
Correct! Always remember the mnemonic 'VIVI' for Volcanic Activity Induced by Humans!
Collapse Earthquakes
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Lastly, let's touch upon collapse earthquakes. What do we know about them?
I think they're caused by underground cave-ins or something, right? Like when a mine collapses?
Exactly! They are typically minor in magnitude, but they still can impact nearby structures. Why might we want to monitor these?
Because even minor earthquakes can cause injuries or damages to nearby areas?
Exactly! It's important to be aware of all types of earthquakes to better prepare and respond to them. Remember, 'Collapses can cause consequences!'
Introduction & Overview
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Quick Overview
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Earthquakes arise from various factors, predominantly from tectonic movements caused by the interactions among tectonic plates. Additional causes include volcanic activity, human-induced seismicity, and minor earthquakes from cave-ins or subsurface collapses. Understanding these causes is crucial for earthquake prediction and engineering.
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Tectonic Movements
Chapter 1 of 4
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Chapter Content
• Caused due to relative motion between tectonic plates.
• Most earthquakes are generated by the sudden release of energy accumulated due to plate movements.
• Boundaries: divergent, convergent, and transform faults.
Detailed Explanation
Tectonic movements are the primary cause of most earthquakes. The Earth's outer layer, known as the lithosphere, is divided into several large and small pieces called tectonic plates. These plates move relative to one another due to the heat from the Earth's interior. When these plates interact, they can either move apart (divergent boundaries), push together (convergent boundaries), or slide past each other (transform faults).
Over time, stress builds up along these plate boundaries as they move. When the stress exceeds the strength of the rocks composing the plates, it results in a sudden release of energy in the form of seismic waves, which we feel as an earthquake. This explains why areas near tectonic plate boundaries are particularly earthquake-prone.
Examples & Analogies
Imagine two cars moving toward each other and suddenly crashing into each other, creating a shockwave. Similarly, when tectonic plates collide or rub against each other, they build up tension until it releases, causing an earthquake. The boundaries between these plates can be thought of as the 'road' where these 'cars' interact.
Volcanic Activity
Chapter 2 of 4
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• Earthquakes that occur due to volcanic eruptions.
• Less frequent but can be highly destructive locally.
Detailed Explanation
Volcanic activity can also cause earthquakes, although these are generally less frequent than those caused by tectonic movements. When magma rises to the Earth's surface, it can cause the surrounding rock to fracture, leading to earthquakes. These earthquakes often occur before, during, or after a volcanic eruption. While they may not happen as often as tectonic earthquakes, they can be particularly destructive because they occur in populated areas near volcanoes.
Examples & Analogies
Think of a balloon being inflated. As it fills up, the skin stretches and can create small ruptures. Similarly, as magma fills a volcanic chamber, it can cause the surrounding rocks to crack and lead to earthquakes before the actual eruption happens.
Induced Seismicity
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• Result of human activities such as:
– Reservoir-induced seismicity.
– Mining explosions.
– Deep well injections and hydraulic fracturing (fracking).
Detailed Explanation
Induced seismicity refers to earthquakes that are caused by human activities rather than natural processes. This can happen in several ways. For example, when large reservoirs are created by damming rivers, the pressure from the water can trigger earthquakes in the surrounding area. Similarly, mining activities can cause explosions that release energy, leading to smaller earthquakes. Another significant contributor is hydraulic fracturing (fracking), a process used to extract oil or gas where high-pressure fluids are injected into the ground, which can also trigger seismic events.
Examples & Analogies
Imagine shaking a soda bottle and then suddenly releasing the cap. Just as the built-up pressure causes a burst of bubbles, human activities can create pressure changes in the Earth that result in earthquakes. Every time we apply stress to the Earth through activities like fracking or building reservoirs, we risk causing these minor tremors.
Collapse Earthquakes
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• Caused by underground cave-ins or mine collapses.
• Usually of minor magnitude.
Detailed Explanation
Collapse earthquakes are the result of underground structures failing, such as caves collapsing or mine shafts giving way. These types of earthquakes generally have a minor magnitude compared to tectonic earthquakes. They occur when the ground underneath a surface structure loses support, causing it to cave in, which can send shockwaves through the surrounding earth. Though smaller, they can still be dangerous, particularly in populated or industrial areas.
Examples & Analogies
Think about when you build a sandcastle on the beach. If you dig too deeply or remove too much sand from the base, the structure may collapse. This is similar to how the ground can cave in and create minor earthquakes when underground support structures fail.
Key Concepts
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Tectonic Movements: Relative motion between tectonic plates causing energy release.
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Volcanic Activity: Earthquakes linked to volcanic eruptions, often localized and destructive.
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Induced Seismicity: Seismic events triggered by human activities.
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Collapse Earthquakes: Minor earthquakes from caverns or mine collapses.
Examples & Applications
Subduction zones, such as the Cascade Range, produce significant earthquakes due to convergent plate boundaries.
Mining activities can lead to small earthquakes, known as induced seismicity, which can destabilize surrounding areas.
Memory Aids
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Rhymes
Tectonic plates shift, creating a rift, causing earthquakes to uplift.
Stories
Imagine two friends, Tectonic and Volcanic, arguing over who caused the last earthquake; Tectonic swears it was their movements, while Volcanic claims it was the eruption!
Memory Tools
Remember 'TVIC' for Tectonic, Volcanic, Induced collapse for types of earthquake causes.
Acronyms
T.E.V.I.C
Tectonic events
Volcanic activities
Induced seismicity
Collapse earthquakes.
Flash Cards
Glossary
- Tectonic Movements
Shifts and interactions between Earth's tectonic plates leading to earthquakes.
- Volcanic Activity
Earthquakes resulting from movements associated with volcanic eruptions.
- Induced Seismicity
Earthquakes resulting from human activities, like mining or hydraulic fracturing.
- Collapse Earthquakes
Minor earthquakes caused by cave-ins or underground collapses.
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