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Following the previous lesson on Plate Boundaries, it is time to dive further. We will begin by investigating one of the world's most devasting natural hazards, Earthquakes. This segment of the syllabus asks that we investigate the formation of earthquakes, the cause of them and breakdown the effects of their destruction.
What are Earthquakes?
What are Earthquakes?
Definition: An earthquake is a sudden release of energy in the Earth's crust that causes the ground to shake.
Earthquakes occur at all plate boundaries (convergent, divergent, and transform). When stresses in the crust exceed the strength of the rock, it breaks along lines of weakness, either a pre-existing or new fault plane. For those who don't quite grasp it yet, when two blocks of the earth suddenly slip past one another, the surface where they slip is called the fault or fault plane. This slipping action is what creates earthquakes. This happens because as tectonic plates move, they cause friction that builds up stress at the edges of the plates. When the stress overcomes the friction, the plates slip past each other, releasing energy in the form of seismic waves. It should be of note that the size and frequency of earthquakes varies across different plate boundaries.
For further reference, at transform boundaries, where two plates slide past each other, the friction here typically produce large, shallow-focus earthquakes. In contrast, subduction zones, where one plate is forced beneath another, are the site of the most powerful earthquakes, tsunamis, and volcanic eruptions. So let's take a look at the anatomy of an earthquake.
Diagram of the Anatomy of an Earthquake
Upon examining the diagram above, it is easy to discern the parts of an earthquake. Just to recap however, the parts of an earthquake are:
Focus (Hypocenter) - the point within the earth where an earthquake originates.
Epicenter - the point at the surface of the earth directly above the focus.
Seismic waves - waves that transmit the energy released by an earthquake.
Plates - Massive rocks that make up the outer layer of the Earth’s surface and whose movement along faults triggers earthquakes
Fault - A fracture in the rocks that make up the Earth’s crust
What happens during an Earthquake?
After the two blocks of the earth suddenly slip past one another, the built up energy explodes at focus of the earthquake. The energy released is turned into seismic waves, which spread out from the initial point of rupture (the focus) like ripples on a pond. These waves are what makes the ground shake and can travel large distances in all directions. Near the focus, the waves can be very large, making them extremely destructive. These seismic waves are felt on the earth's surface at the epicenter and the surrounding areas.
*NOTE*
Sometimes an earthquake has foreshocks. These are smaller earthquakes that happen in the same place as the larger earthquake that follows. The largest, main earthquake is called the mainshock. Mainshocks always have aftershocks that follow. Aftershocks are smaller earthquakes that occur afterwards in the same place as the mainshock. Depending on the size of the mainshock, aftershocks can continue for weeks to months.
How are they recorded?
Earthquakes are recorded by instruments called seismographs. The recording they make is called a seismogram. The seismograph has a base that sets firmly in the ground, and a heavy weight that hangs free. When an earthquake causes the ground to shake, the base of the seismograph shakes too, but the hanging weight does not. Instead the spring or string that it is hanging from absorbs all the movement. The difference in position between the shaking part of the seismograph and the motionless part is what is recorded. It should be further noted that thousands of earthquakes occur worldwide each day, with some being so small that only specialized equipment can detect them (some seismographs may not be able to record them).
An Image of a Seismograph
Diagram of a Seismograph
Alternative Seismograph (Juvenile in development)
Watch this video to see the seismograph in action.
How are they measured?
Earthquakes are measured using the Richter Scale. This scale serves to showcase the magnitude of an earthquake and define the type of energy it releases in comparison to dynamite explosions.
Diagram understanding the Richter Scale
The Effects of Earthquakes
So far, we have discussed the formation of Earthquakes, how they are recorded and how they are measured. Now, let's discuss something truly important, the effects of Earthquakes. In fact, there are very little positive effects and more devasting negative effects.
Positive Effects
Mineral exposure: Earthquakes can fracture the Earth's crust, exposing valuable mineral deposits like gold, silver, and platinum, making them more accessible for mining operations. It also causes fractures that allow for water to rise from underground where it would have previously been inaccessible.
Fossil fuel access: Shifting tectonic plates caused by earthquakes can sometimes bring fossil fuels like natural gas and petroleum closer to the surface, making extraction easier. Almost the same thing as mineral exposure.
Negative Effects
Ground shaking: The most familiar effect of an earthquake, ground shaking can range from gentle to violent. The shaking depends on the distance from the fault line and the soil conditions.
Landslides: Earthquakes can trigger landslides that can cause significant damage to homes. Especially in highland areas where mass movement or soil creep has begun.
Liquefaction: When water-saturated soil is shaken, it can turn to liquid, causing buildings to sink or overturn. This liquefaction turns the surrounding areas into quicksand.
Tsunamis: Earthquakes that occur at sea can generate tsunamis, which are long-wavelength sea waves that can travel thousands of kilometers.
Fires: Because most gas lines are built underground for safety, earthquakes can cause the collapse of pipelines which can create fractures in gas lines. Broken gas lines can fuel fires, which are a secondary effect of earthquakes.
Injuries and death: Falling objects, shattered glass, and collapsing structures can cause injuries and death.
Isolation: Earthquakes can damage roads, bridges, and cause avalanches and landslides, which can isolate communities. Without access to certain areas, authorities cannot ascertain the level of damage caused by the earthquake or send appropriate care to those in need.
Infectious diseases: Earthquakes can create conditions that favor the emergence of infectious diseases. This happens through damage to waterways (In Haiti, Cholera spread because of this) and deterioration of the air quality.
Psychological effects: People may experience symptoms like dizziness, nausea, and vomiting after an earthquake.
Be mindful that these negative effects can be split into two main categories: Human Effects and Physical Effects.
Recently, in the Caribbean, Haiti has been subjected to two devastating earthquakes which caused a complete dismantling of their economy and obstruction of their livelihoods. If you require more clarity on the negative effects explained above, watch the following videos. This video survives as supplemental information to case studies relating to Earthquakes.
This video showcases a news reports of the Haiti Earthquake of 2010.
This video should be listened to very closely. This is a firsthand account of the Haitian Earthquake.
Mitigation Methods for Earthquakes
Earthquakes are natural hazards that occur suddenly and have the potential to become Natural Disasters. This means that we need to find ways to mitigate the effects of Earthquakes. Here are some ways:
Structural improvements: Anchor, brace, or restrain non-structural building components like chimneys, parapets, and windows. This also works for reinforcing buildings with more steel and cement to make them more solid to withstand the shaking caused by earthquakes.
Secure items: Hang heavy items like mirrors and pictures away from beds and couches. Install ledge barriers on shelves and secure breakables and heavy items directly to shelves. Anchor large appliances to walls using safety straps or cables. Install strong latches on cabinet doors, this way nothing has the opportunity to slide out of your and cause harm from falling as cabinet door fly open during an earthquake.
Install anti-shatter coatings: Apply anti-shatter coatings to glass windows.
Plan and zone: Promote land use planning based on known hazards. This means restricting individuals from building homes in areas prone to landslides or areas that have been airmarked as possible landslide sites.
Educate the public: Conduct public outreach projects and research and assessment. Learn about the community. Express to them whether they are well knowledgeable on Earthquakes and the systems put in place to help given the emergency of a devastating Earthquake. Also, educate the youth on Drop, Cover, Holdon (DCH), an acronym for hiding under your tables and other sturdy furniture to protect your head in the event of an Earthquake.
Prepare a disaster plan: Decide how you will communicate in an emergency. Ensure you have radios that utilize batteries to listen for potential evacuation messages over the radio waves.
Stockpile supplies: Have at least three days' worth of drinking water and food.
Prepare a supply kit: Include a flashlight, fire extinguisher, whistle, and enough food and water for several day.
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