Based on Plate Tectonics, we know that the Earth’s surface is made up of various masses of lithosphere known as Tectonic Plates. These plates constantly move but we don’t feel it most of the time as the tremors are too small to reach us. But when they are strong, they can be felt over as far as 1000 miles away. In fact, Earth experiences millions of earthquakes in a year.
San Andreas Fault Zone in California
Formation of an Earthquake
Earthquakes are the sudden shaking of the Earth’s surface. They are also known as quakes or tremors. The Earth is made of four basic layers: a solid crust, a hot nearly solid mantle, a liquid outer core and a solid inner core. The tectonic plates are constantly shifting as they drift around on the viscous, or slow flowing mantle layer below. An Earthquake is caused when masses of rock suddenly fracture and slip when tectonic plates scrapes, bumps or drags along another plate. The surface where these plates slip is called a fault line
Around 80% of Earthquakes and Volcanoes occur at areas where two plates meet. The point of origin of an earthquake is called the Hypocenter or Focus. The point on the Earth’s surface vertically above the focus of an earthquake is known as the Epicenter.
Types of Earthquakes
Earthquakes are a way by which the Earth gets rid of some of its tension by ‘rocking’ the world. They are caused by the passage of vibrations or seismic waves through Earth’s rocks. There are two types of Earthquakes – Natural and Man-Made.
- NATURAL EARTHQUAKES -Natural earthquakes are classified into Tectonic and Volcanic. A) TECTONIC EARTHQUAKE – Tectonic earthquakes are caused when energy stored in Earth’s crust is released suddenly. They occur mostly along geologic faults that lie at the fringes of huge tectonic plates. Most earthquakes are caused by the movement of magma in the mantle. The magma exerts pressure on these plates and releases energy in the form of tremors and vibrations. It can be generated in any of the following boundaries-
1) Divergent boundaries – When two tectonic plates move away from each other, earthquakes are common along these boundaries. Magma erupts along with the opening from the mantle beneath, solidifies, and creates a new crust. When it occurs beneath the oceanic lithosphere, it produces mid-oceanic ridges. The boundary between the North American Plate and the Eurasian Plate is an example of a divergent boundary.
2) Transform boundaries – These are areas where two tectonic plates move past each other horizontally, rubbing along the edges. Since the plates meet near the surface, they cause shallow focus earthquakes and cause portions of rocks to break. E.g. San Andreas transform boundary plate in Western California.
3) Convergent boundaries – When two tectonic plates collide, earthquakes can be deep and very powerful. When the colliding plates are of unequal density like an oceanic and a continental crust, one plate usually sinks beneath the other. This motion of lithosphere as it plunges through the mantle causes the quakes. Convergent plate boundaries produce earthquakes all around the Pacific Ocean basin.
B) VOLCANIC EARTHQUAKES: As magma makes its way through the crust to the surface of the Earth, it breaks apart the surrounding rock thereby generating volcanic earthquakes. It is relatively weaker than a tectonic earthquake and affects a smaller area around the volcano.
II) MANMADE EARTHQUAKES:
Human activities can also trigger earthquakes. Central US has witnessed an increase in earthquakes in the last decade due to the expansion of unconventional oil and gas operations. As they discard wastewater by injecting into the ground, it adds to the stress underneath causing frequent earthquakes. The enormous load exerted by dam reservoirs also adds to its frequency.
The Richter Scale and Seismometer
Developed by Charles F. Richter in 1935, the Richter magnitude scale is the measure of the strength of earthquakes. It was later revised and renamed the local magnitude scale, in short ML or ML. A seismometer is an instrument and a recording device that records ground motions like earthquakes, volcanic eruptions, and explosions.
A powerful Earthquake can be hazardous in many ways:
- Ground Shaking – High intensity shaking due to Earthquakes cause damage to buildings, bridges, and dams. Mild shaking can cause minor damages and objects inside a house may topple injuring people.
- Tsunami – Sudden displacement of seawater by a shallow earthquake can cause oceanic waves with long-wavelength leading to Tsunamis.
- Landslides, Avalanches, and Rockfall – Earthquakes may destabilize cliffs and steep slopes causing landslides, avalanches, and rockfalls.
- Flooding – When Earthquakes cause downward displacement of part of a landmass especially near coastal areas, it may lead to flooding of that area.
Precautions during Earthquake
- Keep your cool and don’t run around.
- Take cover under a strong table or any strong piece of furniture. Stay near the corner of the house.
- Stay away from glass doors and windows and walls with weak structure.
- If outside, move away from tall buildings, trees, streetlights and electrical wires.
- If you are in a vehicle, stop the car near a safe place and avoid taking weak roads and bridges.
- The largest recorded earthquake happened in Chile on May 22, 1960. It had a magnitude of 9.5 on the Richter scale.
- Alaska has recorded the largest earthquake in the US on March 28, 1964, with a magnitude of 9.2 and experiences an average of 24,000 earthquakes a year.
- Southern California records about 10,000 earthquakes a year, though very few are felt.
- Geologists predict that as per plate tectonics, in 70 million years, Los Angeles will meet Alaska and it will be neighbors with San Francisco in 15 million years!
- The Earth’s surface is made up of various masses of lithosphere known as __________.
- At __________boundaries, two tectonic plates move away from each other.
- The sudden displacement of seawater by a shallow earthquake can cause oceanic waves with long-wavelength leading to ________.
- The point on the Earth’s surface vertically above the focus of an earthquake is known as the ________.
- Developed in 1935, the _________ measures the strength of earthquakes.
- The Earth’s surface is made up of various masses of lithosphere known as tectonic plates.
- At divergent boundaries, two tectonic plates move away from each other.
- The sudden displacement of seawater by a shallow earthquake can cause oceanic waves with long-wavelength leading to Tsunamis.
- The point on the Earth’s surface vertically above the focus of an earthquake is known as the epicenter.
- Developed in 1935, the Richter magnitude scale is the measure of the strength of earthquakes.