you may have heard of the epicenter of an earthquake. but what do you call the point that lies below the earth’s surface where the rupture begins?
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The Science of Earthquakes
Originally written by Lisa Wald (U.S. Geological Survey) for “The Green Frog News”
The Science of Earthquakes
By Earthquake Hazards
Originally written by Lisa Wald (U.S. Geological Survey) for “The Green Frog News”
What is an earthquake?
Sources/Usage: Public Domain.
A normal (dip-slip) fault is an inclined fracture where the rock mass above an inclined fault moves down (Public domain.)
An earthquake is what happens when two blocks of the earth suddenly slip past one another. The surface where they slip is called the fault or fault plane. The location below the earth’s surface where the earthquake starts is called the hypocenter, and the location directly above it on the surface of the earth is called the epicenter.
Sometimes an earthquake has foreshocks. These are smaller earthquakes that happen in the same place as the larger earthquake that follows. Scientists can’t tell that an earthquake is a foreshock until the larger earthquake happens. The largest, main earthquake is called the mainshock. Mainshocks always have aftershocks that follow. These 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, months, and even years after the mainshock!
What causes earthquakes and where do they happen?
Sources/Usage: Public Domain.
A simplified cartoon of the crust (brown), mantle (orange), and core (liquid in light gray, solid in dark gray) of the earth. (Public domain.)
The earth has four major layers: the inner core, outer core, mantle and crust. The crust and the top of the mantle make up a thin skin on the surface of our planet.But this skin is not all in one piece – it is made up of many pieces like a puzzle covering the surface of the earth. Not only that, but these puzzle pieces keep slowly moving around, sliding past one another and bumping into each other. We call these puzzle pieces tectonic plates, and the edges of the plates are called the plate boundaries. The plate boundaries are made up of many faults, and most of the earthquakes around the world occur on these faults. Since the edges of the plates are rough, they get stuck while the rest of the plate keeps moving. Finally, when the plate has moved far enough, the edges unstick on one of the faults and there is an earthquake.
Sources/Usage: Public Domain.
The tectonic plates divide the Earth's crust into distinct "plates" that are always slowly moving. Earthquakes are concentrated along these plate boundaries. (Public domain.)
Why does the earth shake when there is an earthquake?
While the edges of faults are stuck together, and the rest of the block is moving, the energy that would normally cause the blocks to slide past one another is being stored up. When the force of the moving blocks finally overcomes the friction of the jagged edges of the fault and it unsticks, all that stored up energy is released. The energy radiates outward from the fault in all directions in the form of seismic waves like ripples on a pond. The seismic waves shake the earth as they move through it, and when the waves reach the earth’s surface, they shake the ground and anything on it, like our houses and us!
How are earthquakes recorded?
Sources/Usage: Public Domain.
The cartoon sketch of the seismograph shows how the insrument shakes with the earth below it, but the recording device remains stationary (instead of the other way around). (Public domain.)
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.
How do scientists measure the size of earthquakes?
Sources/Usage: Public Domain.
An example of a seismic wave with the P wave and S wave labeled. (Public domain.)
Epicenter of an Earthquake
What is the epicenter of an earthquake? Learn the epicenter definition, where to find the epicenter of an earthquake, and why we need to find the...
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What is the Epicenter of an Earthquake?
Tiffany Leonard, David Wood
What is the epicenter of an earthquake? Learn the epicenter definition, where to find the epicenter of an earthquake, and why we need to find the epicenter. Updated: 10/02/2021
Table of Contents
What is the Epicenter of an Earthquake?
Locating the Epicenter of an Earthquake
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What is the Epicenter of an Earthquake?
An earthquake can be a major natural disaster, causing massive amounts of destruction in its wake. The epicenter of an earthquake is the surface expression of the earthquake. It is located directly above the focus or hypocenter of the earthquake, which is where the earthquake begins at depth. Earthquakes typically begin deep within the Earth's surface, with shallow earthquakes still located up to depths of 70 km. Most people are more aware of the epicenter than the hypocenter, as the epicenter is easier to plot on the map
The epicenter tends to be the point on the surface that experiences the most shaking. It is the first place the seismic waves hit on the surface after the initial rupture has occurred. However, this is not always the case. If the rupture of an earthquake is particularly large, the initial hypocenter location may be one of several hotspots where energy is released.
An example of an earthquake where shaking was more severe away from the epicenter is the 2002 Alaskan Earthquake. For that event, most of the damage occurred 330 km east from the epicenter, on the other end of the rupture.
The Hypocenter of an Earthquake
The hypocenter is the location where the earthquake begins deep within the Earth. When an earthquake begins, two sides of a fault move against each other and release pressure. A fault is a fracture zone between two blocks of rock (often two tectonic plates). Due to the fault, the blocks are allowed to move relative to each other. When the motion happens quickly, that is when an earthquake occurs. The hypocenter occurs where this motion or slip first begins.
The depth of the hypocenter determines how shallow or deep the earthquake is. Shallow earthquakes tend to be more destructive, because the energy is released much closer to the surface, causing more severe shaking.
Range of Earthquake Depths
Depth Range
Shallow Earthquakes 0 - 70 km
Intermediate Earthquakes 70 - 300 km
Deep Earthquakes Below 300 km
Despite the shallow range beginning at 0 km, earthquakes cannot occur at the surface of the Earth. Two blocks of rock must slide for an earthquake to be generated.
Differences between the Epicenter and Hypocenter
How can we differentiate between the epicenter and hypocenter? Both are locations that pinpoint earthquake activity. However, remember that epicenter is on the surface and the hypocenter is at depth.
Example from the 1989 Loma Prieta Quake
Latitude Longitude Depth
Hypocenter 37.04° N 121.88° W 19 km
Epicenter 37.04° N 121.88° W 0 km
The epicenter is located on the surface directly above the focus (hypocenter), which is located on the fault at depth.
The latitude and longitude will be the same for the epicenter and hypocenter of the same event.
Quiz Course 97K views
Locating the Epicenter of an Earthquake
Scientists use two different methods to determine the epicenter of an earthquake:
Seismograph triangulation
P- and S-Wave intervals
Often, these two methods will be combined to improve the overall accuracy of the location.
Seismograph Triangulation
Seismographs are instruments that scientists use to measure earthquake signals around the globe. They work by measuring the motion of the ground in the area they are installed. These instruments must be installed in the ground to get accurate readings. Seismographs are typically installed and operated as part of a network of instruments. Many seismographs must be used to get an accurate image of the earthquake.The seismometer is the part inside the seismograph, often a pendulum or mass on a spring, that is used to measure the ground motion. However, people often use the terms seismometer and seismograph interchangeably.
Diagram of a simple seismograph. In this model, the seismometer is a pendulum. As the seismograph moves with ground motion, the seismometer will remain still, recording the motion on the paper roll.
Triangulation Process
In order to use seismographs to locate the epicenter of a quake, at least 3 different instruments in different locations must be used.
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What is an Epicenter?
Earthquakes can cause huge amounts of death and destruction. For this reason, understanding them is vitally important. Like all major events, seismic or otherwise, they have to start somewhere. The epicenter is the place on the Earth's surface under which they start.
The point from where an earthquake originates is called A. CrustB. EarthC. SurfaceD. Focus
The point from where an earthquake originates is called A. CrustB. EarthC. SurfaceD. Focus. Ans: Hint:An earthquake is an intense shaking of the earth’s surface which is the result of a sudden release of stored energy. The point where the earthquake ...
The point from where an earthquake originates is calledA. Crust B. Earth C. Surface D. Focus
Last updated date: 16th Mar 2023
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Hint:An earthquake is an intense shaking of the earth’s surface which is the result of a sudden release of stored energy. The point where the earthquake originates is also known as a hypocenter.Complete answer:An earthquake is the vigorous shaking of the earth’s surface which is due to the sudden release of stored energy in the Earth's crust that creates seismic waves.
Earthquakes occur when large blocks of the Earth's crust move suddenly past one another at a fault. The pieces of the Earth’s crust are caused to move by the force of the plate tectonics but sometimes the pieces do not slide smoothly past one another. This is due to the friction present along the fault - jagged edges that obstructs the blocks of rock, thus making it difficult for them to move past each other. Sometimes they adhere together temporarily. When the pieces of rock overcome the obstruction, energy is released. The release of energy causes an intense shaking at the ground surface.
The point inside the Earth where an earthquake originates is known as the focus (or hypocenter) of the earthquake. The point directly above the focus at the surface of the earth is called the epicenter of the earthquake. At the epicenter, the most vigorous shaking is caused during an earthquake. Sometimes the ground surface breaks across the fault. At times the movement is deep underground and the surface does not break.Hence the correct answer is option D.Note:Earthquakes are measured with the help of a device known as seismometer, commonly known as a seismograph. The magnitude of an earthquake is conventionally reported with the help of a Richter scale or a related Moment scale ( earthquakes having a magnitude of 3 or lower are hard to notice while the ones with a magnitude 7 causing serious damage over large areas).
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