Diagram showing the epicenter and hypocenter.
The epicenter is the point on the earth's surface vertically above the hypocenter (or focus), point in the crust where a seismic rupture begins.
What is a Geoid? Why do we use it and where does its shape come from?
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A geoid is the irregular-shaped “ball” that scientists use to more accurately calculate depths of earthquakes, or any other deep object beneath the earth’s surface. Currently, we use the “WGS84” version (World Geodetic System of 1984).
If Earth were a perfect sphere, calculations of depth and distances would be easy because we know the equations for those calculations on a sphere. However, the Earth more closely approximates an ellipsoid, which is what a ball looks like if you sit on it. Ellipsoid calculations aren’t as easy as spherical calculations, but they’re still well-known and do-able. Be that as it may, we all know that the earth is not really an ellipsoid because there are oceans, and mountains, and valleys, and many other features that are not part of an ellipsoid.
The geoid is an imaginary sea level surface that undulates (has a wavy surface) over all of the earth; it isn’t just for the oceanic areas, it also extends through the land masses.
You can generalize the relationship between the ellipsoid, the geoid, and the actual shape of the earth with this:
geoid + ellipsoid = Earth
Learn More: NOAA - What is the Geoid?
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Epicenter Hypocenter Diagram
Diagram showing the epicenter and hypocenter.
The epicenter is the point on the earth's surface vertically above the hypocenter (or focus), point in the crust where a seismic rupture begins.
Related Content
- FAQ
How are earthquakes recorded? How are earthquakes measured? How is the magnitude of an earthquake determined?
Earthquakes are recorded by a seismographic network . Each seismic station in the network measures the movement of the ground at that site. The slip of one block of rock over another in an earthquake releases energy that makes the ground vibrate. That vibration pushes the adjoining piece of ground and causes it to vibrate, and thus the energy travels out from the earthquake hypocenter in a wave...
Moment magnitude, Richter scale - what are the different magnitude scales, and why are there so many?
Earthquake size, as measured by the Richter Scale is a well known, but not well understood, concept. The idea of a logarithmic earthquake magnitude scale was first developed by Charles Richter in the 1930's for measuring the size of earthquakes occurring in southern California using relatively high-frequency data from nearby seismograph stations. This magnitude scale was referred to as ML , with...
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How do you determine the magnitude for an earthquake that occurred prior to the creation of the magnitude scale?
For earthquakes that occurred between about 1890 (when modern seismographs came into use) and 1935 when Charles Richter developed the magnitude scale, people went back to the old records and compared the seismograms from those days with similar records for later earthquakes. For earthquakes prior to about 1890, magnitudes have been estimated by looking at the physical effects (such as amount of...
How do seismologists locate an earthquake?
When an earthquake occurs, one of the first questions is "where was it?" The location may tell us what fault it was on and where damage (if any) most likely occurred. Unfortunately, Earth is not transparent and we can't just see or photograph the earthquake disturbance like meteorologists can photograph clouds. When an earthquake occurs, it generates an expanding wavefront from the earthquake...
What was the duration of the earthquake? Why don't you report the duration of each earthquake? How does the duration affect the magnitude?
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How can an earthquake have a negative magnitude?
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An earthquake cannot physically occur at a depth of 0 km or -1km (above the surface of the earth). In order for an earthquake to occur, two blocks of crust must slip past one another, and it is impossible for this to happen at or above the surface of the earth. So why do we report that the earthquake occurred at a depth of 0 km or event as a negative depth sometimes? First of all, the depth of an...
How can I make my own seismometer?
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Seismometers, seismographs, seismograms - what's the difference? How do they work?
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The earliest seismoscope was invented by the Chinese philosopher Chang Heng in A.D. 132. This was a large urn on the outside of which were eight dragon heads facing the eight principal directions of the compass. Below each dragon head was a toad with its mouth opened toward the dragon. When an earthquake occurred, one or more of the eight dragon-mouths would release a ball into the open mouth of...
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Epicenter Hypocenter Diagram
Diagram showing the epicenter and hypocenter.
The epicenter is the point on the earth's surface vertically above the hypocenter (or focus), point in the crust where a seismic rupture begins.
Diagram showing the epicenter and hypocenter.
The epicenter is the point on the earth's surface vertically above the hypocenter (or focus), point in the crust where a seismic rupture begins.
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