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 does it mean that the earthquake occurred at a depth of 0 km? How can an earthquake have a negative depth; that would mean it’s in the air. What is the geoid, and what does it have to do with earthquake depth?
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 earthquake is usually the most difficult part of its location to nail down with great accuracy. Since most earthquakes are deep within the crust, an error of +/- 1 or 2 km is irrelevant; in other words, it is a small error when the depth is something like 13 km. If the earthquake depth is relatively shallow, however, it becomes more of an issue. A negative depth can sometimes be an artifact of the poor resolution for a shallow event.
For quarry blasts that are recorded by the seismic network, the depth is fixed at 0 km since we can never determine a precise depth for these, but we know they are very close to the surface.
Sometimes, because of the density of the seismic network and the close proximity of the seismic stations to an earthquake epicenter, we are able to determine a very precise depth. When the earthquake depth is very shallow, it can be reported as a negative depth.
The USGS National Earthquake Information Center uses the World Geodetic System of 1984 (WGS84) geoid as a reference from which earthquake depths are calculated. But other seismic networks may use WGS84, mean sea-level, or the average elevation of the seismic stations that provided arrival-time data for the earthquake location. The choice of reference depth is dependent on the method used to locate the earthquake, which varies by seismic network.
Since the Comprehensive Earthquake Catalog (ComCat) includes data from many different seismic networks, the process for determining the depth is different for different events. The depth is the least-constrained parameter in the earthquake location, and the error bars are generally larger than the variation due to different depth determination methods.
Learn more: Volcano Watch--Why do some earthquakes have negative depths?
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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.
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- 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...
What is the difference between earthquake magnitude and earthquake intensity? What is the Modified Mercalli Intensity Scale?
Magnitude scales, like the moment magnitude, measure the size of the earthquake at its source. An earthquake has one magnitude. The magnitude does not depend on where the measurement is made. Often, several slightly different magnitudes are reported for an earthquake. This happens because the relation between the seismic measurements and the magnitude is complex and different procedures will often...
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?
The duration of an earthquake is related to its magnitude but not in a perfectly strict sense. There are two ways to think about the duration of an earthquake. The first is the length of time it takes for the fault to rupture and the second is the length of time shaking is felt at any given point (e.g. when someone says "I felt it shake for 10 seconds" they are making a statement about the...
How can an earthquake have a negative magnitude?
Magnitude calculations are based on a logarithmic scale, so a ten-fold drop in amplitude decreases the magnitude by 1. If an amplitude of 20 millimetres as measured on a seismic signal corresponds to a magnitude 2 earthquake, then: 10 times less (2 millimetres) corresponds to a magnitude of 1; 100 times less (0.2 millimetres) corresponds to magnitude 0; 1000 times less (0.02 millimetres)...
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A seismometer is the internal part of the seismograph, which may be a pendulum or a mass mounted on a spring; however, it is often used synonymously with "seismograph". Seismographs are instruments used to record the motion of the ground during an earthquake. They are installed in the ground throughout the world and operated as part of a seismographic network. The earliest "seismoscope" was...
How can I make my own seismometer?
It is relatively easy to acquire the necessary materials and build your own seismometer. The links here are to various sources with information on how to build a seismometer. They range from very simple and inexpensive to sophisticated and pricey. Model Seismograph - Classroom Demonstration Build your own Seismograph Station Build Your Own Seismograph Amateur Seismologist Homebuilt Seismograph FAQ
What was the first instrument that actually recorded an earthquake?
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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