On June 6, USGS Hawaiian Volcano Observatory field engineers installed a new seismometer on the Southwest Rift Zone of Kīlauea to monitor earthquakes. The new seismic station is co-located at an existing monitoring station consisting of a GPS and high-resolution gas monitoring equipment.
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 hypocenter at a speed of several kilometers per second.
We observe earthquakes with a network of seismometers on the earth's surface. The ground motion at each seismometer is amplified and recorded electronically at a central recording site. As the wavefront expands from the earthquake, it reaches more distant seismic stations. When an earthquake occurs, we observe the times at which the wavefront passes each station. We must find the unknown earthquake source knowing these wave arrival times. Here is a map of U.S. Geological Survey seismic stations in the San Francisco Bay Area and 6 seismograms from an earthquake:
We want to find the location, depth and origin time of an earthquake whose waves arrive at the times measured on each seismograms. We want a straightforward and general procedure that we can also program in a computer. The procedure is simple to state: guess a location, depth and origin time; compare the predicted arrival times of the wave from your guessed location with the observed times at each station; then move the location a little in the direction that reduces the difference between the observed and calculated times. Then repeat this procedure, each time getting closer to the actual earthquake location and fitting the observed times a little better. Quit when your adjustments have become small enough and when the fit to the observed wave arrival times is close enough.
You can try to fit an earthquake location on the map just to see how the procedure goes. Note that the earthquake arrives first on station C, so C is a good first guess for the location. Many earthquakes in California occur between 2 and 12 kilometers depth and we will guess a 6 kilometer depth. The origin time should be a few seconds before the time of the wave at the first station. Let's guess an origin time of 10 seconds, measured on the same clock that made the time scale at the bottom of the figure and timed the seismograms. Then we can list the tentative travel times by subtracting the origin time from the observed arrival times:
station....................... A B C D E F
observed time.......... 16.5 17.8 11.3 15.2 22.3 18.3
tentative travel time... 6.5 7.8 1.3 5.2 12.3 8.3
Note the scale at the left of the figure. It shows travel times for waves from an earthquake at a depth of 6 kilometers. The scale starts at 1.3 seconds because the wave reaches the surface 1.3 seconds after the earthquake origin time. You can make a tracing of the scale and move the earthquake on the map until the tentative travel times match the travel times from the scale. Where do you think the earthquake was? Are the times for each station systematically early or late, requiring a shift in the origin time?
The earthquake was near station C. The depth was about 6 km and the origin time was about 10 seconds. (We guessed very well!) A real magnitude 3.4 earthquake occurred at this location on April 29, 1992. It was felt by many people who were sitting or at rest.
Mathematically, the problem is solved by setting up a system of linear equations, one for each station. The equations express the difference between the observed arrival times and those calculated from the previous (or initial) hypocenter, in terms of small steps in the 3 hypocentral coordinates and the origin time. We must also have a mathematical model of the crustal velocities (in kilometers per second) under the seismic network to calculate the travel times of waves from an earthquake at a given depth to a station at a given distance. The system of linear equations is solved by the method of least squares which minimizes the sum of the squares of the differences between the observed and calculated arrival times. The process begins with an initial guessed hypocenter, performs several hypocentral adjustments each found by a least squares solution to the equations, and iterates to a hypocenter that best fits the observed set of wave arrival times at the stations of the seismic network.
Learn More: Glossary of earthquake terms
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On June 6, USGS Hawaiian Volcano Observatory field engineers installed a new seismometer on the Southwest Rift Zone of Kīlauea to monitor earthquakes. The new seismic station is co-located at an existing monitoring station consisting of a GPS and high-resolution gas monitoring equipment.
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August 22, 2022 — USGS Hawaiian Volcano Observatory scientist setting up a seismometer on Ta‘ū Island
linkA broadband seismometer was installed on Ta‘ū island on August 22, 2022, by the USGS Hawaiian Volcano Observatory and NOAA National Weather Service team. This is one of two broadband seismometers that have been placed on Ta‘ū island to help detect earthquakes that started to be felt in the region starting in late July. USGS photo by D. Downs.
USGS scientists Nicholas van der Elst and Alan Yong installing a seismometer near the 2019 Ridgecrest earthquakes in southern California in order to record its aftershocks.
USGS scientists Nicholas van der Elst and Alan Yong installing a seismometer near the 2019 Ridgecrest earthquakes in southern California in order to record its aftershocks.
Title: ShakeAlert: The Path to West Coast Earthquake Early Warning ... how a few seconds can save lives and property
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Map of historic seismicity, major faults, and paleoseismic summary of San Andreas Fault system.
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United States Geological Survey (USGS) Natural Hazards Response
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Taking the Earth's pulse
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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?
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How do you determine the magnitude for an earthquake that occurred prior to the creation of the magnitude scale?
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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?
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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...
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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 Seismology 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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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...
On June 6, USGS Hawaiian Volcano Observatory field engineers installed a new seismometer on the Southwest Rift Zone of Kīlauea to monitor earthquakes. The new seismic station is co-located at an existing monitoring station consisting of a GPS and high-resolution gas monitoring equipment.
On June 6, USGS Hawaiian Volcano Observatory field engineers installed a new seismometer on the Southwest Rift Zone of Kīlauea to monitor earthquakes. The new seismic station is co-located at an existing monitoring station consisting of a GPS and high-resolution gas monitoring equipment.
August 22, 2022 — USGS Hawaiian Volcano Observatory scientist setting up a seismometer on Ta‘ū Island
linkA broadband seismometer was installed on Ta‘ū island on August 22, 2022, by the USGS Hawaiian Volcano Observatory and NOAA National Weather Service team. This is one of two broadband seismometers that have been placed on Ta‘ū island to help detect earthquakes that started to be felt in the region starting in late July. USGS photo by D. Downs.
August 22, 2022 — USGS Hawaiian Volcano Observatory scientist setting up a seismometer on Ta‘ū Island
linkA broadband seismometer was installed on Ta‘ū island on August 22, 2022, by the USGS Hawaiian Volcano Observatory and NOAA National Weather Service team. This is one of two broadband seismometers that have been placed on Ta‘ū island to help detect earthquakes that started to be felt in the region starting in late July. USGS photo by D. Downs.
USGS scientists Nicholas van der Elst and Alan Yong installing a seismometer near the 2019 Ridgecrest earthquakes in southern California in order to record its aftershocks.
USGS scientists Nicholas van der Elst and Alan Yong installing a seismometer near the 2019 Ridgecrest earthquakes in southern California in order to record its aftershocks.
Title: ShakeAlert: The Path to West Coast Earthquake Early Warning ... how a few seconds can save lives and property
Title: ShakeAlert: The Path to West Coast Earthquake Early Warning ... how a few seconds can save lives and property
Map of historic seismicity, major faults, and paleoseismic summary of San Andreas Fault system.
Map of historic seismicity, major faults, and paleoseismic summary of San Andreas Fault system.
This video provides a tutorial for anyone interested in interpreting the seismic records on public webicorder displays.
This video provides a tutorial for anyone interested in interpreting the seismic records on public webicorder displays.
USGS volcano seismologist, Seth Moran, describes how seismology and seismic networks are used to mitigate volcanic hazards.
USGS volcano seismologist, Seth Moran, describes how seismology and seismic networks are used to mitigate volcanic hazards.