When 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 the toad sitting below. The direction of the shaking determined which of the dragons released its ball. The instrument is reported to have detected an earthquake 400 miles away that was not felt at the location of the seismoscope. The inside of the seismoscope is unknown: most speculations assume that the motion of some kind of pendulum would activate the dragons.

Learn more: The Early History of Seismometry (to 1900)

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What is a Geoid? Why do they use the geoid, and where does its shape come from?

Geoid illustration

Contrast of the Geoid model with an Ellipsoid and cross-section of the Earth's surface. (Public domain.)

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Seismometers, seismographs, seismograms - what's the difference? How do they work?

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".

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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.

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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

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What are UTC and GMT (in reference to the time of an earthquake)?

UTC stands for Coordinated Universal Time, and GMT stands for Greenwich Mean Time. The time that earthquakes occur around the world is reported in UTC or GMT, which are essentially the same.

Learn more: Time

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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;
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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

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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, the earth is not transparent and we can't just see or photograph the earthquake disturbance like meteorologists can photograph clouds.

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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 the 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,

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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

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What is the difference between magnitude and 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 do not depend on where the measurement is made. Often, several slightly different magnitudes are

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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

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Date published: May 25, 2017

Updated USGS Earthquake Monitoring Strategy Released

The USGS Earthquake Hazards Program recently released a new strategic plan for earthquake monitoring entitled the “Advanced National Seismic System – Current Status, Development Opportunities, Priorities, 2017-2027.”

Date published: January 23, 2012

A 100-year-long History of Earthquakes and Seismic Monitoring in Hawaii

The Hawaiian Volcano Observatory’s 1912–2012 Centennial—100 Years of Tracking Eruptions and Earthquakes

HAWAI‘I ISLAND, Hawaii —The history of earthquakes and seismic monitoring in Hawai‘i during the past century will be the topic of a presentation at the University of Hawai‘i at Hilo on Thursday, January 26, at 7:00 p.m. 

Date published: September 24, 2009

Recovery Act Funds Will Upgrade Earthquake Monitoring

USGS will Grant Universities $5 Million to Beef Up Public Safety Grants totaling $5 million under the American Recovery and Reinvestment Act are being awarded to 13 universities nationwide to upgrade critical earthquake monitoring networks and increase public safety.

Date published: November 20, 2003

Old Japanese Documents Confirm Warnings of Future North American Earthquakes

Guided by Japanese writings from an era of shoguns, an international team of scientists today reported new evidence that an earthquake of magnitude 9 struck the northwestern United States and southwestern Canada three centuries ago. The findings are likely to affect the region’s precautions against future earthquakes and tsunamis.

Date published: January 24, 2000

Giant Earthquake, 300 Years Ago This Week

On January 26, 1700, the largest earthquake known to have occurred in the "lower 48" United States, rocked Cascadia, a region 600 miles long that includes northern California, Oregon, Washington, and southern British Columbia.

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three dimensional conceptual drawing of a tectonic plate being pushed down under another plate.
March 6, 2017

Block diagram illustrating an idealized geological setting offshore the state of Washington. As the subducting Juan De Fuca tectonic plate dives beneath North America, it can generate an earthquake, and trigger a tsunami.

close-up of steel cylinder with cables and bright red and yellow caps coming out of it.
October 27, 2016

Seismometers (instruments for recording earthquakes) are tested and fitted at the USGS Cascades Volcano Observatory before going out into the field.

Seismometer
June 3, 2014

A seismometer deployed near the epicenter of the Greeley earthquake in 2014.

Map showing locations of ANSS stations
November 30, 2000

Map of ANSS free-field seismic stations across the U.S. in 2016 (not shown are additional seismic instruments in buildings and other structures).  Map colors show seismic hazard across the United States derived from the National Seismic Hazard Model.  Background colors indicate the levels of shaking that have a 2% chance of being exceed in a 50-year period.  Shaking is expressed in a percentage of g, which is the acceleration of a falling object due to gravity, with red colors indicating highest shaking and thus higher hazard.  Notice the greater density of stations in regions with either higher hazard, higher risk (e.g., southern California), or both.

Image: Seismic station,  USGS Northern California Seismic Network

Traditional seismic stations such as this one require a source of power (solar here), a poured concrete foundation and several square feet of space. They are not always practical to install in urban areas, and that's where NetQuakes comes in.

Global Seismographic Network illustration

Global Seismographic Network

gray cylindrical instrument on gray table on grass

Three Component Seismometer: The image above shows a three component seismometer used by the USGS for the collection of H/V data (Public domain).