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 in a wave.

There are many different ways to measure different aspects of an earthquake:

  • Magnitude is the most common measure of an earthquake's size. It is a measure of the size of the earthquake source and is the same number no matter where you are or what the shaking feels like. The Richter scale is an outdated method for measuring magnitude that is no longer used by the USGS for large, teleseismic earthquakes. The Richter scale measures the largest wiggle (amplitude) on the recording, but other magnitude scales measure different parts of the earthquake. The USGS currently reports earthquake magnitudes using the Moment Magnitude scale, though many other magnitudes are calculated for research and comparison purposes.
  • Intensity is a measure of the shaking and damage caused by the earthquake; this value changes from location to location.

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What is a Geoid? Why do we use it 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.) 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...

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". 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...

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...

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-...

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...

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...

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...

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...

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 reported for an earthquake. This happens because the relation between the seismic measurements and the...

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...

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...
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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: May 3, 2010

Millions Awarded for Earthquake Monitoring in the United States

More than $7 million in cooperative agreements will be awarded for earthquake monitoring by the U.S Geological Survey in 2010. This funding will contribute to the development and operation of the USGS Advanced National Seismic System (ANSS).

Date published: January 22, 2001

New USGS Map Will Improve Earthquake Hazards Assessment in the Bay Area

A new geologic map of surficial deposits in the nine-county San Francisco Bay region that can be used to evaluate earthquake hazards has been released in digital form by the U.S. Geological Survey in Menlo Park.

Filter Total Items: 9
K2 Seismic Station
February 2, 2017

K2 Seismic Station

A K2 seismograph. The K2 is an accelerometer and data-logger combined into a single, portable package. The unit is the size of a large shoe box. It is usually installed in a corner of the garage or a convenient area on a patio or side walk along the house. A photo of a unit installed in a garage is shown.

Latest Earthquakes
August 31, 2016

Latest Earthquakes

building damaged by an earthquake
December 31, 2011

Earthquake-damaged building

Building damanged by the February 2011 earthquake in Christchurch, New Zealand.

Attribution: Natural Hazards
Image: Installing Antenna and Solar Panel for Seismic Station
February 18, 2009

Installing Antenna and Solar Panel for Seismic Station

With funding from the American Recovery and Reinvestment Act of 2009, The USGS Hawaiian Volcano Observatory recently upgraded its seismic monitoring network.  Here, HVO staff, assisted by an HVO volunteer, installs the solar panel and antenna for one of the upgraded seismic stations on Kīlauea.

A map of ShakeOut scenario shaking in southern California
December 31, 2008

A map of ShakeOut scenario shaking in southern California

A map of ShakeOut scenario shaking in southern California.

Map showing locations of ANSS stations
November 30, 2000

Map of ANSS Stations

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

Installation of seismometers to monitor seismicity

Installation of seismometers to monitor induced seismicity

Bryant Platt digs a hole to install seismometers at a home in southern Kansas. Seismometers are in the foreground.

Attribution: Earthquake Hazards
Interactive map showing earthquake scenario data

Scenario Earthquake Map

Interactive map showing earthquake scenario data

Attribution: Natural Hazards