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Recommended textbook solutions Environmental Science for AP2nd EditionAndrew Friedland, Rick Relyea 965 solutions Environmental Science15th EditionBradley Smith, Eldon D. Enger 427 solutions Natural Disasters11th EditionPatrick L. Abbott 465 solutions Earth Science14th EditionEdward J. Tarbuck 957 solutions 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 invented by the Chinese philosopher Chang Heng in A.D. 132. This did not, however, record earthquakes; it only indicated that an earthquake was occurring. The first seismograph was developed in 1890. A seismograph is securely mounted onto the surface of the earth so that when the earth shakes, the entire unit shakes with it EXCEPT for the mass on the spring, which has inertia and remains in the same place. As the seismograph shakes under the mass, the recording device on the mass records the relative motion between itself and the rest of the instrument, thus recording the ground motion. In reality, these mechanisms are no longer manual, but instead work by measuring electronic changes produced by the motion of the ground with respect to the mass. A seismogram is the recording of the ground shaking at the specific location of the instrument. On a seismogram, the HORIZONTAL axis = time (measured in seconds) and the VERTICAL axis= ground displacement (usually measured in millimeters). When there is NO EARTHQUAKE reading, there is just a straight line except for small wiggles caused by local disturbance or "noise" and the time markers. Seismograms are digital now - there are no more paper recordings. 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. There are many different ways to measure different aspects of an earthquake:
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Earthquakes happen when there is a sudden movement of the rocks along a fault inside the Earth. As the rocks on either side of the fault move past each other, they send seismic waves through the Earth, radiating outwards like the ripples produced when a stone is dropped into a pond. Earthquake detectionA seismogram is a record of the ground motions caused by seismic waves from an earthquake. A seismograph or seismometer is the measuring instrument that creates the seismogram. Almost all seismometers are based on the principle of inertia, that is, where a suspended mass tends to remain still when the ground moves. An illustration of a simple seismometer. The suspended mass tends to stay still, due to its inertia, when the ground moves. The pen records the relative motion. BGS ©UKRI. All rights reserved. Seismometers allow us to detect and measure earthquakes by converting vibrations due to seismic waves into electrical signals, which we can then display as seismograms on a computer screen. Seismologists study earthquakes and can use this data to determine where and how big a particular earthquake is. To record the actual motion of the ground in all three dimensions, seismologists need to use three separate sensors within the same instrument. Each sensor records the vibrations in a different direction:
A three-component seismometer. Z (red) measures up/down motion; E (green) measures east/west motion; N (blue) measures north/south motion. BGS ©UKRI. All rights reserved. Seismic wavesThere are two basic types of seismic wave that travel through the body of the Earth: P-waves and S-waves. P-waves are longitudinal waves that consist of a series of compressions and dilations along the direction of travel. The P stands for primary because they travel the fastest. S-waves are transverse waves, whose motion is perpendicular to the direction of travel. The S stands for shear or secondary since they are slower than P-waves. Where a free surface is present (like the Earth/air interface) these two types of motion can combine to form surface waves, which produce a type of shaking that causes buildings to fail and fall down. There are two types of surface waves: Rayleigh waves and Love waves. Rayleigh waves are generated by the interaction of P- and S-waves at the surface of the Earth, while Love waves are generated by interference of multiple shear waves. The ground motions from surface waves are often much larger than those motions from body waves.
Animation and captions courtesy of L Braile, Purdue University www.ics.purdue.edu/~braile Note the deformation of the black rectangle as the wave propagates through it. Animation and captions courtesy of L Braile, Purdue University www.ics.purdue.edu/~braile How are earthquakes located?Earthquakes generate different types of seismic waves and these travel at different speeds through the Earth. P-waves are fastest and are the first signal to arrive on a seismogram, followed by the slower S-wave, then the surface waves. The arrival times of the P- and S-waves at different seismometers are used to determine the location of the earthquake. Assuming that we know the relative speed of P- and S-waves, the time difference between the arrivals of the P- and S-waves determines the distance the earthquake is from the seismometer.
Seismogram showing the arrival time sequence of P-, S- and surface waves from a distant earthquake. ©UKRI. All rights reserved. By looking at the seismograms from different recording stations, we can find out the epicentre of the earthquake. The signals arrive first at the closest station and last at the one furthest away. The time difference between the P- and S-waves tells us the distance the earthquake is from the seismometer. If we calculate the S minus P time to determine distance from the seismometer at three stations, we can work out where the epicentre of the earthquake is. Imagine A, B and C are three different seismometer stations at distant locations. Once we know the distance to an earthquake from three seismic stations, we can determine the location of the earthquake. Draw a circle around each station with a radius equal to its distance from the earthquake. The earthquake occurred at the point where all three circles intersect. BGS ©UKRI. All rights reserved. How are earthquakes measured?Measurement of the severity of an earthquake can be expressed in several ways, but the two most common scales used by seismologists are intensity and magnitude. Earthquake intensityIntensity is a qualitative measure of the strength of shaking caused by an earthquake determined from the observed effects on people, objects and buildings. For a given earthquake, the intensity normally decreases with distance from the epicentre. There are a number of different intensity scales in use around the world that are all based on the shaking people experience and the effects it has on objects and buildings. It is also possible to estimate intensity from recordings of ground motions. In the UK, we use the European Macroseismic scale (EMS) to quantify the effect of earthquake shaking on people, objects and buildings. Estimates of intensity from different locations can be combined to make macroseismic maps that show how the strength of shaking varies. Macroseismic intensities (EMS) for the magnitude 3.1 ML earthquake on 23 January 2020, near Stockton-on-Tees, UK. The yellow star shows the earthquake epicentre. Intensities are calculated in 2 km grid squares from over 840 reports from people who felt the earthquake. A minimum of five observations are needed in any grid square to calculate a value of intensity, otherwise the value is recorded as ‘Felt’, but no intensity is calculated (shown by grey squares). Blue squares indicate that reports from these locations suggest that the earthquake was not felt. BGS © UKRI. All rights reserved. Since earthquake intensity can be related to what people feel or experience during the shaking from an earthquake it is possible to use people as ‘intensity sensors’ and collect earthquake-felt reports for significant earthquakes. For example:
Scientists can make use of these felt-report surveys to compare the size of recent earthquakes to ones that occurred before modern scientific measurements (from about 1970 in the UK), but for which contemporary written reports are available. In this way earthquake records in the UK can be extrapolated back to the 14th century.
Abstracted from Grünthal, G (editor), Musson, R, Schwarz, J, and Stucchi, M. 1998. European Macroseismic Scale 1998. Cahiers du Centre Européen de Géodynamique et de Séismologie Vol. 15. (Luxembourg: European Center for Geodynamics and Seismology.) Earthquake magnitudeMagnitude is a measure of the amount of energy released during an earthquake and can be estimated from the amplitude of ground motions recorded by seismometers. It is independent of distance from the epicentre. A number of different magnitude scales have been developed based on the amplitude of different parts of the observed record of ground motion with specific corrections for distance. Earthquake magnitude scales are logarithmic, i.e. a one unit increase in magnitude corresponds to a tenfold increase in amplitude. The first magnitude scale was developed by Charles Richter in 1935 using observations of earthquakes in southern California and, although the scale is only strictly applicable there, it has been used all around the world. Press reporters love the Richter scale and will report any earthquake magnitude as a ‘magnitude on the Richter scale’. However, for any large earthquake that has made the news, it is very unlikely that a Richter (or local) magnitude is an appropriate scale. Due to the complexities of the calculations, the reported magnitude of events can change as more data gets analysed. For the devastating 24 December 2004 event in Sumatra, the original magnitude of Mw 9.0 was recalculated several months later to Mw 9.3 as more data was analysed. Fast fact We can’t directly measure the size of an earthquake, because of all the complex and inaccessible geology that lies hidden beneath the surface of the Earth. Seismologists have to use various methods to try and measure earthquakes by the effects they produce at the surface. Moment magnitudeNowadays, the most standard and reliable measure of earthquake size is moment magnitude (Mw), which is based on seismic ‘moment’. Moment is related to the area of the earthquake fault rupture and the amount of slip on the rupture, as well as the strength of the rocks themselves. Richter’s original magnitude scale underestimates the size of large events, so the constants used in the definition of Mw were chosen so that the magnitude numbers for Richter and moment magnitudes match for smaller events. More about magnitudeSeismic moment (Mo) = μ × rupture area × slip length where μ is the shear modulus of the crust (approximately 3 × 1010 N/m) Moment magnitude (Mw) = 2/3(log Mo) – 6.06 Originally earthquake magnitudes were based on the amplitude of ground motion displacement as measured by a standard seismograph. The best known of these is the Richter magnitude, which was defined for local earthquakes in southern California. ML = (log A) + 2.56(log D) – 1.67 Where A is the measured ground motion (in micrometres) and D is the distance from the event (in kilometres). This is still used for measuring the magnitude of shallow events at distances less than 600 km (today called the local magnitude or ML). For events larger than ML 8, this scale saturates and gives magnitude estimates that are too small. For earthquakes measured at distances greater than 600 km, magnitude can be estimated from the formula: Mb = log(A/T) + σ(D,h) Where A is the maximum amplitude (in micrometres) of the P-waves measured at period T (generally about one second) and σ is a calibration term (in the range 6–8) that depends on distance from the event D and depth of the event h (tables of σ are used). For shallow earthquakes (i.e. ones that generate surface waves) magnitudes can be estimated using the formula: MS = log(A/T) + 1.66(log Δ) + 3.3 Where A is the maximum amplitude (in micrometres) of the Rayleigh waves, T is the period (usually about 20 seconds) and Δ is the distance (in degrees). Seismic energyEarthquake energy is a function of magnitude. Both the magnitude and the seismic moment are related to the amount of energy that is radiated by an earthquake.
How do geologists use seismographs to predict earthquakes?Seismometers allow us to detect and measure earthquakes by converting vibrations due to seismic waves into electrical signals, which we can then display as seismograms on a computer screen. Seismologists study earthquakes and can use this data to determine where and how big a particular earthquake is.
How do geologists use data from seismographs?By using wave arrival times recorded on seismographs at multiple stations, seismologists can apply triangulation to pinpoint the location of the epicenter of an earthquake. At least three seismograph stations are needed for triangulation.
What can data from a seismograph tell us about an earthquake?Seismograms are used to determine the location and magnitude of earthquakes. An earthquake's magnitude may be considered to vary as a function of the amount of energy released at the rupture point.
What statement describes a seismograph?Which statement describes a seismograph? It has a flat graphing surface. It is used to locate an earthquake's focus.
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Which statement describes how geologists use data from seismographs to learn about earthquakes?
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