Measuring Earthquakes

 

Q: Where can I buy a Richter scale?

A: The Richter scale is not a physical device, but a mathematical formula. The magnitude of an earthquake is determined from the logarithm of the amplitude of waves recorded on a seismogram at a certain period.

 

Q: How are earthquakes recorded? How are earthquakes measured? How is the magnitude of an earthquake determined?

 

A: 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 block of rock over another in an EQ releases energy that makes the ground vibrate. That vibration pushes the adjoining piece of ground and cause it to vibrate and thus the energy travel out from the EQ 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 measures the largest wiggle on the recording, but other magnitude scales measure different parts of the earthquake. Intensity is a measure of the shaking and damage caused by the earthquake, and this value changes from location to location.

 

Q: What are the different magnitude scales, and why are there so many?

 

A: 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 the L standing for local. This is what was to eventually become known as the Richter magnitude.

As more seismograph stations were installed around the world, it became apparent that the method developed by Richter was strictly valid only for certain frequency and distance ranges. In order to take advantage of the growing number of globally distributed seismograph stations, new magnitude scales that are an extension of Richter's original idea were developed. These include body-wave magnitude, mb, and surface-wave magnitude, Ms. Each is valid for a particular frequency range and type of seismic signal. In its range of validity each is equivalent to the Richter magnitude. Because of the limitations of all three magnitude scales, ML, mb, and Ms, a new, more uniformly applicable extension of the magnitude scale, known as moment magnitude, or Mw, was developed. In particular, for very large earthquakes moment magnitude gives the most reliable estimate of earthquake size. New techniques that take advantage of modern telecommunications have recently been implemented, allowing reporting agencies to obtain rapid estimates of moment magnitude for significant earthquakes.

 

Q: Why are there often different magnitudes reported for the same earthquake?

 

A: When an earthquake occurs, the first information that is processed and relayed is usually based on a small subset of the seismic stations in the network, especially in the case of a larger earthquake. This is done so that some information can be obtained immediately without waiting for all of it to be processed. As a result, the first magnitude reported is usually based on a small number of recordings. As additional data are processed and become available, the magnitude and location are refined and updated. Sometimes the assigned magnitude is "upgraded" or slightly increased, and sometimes it is "downgraded" or slightly decreased.

Sometimes the earthquake magnitude is reported by different networks based on only their recordings. In that case, the different assigned magnitudes are a result of the slight differences in the instruments and their locations with respect to the earthquake epicenter.

 

Q: What is "moment magnitude"?

 

A: Moment is a physical quantity proportional to the slip on the fault times the area of the fault surface that slips; it is related to the total energy released in the EQ. The moment can be estimated from seismograms (and also from geodetic measurements). The moment is then converted into a number similar to other earthquake magnitudes by a standard formula. The results is called the moment magnitude. The moment magnitude provides an estimate of earthquake size that is valid over the complete range of magnitudes, a characteristic that was lacking in other magnitude scales.

 

Q: What are the earthquake magnitude classes?

 

A:

Great; M > =8
Major; 7 < =M < 7.9
Strong; 6 < = M < 6.9
Moderate: 5 < =M < 5.9
Light: 4 < =M < 4.9
Minor: 3 < =M < 3.9
Micro: M < 3

 

Q: How do you give a Richter magnitude to earthquakes that occurred prior to the scale?

 

 A: 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 faulting, landslides, sandblows or river channel changes) plus the human effects (such as area area of damage or felt reports or how strongly a quake was felt) and comparing them to modern earthquakes. Many assumptions have to be made when making these comparisons. For example, how do you compare the shaking for people living in log cabins or tents in the early 1800's with shaking for people living in high-rise steel and concrete buildings (with waterbeds!) in the 1990's? Because different researchers can get widely varying magnitudes from using different assumptions on how to make these comparisons, many of the old earthquakes have big differences in the magnitudes assigned to them. For example, magnitude estimates for the quakes that occurred near New Madrid, Missouri in 1811 and 1812 vary from the upper magnitude 6 range to as high as 8.8, all because of the choices the researchers made about how to compare the data.

 

Q: When was the first instrument that actually recorded an earthquake?

 

A: 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.

Note: original source of this image is unknown

 

Q: What is a P wave? An S wave?

 

A: When an earthquake occurs, it releases energy in the form of waves that radiate from the earthquake source in all directions. The different types of energy waves shake the ground in different ways and also travel through the earth at different velocities. The fastest wave, and therefore the first to arrive at a given location, is called the P wave. The P wave, or compressional wave, alternately compresses and expands material in the same direction it is traveling. The S wave is slower than the P wave and arrives next, shaking the ground up and down and back and forth perpendicular to the direction it is traveling. Surface waves follow the P and S waves.

Note: original source of this image is unknown

 

Q: What was the duration of the earthquake?

 

A: The duration of shaking you feel from an earthquake depends in part on the distance you are from the epicenter of the earthquake. If you are close, the shaking will be more violent, "faster", and may not last as long. If you are further away, the high-frequency "fast" shaking will have been "absorbed" into the earth's crust, you will feel are the longer-period, more rolling motions, and they may be of longer duration. In short, the duration is different in different places.

 

Q: What does an earthquake look like?

 

A: In order to study earthquakes, scientists deploy seismometers to measure ground motion. Seismograms are recordings of ground motion as a function of time and are the basic data which seismologists use to study the waves generated by earthquakes. These data are used to study the earthquakes themselves and to learn more about the structure of the Earth.

Seismologists generally describe earthquakes as local, regional, or teleseismic. These terms refer to distance from the earthquake to the recording instrument. Local events occur within the immediate area less than 100km away. Regional events occur within 10 - 1400km away. Teleseismic events are those which occur at great distances, greater than 1400km away. Local and regional earthquakes are dominated by crustal waves, i.e., by waves which propagate through the crust. At greater distances, the seismic wavefield is dominated by waves which sample the body of the earth - the upper mantle, the lower mantle, and the core.

Earthquake Examples:

Local or Near-Field Earthquake

 

Regional Earthquake

 

Q: How do seismologists locate an earthquake?

 

A: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. 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, thus 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 km. 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.

 

Q: What is intensity? What is the Modified Mercalli Intensity Scale?

 

A:

The Mercalli Scale is based on observable EQ damage. From a scientific standpoint, the Richter scale is based on seismic records while the Mercalli is based on observable data which can be subjective. Thus, the Richter scale is considered scientifically more objective and therefore more accurate. For example a level I-V on the Mercalli scale would represent a small amount of observable damage. At this level doors would rattle, dishes break and weak or poor plaster would crack. As the level rises toward the larger numbers, the amount of damage increases considerably. The top number, 12, represents total damage.

 

Q: What is the difference between intensity scales and magnitude scales?

 

A: Intensity scales, like the Modified Mercalli Scale and the Rossi-Forel scale, measure the amount of shaking at a particular location. So the intensity of an earthquake will vary depending on where you are. Sometimes earthquakes are referred to by the maximum intensity they produce. Magnitude scales, like the Richter magnitude and moment magnitude, measure the size of the earthquake at its source. So they 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 magnitude is complex and different procedures will often give slightly different magnitudes for the same earthquake.

 

Q: How much energy is released in an earthquake?

 

A: Earthquakes release a tremendous amount of energy, which is why they can be so destructive. The table below shows magnitudes with the approximate amount of TNT needed to release the same amount of energy.

Magnitude

Approximate Equivalent TNT Energy

4.0

1010 tons

5.0

31800 tons

6.0

1,010,000 tons

7.0

31,800,000 tons

8.0

1,010,000,000 tons

9.0

31,800,000,000 tons

 

Q: What is acceleration, velocity, and displacement?

 

A: Acceleration is the rate of change in velocity of the ground shaking (how much the velocity changes in a unit time), just as it is the rate of change in the velocity of your car when you step on the accelerator or put on the brakes. Velocity is the measurement of the speed of the ground motion. Displacement is the measurement of the actual changing location of the ground due to shaking. All three of the values can be measured continuously during an earthquake.

 

Q: What is spectral acceleration?

 

A: PGA (peak acceleration) is what is experienced by a particle on the ground. SA is approximately what is experienced by a building, as modeled by a particle on a massless vertical rod having the same natural period of vibration as the building.

 

Q: What are those beachball figures?

 

A: In addition to determining the location and magnitude of earthquakes, seismologists are now routinely determining the "fault plane" solutions or "focal mechanisms" of events. A fault plane solution illustrates the direction of slip and the orientation of the fault during the earthquake. These solutions, which are displayed in lower-hemisphere projections frequently described as "beachballs", can be determined from the first-motion of P-waves and from the inversion of seismic waveforms. These figures help identify the type of earthquake rupture: strike-slip, normal, or thrust. Strike-slip earthquakes are typical of the San Andreas fault zone, which forms part of the boundary between the North American and Pacific plates. Normal earthquakes are associated with extension, particularly with formation of plates at mid-ocean ridges. Thrust or reverse earthquakes are associated with compression, particularly with the subduction of one plate under another as in Japan.

 

Q: What are UTC and GMT (in reference to the time of an EQ)?

 

A: 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.

 

Q: What does it mean that the earthquake occurred at a depth of 0 km?

 

A: An earthquake cannot occur at depth of 0 km. In order for an earthquake to occur, two blocks of crust must slip past one another, and it is physically impossible for this to happen at the surface of the earth. So why do we report that the earthquake occured at a depth of 0 km sometimes? Sometimes it is simply a very shallow event with poor depth resolution, but more often it is not actually an earthquake, but a quarry blast. These explosions are recorded by the seismic network and located by the software. When they are reviewed by a seismic analyst, they are labaled as a quarry blast in the earthquake catalog.

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