# measuring earthquake magnitude

## measuring magnitude

A seismograph produces a graph-like representation of the seismic waves it receives and records them onto a seismogram (Figure 1.1). Seismograms contain information that can be used to determine how strong an earthquake was, how long it lasted, and how far away it was. Modern seismometers record ground motions using electronic motion detectors. The data are then kept digitally on a computer. If a seismogram records P-waves and surface waves but not S-waves, the seismograph was on the other side of the Earth from the earthquake. The amplitude of the waves can be used to determine the magnitude of the earthquake, which will be discussed in a later section.

## interpreting a seismogram

The seismogram in the introduction shows: foreshocks. the arrival of the P-waves. the arrival of the S-waves. the arrival of the surface waves (very hard to pick out). aftershocks. the times when all of these things occur. These seismograms show the arrival of P- waves and S-waves. The surface waves arrive just after the S-waves and are diffi- cult to distinguish. Time is indicated on the horizontal portion (or x-axis) of the graph. Click image to the left or use the URL below. URL:

## instructional diagrams

No diagram descriptions associated with this lesson

## questions

a seismograph from an earthquake with an epicenter about 200 miles away shows

``````a) a large arrival for the first p-waves and then a tapering off.

-->  b) a large arrival for the first p-waves, then a falling off, then a large arrival for the first s-

c) a small arrival for the first p-waves then a large arrival for the first s-waves.

d) random arrivals of waves.
``````

the first wave produced by an earthquake is called

``````-->  a) a primary wave

b) a secondary wave

c) a first wave

d) none of the above
``````

a seismogram produces a graph-like representation of the seismic waves it receives and records them onto a seismograph.

``````a) true

-->  b) false
``````

surface waves are hard to find on a seismograph because they

``````a) dont travel through liquid.

b) are very small.

c) arrive before the primary waves when no one is looking.

-->  d) arrive very shortly after the secondary waves.
``````

to locate an earthquakes epicenter, you need to know the distance to the epicenter from at least ____ seismic station.

``````a) one

b) two

-->  c) three

d) four or more
``````

seismograms contain information that can be used to determine

``````a) how strong an earthquake was.

b) how long an earthquake lasted.

c) how far the earthquake was.

-->  d) all of the above.
``````

the amplitude is

``````a) the distance of the top of one wave to the top of the next wave.

-->  b) between the highest peak and lowest trough of a wave.

c) the length of time that it took for the seismic waves came in.

d) the distance between the p-wave and s-wave first arrival.
``````

the wavelength of the waves can be used to determine the magnitude of an earthquake.

``````a) true

-->  b) false
``````

seismograph that records p-waves but not s-waves is

``````a) malfunctioning.

b) very far away from the earthquake.

-->  c) on the other side of the planet from the earthquake.

d) the wrong type of recorder to use on this type of earthquake.
``````

what is the difference between a seismograph and a modern seismometer?

``````-->  a) a seismograph records on a seismogram; a seismometer records digitally.

b) a seismograph needs to record more seismic stations than a seismometer.

c) a seismograph does not give the location of the epicenter but a seismometer does.

d) all of the above
``````

## diagram questions

No diagram questions associated with this lesson