grade levels: 9-12
achievement standards for this lesson
Earthquakes release energy
in two different forms that travel through the earth differently, and which have
different energies associated with them. Earthquake energy is measured by an instrument
known as a seismograph which measures the back-and-forth and side-to-side motion
of the earth's surface as seismic waves pass by. Seismic body waves move through
rock in a compression - relaxation mode, analogous to sound waves traveling through
air. These waves travel fastest through rock. Earthquake magnitude determined
from measurement of body wave motion (MB) is based on the maximum observed ratio
of wave amplitude to period for body waves arriving in the first 5 seconds of
a record (U.S. Geological Survey practice). Seismic surface wave energy is carried
in a side-to-side motion and arrives slightly later than body waves. Large earthquakes
tend to generate more surface wave amplitude relative to body wave amplitude.
The opposite is true for small events. Thus, earthquake magnitude determined from
surface wave energy (MS) is dependent on the amount of total energy released at
the source or epicenter. Local magnitude (ML) is the commonly quoted earthquake
magnitude originally defined by Richter. Because this measure of magnitude was
originally defined from the amplitude measured on a specific type of seismograph
at a specific distance (100 km) from an earthquake and for a specific region (California),
empirical calibration curves must be used to convert seismograph information at
an arbitrary distance in any given region to that expected at 100 kilometers from
a California quake, for different types of seismographs. For a given event, MB,
MS, and ML are generally different. Approximate relationships between these three
scales of earthquake magnitude have been deduced as follows: MB = 1.7 + 0.8 ML
- 0.01ML2 MB = 0.56 MS + 2.9 The importance of determining magnitude is that it
permits earthquakes to be classified on the basis of the elastic energy released
at the epicenter. The relationship between magnitude and energy released (E) is:
logE = 12.24 + 1.44 MS Thus, a one-unit increase in magnitude implies a 30-fold
increase in total elastic energy release!
Print the map, by clicking
below, which shows Idaho earthquake magnitudes measured by seismographs between
1935 and 1993. Turn off the appropriate layers to show earthquakes of greater
than magnitude 3.5 but less than 4.5 (expressed as body wave magnitude, MB);
of greater than magnitude 4.5 and less than 5; and of greater than magnitude
5. Magnitudes greater than 6 are shown as large symbols. Note that because body
wave magnitude is reported, the maximum values for the largest events may be
smaller than commonly reported as Ms. Print
& Figure 7,
then complete the activities below.
Table 1 lists
Idaho earthquakes with surface wave magnitudes greater than 5 that have occurred
between 1884 and 1994; in Table 2, enter the number of earthquakes having magnitudes
greater than or equal to each value listed; plot these values against magnitude
in a graph like that shown in Figure 7. As an example, the first and last two
values have already been entered in Table 2 and plotted in Figure 7.
These are links to access
the handouts and printable materials.
geol3ho.pdf | table1
| Figure 7
| CAD map6
the map, which county and which geologic province have had the most earthquakes?
2. Excluding areas outside the state, where do most felt earthquakes (M>4)
occur in Idaho?
3. Considering only earthquakes with magnitudes greater than 4.5, where do
these occur relative to Holocene fault activity?
Lesson Plan provided by Vita
Idaho Achievement Standards (as of 7/2001) met
by completing this activity: