Idaho Earthquakes
suggested grade levels: 9-12

view Idaho achievement standards for this lesson

Overview:
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!

Procedure:
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 Table 1, Table 2 & 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.

Handouts/Activities:
These are links to access the handouts and printable materials.
geol3ho.pdf | table1 | table2 | Figure 7 | CAD map6

Handout Sample:
Answer the questions below.

1. Using the map, which county and which geologic province have had the most earthquakes? the fewest?
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?

Related Lesson Topics:
Geology: Geology Topics

Lesson Plan provided by Vita Taube, 2000
Idaho Achievement Standards (as of 7/2001) met by completing this activity:

351.04
352.01
357.01
357.02