EARTHQUAKE INTENSITY

To complete this assignment, you should read the information in your textbook pertaining to the 1994 Northridge earthquake (see Figure 4.10 on page 80, the text on pages 89-91, and Figure 4.24 on page 91).  Note that alluvium (sediment that was deposited over the sedimentary rocks by streams flowing out of the mountains) is not shown in Figure 4.24. The thickness of alluvium varies from 0 (in the mountains) to over 1,000 feet (0.3 km) towards the center of the valley.

The U.S.G.S. collected ground motion and S-wave velocity data for the 1994 earthquake from a number of seismic stations in and around the Northridge area.  Table 1 summarizes the pertinent information from their study.  The amplification numbers represent ground motion relative to the igneous and metamorphic rocks, which experienced the least amount of motion.  Thus, the larger the amplification number, the greater was the ground motion.
 

TABLE 1:  Relative Ground Motions and S-Wave Velocities Measured at Seismic Stations
                   on Different Geologic Materials During the 1994 Northridge Earthquake

1.  Based on the data given in Table 1, what effect does geologic material appear to have on the
     magnitude of ground motion during an earthquake?

2.  Based on the descriptions in Table 1, why is there is a greater range in S-wave amplitudes for
     alluvium than there is for the sedimentary rocks?
 

3.  Based on the data given in Table 1, what relationship exists between the speed at which shock
     waves travel through rock or sediment and the degree to which shock waves are amplified? 
 

4.  Though there are a number of factors involved, shock-wave velocities are generally inversely
     proportional to their periods (the time intervals between successive wave crests).  According
     to the discussion of resonance in your textbook (on page 82), which type of geologic material
     in Table 1 would cause high-rise buildings to suffer the most damage during an earthquake?
 

5.  Shock-wave periods in sediments are also positively correlated with the sediment's thickness.
     Given this relationship, predict how the intensity of earthquake damage might vary in an area
     where the thickness of underlying sediment is quite variable.      

6.  How do the influences of geology on shock waves (your answers to the questions above) help
     to explain the pattern of earthquake intensity shown in Figure 4.10?  (To better understand the
     geography of this region, refer to these maps).