Investigating Earthquakes through Regional Seismicity

Section 1 Answer Key

What is an Earthquake?

The basic facts of seismology

Activity #1: Rescue in Pinevale

Activity #2: Fault Rupture Analogies

Activity #3: Rupture Models

Activity #4: Strike and Dip

Activity #5: Revealing a Fault Plane with Hypocenters

  1. The points plotted on the cross-section should be in a roughly linear orientation -- a "fuzzy" line.

  2. Answers will vary. Most should find an angle of about 45°, dipping toward the northeast.

  3. Depending on how you drew your line, you should find it comes closest to either the Garnet Hill fault zone or the Banning fault zone (South Branch San Andreas fault). The assumption that this is the fault that produced the mainshock may be invalid if that fault changes dip before it reaches the surface, or if that fault is truncated by another fault.

  4. Answers will vary. Some possibilities are: inaccuracies from the crudeness of this cross-section, inaccuracies in the data due to uncertainties in locating hypocenters, choosing the cross-section in an orientation not exactly perpendicular to the strike of the fault, and/or that the aftershocks are not actually occurring on the mainshock fault, but on small faults adjacent to it.

Activity #6: Scarp Formation

  1. Lateral offset created a cliff by cutting a pre-existing change in topography. Answers will vary. Yes, the solution would probably have been more obvious if you had been able to see the entire area around the scarp in advance.

  2. Some examples of correct answers:
    Scarps can be formed by water: e.g., wave-cut platforms and marine terraces, or the edges of river and stream channels. This kind of feature could also be the edge of a landslide (a debris flow) or a glacial deposit (a moraine). It could even have been man-made.

  3. [no answer required]

Activity #7: Changing the Dimensions of the Crust

  1. You would expect reverse slip in a zone of tectonic compression; and area experiencing tectonic rifting should exhibit normal slip.

  2. You could drill through the crust near the normal fault more easily; the crust is thinner here.

  3. Yes, changing the dip of these faults would make a difference in how they lengthen or shorten the crust -- as the angle of dip decreases, the amount of lengthening or shortening increases. If you change the angle of dip to increase the change in length, the change in thickness will decrease. This kind of change will also tend to increase the area over which crustal thickness is altered.

  4. You would expect faults in an area of strong tectonic compression or extension to be low-angle faults.

  5. Tectonic extension (rifting) should allow volcanism to take place more easily than would tectonic compression, since extension decreases the distance between the top of the mantle and the surface of the Earth.

Activity #8: Oblique Slip

  1. Answers will vary.

  2. Yes, all the examples displayed nearly equal dip-slip and strike-slip components.

Activity #9: Recurrence Interval

Activity #10: Different Data, Different Rates?

Activity #11: How Tectonic Forces Affect Faults

Activity #12: "Find That Fault Slip!"

Activity #13: Regional Distribution of Slip

Activity #14: Partitioning Slip

Additional Resources: