Investigating Earthquakes through Regional Seismicity

Section 3 Answer Key

Measuring Earthquakes The tricks and tools of seismology

Activity #1: Intensity Maps

Exercise 1
1. Answers will vary, but most of the isoseisms should not have been simple circles or ellipses.

2. 1. You would expect the epicenter to be located at the center of the innermost circle or ellipse -- the "bull's-eye".

   2. Answers will vary.

   3. Answers will vary.

3. 1. Answers will vary.

   2. No, the map you drew needs not be just like the example to be valid. The set of data is limited and probably contains significant errors, so many somewhat-different interpretations of it are equally valid; there is no single, "correct" answer.

4. Answers will vary for all questions; suggested responses are given below.
   1. Probably not.

   2. They'd almost certainly be more likely to notice, since the mattress would give as you sat down.

   3. The building sitting atop soft alluvium would probably experience greater shaking.

   4. Yes, there do appear to be some correlations between the terrain of southern California and the irregularities in the isoseisms on the map, and they seem to match the assessment made using the analogy above.

Exercise 2
1. Answers will vary.

2. Answers will vary, but most people probably will find one or more intensity reports that were not easily placed in one intensity rating or another.

3. Answers will vary.

4. Answers will vary.

Activity #2: Seismic Waves

1. Answers will vary, but there should have been little problem distinguishing the waves. The main confusion may result from perspective, which could make S waves and Love waves look similar.
   1. P waves produce a back-and-forth motion in the direction of propagation. This motion does not vary with depth.
      S waves produce a side-to-side motion perpendicular to their direction of propagation. This motion does not vary with depth.
      Love waves generate a side-to-side motion perpendicular to their direction of propagation. The amplitude of this motion decreases with depth.
      Rayleigh waves produce a rolling motion that consists of a combination of motion perpendicular and parallel to the direction in which they propagate. The amplitude of this motion decreases with depth.
      

   2. Answers will vary, but the seismograms should have helped.

   3. With only a single all-component seismogram, it would be nearly impossible to tell the difference between the wave types.

2. 1. The S wave could have shown up on the seismograms generated by either the north-south (N) component or the vertical (Z) component of the seismometers, since both directions are perpendicular to the direction of propagation (west to east).

   2. No; theoretically, Love waves should be invisible to the vertical component of a seismometer.

3. 1. The Love waves produced greater oscillations at the surface than they did deeper into the interior of the block; S waves produced the same amount of oscillation at every depth.

   2. Rayleigh waves also displayed this limitation.

   3. P and S waves are called body waves.

4. 1. Surface waves should always propagate horizontally.

   2. Like other waves do, seismic waves should reflect and refract when they reach a boundary between two different types of material.

   3. The Rayleigh wave exhibits motion similar to a P wave; it is the polarized form of the P wave. Love waves are similar in motion to S waves; they are the polarized form of S waves.

5. 1. No. Amplitude might be helpful if there were a way to know how great the amplitude of the oscillation was when the wave first began, but you don't have that information.

   2. Yes; the waves have different velocities, which causes them to spread out as they travel. Assuming the two waves began at the same time and place, and also assuming you know their velocities, you could determine the distance to their source by noting the difference in the time each wave passed you, and calculating their origin time, which you could then use to figure out the distance the waves travelled to reach you.

   3. Being able to determine the distance from a seismometer to the source of a seismic wave -- the fault rupture (earthquake) itself -- would be fundamental to locating where, exactly, earthquakes originate inside the Earth.

Activity #3: Locating Earthquakes

1. 1. Answers will vary.

   2. Answers will vary.

   3. Answers will vary.

   4. Answers will vary.

2. 1. Answers will vary.

   2. There would be 9 km of uncertainty in the radius of the travel-time circle (which should be 90 km in radius).

   3. A better velocity model for calculating travel times could reduce the error. However, to use a better velocity model would require knowing the location of the hypocenter before you find its location using the travel-time circles!

   4. [No answer required; this point settles the apparent contradiction above.]

Activity #4: The Richter Scale

Answers will vary for all questions; suggested responses are given below.
1. 1. Though there is room for some error in the methods used, hopefully your answers were reasonably similar to the ones given in this list.

2. The arrow marking the S-wave arrival was the most difficult to position correctly. By comparison, the arrival of the P wave was generally pretty obvious, and marking the maximum amplitude was trivial.

3. The maximum amplitude on each trace generally occurred very soon after the arrival of the S wave.
   1. Yes; the P wave could be thought of as a warning sign, giving a person a few seconds to recognize the disturbance as an earthquake and take cover appropriately.

4. 1. No, the best you could do would be to find a minimum Richter magnitude for the earthquake.

   2. Answers will vary greatly; a few possibilities are: Use records from stations outside of your network, where amplitudes were smaller. Project the likely amplitude of the stylus, were it long enough to trace out the deflection properly (without "pinning"). Develop a different means of measuring the magnitude of very large earthquakes, one that is not dependent upon amplitude, but perhaps on duration of signal, or amplitude of the P wave only, etc.

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