Number 14 Feeling good? Maybe... you know where, when, and how big that earthquake was. But wait a minute! You measured themagnitude of a single earthquake at three official seismic stations and you got three different magnitudes. How can that be? A singleearthquake has a single magnitude. Logically, that is the only way it can be. So what gives? To figure out this conundrum, start byreading this excerpt from the official USGS website that is devoted to seismology:Soil Type and Shaking Hazard in the San Francisco Bay Areaa)b)c)Ground shaking is the primary cause of earthquake damage to man-made structures.When the ground shakes strongly, buildings can be damaged or destroyed and theiroccupants may be injured or killed.12. Given this information, which of the following answers is the most-likely explanation of why your analysis of theOctober 18, 1989 earthquake yielded three different estimates of the Richter scale magnitude of the earthquake?d)Seismologists have observed that some districts tend to repeatedly experiencestronger seismic shaking than others. This is because the ground under thesedistricts is relatively soft. Soft soils amplify ground shaking. If you live in an area thatin past earthquakes suffered shaking stronger than that felt in other areas atcomparable distance from the source, you are likely to experience relatively strongshaking in future earthquakes as well. An example of this effect was observed in SanFrancisco, where many of the same neighborhoods were heavily damaged in both the1906 and 1989 earthquakes. The influence of the underlying soil on the localamplification of earth- quake shaking is called the site effect.Other factors influence the strength of earthquake shaking at a site as well, includingthe earthquake's magnitude and the site's proximity to the fault. These factors varyfrom earthquake to earthquake. In contrast, soft soil always amplifies shear waves. Ifan earthquake is strong enough and close enough to cause damage, the damage willusually be more severe on soft soils.The nomograph method is not valid in California.The earth upon which the three seismic station is composed of different types of rock and soil and becauseof the site effect, the amount of shaking experienced at each station is not truly reflective of the size of theearthquake at its place of origin.The seismic stations in Pasadena and Elko are located near faults that amplify the shaking experienced bythe seismometer and invalidate Richter's method of estimating earthquake magnitude.Because the seismic waves took longer to travel to Pasadena and even longer to travel to Elko, theiramplitude was diminished by the length of their travel time and thus they appear smaller at those locationsand the resulting magnitudes estimated from the ground shaking at those sites is incorrectly too small.13. So what do you do? What do scientists do? What makes the most common sense? If you have three separatemeasurements of the same thing and the three numbers are all different, what is the answer?a)the maximum value is the correct answerb)the minimum value is the correct answerthe average of the maximum and minimum values is the correct answerd)the average of all three values is the most correct answer.14. Therefore, the magnitude of this earthquake was...Now you are finished. You know where, when, and how big this earthquake was. You know all the processes necessary to study thedistribution of earthquakes on earth. You are a nascent seismologist.8

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