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He of a single earthquake at three official seismic stations and you got three different magnitudes. How can that be? A sins ke has a single magnitude. Logically, that is the only way it can be. So what gives? To figure out this conundrum, start b his excerpt from the official USGS website that is devoted to seismology: Soil Type and Shaking Hazard in the San Francisco Bay Area 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 their occupants may be injured or killed. Seismologists have observed that some districts tend to repeatedly experience TL1

He of a single earthquake at three official seismic stations and you got three different magnitudes. How can that be? A sins ke has a single magnitude. Logically, that is the only way it can be. So what gives? To figure out this conundrum, start b his excerpt from the official USGS website that is devoted to seismology: Soil Type and Shaking Hazard in the San Francisco Bay Area 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 their occupants may be injured or killed. Seismologists have observed that some districts tend to repeatedly experience TL1

Applications and Investigations in Earth Science (9th Edition)
Applications and Investigations in Earth Science (9th Edition)
9th Edition
ISBN:9780134746241
Author:Edward J. Tarbuck, Frederick K. Lutgens, Dennis G. Tasa
Publisher:Edward J. Tarbuck, Frederick K. Lutgens, Dennis G. Tasa
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Feeling good? Maybe... you know where, when, and how big that earthquake was. But wait a minute! You measured the magnitude of a single earthquake at three official seismic stations and you got three different magnitudes. How can that be? A single earthquake has a single magnitude. Logically, that is the only way it can be. So what gives? To figure out this conundrum, start by reading this excerpt from the official USGS website that is devoted to seismology: Soil Type and Shaking Hazard in the San Francisco Bay Area a) 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 their occupants may be injured or killed. 12. Given this information, which of the following answers is the most-likely explanation of why your analysis of the October 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 experience stronger seismic shaking than others. This is because the ground under these districts is relatively soft. Soft soils amplify ground shaking. If you live in an area that in past earthquakes suffered shaking stronger than that felt in other areas at comparable distance from the source, you are likely to experience relatively strong shaking in future earthquakes as well. An example of this effect was observed in San Francisco, where many of the same neighborhoods were heavily damaged in both the 1906 and 1989 earthquakes. The influence of the underlying soil on the local amplification of earth- quake shaking is called the site effect. Other factors influence the strength of earthquake shaking at a site as well, including the earthquake's magnitude and the site's proximity to the fault. These factors vary from earthquake to earthquake. In contrast, soft soil always amplifies shear waves. If an earthquake is strong enough and close enough to cause damage, the damage will usually 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 because of the site effect, the amount of shaking experienced at each station is not truly reflective of the size of the earthquake at its place of origin. The seismic stations in Pasadena and Elko are located near faults that amplify the shaking experienced by the 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, their amplitude was diminished by the length of their travel time and thus they appear smaller at those locations and 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 separate measurements of the same thing and the three numbers are all different, what is the answer? a) the maximum value is the correct answer b) the minimum value is the correct answer the average of the maximum and minimum values is the correct answer d) 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 the distribution of earthquakes on earth. You are a nascent seismologist. 8
Transcribed Image Text:Feeling good? Maybe... you know where, when, and how big that earthquake was. But wait a minute! You measured the magnitude of a single earthquake at three official seismic stations and you got three different magnitudes. How can that be? A single earthquake has a single magnitude. Logically, that is the only way it can be. So what gives? To figure out this conundrum, start by reading this excerpt from the official USGS website that is devoted to seismology: Soil Type and Shaking Hazard in the San Francisco Bay Area a) 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 their occupants may be injured or killed. 12. Given this information, which of the following answers is the most-likely explanation of why your analysis of the October 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 experience stronger seismic shaking than others. This is because the ground under these districts is relatively soft. Soft soils amplify ground shaking. If you live in an area that in past earthquakes suffered shaking stronger than that felt in other areas at comparable distance from the source, you are likely to experience relatively strong shaking in future earthquakes as well. An example of this effect was observed in San Francisco, where many of the same neighborhoods were heavily damaged in both the 1906 and 1989 earthquakes. The influence of the underlying soil on the local amplification of earth- quake shaking is called the site effect. Other factors influence the strength of earthquake shaking at a site as well, including the earthquake's magnitude and the site's proximity to the fault. These factors vary from earthquake to earthquake. In contrast, soft soil always amplifies shear waves. If an earthquake is strong enough and close enough to cause damage, the damage will usually 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 because of the site effect, the amount of shaking experienced at each station is not truly reflective of the size of the earthquake at its place of origin. The seismic stations in Pasadena and Elko are located near faults that amplify the shaking experienced by the 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, their amplitude was diminished by the length of their travel time and thus they appear smaller at those locations and 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 separate measurements of the same thing and the three numbers are all different, what is the answer? a) the maximum value is the correct answer b) the minimum value is the correct answer the average of the maximum and minimum values is the correct answer d) 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 the distribution of earthquakes on earth. You are a nascent seismologist. 8
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