Geology Assignment. Loma Prieta Earthquake San Francisco is located in a tectoni

Question

Geology Assignment.

Loma Prieta Earthquake San Francisco is located in a tectonically active region, so it occasionally experiences strong earthquakes. Figure 1 is a map showing the kinds of Earth materials upon which buildings have been constructed in a portion of San Francisco. These materials include hard compact Franciscan Sandstone, uncompacted beach and dune sands, river gravel, and artificial fill. The artificial fill is mostly debris from buildings destroyed in the great 1906 earthquake that reduced large portions of the city to blocks of rubble. Also note that three locations have been labeled X, Y, and Z on Figure 1. Imagine that you have been hired by an insurance company to asses what risk there may be in building newly constructed apartment buildings located at X, Y, and Z on Figure 1. Your job is to infer whether the risk of property damage during strong earthquakes is low (little or no damage expected) or high (damage can be expected). All that you have as a basis for reasoning is Figure 1 and knowledge of your experiments with the liquefaction models San Francisco Bay Marina Green 0.2 0 500 m River gravel XFranciscan sandstone member Z Artificial fill A Seismic station 10 20 30 Y Modern beach and dune sands Time(s) Figure 1 Map ofthe nature and distribution of Earth materials on which buildings and roads h ave been constructed for a portionof San Francisco, California. Figure 2 Seismograms recorded at Stations X, Y, and Z, foir a strong (Richter Mag nitude 4.6) aftershock ofthe Loma Prieta, California, earthquake. During the earthquake, little damage occurred at X, but sig nificant damage to houses occurred at Y and Z. 1. In relation to the seismograms (Figure 2), rank the seismograms according to the amount of shaking

Explanation / Answer

1. In relation to the seismograms shown in the figure 2, the localities can be ranked as:

X - 3, Y - 1, Z- 2 .

2. The risk at location X is the least.

Explanation : Location X is underlain by hard and compacted Franciscan sandstone which lessens the shaking of the ground to some extent.

3. The risk at location Y is greater than X but less than that at Z.

Explanation : Location Y being situated in a region of uncompacted beach and dune sands, river gravel is at a greater risk than location X because here the soft subsurface materials amplify ground shaking which is the primary cause of damage caused during an earthquake.

4. The risk at location Z is the greatest.

Explanation : Greatest risk is at location Z because shaking is stronger where the shear wave velocity is lower. Location Z being underlain by artificial fill has the strongest amplification of shaking which results in maximum damage.

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