Spoiler alert: nothing good!
As Carol described in their totally awesome blog post that you should check out, the city of Philadelphia is a relatively average place when it comes to weather, and has a pretty low risk of being affected by any serious geological hazards. That being said, if even a minor geohazard were to hit this city in the right way, the damage would be catastrophic.
According to my super-cool aunt, Rachel Davidson, a professor in Civil and Environmental Engineering and a core faculty member in the Disaster Research Center at the University of Delaware, the risk associated with geohazards can be determined through three major factors. First, you have the hazard and its size, which in it of itself does not have to be a problem. Then, you have the exposure of that hazard, or the factor of the valuable assets involved (ie. people and property). This factor is crucial because if you have an earthquake in the middle of a desert where no one lives, that earthquake is likely to have little effect on any valuable assets. Lastly, the vulnerability of the assets affected plays a huge role in the risk that a certain location takes on.
Now, in the case of Philadelphia, when considering earthquakes in particular, we know that the probability of any hazardous seismic event occurring is very low. However, given the concentration of the assets involved as well as the fragility of those assets, Philadelphia would undoubtedly be considered a high risk city.
Figure 1: Earthquakes M3 and above in the United States between 1973 and 2014
To start, the concentration of assets in Philadelphia is extremely high, which comes as no surprise given that it is the sixth largest city in the United States. Within Philadelphia County alone, the population is 1,591,800, there are 11,379.5 people per square mile, and there are 594,778 occupied households. Additionally, Philadelphia County is home to 30 hospitals, 63 fire stations, 44 hazardous material sites, and over 320 bridges.
Given that Philadelphia is such an old city, these assets (buildings in particular) are highly vulnerable. The buildings that are typically the most susceptible to damage in the case of an earthquake are unreinforced masonry buildings that lack steel support. Philadelphia is very much characterized by this style of architecture as many of the buildings in this city were built long before people worried about hazards such as earthquakes. To name a few important buildings of this nature that would likely be affected by a significant seismic event, there is Independence Hall (where the Declaration of Independence and U.S. Constitutions were signed), Philadelphia City Hall (the largest free standing masonry building in the world), and my house (which is now just over 200 years old).
Figure 2: Independence Hall (Left); Figure 3: City Hall (Center); Figure 4: My house (Right)
As shown plainly in Figure 5 below, where all of the shaded buildings are designated as historic, the concentration of such buildings that are likely to be vulnerable is very high, especially in the city center (which this map displays). This combination of high vulnerability and high density can only mean bad things for the city of Philadelphia, if an earthquake were ever to occur.
Figure 5: Designated Historic Buildings At Risk
References
Tantala, Michael, et al. “Earthquake Loss Estimation Study for the New York City Area.” NYCEM: The New York City Area Consortium for Earthquake Loss Mitigation, 2003, www.nj.gov/drbc/library/documents/Flood_Website/NJmitigation/FEMAToolkit072806draft/NYCEM_report.pdf.
Eckstein, Carol. Philadelphia, PA: The Land of Mild Weather and Homicidal People, Blogger, 2 Nov. 2020, hometownhazards.blogspot.com/2020/11/philadelphia-pa-land-of-mild-weather.html.
“U.S. Geological Survey.” USGS.gov | Science for a Changing World, www.usgs.gov/.
Potter, Ned, and Jane E Allen. “Earthquake: East Coast Cities Not Prepared, Say Experts.” ABC News, ABC News Network, 23 Aug. 2011, 2:18 PM, abcnews.go.com/Technology/earthquakes-east-coast-states-unprepared-emergency-management-experts/story?id=14365448.
Jaffe, Alan. “Philadelphia to Be First Big City with Disaster Plan to Protect Historic Buildings.” WHYY, WHYY, 25 Apr. 2016, whyy.org/articles/philadelphia-to-be-first-big-city-with-disaster-plan-to-protect-historic-buildings/.
“U.S. Census Bureau QuickFacts: Philadelphia County, Pennsylvania.” Census Bureau QuickFacts, www.census.gov/quickfacts/philadelphiacountypennsylvania.
Davidson, Noah M, and Rachel A Davidson. “Geohazards Interview.” 22 Nov. 2020.
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