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* Damage at and near tunnel portals may be significant due to slope instability.<ref>{{Cite journal |author1=Hashash |first=Youssef M.A. |author2=Hook |first2=Jeffrey J. |author3=Schmidt |first3=Birger |author4=Yao |first4=John I-Chiang |date=2001 |title=Seismic design and analysis of underground structures |url=https://www.sciencedirect.com/science/article/abs/pii/S0886779801000517 |journal=Tunnelling and Underground Space Technology |volume=16 |issue=4 |pages=247–293 |bibcode=2001TUSTI..16..247H |doi=10.1016/S0886-7798(01)00051-7 |s2cid=108456041 |via=[[Science Direct]]}}</ref>
Earthquakes are one of nature's most formidable threats. A magnitude 6.7 earthquake shook the San Fernando valley in Los Angeles in 1994. The earthquake caused extensive damage to various structures, including buildings, freeway overpasses and road systems throughout the area. The National Center for Environmental Information estimates total damages to be 40 billion dollars.<ref>{{cite web|url=https://www.ngdc.noaa.gov/
This same trend of limited subway damage following an earthquake can be seen in many other places. In 1985 a magnitude 8.1 earthquake shook Mexico City; there was no damage to the subway system, and in fact the subway systems served as a lifeline for emergency personnel and evacuations. A magnitude 7.2 ripped through Kobe Japan in 1995, leaving no damage to the tunnels themselves. Entry portals sustained minor damages, however these damages were attributed to inadequate earthquake design that originated from the original construction date of 1965. In 2010 a magnitude 8.8, massive by any scale, afflicted Chile. Entrance stations to subway systems suffered minor damages, and the subway system was down for the rest of the day. By the next afternoon, the subway system was operational again.<ref>Hymon, Steve. "Designing A Subway to Withstand an Earthquake." The Source. N.p., 2017. Web. 11 November 2017. http://thesource.metro.net/2012/08/10/designing-a-subway-to-withstand-an-earthquake/
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