What makes Los Angeles prone to earthquakes?

Understanding Plate Tectonics

Los Angeles is extremely prone to earthquakes, and earthquakes present a number of unique hazards which include property damage, loss of life, tsunamis, coastal flooding, mudslides, landslides, brush fires, and power outages. The damages are always worsened by the fact that most public utilities will not be working after a large quake. Power and water are often crippled due to the damage to infrastructure (Petak & Elahi, 2001).

A study from the US Geological Survey says the chance of a magnitude 8 quake, or larger, in the next 30 years has doubled since 2008. Experts say a major quake along the San Andreas fault in southern California could kill 1800 people, injure 53,000, destroy 1500 buildings and damage 300,000 more. A serious earthquake could also cripple the city’s water supply and take as long as six months to repair (Euronews, 2017).

When earthquakes strike, there is always damage and as such, the cost of living in these areas is always higher than non-earthquake prone areas. Despite the risk of earthquakes in LA, there has been surprisingly little done to restructure many areas that were built prior to the understanding of the risk posed by earthquakes in LA. Much of this is due to lack of understanding the origin and causes of earthquakes in LA. 

LA is built along the San Andreas fault. In accordance with plate tectonics, this fault where the two tectonic plates of North American and Pacific meet. Los Angeles rests on the Pacific Plate (Murck, Skinner, & Mackenzie, 2010). These plates move approximately 2 centimeters per year (Murck, Skinner, & Mackenzie, 2010). It is the motion of these plates which allows for geographic features such as mountains to be created. However, the motion of these plates also fuels the risk of earthquakes. As the North American and Pacific Plates move they often grind at each other which causes an earthquake (Murck, Skinner, & Mackenzie, 2010). What makes LA and California prone to so many earthquakes are the presence of hundreds of fault lines similar to the San Andreas. As a result of these large numbers of faults there are over 10,000 earthquakes per year in California. Most of these quakes are small under 3.0 magnitude, but large earthquakes happen frequently enough to give cause for alarm.

Courtesy California Government Caltrans

This timeline of earthquakes represents the most impactful quakes since 1857. The 1857 Fort Tejon earthquake Most notable was a 7.9 magnitude and altered the landscape, stream flows, as well as causing considerable damage (USGS, 2018). This quake was the last major quake of this magnitude. To put this quake into context, the 1994 earthquake in Northridge which was a 6.7 magnitude quake (Petak & Elahi, 2001). The Northridge quake was so strong that it could be felt in Las Vegas (Petak & Elahi, 2001). The quake killed 57 people and injured 8,700. Damage from the quake was estimated at $50 billion (Petak & Elahi, 2001). Because of the close proximity of the tectonic plates to LA, these disasters are a reality and have a high potential for occurrence. It is highly likely that another quake of this magnitude will occur in LA in the future. The real problem is that many of the older buildings and infrastructure have not been built or restructured to withstand high magnitude quakes. The less prepared LA is for the larger magnitude quakes- the higher the damage and death toll. A 7.8 magnitude quake struck San Francisco in 1906 and the city was not structured for such an event. The quake killed 3,000 people left 200,000 people homeless (California Academy of Sciences, 2018). If LA were struck with any quake above 7.0 it is likely that thousands of people will die and the damage cost would be in the billions.

References

California Academy of Sciences. (2018). The Great San Francisco Earthquake of 1906. Retrieved from California Academy of Sciences: https://www.calacademy.org/explore-science/the-great-san-francisco-earthquake-of-1906

Euronews. (2017, September 21). Is California ready for the “Big One”? . Retrieved from Euronews: http://www.euronews.com/2017/09/21/is-california-ready-for-the-big-one

Murck, Skinner, & Mackenzie. (2010). Visualizing Geology. Wiley.

Petak, W. J., & Elahi, S. (2001). The NORTHRIDGE EARTHQUAKE, USA and its ECONOMIC AND SOCIAL IMPACTS. EuroConference on Global Change and Catastrophe Risk Management Earthquake Risks in Europe, IIASA, School of Policy Planning and Development. Los Angeles: University of Southern California.

USGS. (2018). The Great M7.9 1857 Fort Tejon Earthquake. Retrieved from USGS: https://earthquake.usgs.gov/earthquakes/events/1857sca/

Golden State Freeway Failure January 17, 1994 By Robert A. Eplett — This image is from the FEMA Photo Library., Public Domain