Designing a subway to withstand an earthquake

Metro’s subway rode out the 1994 Northridge earthquake, but a section of the Santa Monica Freeway collapsed and other area roads suffered serious damage.

After a pair of 4.5 magnitude earthquakes were felt throughout the Los Angeles area earlier this week, a Source reader asked this question:

What magnitude are the tunnels or stations designed to withstand?

Here is the answer from Metro’s engineering and operations staff, as well as consultants who work with Metro to design projects:

There is no specific magnitude that subways are designed to universally withstand. The strength and flexibility the subway is designed for depends on the characteristics of earthquake faults in the area and their proximity to the structure being designed. In other words, the main question engineers ask is this: how strong is the ground shaking likely to be at the tunnels and stations?

A chart from a Westside Subway Extension community presentation.

The forecasted level of ground shaking at a particular location is garnered from seismic hazard maps published by the United States Geological Survey.  Building designers and engineers use these same maps to design their projects.

Obviously, Southern California sits in the midst of well-known earthquake country (here is a list of notable earthquakes in California in the past 200 years; the largest was a 7.9-magnitude quake near Fort Tejon in 1857).  Metro’s design criteria requires that its facilities are designed to ensure both life safety and the ability to be repaired after larger earthquakes – the ones that are predicted to occur every 2500 years. At the same time, Metro’s facilities are designed to ensure continuous operation in smaller earthquakes that have a probability of recurring every 150 years.

The Metro subway system first opened in 1993. In the earthquakes since then — including the 6.7-magnitude Northridge quake in 1994 — the subway tunnels have not suffered any damage and train service was quick to return. Generally speaking, subways in many other areas have survived earthquakes with minimal or no damage — and often far less damage than is suffered by buildings and roads.

What happens when an earthquake occurs in Los Angeles County?

Metro’s Rail Operations Center quickly notifies train operators that there has been an earthquake. All trains are stopped at the next station (if not already in one) and then they are put on restricted speed to allow the train operator to inspect the tunnel for any signs of trouble or damage — in addition operations staff ensure the system works from a signaling and electronics standpoint. Once operators and Metro operations staff are positive no damage has occured, trains are allowed to return to their normal speeds.

If a larger magnitude earthquake should occur that is closer to the actual subway, additional steps may be taken. This could include suspending service to allow structural engineers to thoroughly inspect the tracks and tunnels.

5 replies

  1. I have to wonder what other words in the English language use the same spelling for both the present and past tenses (forecast; forecast). It seems to me that the developing use of the word “forecasted,” though not traditional, makes complete sense. [At the moment, the only example I can cite applies to fishing: “I will cast the line and see what I can catch. I cast the line yesterday and caught nothing.”]

    Not that any of this has anything to do with subway seismic safety!

  2. Since the use of “affect” rather than “effect” was on a slide developed by MTA (or its engineers) you are absolved from any responsibility. However, while using “forecasted” as the past tense of forecast is accepted and is unfortunately becoming more common, the traditional past tense of forecast is “forecast.” I am trying to get planners and forecasters to retain the more traditional use which truly sounds far better.

  3. I wish people would get it right the subway opened in 92 for christmas time and not in 93

  4. It’s EFFECT, not AFFECT. The table title should be “The Effect of Seismic Activity on Subway Tunnels”. Effect = result, affect = cause/influence. For example: “The EFFECTS of seismic activity may AFFECT the timely running of scheduled trains.”

    Please change this. Your articles are being read the world over (I’m in Australia, for goodness’ sake) and people notice theses things. Your articles – and their writers – lose credibility because of errors like this.

  5. One of the great things about life is that you learn something new everyday. Based on the unexpected Mineral, VA, earthquake that was felt strongest in the Washington, DC area last year, the Metro had their trains run at much slower speeds for the rest of the day’s operation, making sure that there were no structural damage to its tunnels as well as the elevated and at-grade sections and stations. Amtrak also ran its trains at limited speed south of Washington that day of the earthquake for the same reason. Service returned to normal the following morning in both cases. There were no effects to subway, commuter and light rail service in New York City (where I live), Philadelphia (where I went the day after the ‘quake), Baltimore and Boston.