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?
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.