Everyone by now has probably already heard of the earthquake that took place on Friday. See USGS data for information on our little friend. It was centered about 150 miles away from Indianapolis, but still quite powerful when it arrived. The attached map from USGS shows the event intensity and peak ground motion.
First, it brings earthquake and emergency preparedness back to the forefront of media attention, and that is always good. Next, it reminds building owners that they have an obligation to build and maintain safe structures. But most importantly (from my point of view), harmless earthquakes give structural engineers a great opportunity.
I think going through an earthquake can give just about any engineer a better understanding of how the ground moves. Looking at seismic data for historic events, it just doesn’t affect you the same way as experiencing an actual event. What I learned was this – the ground (and building by extension) does indeed move. I can imagine that a bad earthquake would be truly terrifying. I also learned that earthquakes make noise. It was a low rumbling noise, not loud at all. I also learned that nothing I had experienced in my life so far resembled what happened, but I have to say the Modified Mercalli Intensity measurements are pretty accurate if you need a comparison. See Wikipedia Intensity article for a good explanation.
Now for a little science lesson… Earthquake magnitude is measured by the “Richter Scale” typically. This represents the energy released during an event, and is related to the waveform of movement radiating from the epicenter. It is a mathematical measurement, and completely free from objective interference by people. A 5.2 magnitude earthquake somewhere else in the world releases the same amount of energy as the one in Illinois did. One note about the Richter scale is that is a logarithmic measurement. Thus, a 6.2 EQ would indicate of release of 32 times more energy than the 5.2, while a 7.2 EQ would be 1000 times more energy. In other words, if you thought the 5.2 was scary you ain’t seen nothing yet. See Wikipedia Richter Scale article for more info.
Now, the effect of the earthquake has a lot to do with the specific area it occurs in. Here in the midwest, we have a wonderful rock layer that transfers and amplifies the waveform quite easily. So it’s not surprising that people all over the Eastern US felt this. The Modified Mercalli intensity scale includes the actions of the soil and rock, so it’s more useful for discussion when talking about structural engineering. We experienced a III-IV level intensity here in Indianapolis. Which is pretty amazing considering how far away the epicenter was. If a large earthquake were to hit the New Madrid fault in Northeast Arkansas, like many geologists say could happen, it is likely that many places in the Eastern US will be affected. It may not happen for 2500 years, it may happen tomorrow.
Back to the post at hand – This EQ gives structural engineers an opportunity to discuss seismic events with clients, business owners, and the public. It is essential that they understand the real dangers of earthquakes in the Midwest. We should not misrepresent the danger or try to scare people, but do let them know that they have choices when it comes to EQ design. Any building can be made more robust. Masonry parapets can be secured against toppling, gas and water pipelines can installed with flexible connections, and hospitals and important bridges can be designed such that only an atomic bomb will close them down. It’s a lot of work, it’s expensive, and construction proceeds slowly. However, they say the best time to patch a roof is when the sun is shining.