Tale of Two Quakes

The U.S. Geological Survey has wasted no time warning folks that Friday's Illinois earthquake was a "wake-up call." In a press release and podcast they reminded folks that no place is quake-free. They also pointed out something a tad more subtle that is often lost in the news coverage: That quakes in the Midwest are felt far more widely than in places like California.

These two shake maps  demonstrate this. The on left shows how the shaking was felt last Friday in Illinois. On the Right is a comparable quake in California. I've deliberately shrunk the California map to make it appear at a similar scale to the Illinois map (click on them to view at full original size -- and very different scales). Notice how far the shaking is felt in Illinois compared to the white zones of no shaking felt in California.

What did Midwesterners do to deserve this more jittery ground? They simply settled on geological Jello. It turns out that all those sands, silts and other river, lake and glacier sediments that span such huge areas of the Midwest are very easily jiggled by earthquakes and they transmit that energy far and wide. Now throw into that Jello a few mountain ranges and major faults to deflect, reflect and otherwise mess with the seismic energy -- as is the case in California -- and the shaking doesn't reach nearly as far. Something to think about before labeling any place "earthquake country."