An Earthquake in Indiana?
Yes, you heard right. A magnitude 3.8 earthquake rocked (gently jiggled, really) Indiana at 7:55am local time. The mild quake was centered 15 miles (20 km) southeast of the city of Kokomo, about 50 miles (75 km) northwest of the state capital Indianapolis.
The USGS has received reports from people who say they felt the shaking as far away as southern Wisconsin and eastern Ohio. Reports of damage to people or property are not expected from such a minor event, but I will update this statement if that changes.
According to the U.S. Geological Survey (USGS), this earthquake did NOT occur in the famous New Madrid fault zone. The the New Madrid zone is to the southwest of Indiana, in the area of the Missouri/Illinois/Kentucky border (just barely visible in the lower left-hand corner of the detail map to the right). Reading the statement from the USGS, it seems that the Kokomo earthquake likely occurred on a small, unnamed fault deep within the bedrock that underlies much of the upper Midwest. Geologists know these types of faults exist, so an earthquake in Indiana isn’t a total shock. But given that they don’t slip and cause earthquakes very often, and they tend to be deep within the ground, these faults are not well documented or named.
Added 1/3/2011 – The Indiana Geological Survey at Indiana University has more information on historic earthquake activity and fault systems in Indiana. They are a great resource if you’re interested in learning more.
So why is this earthquake newsworthy? I would guess mainly because earthquakes large enough to be felt are rare events in the eastern United States. This part of the continent is far from the active fault zones at the tectonic plate boundaries (read about those in my Earthquake in Haiti post) and with the exception of the New Madrid earthquakes of the early 1800’s there haven’t been any major earthquakes east of the Rockies in modern times. Another reason could be that after the earthquake in Haiti and the Christmastime earthquakes in Vanuatu and Japan (a post on these later) we all have earthquakes on the brain. This could be why people are guessing that the number of earthquakes worldwide has been unusually high, even though geologists say that the number of earthquakes in 2010 is normal when compared to historic averages (see Earthquake Frequency Has NOT Increased in 2010 — I seem to write a lot about earthquakes).