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Michael Hamburger
Geological Sciences

Last modified: Monday, April 21, 2008

Active aftershocks

April 21, 2008

BLOOMINGTON, Ind. -- In the wake of the earthquake that shook the Midwest early last Friday, there have been a series of aftershocks associated with the event. Much can be learned about the processes of seismically active zones in the Midwest by studying these aftershocks. Indiana University Professor of Geological Studies Michael Hamburger answers some commonly asked questions regarding the aftershocks, how they are being studied, what can be learned from them, and the area and event from which they originate.


Photo by: Jacob Kriese

Early last Friday morning a magnitude 5.2 earthquake struck near the Indiana/Illinois border just west of Mt. Carmel, Ill.

Print-Quality Photo

Q: What exactly happened on Friday morning? What do we know about the earthquake now?

HAMBURGER: Early last Friday morning, at 5:38 a.m. local time, a magnitude 5.2 earthquake struck near the Indiana/Illinois border just west of Mt. Carmel, Ill. The earthquake was widely felt throughout the Midwest with some minor damage in areas surrounding the epicenter, mostly occurring in small towns west of Mt. Carmel. The earthquake occurred in an area known as the Wabash Valley Seismic Zone, the site of a number of moderate-sized earthquakes over the last several decades. This event was the largest to have occurred in the area since a magnitude 5.5 earthquake that occurred near Carbondale, Ill. in 1968.

Q: What are aftershocks, how many have we experienced, and how long will they last?

HAMBURGER: Aftershocks are small earthquakes that occur in the aftermath of a major event, like the one experienced Friday morning. It is very common for dozens or more to occur in the days to weeks following an earthquake, but their number and intensity is highly variable. This event is proving to be a very active sequence, including at least two substantial aftershocks felt throughout the region. An aftershock of magnitude 4.6 occurred at 11:14 a.m. last Friday morning, and more recently a magnitude 4.0 aftershock occurred at 1:38 a.m. Monday morning. Over two dozen aftershocks have been reported thus far.

Q: How are the aftershocks being studied?

HAMBURGER: Indiana University, along with several other institutions, sent several teams to the region of the epicenter and is monitoring aftershock activity. The teams set up five Global Positioning System (GPS) instruments and six seismographs to monitor the movement of the earth's crust associated with the earthquake and aftershock activity in the days following the event. (A map detailing the locations of each earthquake, aftershock and instrumentation setup is available upon request.)

Additionally, IU has been involved for many years in running an outreach program placing seismographs at schools throughout the region. By virtue of that network, known as the Indiana PEPP Earthquake Science Program (see for additional information about the program), we now have an Indiana state seismograph network that has provided high-quality scientific data with which to study the earthquake.

Q: What can be learned by studying the aftershocks?

HAMBURGER: Because earthquakes are relatively rare in the Midwest, each time one occurs seismologists tend to rush out to gather as much field-based information as possible. The detailed study of aftershocks and their effects help teach us about the structures and processes of earthquakes in the Midwest. This particular earthquake occurred in the Wabash Valley Seismic Zone, which contains its own fault system that runs close to and parallels the Wabash River. Studying the aftershocks may also help better understand the nature of the system and to forecast possible future seismic activity.

Q: What is this earthquake's connection to the New Madrid Fault?

HAMBURGER: The New Madrid Seismic Zone refers to area of intense seismic activity along the Mississippi River Valley, located well south of the Wabash Valley Seismic Zone. Although they are different areas, the two seismic zones have many similarities, and understanding the structures and processes of one will help us understand the origins and relationships between these two enigmatic geological structures.