OIINK!: A Telescope to Peer into Earth's Interior!
What the heck is OIINK!??? It is a new regional research project to study the details of the Earth's structure beneath the central United States. Three university research teams' from the University of Illinois, Indiana University, and Purdue University, together with the Illinois and Indiana state geological surveys, have teamed up to take on an ambitious project to better understand earthquakes and the geological structures that trigger them. This new research project, supported by the National Science Foundation, is playfully dubbed "OIINK!" after its principal study area: Ozarks Illinois INdiana Kentucky. The project involves the operation of a large seismic array centered over the Illinois Basin. It is being conducted in coordination with the deployment of a nationwide array of seismometers called the USArray The USArray is a component of a large experimental program called EarthScope, a national scientific experiment directed at understanding the geologic processes that create earthquakes and their relation to continental-scale geologic structures.
Location map of planned OIINK! seismometer deployment in two phases. The OIINK! array will fit within the larger EarthScope Transportable Array (TA) configuration. Inset map (lower right) is an image of the structure of the Precambrian surface in the study area and shows how OIINK! is designed to investigate the boundaries between the Ozark Plateau, the Illinois Basin, the Rough Creek Graben, and the Grenville Front.
Our study area includes some of the world's best examples of geologic structures typical of the Earth's continental interiors (or "cratons"). The study will help us understand the structure of the Earth's crust and under-lying mantle, the causes of earthquakes within the interior of an otherwise stable craton, and the geologic history of the North American continent. Specifically, our array will span four of the major geologic structures that characterize the mid-continental area of the central US: the Ozark Dome of southeastern Missouri, the Illinois Basin of southern Illinois and Indiana, the Rough Creek Graben of western Kentucky, and the Grenville Front of central Kentucky. These features also include major fault zones such as the Wabash Valley Seismic Zone, the source of many earthquakes in the Midwest.
Beginning in the summer of 2011, our researchers visited dozens of sites in southern Illinois and southeastern Missouri, selecting sites for the first 60 of the OIINK! array stations (violet and yellow symbols on the 2nd figure). We installed seismic vaults at 38 of the 60 sites. With only a fraction of the network instruments available, we elected to install a "pilot network", consisting of 23 seismic stations in the western half of the network. The pilot network consists of:
- 13 "short-period" seismometers (shown by yellow circles on map) clustered along the Illinois-Missouri border that are designed for recording local earthquakes, and
- 10 "broadband" seismometers (yellow triangles) along an east-west line through the network that are designed for recording both local and global earthquakes.
The pilot network, together with neighboring sites of the national USArray network (red triangles), are providing us with detailed data on earthquakes and earth structure along the Ozark Dome/Illinois Basin transition.
OIINK! array as it exists in May 2012. OIINK! stations are shown in violet circles and yellow circles and triangles. Stations shown as violet circles have been permitted. Vaults have been constructed at many of these stations. Stations shown as yellow have operating instruments: yellow circles are short period seismometers, yellow triangles are long period seismometers. OIINK! also uses data from the Transportable Array (red triangles and the existing network of permanently installed broadband seismometers (blue triangles).
In its first several months of recording, the OIINK! network has already recorded several thousand seismic events. The recordings are dominated by the "artificial earthquakes" produced by blasting from neighboring quarries and surface mines, which occur at a rate of about 25 events/day. A major technical challenge is the discrimination of quarry and mine blasts from naturally occurring seismicity. The blasts are located in belts associated with coal mining along the eastern and western margins of the Illinois Basin and in the Rough Creek Graben.
We have recorded a small number of local natural seismic events and a much larger number of regional earthquakes from the New Madrid earthquake zone, as well as earthquakes from Oklahoma, Arkansas, Virginia, and Texas. We are able to record earthquakes as small as magnitude 2.0 for local earthquakes and magnitude 3.0 elsewhere in the region. Data from the OIINK! array are also being used to help us understand the geologic structures that produce these small earthquakes. Initial results from a sophisticated imaging technique called "teleseismic receiver function analysis" suggests that the Earth's crust and underlying mantle beneath the Ozark Plateau and the Illinois Basin vary considerably both in thickness and in physical properties.