LOCATING BACKGROUND INTRAPLATE MICROSEISMICITY IN SOUTHERN ILLINOIS FROM A SMALL, SHORT DURATION,SEISMOGRAPH ARRAY

Seismicity occurs mainly around plate boundaries, but there are rare occurrences of large magnitude earthquakes with a plate. When and why these events occur is not well understood but they are capable of producing significant damage to these regions. Where these earthquakes occur, are known as intraplate seismic zones. In the American Midwest there are two seismically active intraplate seismic zones, the New Madrid and Wabash Valley seismic zones. Each of these zones is capable of producing large magnitude earthquakes though it has proven somewhat difficult to study intraplate seismicity because the reoccurrence intervals for intraplate events are much longer than interplate earthquakes, requiring a much longer timescale for study. One approach to avoid the longer timescale of observation is to focus on studying smaller events, which occur more often. However, these events will have their own disadvantages with less optimal signal-to-noise ratios which does not allow events to be located by the minimum three seismometers needed to triangulate the events epicenter. A potential solution to remedy this difficulty is to use methods using one-station events that go beyond the conventional ways of locating earthquakes and possibly improve locating microseismic events that normally go unnoticed. A small seismograph array was set up around Cedar Lake, in Makanda, Illinois. This location is situated southwest of the Wabash Valley seismic zone and north of the New Madrid seismic zone but is typically viewed as aseismic. The conventional method was used for finding events by triangulation on three months data from January 1, 2011 to March 31, 2011. Through triangulation there 14 events located in January, 10 in February, and 74 in March. In addition to triangulated events, event probabilities for one-station also applied events and located a few of these events via azimuth by utilizing three component seismometers. As a result the area closest to Cedar Lake exhibited a measurable amount of activity that was not detected through triangulation. Along the Pomona Fault there were numerous azimuth events located and a few triangulated events. A linear trend of events, both azimuth and triangulated, were depicted to possibly be an unknown fault in the areas that trend NW-SE through the north portion of Cedar Lake. The comparison of rainfall and seismicity suggest the possibility of a seasonal component in background seismicity. Through our study, utilizing the one-station events to locate areas of probability for events, and locating through azimuth, methods are able to improve upon when examining microseismicity in an intraplate setting