A six-element, short-period circular array (diameter, 27 km) has been operated in east-central Minnesota since January 1977. The goal is to monitor low-level seismicity in the Upper Great Lakes Precambrian Shield Province. Special reference is given to the Midcontinent Gravity High and to a newly recognized tectonic hinge line across central Minnesota which has been the site of several earthquakes of magnitude up to 5.
Data from the array are telemented 100 km to Minneapolis for tape recording. Flexible playback arrangements permit very compressed to very expanded time scales, using common time-base display with WWVB time code.
Detection capability is estimated at mb = 2.0 within 100 km and 2.5 within 300 km, although three local events of mb = 1.5, 2.6, and 3.0 were clearly recorded well above noise level. A companion paper (Mooney and Morey, 1980) sets these events in the context of historical seismicity and regional tectonics.
Based upon an analysis of hundreds of events, it is shown that the array is capable of determining azimuth for near-regional events within 1° to 4° and for teleseisms within 2° to 5°. Capability for determining distance from apparent horizontal slowness is less good but in the case of local and regional events, can be improved through measurements of S minus P time and Rg minus P time.