Assessing Microseismicity of the Northern Mid-Continent Rift Zone and Surrounding Regions

Monday, 15 December 2014
Daniel Bartz1, Michael Edward Wysession1, Douglas A Wiens1, Ghassan I Aleqabi1, Patrick Shore1, Suzan van der Lee2, Donna M Jurdy2, Seth A Stein2, Justin Revenaugh3, Emily Wolin2, Trevor A Bollmann2, Andrew W Frederiksen4 and Fiona Ann Darbyshire5, (1)Washington University in St Louis, Department of Earth and Planetary Sciences, St. Louis, MO, United States, (2)Northwestern University, Evanston, IL, United States, (3)Univ Minnesota, Minneapolis, MN, United States, (4)University of Manitoba, Winnipeg, MB, Canada, (5)University of Quebec at Montreal UQAM, Centre de recherche GEOTOP, Montreal, QC, Canada
SPREE is a flexible array (FA) deployment associated with the EarthScope project with the aim of better understanding the Mid-Continent Rift Zone (MCRZ). We have used data from SPREE FA and TA stations to detect and locate small earthquakes in the vicinity of the northwestern arm of the Mid-Continent Rift Zone, covering Minnesota, Wisconsin, and parts of Iowa, Michigan, and Ontario. This region, now stable craton, is a failed Precambrian rift marked by low levels of recorded seismicity, perhaps resulting in part from a historic dearth of stations deployed in the region. We assessed this possibility by taking advantage of the densest array that has been deployed in the region. Processing has allowed for regional stress constraints and a means of distinguishing earthquakes from mine blasts, and a quantitative method to differentiate natural earthquakes from mine blasting events, frequent here. After automated event identification, a manual review confirmed 14 earthquakes and several hundred blasting events with magnitudes ranging from M1 - M3. Observed seismicity is in reasonable agreement with available historical data. We use these earthquakes in conjunction with historical catalogs to estimate regional recurrence intervals for events of greater magnitudes. While initial earthquake/blast discrimination was done manually, the ratio of Rayleigh to P-wave amplitude appears to be a reliable discriminant for distinguishing between earthquakes and mine blasting in this region in a systematic way, with earthquakes displaying a systematically depressed ratio. Additionally, P-wave first-motion data and S/P amplitude ratios from natural events constrain focal mechanisms and regional stresses. These methods indicate roughly WSW-ENE compression, consistent with other determinations and absolute plate motion. The majority of events detected lie some distance from the actual rift; we thus conclude current seismicity bears no preferred association with the MCR.