Preliminary Focal Mechanism Analysis of the 6 November 2011 M 5.7 Oklahoma sequence


Meeting2012 Fall Meeting
SessionUnderstanding Recent and Historical Seismicity in the Central and Eastern U.S. I Posters
Authors Wei, M*, Geological Sciences, Brown University, Fairfax, VA, USA
Sumy, D F, NSF Postdoctoral Scholar at the United States Geological Survey, Pasadena, CA, USA
Cochran, E S, United States Geological Survey, Pasadena, CA, USA
Keranen, K M, School of Geology and Geophysics, University of Oklahoma, Norman, OK, USA
Abers, G A, Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY, USA
Savage, H M, Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY, USA
Index Terms Seismicity and tectonics [7230]
General or miscellaneous [7299]
Stresses: crust and lithosphere [8164]


A M5.7 strike-slip earthquake occurred on 6 November 2011 near Prague, Oklahoma and was followed by hundreds of aftershocks in the subsequent months. While earthquakes are not unknown to Oklahoma, seismicity rates in the region have risen steadily since 2008, with increases in both the frequency and intensity of the earthquakes observed. The M5.7 earthquake is the largest quake recorded during this recent period of increased seismicity. Prior to the mainshock, 19 seismometers were located within approximately 100 km of the event. An additional 28 seismometers were temporarily deployed after the mainshock to record the aftershock sequence. We use data collected from these seismometers to calculate the focal mechanisms for a subset of the aftershocks. Here, we examine the 80 largest aftershocks that occur prior to 31 December 2011. P-wave arrivals and polarities are manually identified on the vertical component of each station. Polarities are marked as impulsive or emergent and the pick is given a quality rating (0-4). We then use HASH (Hardebeck and Shearer, 2002) and a 1-D velocity model to calculate the focal mechanisms. For each event, HASH outputs a set of acceptable mechanisms and, based on how clustered the set of acceptable mechanisms is, a quality and uncertainty is assigned. The early aftershock locations suggest that the 5 November 2011 M5.0 foreshock, 6 November 2011 M5.7 mainshock, and the largest (8 November 2011 M5.0) aftershock may have occurred on faults with strikes of 34°, 55°, and 90°, respectively. Given this change in fault strike for the largest events in the sequence, we will investigate whether there is also a systematic variation in the aftershock focal mechanisms with time. We will also investigate spatial variation in focal mechanism type (e.g. strike-slip, normal, or thrust) and inferred fault strike.

Cite as: Author(s) (2012), Title, Abstract S51E-2455 presented at 2012 Fall Meeting, AGU, San Francisco, Calif., 3-7 Dec.