THE 6 NOVEMBER 2011 M5.6 PRAGUE, OKLAHOMA AFTERSHOCK SEQUENCE STUDIED USING SUBSPACE DETECTION

The 6 November 2011 M5.6 earthquake near Prague, Oklahoma is the largest earthquake instrumentally recorded in Oklahoma history. A M4.8 foreshock on 5 November 2011 and the M5.6 mainshock triggered tens of thousands of detectable aftershocks along a 20 km splay of the Pennsylvanian-aged Wilzetta Fault Zone (WFZ) known as the Meeker- Prague fault. In response to this unprecedented earthquake, 21 temporary seismic stations were deployed surrounding the seismic activity. This sequence, with its prolific number of events and dense station coverage, provides an ideal target for subspace detection and subsequent microseismic characterization. We utilized a catalog of 767 previously located aftershocks to construct subspace detectors on 31 seismic stations. In the month following the main shock, more than 7,000 events with epicentral uncertainties less than 400 m were located with a b-value of -1. The event locations provide unique insight into the spatio- temporal evolution of the aftershock sequence along the WFZ and its associated structures. We find that the crystalline basement and overlying sedimentary Arbuckle formation accommodate the majority of aftershocks. While we observe aftershocks along the entire 20 km length of the Meeker-Prague fault, the vast majority of earthquakes were confined to a 9 km wide by 9 km deep surface striking N54°E and dipping 83° to the northwest. Contrary to previous studies, we find a large number of events in the sedimentary Arbuckle formation above the basement. Event locations reveal structures at depth congruent with published regional velocity models and finite slip models. Our study demonstrates subspace detectors running on multiple stations can significantly increase the number of locatable events, thereby lowering the catalog’s magnitude of completeness and providing extraordinary detail on the kinematics of the aftershock process and potential hazards for another large induced event.