The Spatio-Temporal Evolution of Induced Seismicity Clusters in Oklahoma and Southern Kansas

Martin Schoenball, Stanford University

Thursday, June 29, 2017 at 10:30 AM

Location:
Building 3, Rambo Auditorium

For much of Oklahoma, augmentation of the seismic network with new public stations in the activated areas has followed rather than preceded the spread of seismicity across the state, and consequently the network geometry is often unfavorable for resolving the underlying fault structures. With this study, we reanalyze the existing earthquake catalog with additional data from two industry operated networks for the period May 2013 to November 2016. These networks include 40 seismic stations and cover seismically active north-central Oklahoma with a station spacing on the order of 25 km. Relative locations obtained from waveform cross-correlation reveal a striking pattern of seismicity illuminating many previously unmapped faults. Absolute depths are usually well constrained to within 1 km. Relative locations provide about one order of magnitude better precision for resolving the structure of seismicity clusters. Relocated epicenters tend to cluster in linear trends of less than 1 km to more than 20 km in length. In areas with stations closer than about 10 km, we can resolve fault planes not only by strike but also by dip. These are generally in agreement with surface wave-derived moment tensor solutions.
We identify 93 sequences with at least 30 events that we use for detailed analysis of the spatio-temporal evolution and clustering of these sequences. For most awakened faults, seismicity tends to initiate at shallower depth and migrates deeper along the faults as the sequence proceeds. We find that no sequence starts with the largest earthquake and many sequences initiate months before they rise to peak activity. We study temporal clustering as a means to quantify earthquake interactions. Some sequences show no temporal clustering similar to Poissonian background seismicity but at much higher rate. Other sequences exhibit strong temporal clustering akin of mainshock-aftershock sequences.
We conclude that once initiated by anthropogenic stressing, faults in the Oklahoma/Kansas area are close enough to failure to sustain rupturing through earthquake interaction. Furthermore, we show that the largest earthquakes occur as a result of continued stressing, rather than by growing the reactivated part of a pre-existing fault. Therefore, improve monitoring can provide us with warning time to actively mitigate the occurrence of larger earthquakes.

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