Varied Fault Orientations in a Uniform Stress Field: The 2011 M5.7 Prague Earthquake Sequence

Elizabeth S. Cochran, Rob Skoumal, Devin McPhillips, Zachary E. Ross, & Kathleen M. Keranen

Submitted August 12, 2020, SCEC Contribution #10400, 2020 SCEC Annual Meeting Poster #050

The role of pore pressure changes in induced earthquake sequences can be inferred by the set of faults activated relative to the local stress orientations. Here, we examine the 2011 M5.7 Prague earthquake sequence that was induced by nearby wastewater disposal. While the regional stress directions in Oklahoma are relatively well constrained, we look for evidence of local stress rotations near the sequence and we map the detailed fault strands activated in the sequence. We use shear wave splitting measurements to estimate the local horizontal compressive stress orientation (SHmax) at spatial resolutions as small as 750 m. We find that the predominant SHmax azimuth is consistent with previous estimates of the regional compressive stress, with some secondary azimuths oriented sub-parallel to the strike of the major fault structures. Using a template matching earthquake catalog, we map ten distinct fault segments activated during the sequence that exhibit a wide array of orientations. We assess whether the five near-vertical fault planes are optimally oriented to fail in the determined stress field. Only the M5.7 mainshock fault and one other fault plane are optimally oriented to slip in the local stress field. Both the M4.8 foreshock and M4.8 aftershock occur on fault planes that deviate 20-29 degrees from the optimal orientation for slip. Our results confirm that induced event sequences can occur on faults at a range of orientations relative to the local stress field. We interpret the results to suggest elevated pore fluid pressures induce failure along unfavorably-oriented faults activated in the 2011 Prague sequence.

Key Words
Induced Seismicity, Stress, Fault Slip, Pore Pressure

Cochran, E. S., Skoumal, R., McPhillips, D., Ross, Z. E., & Keranen, K. M. (2020, 08). Varied Fault Orientations in a Uniform Stress Field: The 2011 M5.7 Prague Earthquake Sequence. Poster Presentation at 2020 SCEC Annual Meeting.

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