A Decade of Induced Slip on the Causative Fault of the 2015 Mw 4.0 Venus Earthquake, Northeast Johnson County, Texas

On 7 May 2015, a MW 4.0 earthquake occurred near Venus, northeast Johnson County, Texas, in an area of the Bend Arch-Fort Worth Basin that reports long-term, high-volume wastewater disposal and has hosted felt earthquakes since 2009. Scientists at SMU deployed a local seismic network and purchased nearby seismic reflection data to capture additional events, identify and image the causative fault, and explore potential links between ongoing industry activity and seismicity. Double-difference derived hypocenter relocations of the local earthquake catalog indicate a fault striking ~230oN, dipping to the west, consistent with a nodal plane of the MW 4.0 regional moment tensor. Fault plane solutions, calculated using a combination of P-wave first motions and S to P amplitude ratios, indicate normal faulting, with B-axes oriented parallel to maximum horizontal stress. Based on seismic reflection data, the reactivated basement fault penetrates the Ordovician disposal layer and Mississippian production layer, but does not displace post-Lower Pennsylvanian units. The fault rotates counter-clockwise north of current seismicity to become non-critically oriented within the modern stress field. Template matching at regional stations indicates that low magnitude earthquakes with similar waveforms began in April 2008. Pressure data from five saltwater disposal wells within 5 km of the active fault indicate a disposal formation that is 0.9-4.8 MPa above hydrostatic. I suggest that the injection of 28,000,000 m3 of wastewater between 2006 and 2016 at these wells led to an increase in subsurface pore fluid pressure that contributed to the triggering of this long-lived earthquake sequence. The 2015 MW 4.0 event represents the largest event of a continuing evolution of slip on a causative fault, with increasing magnitude over time

A Decade of Induced Slip on the Causative Fault of the 2015 Mw 4.0 Venus Earthquake, Northeast Johnson County, Texas

On 7 May 2015, a MW 4.0 earthquake occurred near Venus, northeast Johnson County, Texas, in an area of the Bend Arch-Fort Worth Basin that reports long-term, high-volume wastewater disposal and has hosted felt earthquakes since 2009. Scientists at SMU deployed a local seismic network and purchased nearby seismic reflection data to capture additional events, identify and image the causative fault, and explore potential links between ongoing industry activity and seismicity. Double-difference derived hypocenter relocations of the local earthquake catalog indicate a fault striking ~230oN, dipping to the west, consistent with a nodal plane of the MW 4.0 regional moment tensor. Fault plane solutions, calculated using a combination of P-wave first motions and S to P amplitude ratios, indicate normal faulting, with B-axes oriented parallel to maximum horizontal stress. Based on seismic reflection data, the reactivated basement fault penetrates the Ordovician disposal layer and Mississippian production layer, but does not displace post-Lower Pennsylvanian units. The fault rotates counter-clockwise north of current seismicity to become non-critically oriented within the modern stress field. Template matching at regional stations indicates that low magnitude earthquakes with similar waveforms began in April 2008. Pressure data from five saltwater disposal wells within 5 km of the active fault indicate a disposal formation that is 0.9-4.8 MPa above hydrostatic. I suggest that the injection of 28,000,000 m3 of wastewater between 2006 and 2016 at these wells led to an increase in subsurface pore fluid pressure that contributed to the triggering of this long-lived earthquake sequence. The 2015 MW 4.0 event represents the largest event of a continuing evolution of slip on a causative fault, with increasing magnitude over time

On the performance of ML-MC as a depth discriminant for small seismic events recorded at local distances in Yellowstone, Oklahoma, and Italy

A recent study by Koper et al. (2016) found that the difference between local magnitude (ML) and coda duration magnitude (MC) successfully distinguished shallow seismic events (mining blasts, mining-induced seismicity, and shallow tectonic earthquakes) from deeper seismic events (tectonic earthquakes) in the Utah region and could therefore be helpful for blast discrimination. Here we present tests of the performance of ML-MC as a depth discriminant in three regions and show that it is effective in all of them. Initially, we investigated ML-MC as a function of depth for seismicity in and around Yellowstone National Park recorded by the University of Utah Seismograph Stations. For 2,845 Yellowstone earthquakes with well-constrained depths varying from 0-25 km, we found that ML-MC decreases 0.036 ± 0.014 magnitude units (m.u.) per 1 km in depth over the depth range of 0-8 km. Then, we examined ML-MC values for anthropogenic seismicity recorded by the National Earthquake Information Center in northern Oklahoma and southern Kansas. We found that for 1,692 events with well-constrained depths, the slope of ML-MC for the shallowest 10 km in depth is 0.037 ± 0.016 m.u. per 1 km depth. Finally, we analyzed ML-MC for 28,721 well-located earthquakes in Italy and Sicily recorded by Istituto Nazionale di Geofisica e Vulcanologia. This region showed an increase of 0.017 ± 0.001 m.u. per 1 km depth, up to 30 km in depth. In each case, the quoted error bounds represent 99% confidence regions. We performed several robustness tests in which we varied the depth bin size, the criterion used to define a well-constrained depth, and the depth range used in the linear fit. In nearly all cases we found a positive slope for ML-MC vs. depth at a confidence level above 99%. Our results provide further evidence that ML-MC is useful as a depth discriminant for events recorded at local distances in different physiographic regions.peer-reviewe