ALBERT

Description: Induced seismicity exhibits diverse source mechanisms that are often difficult to constrain for small events. Here, we use data from the in-mine seismic network, the Natural Earthquake Laboratory in South African Mines network, and a temporary Program for the Array Seismic Studies of the Continental Lithosphere deployment in TauTona Mine, South Africa, to determine full moment tensors of 100 mining-induced earthquakes in the magnitude range –2.7〈 M w 〈2.5. Ground displacement derived from velocity and acceleration data show clear near-field effects, indicating that the lowest frequencies are well resolved. Phase amplitudes of between 11 and 77 picks per event were inverted to obtain the six independent moment tensor components. The quality of each moment tensor solution is quantified using (1) the misfit between observed and synthetic waveforms, (2) bootstrap resampling to estimate uncertainties, and (3) the F -test to determine the need for including an isotropic component with an extra degree of freedom in the solution. The results indicate 82% of the events have well-constrained solutions, and 45% of the well-constrained events require an isotropic source term. Throughout the magnitude range, both deviatoric and implosive mechanisms are observed, with implosive ratios of volume change to shear deformation (V/ ) of –1.03 to –0.15. Two explosive events are observed at M w –0.5 and –0.2, withV/ =0.15 and 0.51, respectively. For the largest events, we determine maximum slip and apparent stress ( a ) and find values consistent with those of natural tectonic earthquakes, with 0.1≤ a ≤9.2 MPa. Our results support previous speculation on the nature of isotropic components of mining-induced earthquakes, in which events of all sizes begin as shear failure that may intersect a void (tunnel or stope) and cause collapse, whereas only small events result in explosive sources.

Description: 〈span〉〈div〉Abstract〈/div〉Hydraulic fracturing has been inferred to trigger the majority of injection‐induced seismicity in the Zhaotong and Changning shale gas field, Sichuan basin of China, in contrast to the Midwestern United States, where massive wastewater disposal has been the dominant triggering mechanism. More than 15,000 earthquakes, with magnitudes ranging up to Mw 4.7, were recorded by a temporary network in the Sichuan basin for three years, with a completeness magnitude of ML 1.1. A catalog of earthquakes relocated with code tomoDD, combined with comprehensive injection data during an eight day period, shows that the earthquakes were highly correlated spatiotemporally with hydraulic fracturing activities mostly from a single well pad. Three ML≥4.0 events occurred during hydraulic fracturing operations from 12 to 19 January 2017, followed by the fourth and largest event, with moment magnitude (Mw) 4.7, on 28 January. The hypocenters of the four largest events were located in dolomite of Cambrian age, between a shale gas reservoir and the top of the crystalline basement rocks. This was found to be similar to 60% of the smaller earthquakes in this cluster, at depths from 2.5 to 4.0 km.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Hydraulic fracturing has been inferred to trigger the majority of injection‐induced seismicity in the Zhaotong and Changning shale gas field, Sichuan basin of China, in contrast to the Midwestern United States, where massive wastewater disposal has been the dominant triggering mechanism. More than 15,000 earthquakes, with magnitudes ranging up to Mw 4.7, were recorded by a temporary network in the Sichuan basin for three years, with a completeness magnitude of ML 1.1. A catalog of earthquakes relocated with code tomoDD, combined with comprehensive injection data during an eight day period, shows that the earthquakes were highly correlated spatiotemporally with hydraulic fracturing activities mostly from a single well pad. Three ML≥4.0 events occurred during hydraulic fracturing operations from 12 to 19 January 2017, followed by the fourth and largest event, with moment magnitude (Mw) 4.7, on 28 January. The hypocenters of the four largest events were located in dolomite of Cambrian age, between a shale gas reservoir and the top of the crystalline basement rocks. This was found to be similar to 60% of the smaller earthquakes in this cluster, at depths from 2.5 to 4.0 km.〈/span〉

Description: The U.S. Geological Survey (USGS) has produced a one-year (2016) probabilistic seismic-hazard assessment for the central and eastern United States (CEUS) that includes contributions from both induced and natural earthquakes that are constructed with probabilistic methods using alternative data and inputs. This hazard assessment builds on our 2016 final model ( Petersen et al. , 2016 ) by adding sensitivity studies, illustrating hazard in new ways, incorporating new population data, and discussing potential improvements. The model considers short-term seismic activity rates (primarily 2014–2015) and assumes that the activity rates will remain stationary over short time intervals. The final model considers different ways of categorizing induced and natural earthquakes by incorporating two equally weighted earthquake rate submodels that are composed of alternative earthquake inputs for catalog duration, smoothing parameters, maximum magnitudes, and ground-motion models. These alternatives represent uncertainties on how we calculate earthquake occurrence and the diversity of opinion within the science community. In this article, we also test sensitivity to the minimum moment magnitude between M  4 and M  4.7 and the choice of applying a declustered catalog with b =1.0 rather than the full catalog with b =1.3. We incorporate two earthquake rate submodels: in the informed submodel we classify earthquakes as induced or natural, and in the adaptive submodel we do not differentiate. The alternative submodel hazard maps both depict high hazard and these are combined in the final model. Results depict several ground-shaking measures as well as intensity and include maps showing a high-hazard level (1% probability of exceedance in 1 year or greater). Ground motions reach 0.6 g horizontal peak ground acceleration (PGA) in north-central Oklahoma and southern Kansas, and about 0.2 g PGA in the Raton basin of Colorado and New Mexico, in central Arkansas, and in north-central Texas near Dallas–Fort Worth. The chance of having levels of ground motions corresponding to modified Mercalli intensity (MMI) VI or greater earthquake shaking is 2%–12% per year in north-central Oklahoma and southern Kansas and New Madrid similar to the chance of damage at sites in high-hazard portions of California caused by natural earthquakes. Hazard is also significant in the Raton basin of Colorado/New Mexico; north-central Arkansas; Dallas–Fort Worth, Texas; and in a few other areas. Hazard probabilities are much lower (by about half or more) for exceeding MMI VII or VIII. Hazard is 3- to 10-fold higher near some areas of active-induced earthquakes than in the 2014 USGS National Seismic Hazard Model (NSHM), which did not consider induced earthquakes. This study in conjunction with the LandScan TM Database (2013) indicates that about 8 million people live in areas of active injection wells that have a greater than 1% chance of experiencing damaging ground shaking (MMI≥VI) in 2016. The final model has high uncertainty, and engineers, regulators, and industry should use these assessments cautiously to make informed decisions on mitigating the potential effects of induced and natural earthquakes.

Description: We produce a one-year 2017 seismic-hazard forecast for the central and eastern United States from induced and natural earthquakes that updates the 2016 one-year forecast; this map is intended to provide information to the public and to facilitate the development of induced seismicity forecasting models, methods, and data. The 2017 hazard model applies the same methodology and input logic tree as the 2016 forecast, but with an updated earthquake catalog. We also evaluate the 2016 seismic-hazard forecast to improve future assessments. The 2016 forecast indicated high seismic hazard (greater than 1% probability of potentially damaging ground shaking in one year) in five focus areas: Oklahoma–Kansas, the Raton basin (Colorado/New Mexico border), north Texas, north Arkansas, and the New Madrid Seismic Zone. During 2016, several damaging induced earthquakes occurred in Oklahoma within the highest hazard region of the 2016 forecast; all of the 21 moment magnitude ( M ) ≥4 and 3 M ≥5 earthquakes occurred within the highest hazard area in the 2016 forecast. Outside the Oklahoma–Kansas focus area, two earthquakes with M ≥4 occurred near Trinidad, Colorado (in the Raton basin focus area), but no earthquakes with M ≥2.7 were observed in the north Texas or north Arkansas focus areas. Several observations of damaging ground-shaking levels were also recorded in the highest hazard region of Oklahoma. The 2017 forecasted seismic rates are lower in regions of induced activity due to lower rates of earthquakes in 2016 compared with 2015, which may be related to decreased wastewater injection caused by regulatory actions or by a decrease in unconventional oil and gas production. Nevertheless, the 2017 forecasted hazard is still significantly elevated in Oklahoma compared to the hazard calculated from seismicity before 2009.