SPECTRAL CHARACTERISTICS OF GROUND MOTION FROM INDUCED EARTHQUAKES IN THE FORT WORTH BASIN, TEXAS, USING THE GENERALIZED INVERSION TECHNIQUE

Since the beginning of Barnett shale gas production in the Fort Worth Basin (FWB), there has been an abrupt increase in the number of earthquakes, including multiple magnitude 3.0+ events. The proximity of the earthquakes to a large population center motivates an understanding of ground shaking to assess a hazard associated with potential earthquakes in the FWB. The amplitude of ground motion in a given area is dependent upon propagation path attenuation, site amplification related to near-surface geological structures, and the fault rupture process, which can be interpreted using a stress drop representation that characterizes energy release due to earthquake rupture. Here, we use the generalized inversion technique (GIT) to separate these three terms for a dataset generated by 90 induced earthquakes with local magnitude ranging from 2.0 to 3.8. Since the GIT requires one or more hard rock sites, we modify the method to suit the conditions of the sedimentary basin. We observe a change at about 30 km in amplitude attenuation along propagation paths. In addition, we find that the anelastic attenuation factor of S-waves is slightly larger than those of P-wave, which can be interpreted as a concentration of crustal pore fluids or partial fluid-saturated material along the path. The site effects document an average amplification factor of 5 of horizontal ground motions, which may reflect thick unconsolidated sediments in the FWB, while a limited number of sites exhibit amplification or de-amplification only on the vertical component. Taking these effects into account results in average stress drop estimates of ~5 MPa for all events with individual values ranging from 1.4 to 27.4 MPa. These results suggest that ground motions and seismic hazard from the injection-induced earthquake are similar to those for tectonic earthquakes once the path characteristics of seismic waves and variable local geology are taken into account.