Earthquake Source Characterization of Induced Earthquakes in the Fort Worth Basin, north Texas
Abstract
Increased seismicity rates in the Fort Worth Basin have been linked to increased wastewater disposal, and understanding the physical mechanisms is need to inform mitigation strategies. In the Dallas-Fort Worth metroplex, deployment of local seismic networks has increased the number of small magnitude events with waveform data to over 1300. We use the local earthquake dataset to constrain source processes in order to better understand the similarities and/or differences between natural and induced events and assess strong ground motions. We present local magnitudes, b-values, seismic moments, moment magnitudes, stress drops, and stress orientations derived from north Texas earthquakes. The Brune earthquake model is fit to both local and regional displacement spectra to estimate moment, source length and stress drop. A second time domain procedure is also used to estimate the same parameters for comparison and found to be consistent with the frequency domain estimates. Corrections for path effects including attenuation and geometrical spreading are applied prior to making the source estimates. Local average stress drops from SH waves measured at local distances (< 25 km) range from 24 bars for events to the west to 53 bars for events in the eastern part of the basin. Stress drop estimates at regional distances (>25 km) are 70 bars and 150 bars, respectively. These stress drops suggest a breakdown in self-similarity with increasing moment magnitude, but the overall values are similar to those expected for tectonic earthquake. We use the seismic moments, converted to moment magnitude, to find a conversion equation for the local magnitudes, also calibrated for north Texas, reported in the SMU earthquake catalog. The preliminary b-value for North Texas is consistent with tectonic earthquake regions ( 1), but we will explore spatial variability using cross-correlation approaches to reduce magnitude completeness. Finally, we show focal mechanisms constrained using P first motions and S/P amplitude ratios. First motion mechanisms reported for larger earthquakes in the basin have been consistent with NE-SW striking normal faults, but some strike-slip and thrust events have also been identified. Use of S/P amplitudes improves constraints on fault plane orientation and stress directions.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2016
- Bibcode:
- 2016AGUFM.S43C2878D
- Keywords:
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- 4475 Scaling: spatial and temporal;
- NONLINEAR GEOPHYSICSDE: 7209 Earthquake dynamics;
- SEISMOLOGYDE: 7223 Earthquake interaction;
- forecasting;
- and prediction;
- SEISMOLOGYDE: 8164 Stresses: crust and lithosphere;
- TECTONOPHYSICS