Moment Tensor Inversion of Seismic Events Associated with the Sinkhole at Napoleonville Salt Dome, Louisiana

The formation of a large sinkhole at the Napoleonville salt dome, Assumption Parish, Louisiana, in August 2012 was accompanied by a rich sequence of complex seismic events. We implemented a grid-search approach for automatic detection, location, and full moment tensor (MT) inversion of these events using 0.1-0.2 Hz displacement waveforms and 1D velocity models for the salt dome and the surrounding sedimentary strata. We were able to detect 62 events, with a 70% variance reduction (VR) detection threshold, during the one-day period (19:00 hours, 1 August to 19:00 hours, 2 August, 2012) just before the discovery of the sinkhole. The source mechanisms of these events show large isotropic volume-increase components (61%-82%), with magnitudes varying from M-w 1.3 to 1.6 and good waveform fits (71%-86% VR). Locations are well constrained to an approximate depth of 470 m at the western edge of the salt dome, close to the sinkhole. For one representative event, the large volume-increase component in the full MT solution is statistically significant over the deviatoric MT solution and stable with respect to: (1) the velocity models and stations used in the inversion, and (2) the uncertainties in the hypocenter and the MT solution itself. The network sensitivity solution computed for this event using both waveforms and P-wave first motion polarities provides greater confidence in the dominantly explosive source mechanism, which can be attributed to high-pressure flow of natural gas or gas-water mixture through the disturbed rock zone below the sinkhole or pre-existing zones of weaknesses in the source region.