Abstract The formation of a large sinkhole at the Napoleonville salt dome (NSD), Assumption Parish, Louisiana, caused by the collapse of a brine cavern, was accompanied by an intense and complex sequence of seismic events. We implement a grid-search approach to compute centroid locations and point-source moment tensor (MT) solutions of these seismic events using ∼0.1–0.3 Hz displacement waveforms and synthetic Green's functions computed using a 3D velocity model of the western edge of the NSD. The 3D model incorporates the currently known approximate geometry of the salt dome and the overlying anhydrite-gypsum cap rock, and features a large velocity contrast between the high velocity salt dome and low velocity sediments overlying and surrounding it. For each possible location on the source grid, Green's functions (GFs) to each station were computed using source-receiver reciprocity and the finite-difference seismic wave propagation software SW4. We also establish an empirical method to rigorously assess uncertainties in the centroid location, MW and source type of these events under evolving network geometry, using the results of synthetic tests with hypothetical events and real seismic noise. We apply the methods on the entire duration of data (∼6 months) recorded by the temporary US Geological Survey network. During an energetic phase of the sequence from 24–31 July 2012 when 4 stations were operational, the events with the best waveform fits are primarily located at the western edge of the salt dome at most probable depths of ∼0.3–0.85 km, close to the horizontal positions of the cavern and the future sinkhole. The data are fit nearly equally well by opening crack MTs in the high velocity salt medium or by isotropic volume-increase MTs in the low velocity sediment layers. We find that data recorded by 6 stations during 1–2 August 2012, right before the appearance of the sinkhole, indicate that some events are likely located in the lower velocity media just outside the salt dome at slightly shallower depth ∼0.35–0.65 km, with preferred isotropic volume-increase MT solutions. We find that GFs computed using the 3D velocity model generally result in better fits to the data than GFs computed using 1D velocity models, especially for the smaller amplitude tangential and vertical components, and result in better resolution of event locations. The dominant seismicity during 24–30 July 2012 is characterized by steady occurrence of seismic events with similar locations and MT solutions at a near-characteristic inter-event time. The steady activity is sometimes interrupted by tremor-like sequences of multiple events in rapid succession, followed by quiet periods of little of no seismic activity, in turn followed by the resumption of seismicity with a reduced seismic moment-release rate. The dominant volume-increase MT solutions and the steady features of the seismicity indicate a crack-valve-type source mechanism possibly driven by pressurized natural gas. Earthquake source observations, Induced seismicity, Wave propagation, Volcano seismology, Earthquake monitoring and test-ban treaty verification, Computational seismology © The Author(s) 2018. Published by Oxford University Press on behalf of The Royal Astronomical Society. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/about_us/legal/notices)
Geophysical Journal International – Oxford University Press
Published: May 21, 2018
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