EarthScope "Special Report": Sinkhole Formation Captured by USArray Station

On July 16, 2008, USArray Transportable Array station TA-126 northeast of Carlsbad, New Mexico recorded mysterious signals. Six hours after the onset of these signals, a 100-m diameter sinkhole formed ~13 km NW of TA-126 (Figure 1). This is probably the first documented seismic record of catastrophic sinkhole formation, and will be studied in months to come by seismologists and cave experts.

Figure 1. Sinkhole photographed on 10:44 am on July 20, 2008. Courtesy of National Cave and Karst Research Institute.

The sinkhole formed at the site of a brine-well in Eddy Co., NM (Figure 2) where fresh water was being injected into salt beds; the resulting brine was being pumped out for use as an oil field drilling fluid. Borehole problems prevented regular downhole acoustic surveys to assess the dimensions of resulting subsurface voids, and the ensuing collapse was unexpected.

Figure 2. Location of sinkhole (red circle) northwest of TA site TA_126A (green). Google maps.

The TA site TA-126, located about 13 km southeast of the brine well, started recording anomalous short-period seismic signals about 6 hours before sinkhole formation became visible at the surface (Figure 3). The seismic signals probably reflect subsurface spalling and upward stoping of the cavern roof. These recordings demonstrate that precursory seismic activity related to similar size sinkholes may be detectable by seismic stations at close ranges.

Figure 3. Seismogram from TA 126A showing short-period signal preceding sinkhole formation.

Information provided by L. Land (lland@gis.nmt.edu), National Cave and Karst Research Institute and New Mexico Bureau of Geology and Mineral Resources, and R. Aster, Department of Earth and Environmental Sciences, New Mexico Tech (aster@ees.nmt.edu.)