Lee K. Steck

Enhanced DNAPL Transport in a Sand Core during Dynamic Stress Stimulation

Extraction of dense, nonaqueous-phase liquid (DNAPL) contaminants trapped in groundwater aquifers is a major problem in environmental remediation because existing field techniques, such as pump and...

Teleseismic P-wave image of crust and upper mantle structure beneath the Valles Caldera, New Mexico: Initial Results from the 1993 JTEX Passive Array

Teleseismic P-wave relative arrival-time data, collected from a temporary array during the 1993 Jemez Tomography Experiment (JTEX), have been inverted to image velocity anomalies beneath the Valles caldera in northern New Mexico. Instruments were deployed in two 30-km-long profiles, one of 8 and one of 9 stations. These profiles crossed the caldera trending at azimuths of N46°W and N60°E, respectively. Two-dimensional teleseismic relative arrival time inversion of the 1993 data set, supplemented with data from an overlapping 1987 profile, confirms the existence of a mid-crustal low velocity region (-30%) beneath the Valles caldera in the depth range of 8 to 13 km (below sea level), with about a 6 km horizontal extent. This feature is interpreted to be the seismic expression of the remnant magma chamber. A shallow low velocity anomaly beneath San Antonio Mountain coincides with the region of highest thermal gradient values in the caldera. A lower crust/upper mantle low velocity anomaly is imaged but is not as well constrained due to the limited length of the profile. We tentatively correlate this anomaly with the thermal effects of basaltic magmas ponded at the crust-mantle boundary.

Improving Regional Seismic Event Location in China

Abstract — In an effort to improve our ability to locate seismic events in China using only regional data, we have developed empirical propagation path corrections and applied such corrections using traditional location routines. Thus far, we have concentrated on corrections to observed P arrival times for crustal events using travel-time observations available from the USGS Earthquake Data Reports, the International Seismic Centre Bulletin, the preliminary International Data Center Reviewed Event Bulletin, and our own travel-time picks from regional data. Location ground truth for events used in this study ranges from 25 km for well-located teleseimic events, down to 2 km for nuclear explosions located using satellite imagery. We also use eight events for which depth is constrained using several waveform methods. We relocate events using the EvLoc algorithm from a region encompassing much of China (latitude 20°–55°N; longitude 65°–115°E). We observe that travel-time residuals exhibit a distance-dependent bias using IASPEI91 as our base model. To remedy this bias, we have developed a new 1-D model for China, which removes a significant portion of the distance bias. For individual stations having sufficient P-wave residual data, we produce a map of the regional travel-time residuals from all well-located teleseismic events. Residuals are used only if they are smaller than 10 s in absolute value and if the seismic event is located with accuracy better than 25 km. From the residual data, correction surfaces are constructed using modified Bayesian kriging. Modified Bayesian kriging offers us the advantage of providing well-behaved interpolants and their errors, but requires that we have adequate error estimates associated with the travel-time residuals from which they are constructed. For our P-wave residual error estimate, we use the sum of measurement and modeling errors, where measurement error is based on signal-to-noise ratios when available, and on the published catalog estimate otherwise. Our modeling error originates from the variance of travel-time residuals for our 1-D China model. We calculate propagation path correction surfaces for 74 stations in and around China, including six stations from the International Monitoring System. The statistical significance of each correction surface is evaluated using a cross-validation technique. We show relocation results for nuclear tests from the Balapan and Lop Nor test sites, and for earthquakes located using interferometric synthetic aperture radar. These examples show that the use of propagation path correction surfaces in regional relocations eliminates distance bias in the residual curves and significantly improves the accuracy and precision of seismic event locations.

Precise Relative Location of 25-ton Chemical Explosions at Balapan Using IMS Stations

Abstract — We test how well low-magnitude (mbLg 1.8 to 2.6), 25-ton chemical explosions at Balapan, Kazakhstan, can be located using IMS stations and standard earth models, relying on precisely determined relative arrival times of nearly similar, regional and teleseismic waveforms. Three 1997 Balapan explosions were recorded by a number of currently reporting and surrogate IMS stations. Three regional stations and two teleseismic arrays yielded consistent waveforms appropriate for relative picking. Master-event locations based on the AK135 model and ground-truth information from the first, shallowest and best-recorded explosion, fell under 1 km from known locations, for depths constrained to that of the master event. The resulting 90% confidence ellipses covered 12–13 km2 and contained the true locations; however, results for depth constrained to true depth were slightly less satisf actory. From predictions based on ground truth, we found a Pg-coda phase at Makanchi, Kazakhstan to be misidentified and poorly modeled. After accounting for this, 90% ellipses shrank to 2–3 km2 and true-depth mislocation vectors became more consistent with confidence-ellipse orientations. These results suggest that a high level of precision could be provided by a tripartite array of calibration shots in cases where models are poorly known. We hope that the successful relocation of these small Balapan shots will support the role of calibration explosions in verification monitoring and special event studies, including on-site inspection.

Simulated annealing inversion of teleseismic P‐wave slowness and azimuth for crustal velocity structure at Long Valley Caldera

Simulated annealing, a non-linear global search algorithm, is used to invert teleseismic P-wave slowness and azimuth data for crustal velocity structure. Synthetic tests show that simulated annealing is able to locate low P-wave velocity zones with reasonable accuracy in space, and recovers almost 70% of the target velocity perturbations. Testing suggests that significant results can be obtained with as few as two stations. Inversion of ray direction data from Long Valley caldera finds an asymmetrical low velocity zone beneath the resurgent dome at 8km depth, having a perturbation of −25% from the background. This zone extends to the east and south to depths of at least 24km, with a perturbation of about −10%. These features generally agree with previous results from teleseismic travel-time inversion and forward modeling of ray direction, and are probably associated with the residual Long Valley caldera magma chamber.