Sean R. Ford

Source Analysis of the Crandall Canyon, Utah, Mine Collapse

Analysis of seismograms from a magnitude 3.9 seismic event on 6 August 2007 in central Utah reveals an anomalous radiation pattern that is contrary to that expected for a tectonic earthquake and which is dominated by an implosive component. The results show that the seismic event is best modeled as a shallow underground collapse. Interestingly, large transverse surface waves require a smaller additional noncollapse source component that might represent either faulting in the rocks above the mine workings or deformation of the medium surrounding the mine. Seismic moment tensor results for nuclear explosions, explosion and other mining cavity collapses, and tectonic earthquakes are compared, and the separation of the different populations indicates that the seismic moment tensor may be used for source-type discrimination.

Partitioning of Seismoacoustic Energy and Estimation of Yield and Height‐of‐Burst/Depth‐of‐Burial for Near‐Surface Explosions

Explosions near the Earths surface excite both seismic ground motions and atmospheric overpressure. The energy transferred to the ground and atmosphere from a near-surface explosion depends on yield (W) as well as the height-of-burst/ depth-of-burial(HOB/DOB)forabove/belowgroundemplacements.Wereportanalyses of seismic and overpressure motions from the Humble Redwood series of low-yield, near-surface chemical explosions with the aim of developing quantitative models of energy partitioning and a methodology to estimate W and HOB/DOB. The effects of yield, HOB, and range on amplitudes can be cast into separable functions of range andHOBscaledbyyield.WefindthatdisplacementoftheinitialPwaveandtheintegral of the positive overpressure (impulse) are diagnostic of W and HOB with minimal scat- ter. An empirical model describing the dependence of seismic and air-blast measure- ments on W, HOB/DOB, and range is determined and model parameters are found by regression. We find seismic amplitudes for explosions of a given yield emplaced at or above the surface are reduced by a factor of 3 relative to fully contained explosions below ground. Air-blast overpressure is reduced more dramatically, with impulse reduced by a factor of 100 for deeply buried explosions relative to surface blasts. Oursignalmodelsare usedtoinvertseismicandoverpressure measurementsforW and HOB and we find good agreement (W errors <30%, HOB within meters) with ground- truth values for four noncircular validation tests. Although there is a trade-off between W and HOB for a single seismic or overpressure measurement, the use of both meas- urement types allows us tolargelybreak this trade-off and better constrainW and HOB. However, both models lack resolution of HOB for aboveground explosions.

Event Discrimination using Regional Moment Tensors with Teleseismic‐P Constraints

Determining whether a seismic event is an earthquake, explosion, collapse, or something more complex can be done using regional (Δ 10  s) full waveform moment tensors down to low magnitudes ( M ∼3.5). The moment tensor results can be improved for sparse station configurations when teleseismic (Δ>30°) array‐based short‐period ( T <1  s) P constraints are added. The inclusion of teleseismic‐ P aids in event discrimination because it samples the lower region of the focal‐sphere, a region where intermediate‐period waveforms recorded at the surface have low‐sensitivity for shallow event depths. The teleseismic‐ P constraint is particularly useful in reducing the trade‐off between a shallow explosion and a shallow volume‐compensated linear‐vector dipole with a vertical axis in compression. This trade‐off can complicate discrimination. The teleseismic‐ P constraint is applied to the source‐type analysis of the announced nuclear test of the Democratic People’s Republic of Korea on 25 May 2009, resulting in greater confidence in a dominantly explosive solution.

Determining the source characteristics of explosions near the Earth's surface

We present a method to determine source characteristics of explosions near the Earths surface. The technique accounts for the reduction in amplitudes as the explosion depth approaches the free surface and less energy is coupled into the ground. We apply the method to the Humming Roadrunner series of shallow explosions in New Mexico where the yields and depths are known. Knowledge of the material properties is needed for both source coupling/excitation and the free surface effect. Although there is the expected trade-off between depth and yield, the estimated yields are close to the known values when the depth is constrained to the free surface. We then apply the method to a regionally recorded explosion in Syria. We estimate an explosive yield less than the 60 t claimed by sources in the open press. The modifications to the method allow us to apply the technique to new classes of events.

Identifying Isotropic Events Using an Improved Regional Moment Tensor Inversion Technique

Using a regional time-domain waveform inversion for the complete moment tensor we calculate the deviatoric and isotropic source components for several explosions at the Nevada Test Site as well as earthquakes, and collapses in the surrounding region of the western US. The events separate into specific populations according to their deviation from a pure double-couple and ratio of isotropic to deviatoric energy. The separation allows for anomalous event identification and discrimination between explosions, earthquakes, and collapses. Error in the moment tensor solutions and source parameters is also calculated. We investigate the sensitivity of the moment tensor solutions to Greens functions calculated with imperfect Earth models, inaccurate event locations, and data with a low signal-to-noise ratio. We also test the performance of the method under a range of recording conditions from excellent azimuthal coverage to cases of sparse station availability, as might be expected for smaller events. Finally, we assess the depth and frequency dependence upon event size. This analysis will be used to determine the range where well-constrained solutions can be obtained.

Dynamics of the Bingham Canyon rock avalanches (Utah, USA) resolved from topographic, seismic, and infrasound data

The 2013 Bingham Canyon mine rock avalanches represent one of the largest cumulative landslide events in recorded U.S. history, and provide a unique opportunity to test remote analysis techniques for landslide characterization. Here we combine aerial photogrammetry surveying, topographic reconstruction, numerical runout modeling, and analysis of broadband seismic and infrasound data to extract salient details of the dynamics and evolution of the multi-phase landslide event. Our results reveal a cumulative intact-rock source volume of 52 Mm3, which mobilized in two main rock avalanche phases separated by 1.5 h. We estimate the first rock avalanche had 1.5-2 times greater volume than the second. Each failure initiated by sliding along a gently-dipping (21°), highly-persistent basal fault before transitioning to a rock avalanche and spilling into the inner pit. The trajectory and duration of the two rock avalanches were reconstructed using runout modeling and independent force-history inversion of intermediate-period (10-50 s) seismic data. Intermediate- and shorter-period (1-50 s) seismic data were sensitive to intervals of mass redirection and constrained finer details of the individual slide dynamics. Back-projecting short-period (0.2-1 s) seismic energy, we located the two rock avalanches within 2 and 4 km of the mine. Further analysis of infrasound and seismic data revealed that the cumulative event included an additional 11 smaller landslides (volumes ~104-105 m3), and that a trailing signal following the second rock avalanche may result from an air-coupled Rayleigh wave. Our results demonstrate new and refined techniques for detailed remote characterization of the dynamics and evolution of large landslides.

An Explosion Aftershock Model with Application to On-Site Inspection

An estimate of aftershock activity due to a theoretical underground nuclear explosion is produced using an aftershock rate model. The model is developed with data from the Nevada National Security Site, formerly known as the Nevada Test Site, and the Semipalatinsk Test Site, which we take to represent soft-rock and hard-rock testing environments, respectively. Estimates of expected magnitude and number of aftershocks are calculated using the models for different testing and inspection scenarios. These estimates can help inform the Seismic Aftershock Monitoring System (SAMS) deployment in a potential Comprehensive Test Ban Treaty On-Site Inspection (OSI), by giving the OSI team a probabilistic assessment of potential aftershocks in the Inspection Area (IA). The aftershock assessment, combined with an estimate of the background seismicity in the IA and an empirically derived map of threshold magnitude for the SAMS network, could aid the OSI team in reporting. We apply the hard-rock model to a M5 event and combine it with the very sensitive detection threshold for OSI sensors to show that tens of events per day are expected up to a month after an explosion measured several kilometers away.

mb:Ms Screening Revisited for Large Events

Abstract Event screening of large‐magnitude events ( M w ≳5) based on m b : M s is revisited to account for the effect of the source corner frequency relative to the fixed frequencies of the long‐period M s and short‐period m b . For large events this source effect increases the slope of m b : M s relative to the 1:1 value expected for small events. The effect is demonstrated in the large earthquake m b : M s population and in the behavior of large theoretical explosions that are consistent with the more limited explosion population. The behavior is used to create a more conservative screening criterion that ensures large explosions are not inadvertently screened out by m b : M s , while not appreciably decreasing the number of screened earthquakes. This change also makes the variance of the earthquake and explosion populations more equal, which is of utility in statistical analysis. A slight trend in the explosion population and a case study of two large U.S. underground nuclear tests provide support for adopting a more conservative approach.

Seismic Models for Near‐Surface Explosion Yield Estimation in Alluvium and Sedimentary RockSeismic Models for Near‐Surface Explosion Yield Estimation in Alluvium and Sedimentary Rock

Author(s): Templeton, DC; Rodgers, AJ; Ford, SR; Harben, PE; Ramirez, AL; Foxall, W; Reinke, RE | Abstract:

Parametric Explosion Spectral Model

Small underground nuclear explosions need to be confidently detected, identified, and characterized in regions of the world where they have never before occurred. We develop a parametric model of the nuclear explosion seismic source spectrum derived from regional phases that is compatible with earthquake-based geometrical spreading and attenuation. Earthquake spectra are fit with a generalized version of the Brune spectrum, which is a three-parameter model that describes the long-period level, corner-frequency, and spectral slope at high-frequencies. Explosion spectra can be fit with similar spectral models whose parameters are then correlated with near-source geology and containment conditions. We observe a correlation of high gas-porosity (low-strength) with increased spectral slope. The relationship between the parametric equations and the geologic and containment conditions will assist in our physical understanding of the nuclear explosion source.