Brian W. Stump

Causal factors for seismicity near Azle, Texas

In November 2013, a series of earthquakes began along a mapped ancient fault system near Azle, Texas. Here we assess whether it is plausible that human activity caused these earthquakes. Analysis of both lake and groundwater variations near Azle shows that no significant stress changes were associated with the shallow water table before or during the earthquake sequence. In contrast, pore-pressure models demonstrate that a combination of brine production and wastewater injection near the fault generated subsurface pressures sufficient to induce earthquakes on near-critically stressed faults. On the basis of modelling results and the absence of historical earthquakes near Azle, brine production combined with wastewater disposal represent the most likely cause of recent seismicity near Azle. For assessing the earthquake cause, our research underscores the necessity of monitoring subsurface wastewater formation pressures and monitoring earthquakes having magnitudes of ∼M2 and greater. Currently, monitoring at these levels is not standard across Texas or the United States.

Small-Aperture Seismo-Acoustic Arrays: Design, Implementation, and Utilization

A four-element, 1-km-aperture seismo-acoustic array has been designed and installed northeast of Seoul, Korea. Each element of the array consists of a GS-13 vertical seismometer (1 Hz) in a shallow borehole (∼10 m) and a low-frequency acoustic gauge connected to an 11-element hose array (7.6-m hoses) at the surface. The array is being used to assess the importance of colocated seismic and acoustic sensors for the purposes of (1) quantifying wind as a source of seismic and acoustic noise, (2) constraining propagation path effects in the atmosphere and solid earth, (3) locating the sources of the waves, and (4) characterizing the source type. Seismic noise estimates illustrate a level that is only slightly above the low-noise model on average. Acoustic noise levels resolve the microbaroms during low-noise times but document a nearly 50-dB increase in noise during the windiest periods. Infrasonic noise in the 0.01- to 5-Hz band increases rapidly with wind velocity. The seismic noise shows little or no dependence on wind velocity. Analysis of the data from a 2-month time period suggests that there are many more acoustic signals than seismic (4–10 times as many). Approximately 1/4 of all seismic signals are associated with an acoustic arrival. The vast majority of seismo-acoustic observations come from sources in the 30- to 200-km range and occur during working hours, local time. The 30- to 200-km observation distance is surprising in that average atmospheric velocity models predict no acoustic returns in this range. Average atmospheric models modified by meteorological data for the troposphere indicate the possibility of ducting in the troposphere as an explanation for these arrivals. Event location is based upon regional seismic phase identification ( P n, P g, P m P, L g, R g) using the array and backazimuth estimates from both the seismic and acoustic data. Many of the infrasound signals have good signal-to-noise ratios from 1 to beyond 4 Hz. Despite the small size of the array, event clusters are identified at regional distances. Events associated with acoustic signals are presumed to be from mining regions. The existence of Rg arrivals and dominance of P energy at high frequency are consistent with this interpretation. Ground truth in the form of in-mine observations has validated that two of the clusters come from construction and mine blasts.

Dallas-Fort Worth earthquakes coincident with activity associated with natural gas production

On 31 October 2008 and the following day, numerous Dallas-Fort Worth (DFW) residents called 911 to report experiencing several small earthquakes, accompanied by loud booming noises and the shaking of walls and furniture. Using data recorded by regional seismic stations, the USGS National Earthquake Information Center (NEIC) located nine earthquakes with magnitudes between 2.5 and 3.0. On 16 May 2009, this scenario repeated itself, as local residents felt three earthquakes and the NEIC located four (largest = magnitude 3.3). A third sequence of felt events began on 2 June 2009, approximately 65 km southwest near the city of Cleburne, Texas, but has not yet been studied in detail (Figure 1).

Tele-infrasonic studies of hard-rock mining explosions

The Lac-du-Bonnet infrasound station, IS-10, and the Minnesota iron mines 390 km to the southeast are ideally located to assess the accuracy of atmospheric profiles needed for infrasound modeling. Infrasonic data from 2003 associated with explosions at the iron mine were analyzed for effects of explosion size and atmospheric conditions on observations with well-constrained ground truth. Noise was the determining factor for observation; high noise conditions sometimes prevented unequivocal identification of infrasound arrivals. Observed arrivals had frequencies of 0.5 to 5 Hz, with a dominant frequency of 2 Hz, and generally had durations on the order of 10 s or less. There was no correlation between explosive amount and observability. Tele-infrasonic propagation distances (greater than 250 km) produce thermospheric ray paths. Modeling is based upon MSIS/HWM (Mass Spectrometer Incoherent Scatter/Horizontal Wind Model) and NRL-G2S (Naval Research Laboratory Ground to Space) datasets. The NRL-G2S dataset provided more accurate travel time predictions that the MSIS/HWM dataset. PE modeling for the NRL-G2S dataset indicates energy loss at higher frequencies (around 4 Hz). Additionally, applying the Sutherland/Bass model through the NRL-G2S realization of the atmosphere in InfraMAP results in predicted amplitudes too small to be observed.

Discrimination of Delay-Fired Mine Blasts in Wyoming Using an Automatic Time-Frequency Discriminant

Delay-fired mine blasts, which consist of a series of individual shots arranged in a grid pattern and detonated in sequence, can introduce spectral modu- lations into recorded seismograms. We can exploit spectral modulations to separate delay-fired mine blasts from the remaining event population, which includes single- fired mine blasts and earthquakes. Here, we enhance an existing algorithm (Hedlin, 1998) for the automatic discrimination of delay-fired mine blasts. A total of seven separate discriminants are computed, based on the spectrograms of recorded events. A feature-selection procedure is used to ensure that each discriminant is significant and contributes to the overall performance of the discrimination algorithm. The effect of input parameters on the methodology is explored. The choice of input parameters is made to maximize the mean Mahalanobis distance between the earthquake and delay-fired mine-blast populations. The technique is then applied to a dataset con- sisting of regional earthquakes and delay-fired mine blasts recorded at a station in Wyoming. The results show that the larger delay-fired mine blasts, the cast blasts, can be identified successfully by using this technique. The smaller mine blasts are not identified with this technique, although such events are of less interest in a nuclear-monitoring perspective. In a drop-one test, 89.5% of the events studied are successfully identified. Of the events that are misclassified, one is a cast blast and seven are earthquakes. The cast blast is misclassified because of noise on one com- ponent, which biased the value of a single discriminant. The earthquakes are mis- classified because of a greater variance of the seven discriminants for the mine-blast population. The results suggest that this methodology is very successful atidentifying cast blasts in Wyoming, and would be an extremely useful method to use as part of an integrated set of discriminants for the identification of small-magnitude regional events.

Automatic infrasound detection and location of sources in the western United States

A catalog that characterizes sources of regional infrasound observed in the western U.S. (1 November 2010 to 31 October 2012) is produced. Data from nine University of Utah Seismograph Stations infrasonic arrays are supplemented by three additional arrays in Nevada, operated by Southern Methodist University. The detection procedure using an adaptive F-detector provides input into the Bayesian Infrasonic Source Location procedure. The catalog consists of 1510 events with indication of repeated events from many locations such as Dugway Testing Ground, Utah Test and Training Range, and New Bomb. We analyzed the relationship between seasonal variations in the event locations and wind conditions using the Ground-to-Space specifications based on publicly available operational numerical weather prediction data analysis products supplement by empirical models above 80 km. There is significant commonality between this studys bulletin and the Western United States Infrasonic Catalog published by Walker et al. (2011). A previous study utilized infrasound signals detected on the USArray Transportable Array seismic stations (2007–2008). Both results document the vast majority of events that occur during working hours, suggesting a human cause. To illustrate the utility of the event bulletin for exploring atmospheric dynamics, propagation paths of an event detected during the equinox period, when the stratospheric wind is low, were generated using a ray-tracing algorithm. We found that the observations contain stratospheric arrivals, not predicted by ray theory, possibly due to gravity waves increasing the effective jet speed.

Source Scaling of Contained Chemical Explosions as Constrained by Regional Seismograms

Quantification of source coupling and characterization parameters for contained, single-fired chemical explosions observed at a regional seismic array are quantified. The explosions were conducted in a mine in northeast Wyoming where delay-fired explosions occur regularly. The primary observational data sets consist of close-in measurements in the mine and regional observations at the International Monitoring System Primary array at Pinedale, Wyoming (PDAR). The single-fired explosions ranged in size from 5500 to 50,000 lb and were separated from one another from 30 m to over 4 km. Peak amplitude measures at a single element of the regional array are well matched with a power law dependence on explosive weight. Similar scaling constants were determined for each of the dominant regional phases: P n, 0.84 ± 0.14; P g, 0.84 ± 0.09; and L g, 0.91 ± 0.08. Peak amplitudes across PDAR show a factor of 3 variation for an individual explosion across all array elements. This amplitude variation is accompanied by a decrease in spatial correlation with increasing station separation indicating the importance of near-receiver structure. The regional signals were further characterized by computing frequency-dependent envelope functions at each array element. Consistent source scaling factors at a single array element were obtained from the envelope functions. A high degree of spatial variability as a function of frequency was documented for the envelope functions. Spectral ratios are used to investigate source scaling relations for the different explosions. Four similarly prepared shots with total explosive weights between 5500 and 6000 lb were detonated to investigate source repeatability. Spectral ratio analysis identified two of the four shots as only partially detonating, resulting in a reduction of long-period spectral level by factors of 10 and 50. These long-period reductions were accompanied by increases in source corner frequency, consistent with a smaller source size. Spectral ratios of waveforms from the larger explosions that fully detonated illustrate that P g and L g source functions are similar. The empirical source ratio is well modeled with a Mueller–Murphy source model. These experiments and the resulting data analysis suggest that simple, single-fired explosions of modest explosive weight can be used to calibrate regional seismic arrays, networks, and individual stations.

Identification of Mining Blasts at Mid- to Far-regional Distances Using Low Frequency Seismic Signals

Abstract — This paper reports results from two recent monitoring experiments in Wyoming. Broadband seismic recordings of kiloton class delay-fired cast blasts and instantaneous calibration shots in the Black Thunder coal mine were made at four azimuths at ranges from 1° to 2°. The primary focus of this experiment was to observe and to explain low-frequency signals that can be seen at all azimuths and should routinely propagate above noise to mid-regional distances where most events will be recorded by International Monitoring System (IMS) stations.¶The recordings clearly demonstrate that large millisecond delay-fired cast blasts routinely produce seismic signals that have significant spectral modulations below 10 Hz. These modulations are independent of time, the azimuth from the source and the orientation of the sensor. Low-frequency modulations below 5 Hz are seen beyond 9°. The modulations are not due to resonance as they are not produced by the calibration shots. Linear elastic modeling of the blasts that is guided by mine-blast reports fails to reproduce the fine detail of these modulations but clearly indicates that the enhanced “spectral roughness” is due to long interrow delays and source finiteness. The mismatch between the data and the synthetics is likely due to source processes, such as nonlinear interactions between shots, that are poorly understood and to other effects, such as variations of shot time and yield from planned values, that are known to be omnipresent but cannot be described accurately. A variant of the Automated Time-Frequency Discriminant (Hedlin, 1998b), which uses low-frequency spectral modulations, effectively separates these events from the calibration shots.¶The experiment also provided evidence that kiloton class cast blasts consistently yield energetic 2–10 second surface waves. The surface waves are strongly dependent on azimuth but are seen beyond 9°. Physical modeling of these events indicates that the surface waves are due mainly to the extended source duration and to a lesser extent to the slap-down of spalled material. The directionality is largely a path effect. A discriminant that is based on the partitioning of energy between surface and body waves routinely separates these events from the calibration shots.¶The Powder River Basin has essentially no natural seismic activity. How these mining events compare to earthquake observations remains to be determined.

Infrasonic Signals from Large Mining Explosions

We study infrasonic signals from large surface mining explosions in Wyoming. Detections at the Pinedale Infrasound Array (PDIAR) (obtained using a conventional array-processing technique) are automatically associated with ground- truth mining explosions at a range of 368 km. We then focus on four clear signals from mining explosions. By performing a detailed noise study and modeling the propaga- tion of infrasound using a raytracing algorithm and ground-to-space (G2S) atmo- spheric models, we assess the factors that contribute to the detectability of mining explosions. We find that we can explain most of the observations by propagation and noise effects alone, but that there are at least two notable outliers. Because of high noise levels at the PDIAR array, which places significant constraints on the sizes of mining explosions that can be detected, these results are strongly biased and must be reassessed for lower-noise infrasound arrays.

Microseismicity induced by a controlled, mine collapse at White Pine, Michigan

Abstract We recorded an, explosively induced, 320 m deep, mine collapse and subsequent aftershocks, at White Pine, Michigan, using an array of 12 seismic stations. The collapse resulted from the rubblization of a 3×10 4 m 2 panel of a room-and-pillar copper mine, performed to facilitate leaching operations. The explosions produced little seismic energy. However, the collapse generated strong tension-crack, free-fall and slap-down phases. Regional data indicate a magnitude (m bLg ) of 3.1, leading to estimates of displaced mass, 5.6×10 8 kg and thickness, 11 m. Peak acceleration was 300 cm/s 2 at ground zero and dropped to 20 cm/s 2 at 1.1 km. Most of the locatable affershocks, (90%) occured in the first 2 hours following collapse. At ground zero, the occurrence rate followed the modified Omori, law: Rate=560·(time-0.01) −1.3 , with time in hours. The largest aftershock generated a moment magnitude of 1.0. We obtained locations of 135 aftershocks. The aftershock zone was less than 100 m thick, situated on top of the collapsed panel. The shallowest aftershocks occurred at depths of 200 m, giving no indication of collapse-related deformation extending to the surface. Aftershocks concentrated along the only edge of the collapsed panel open to the room-and-pillar mine. If the seismically active area reflects the extent of the de-stressed zone used in modeling stress redistribution, the asymmetrical distribution with respect to the collapsed panel was consistent with lower-than-predicted stresses measured in the first row of intact pillars.