ALBERT

Description: Seismic spectral models for chemical and nuclear explosions are used in many applications including network modeling and yield estimation. Here we compare the models presented in Denny and Johnson (1991) and Mueller and Murphy (1971) with each other and with new results from the Source Physics Experiments (SPE). We demonstrate analytically the two models are in substantial agreement for large and normally buried explosions, consistent with much of the historic data collected during American and Soviet nuclear testing. However, for small and/or deeply buried explosions, the spectral predictions of the two models can differ significantly. For example, the predicted yield of a 1 km deep, M w  2 nuclear explosion differs by more than a factor of 5; and, for the same moment and depth chemical explosion, the difference is greater than a factor of 10. We compare the models with initial data from the SPE, which include small and overburied chemical explosions. The corner frequency of the one-ton SPE explosion (SPE-2) is slightly higher than the Mueller and Murphy (1971) model and approximately double the Denny and Johnson (1991) model prediction. The absolute moment of the one-tenth ton SPE explosion (SPE-1) is near the Denny and Johnson (1991) prediction and an order of magnitude smaller than the Mueller and Murphy (1971) prediction. The low-frequency moment ratio for SPE-2/SPE-1 is more consistent with the Denny and Johnson (1991) model. The results presented here show the need for an improved explosion source model that can accommodate a wider range of yields and emplacement conditions. Online Material: Moment magnitude, corner frequency, and yield for all geologic media.

Description: In this work, we cross-correlated waveforms in a global dataset consisting of over 310 million waveforms from nearly 3.8 million events recorded between 1970 and 2013 for two purposes: to better understand the nature of global seismicity and to evaluate correlation as a technique for automated event processing. We found that about 14.5% of the events for which we have at least one waveform correlated with at least one other event at the 0.6 or higher level. Within the geographic regions where our waveform holdings are complete or nearly complete, that fraction rose to nearly 18%. Moreover, among the events for which we had one or more seismograms recorded at distances less than 12°, the fraction of correlated events was much higher, often exceeding 50%. These results imply that global seismicity contains a large number of repeating events, that is, events that are sufficiently similar to each other to have correlated waveforms over the time period spanned by our dataset. These results are very encouraging for using correlation in aspects of automated event processing. It is well known that because of the strongly implied similarity of the sources of correlated signals, they can be used as empirical signal detectors (ESD) to detect, locate, and identify an event using as few as one channel. Our results are very encouraging for using correlation and perhaps other forms of ESD for regional network processing and continental global processing because, for example, nearly all continental seismicity (99%) is within 12° of at least one International Monitoring System station.

Description: We introduce a new method to use narrowband regional amplitude envelopes for event analysis. Building on the success of the coda-wave method, we construct synthetic template envelopes that attempt to fit the entire waveform, including multiple direct phases and their coda, across a broad frequency band. The method makes use of our understanding of earthquake and explosion source models, regional wave propagation, and the relationship between direct amplitudes and their respective codas. We demonstrate the power of the method by examining earthquake and nuclear explosions in the Korean Peninsula at regional distance stations MDJ (Mudangjing, China) and TJN (Taejon, South Korea). In order to implement the method, however, we need to account for propagation through the use of an attenuation model for the region, which we have developed, in addition to an empirical correction to provide for unaccounted effects in the direct-to-coda transfer functions. Under the assumption that our explosion and attenuation models and the empirically obtained P -to- P -coda and S -to- S -coda transfer functions are correct, we determine that the 2006 test by the Democratic People’s Republic of Korea (DPRK) is consistent with a yield between 200 and 800 tons and a depth between 20 and 300 m, with our best fit at 500 tons at a depth of 100 m. Similarly, the 2009 DPRK test is consistent with a yield range of 1–5 kt and a depth range of 70–600 m, with our best fit at 2 kt at a depth of 200 m.

Description: We investigate the excitation and propagation of far-field seismic waves from the 905 kg trinitrotoluene equivalent underground chemical explosion SPE-3 recorded during the Source Physics Experiment (SPE) at the Nevada National Security Site. The recorded far-field ground motion at short and long distances is characterized by substantial shear-wave energy, and large azimuthal variations in P - and S -wave amplitudes. The shear waves observed on the transverse component of sensors at epicentral distances 〈50 m suggests they were generated at or very near the source. The relative amplitude of the shear waves grows as the waves propagate away from the source. We analyze and model the shear-wave excitation during the explosion in the 0.01–10 Hz frequency range, at epicentral distances of up to 1 km. We used two simulation techniques. One is based on the empirical isotropic Mueller–Murphy (MM) ( Mueller and Murphy, 1971 ) nuclear explosion source model, and 3D anelastic wave propagation modeling. The second uses a physics-based approach that couples hydrodynamic modeling of the chemical explosion source with anelastic wave propagation modeling. Comparisons with recorded data show the MM source model overestimates the SPE-3 far-field ground motion by an average factor of 4. The observations show that shear waves with substantial high-frequency energy were generated at the source. However, to match the observations additional shear waves from scattering, including surface topography, and heterogeneous shallow structure contributed to the amplification of far-field shear motion. Comparisons between empirically based isotropic and physics-based anisotropic source models suggest that both wave-scattering effects and near-field nonlinear effects are needed to explain the amplitude and irregular radiation pattern of shear motion observed during the SPE-3 explosion.

Description: The Source Physics Experiment (SPE) is a series of chemical explosions at the Nevada National Security Site (NNSS, formerly the Nevada Test Site) designed to improve our understanding of explosion physics. A future SPE will place an explosion at the hypocenter of a small, shallow earthquake, providing a direct earthquake-to-explosion experiment. Candidate earthquakes for this novel experiment come from a sequence of over 200 unusually shallow events that occurred in Rock Valley, Nevada, in the southeastern portion of the NNSS during 1993. We apply the Bayesloc multiple-event location algorithm ( Myers et al. , 2007 , 2009 ) to determine the best possible locations and depths for these events. Past nuclear tests in the nearby Yucca Flat on the NNSS are relocated with the same method to provide insight into the accuracy and uncertainties associated with the Bayesloc location results for the Rock Valley earthquakes. This test suggests that we can accurately pinpoint the location of the Rock Valley events within approximately 1 km of their true locations using direct arrival times only. The incorporation of differential arrival times and a potential ground-truth event can significantly decrease the already small uncertainties associated with the epicenter locations. Depth determinations have uncertainties of a few kilometers. Depth uncertainty may be reduced by developing an accurate 3D model of P -wave and S -wave velocity for Rock Valley.

Description: In this study, we investigate the 14 September 1988 U.S.–Soviet Joint Verification Experiment nuclear test at the Semipalatinsk test site in eastern Kazakhstan and two nuclear explosions conducted less than 10 years later at the Chinese Lop Nor test site. These events were very sparsely recorded by stations located within 1600 km, and in each case only three or four stations were available in the regional distance range. We have utilized a regional distance seismic waveform method fitting long-period, complete, three-component waveforms jointly with first-motion observations from regional stations and teleseismic arrays. The combination of long-period waveforms and first-motion observations provides a unique discrimination of these sparsely recorded events in the context of the Hudson et al. (1989) source-type diagram. We demonstrate through a series of jackknife tests and sensitivity analyses that the source type of the explosions is well constrained. One event, a 1996 Lop Nor shaft explosion, displays large Love waves and possibly reversed Rayleigh waves at one station, indicative of a large F -factor. We show the combination of long-period waveforms and P -wave first motions are able to discriminate this event as explosion-like and distinct from earthquakes and collapses. We further demonstrate the behavior of network sensitivity solutions for models of tectonic release and spall-based tensile damage over a range of F -factors and K -factors.

Description: An issue for moment tensor (MT) inversion of shallow seismic sources is that some components of the Green’s functions have vanishing amplitudes at the free surface, which can result in bias in the MT solution. The effects of the free surface on the stability of the MT method become important as we continue to investigate and improve the capabilities of regional full MT inversion for source-type identification and discrimination. It is important to understand free-surface effects on discriminating shallow explosive sources for nuclear monitoring purposes. It may also be important in natural systems that have very shallow seismicity, such as volcanic and geothermal systems. We examine the effects of the free surface on the MT via synthetic testing and apply the MT-based discrimination method to three quarry blasts from the HUMMING ALBATROSS experiment. These shallow chemical explosions at ~10 m depth and recorded up to several kilometers distance represent rather severe source–station geometry in terms of free-surface effects. We show that the method is capable of recovering a predominantly explosive source mechanism, and the combined waveform and first-motion method enables the unique discrimination of these events. Recovering the design yield using seismic moment estimates from MT inversion remains challenging, but we can begin to put error bounds on our moment estimates using the network sensitivity solution technique ( Ford et al. , 2010 ). Online Material: Figures showing synthetic tests for a pure explosion and a composite source at local distances and table of moment tensor components.

Description: 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.

Description: Reliable moment magnitude estimates for seismic events in the Middle East region can be difficult to obtain due to the uneven distribution of stations, the complex tectonic structure, and regions of high attenuation. In this study, we take advantage of the many new broadband seismic stations that have become available through improved national networks and numerous temporary deployments. We make coda envelope-amplitude measurements for 2247 events recorded by 68 stations over 13 narrow frequency bands ranging between 0.03 and 8 Hz. The absolute scaling of these spectra was calculated based on independent waveform modeling solutions of the moment magnitudes for a subset of these events to avoid circularity. Using our 1D path calibrations, we determined coda-based magnitudes for a majority of the events. We obtain fairly good agreement with waveform-modeled seismic moments for the larger events ( M w 〉4.5) at low frequencies (〈0.7 Hz). As expected, the coda-derived source spectra become increasingly scattered at higher frequencies (〉0.7 Hz) because of unaccounted 2D path effects, as well as mixing of both Sn coda and Lg coda, which have different attenuation behavior. This scatter leads to increased variance in the magnitudes estimated for smaller events in which low-frequency amplitudes are below the noise levels and the higher frequencies are the only signals available. We quantify the expected variance in coda envelope amplitudes as a function of frequency using interstation scatter as our metric. The net results of this study provide thousands of new 1D coda magnitude estimates for events in the broad region, as well as the necessary initial starting model for use in a new related 2D coda study ( Pasyanos et al. , 2016 ). Online Material: Table of site terms and moment magnitudes.