ALBERT

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: We performed relative locations of six event-pairs based on surface-wave (SW) and body-wave (BW) differential travel-times of the 2016-09-09, 2016-01-06, 2013-02-12, and 2009-05-25 announced North Korea nuclear explosions. The SW relative locations for the 2009-05-25 and 2013-02-12 events were inconsistent with the BWs when paired with other events and only the 2016-01-06/2016-09-09 pair was consistent. Apparent SW phase shift is investigated with respect to the BW relative locations. The pairs formed with the 2009-05-25 and 2013-02-12 events, beneath the southeast slope of Mount Mant'ap, have the largest phase shifts and amplitude ratio deviations, whereas the least deviation was from the 2016-01-06 and 2016-09-09 event-pair beneath the mountain peak. Regional moment tensors (MTs) predict the amplitude ratios but do not resolve the relative phase. We find MTs with 10% difference in isotropic and rotated +CLVD can fit both relative phase and amplitude ratios. SW relative locations of highly isotropic and correlated explosion clusters can be affected by topography and small differences in MT.

Description: Coda amplitudes have proven to be a stable feature of seismograms, allowing one to reliably measure magnitudes for moderate-to-large ( M ≥3) earthquakes over broad regions. Because smaller ( M 〈3) earthquakes are only recorded at higher frequencies for which we find larger interstation scatter, amplitude and magnitude estimates for these events are more variable, regional, and path dependent. In this article, we investigate coda amplitude measurements in the Middle East for 2D variations in attenuation structure. One critical aspect of this effort is characterizing the propagation term to include scattering, which allows us to use amplitudes out to longer distances and later in the coda. We perform a tomographic inversion and find that the recovered attenuation structure is both very similar to the attenuation structure derived from direct phases and also reflective of the tectonic structure of the region. We then apply the 2D attenuation corrections to several hundred events in the region and find marked improvements to our magnitude estimates, as measured by interstation scattering, resulting in standard deviations of less than 0.025 magnitude units at all frequencies. The improvements are greatest at high frequencies, which will have the largest effect on smaller magnitude events.

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 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: 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.

Description: We performed relative locations of six event pairs based on surface wave (SW) and body wave (BW) differential traveltimes of the 9 September 2016, 6 January 2016, 12 February 2013, and 25 May 2009 announced North Korea nuclear explosions. The SW relative locations for the 25 May 2009 and 12 February 2013 events were inconsistent with the BWs when paired with other events, and only the 6 January 2016/9 September 2016 pair was consistent. Apparent SW phase shift is investigated with respect to the BW relative locations. The pairs formed with the 25 May 2009 and 12 February 2013 events, beneath the southeast slope of Mount Mant'ap, have the largest phase shifts and amplitude ratio deviations, whereas the least deviation was from the 6 January 2016 and 9 September 2016 event pair beneath the mountain peak. Regional moment tensors (MTs) predict the amplitude ratios but do not resolve the relative phase. We find that MTs with 10% difference in isotropic and rotated +CLVD can fit both relative phase and amplitude ratios. SW relative locations of highly isotropic and correlated explosion clusters can be affected by topography and small differences in MT. ©2017. American Geophysical Union. All Rights Reserved.

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.