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  • 1
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    Moment Magnitudes of Local/Regional Events from 1D Coda Calibrations in the Broader Middle East Region (2016)
    Seismological Society of America (SSA)
    Details
    Publication Date: 2016-10-08
    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.
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences , Physics
    Published by Seismological Society of America (SSA)
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  • 2
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    Monitoring of strain release in central and northern California using broadband data (1993)
    In:  Geophys. Res. Lett., Oslo, Wiley, vol. 20, no. 20, pp. 1643-1646, pp. L02304, (ISSN: 1340-4202)
    Details
    Publication Date: 1993
    Keywords: Seismology ; Fracture ; Source parameters ; Broad-band ; GRL
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    BIBLIOGRAPHY SEISMOLOGY
  • 3
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    Seismic experiment paves way for long-term seafloor observatories (1998)
    In:  Eos, Trans., Am. Geophys. Un., Houston, Akademie-Verlag, vol. 79, no. 26, pp. 301, 308 & 309, pp. L08310, (ISSN: 1340-4202)
    Details
    Publication Date: 1998
    Keywords: Seismology ; OBS ; telemetry ; Signal to noise ratio ; NOISE ; Instruments ; cable ; coupling ; bottom ; currents ; real-time
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    BIBLIOGRAPHY SEISMOLOGY
  • 4
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    Lithospheric velocity structure of the Anatolian plateau-Caucasus-Caspian region (2011)
    Wiley
    Details
    Publication Date: 2011-05-07
    Description: The Anatolian plateau-Caucasus-Caspian region is an area of complex lithospheric structure accompanied by large variations in seismic wave velocities. Despite the complexity of the region, little is known about the detailed lithospheric structure. Using data from 31 new, permanent broadband seismic stations along with results from a previous 29 temporary seismic stations and 3 existing global seismic stations in the region, a 3-D velocity model is developed using joint inversion of teleseismic receiver functions and surface waves. Both group and phase dispersion curves (Love and Rayleigh) were derived from regional and teleseismic events. Additional Rayleigh wave group dispersion curves were determined using ambient noise correlation. Receiver functions were calculated using P arrivals from 789 teleseismic (30°–90°) earthquakes. The stacked receiver functions and surface wave dispersion curves were jointly inverted to yield the absolute shear wave velocity to a depth of 100 km at each station. The depths of major discontinuities (sediment-basement, crust-mantle, and lithosphere-asthenosphere) were inferred from the velocity-depth profiles at the location of each station. Distinct spatial variations in crustal and upper mantle shear velocities were observed. The Kura basin showed slow (∼2.7–2.9 km/s) upper crustal (0–11 km) velocities but elevated (∼3.8–3.9 km/s) velocities in the lower crust. The Anatolian plateau varied from ∼3.1–3.2 in the upper crust to ∼3.5–3.7 in the lower crust, while velocities in the Arabian plate (south of the Bitlis suture) were slightly faster (upper crust between 3.3 and 3.4 km/s and lower crust between 3.8 and 3.9 km/s). The depth of the Moho, which was estimated from the shear velocity profiles, was 35 km in the Arabian plate and increased northward to 54 km at the southern edge of the Greater Caucasus. Moho depths in the Kura and at the edge of the Caspian showed more spatial variability but ranged between 35 and 45 km. Upper mantle velocities were slow under the Anatolian plateau but increased to the south under the Arabian plate and to the east (4.3–4.4 km/s) under the Kura basin and Greater Caucasus. The areas of slow mantle coincided with the locations of Holocene volcanoes. Differences between Rayleigh and Love dispersions at long wavelengths reveal a pronounced variation in anisotropy between the Anatolian plateau and the Kura basin.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 5
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    Significantly Improving Regional Seismic Amplitude Tomography at Higher Frequencies by Determining S-Wave Bandwidth (2016)
    Seismological Society of America (SSA)
    Details
    Publication Date: 2016-06-10
    Description: Characterizing regional seismic signals continues to be a difficult problem due to their variability. Calibration of these signals is very important to many aspects of monitoring underground nuclear explosions, including detecting seismic signals, discriminating explosions from earthquakes, and reliably estimating magnitude and yield. Amplitude tomography, which simultaneously inverts for source, propagation, and site effects, is a leading method of calibrating these signals. A major issue in amplitude tomography is the data quality of the input amplitude measurements. Pre-event and prephase signal-to-noise ratio (SNR) tests are typically used but can frequently include bad signals and exclude good signals. The deficiencies of SNR criteria, which are demonstrated here, lead to large calibration errors. To ameliorate these issues, we introduce a semiautomated approach to assess the bandwidth of a spectrum where it behaves physically. We determine the maximum frequency (denoted as F max ) where it deviates from this behavior due to inflections at which noise or spurious signals start to bias the spectra away from the expected decay. We compare two amplitude tomography runs using the SNR and new F max criteria and show significant improvements to the stability and accuracy of the tomography output for frequency bands higher than 2 Hz by using our assessments of valid S -wave bandwidth. We compare Q estimates, P / S residuals, and some detailed results to explain the improvements. For frequency bands higher than 4 Hz, needed for effective P / S discrimination of explosions from earthquakes, the new bandwidth criteria sufficiently fix the instabilities and errors so that the residuals and calibration terms are useful for application.
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences , Physics
    Published by Seismological Society of America (SSA)
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  • 6
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    Exploiting Regional Amplitude Envelopes: A Case Study for Earthquakes and Explosions in the Korean Peninsula (2012)
    Seismological Society of America (SSA)
    Details
    Publication Date: 2012-10-01
    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.
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences , Physics
    Published by Seismological Society of America (SSA)
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  • 7
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    Analysis of Ground Motion from An Underground Chemical Explosion (2015)
    Seismological Society of America (SSA)
    Details
    Publication Date: 2015-10-02
    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.
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences , Physics
    Published by Seismological Society of America (SSA)
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  • 8
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    A comprehensive dispersion model of surface wave phase and group velocity for the globe (2014)
    Oxford University Press
    Details
    Publication Date: 2014-08-02
    Description: A new method is developed to measure Rayleigh- and Love-wave phase velocities globally using a cluster analysis technique. This method clusters similar waveforms recorded at different stations from a single event and allows users to make measurements on hundreds of waveforms, which are filtered at a series of frequency ranges, at the same time. It also requires minimal amount of user interaction and allows easy assessment of the data quality. This method produces a large amount of phase delay measurements in a manageable time frame. Because there is a strong trade-off between the isotropic part of the Rayleigh-wave phase velocity and azimuthal anisotropy, we include the effect of azimuthal anisotropy in our inversions in order to obtain reliable isotropic phase velocity. We use b-splines to combine these isotropic phase velocity maps with our previous group velocity maps to produce an internally consistent global surface wave dispersion model.
    Keywords: Seismology
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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  • 9
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    A Lithospheric Attenuation Model of North America (2013)
    Seismological Society of America (SSA)
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    Publication Date: 2013-11-22
    Description: Recent moderate-sized, but strongly felt, earthquakes in eastern and central North America have highlighted the important role of the Earth’s attenuation structure in estimating and predicting local and regional ground motions. Over the past several years, we have been developing methods to use the amplitudes of regional phases Pn , , Sn , and to invert for the crust and upper mantle attenuation structure in Eurasia, and have recently started transporting the methodology to North America. We now have path coverage for most of North America, including Canada, the United States, Mexico, and portions of the Caribbean, with the best coverage in the United States. After describing the development of the model, we discuss the results in the context of the tectonics of the region, most notably the large differences between western North America and areas east of the Rockies. We will then demonstrate the use of the model in a number of applications including estimating reliable moment magnitudes for the Wells, Nevada, earthquake sequence, the use of the models in strong ground motion prediction for the Mineral, Virginia, mainshock, and in both discriminating and estimating explosion characteristics (depth, yield) of events at the Nevada Test Site.
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences , Physics
    Published by Seismological Society of America (SSA)
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  • 10
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    2D Variations in Coda Amplitudes in the Middle East (2016)
    Seismological Society of America (SSA)
    Details
    Publication Date: 2016-10-08
    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.
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences , Physics
    Published by Seismological Society of America (SSA)
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