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▲ USGS 3D geologic and seismic model of the San Francisco Bay area. Colors denote distinct geological units. Figure from Brocher 2005.
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... Lawrence Livermore National Laboratory hosted a workshop in 2007 June in Berkeley, California that brought together academic, government and industry researchers to explore the potential for 3-D seismic models to contribute to applications of societal importance including seismic monitoring. The summary and conclusions resulting from this workshop were documented in Zucca et al. (2009). Following the guidance in Zucca et al. (2009), our objectives were to first construct global models that could accurately predict traveltimes at teleseismic and regional distances for improved event location (Simmons et al. , 2012. ...
... The summary and conclusions resulting from this workshop were documented in Zucca et al. (2009). Following the guidance in Zucca et al. (2009), our objectives were to first construct global models that could accurately predict traveltimes at teleseismic and regional distances for improved event location (Simmons et al. , 2012. Up to this point, our emphasis has been placed on the location of the events used in the inversions and the treatment of the shallow upper mantle (<200 km depth) needed to predict accurate regional traveltimes for monitoring smaller events (with limited teleseismic signal). ...
... Our progression of global model development has followed the guidance in Zucca et al. (2009) where experts laid out an effective direction forward for constructing 3-D models that addressed near-and long-term societal needs including improved seismic event monitoring. constructed with crustal, regional and teleseismic traveltime data sets with a goal of improving event location. ...
SPiRaL is a joint global-scale model of wave speeds (P and S) and anisotropy (vertical transverse isotropy) variations in the crust and mantle. The model is comprised of >2.1 million nodes with five parameters at each node that capture velocity variations for P- and S-waves traveling at arbitrary directions in transversely isotropic media with a vertical symmetry axis (VTI). The crust (including ice, water, sediments, crystalline layers) is directly incorporated into the model. The default node spacing is approximately 2° in the lower mantle and 1° in the crust and upper mantle. The grid is refined with ∼0.25° minimum node spacing in highly sampled regions of the crust and upper mantle throughout North America and Eurasia. The data considered in the construction of SPiRaL includes millions of body wave travel times (crustal, regional, and teleseismic phases with multiples) and surface wave (Rayleigh and Love) dispersion. A multi-resolution inversion approach is employed to capture long-wavelength heterogeneities commonly depicted in global-scale tomography images as well as more localized details that are typically resolved in more focused regional-scale studies. Our previous work has demonstrated that such global-scale models with regional-scale detail can accurately predict both teleseismic and regional body wave travel times, which is necessary for more accurate location of small seismic events that may have limited signal at teleseismic distances. SPiRaL was constructed to predict travel times for event location and long-period waveform dispersion for seismic source inversion applications in regions without sufficiently tuned models. SPiRaL may also serve as a starting model for full-waveform inversion (FWI) with the goal of fitting waves with periods 10–50 seconds over multiple broad regions (thousands of kilometers) and potentially the globe. To gain insight to this possibility, we simulated waveforms using SPiRaL and independent waveform-based models for comparison. The performance of the travel-time-based SPiRaL model is shown to be on par with regional 3-D waveform-based models in three regions (western United States, Middle East, Korean Peninsula) suggesting SPiRaL may serve as a starting model for FWI over broad regions.
... As a result of a workshop, Zucca et al. (2009) outlined a plan to develop and implement 3D models in an operational system, and several models of earth structure have been subsequently developed with a focus on improving regional and teleseismic traveltime predictions and event locations (e.g., Phillips et al., 2007;Myers et al., 2010;Simmons et al., 2011Simmons et al., , 2012Simmons et al., and 2015Ballard et al., 2016b) (Figures 29 and 30). It has also been shown that global and regional 3D models improve traveltime prediction and, therefore, event location over 1D models for broad areas (e.g., Ritzwoller et al., 2003;Yang et al., 2004a;Bondár et al., 2014;Myers et al., 2015) (Figure 31). Figure 32 shows relocation results using a 3D model from joint inversion of seismic and gravity observations for the area of Iran showed in the inset plot. ...
This document entitled “Trends in Nuclear Explosion Monitoring Research and Development – A Physics Perspective” reviews the accessible literature, as it relates to nuclear explosion monitoring and the Comprehensive Nuclear-Test-Ban Treaty (CTBT, 1996), for four research areas: source physics (understanding signal generation), signal propagation (accounting for changes through physical media), sensors (recording the signals), and signal analysis (processing the signal). Over 40 trends are addressed, such as moving from 1D to 3D earth models, from pick-based seismic event processing to full waveform processing, and from separate treatment of mechanical waves in different media to combined analyses. Highlighted in the document for each trend are the value and bene t to the monitoring mission, key papers that advanced the science, and promising research and development for the future.
... However, more accurate estimates of epicenters are needed to access the proximity of earthquakes to metropolitan areas and to guide an on-site inspection under the Comprehensive Nuclear-Test-Ban Treaty (CTBT). Zucca et al. (2009) outline a plan to develop and implement 3D models in an operational system, and several models of Earth structure have been subsequently developed. ...
A global validation dataset of 116 seismic events and 20,977 associated Pn and P arrivals is used to assess travel-time prediction and event location accuracy for the global-scale, 3D, P-wave velocity model called LLNL-G3Dv3 (Simmons et al., 2012). Strong regional trends that are observed for ak135 travel-time residuals are largely removed when LLNL-G3Dv3 is used for prediction. The 25th-75th quantile spread of travel-time residuals is reduced by 30%-40% at teleseismic distances, and the spread is reduced by ∼60% at regional distances (<16°). Epicenter error decreases when more data are used to constrain event locations until more than ∼40 arrivals times are used. At which point, epicenter error reduction tends to plateau. Median epicenter errors for the ak135 and LLNL-G3Dv3 models plateau at ∼8:0 and ∼5:5 km, respectively, for teleseismic P datasets. Median epicenter errors for the ak135 and LLNL-G3Dv3 models plateau at ∼12:0 and ∼4:0 km, respectively, for regional Pn datasets. We demonstrate that spatially correlated travel-time residual errors for the ak135 model lead to increasing epicenter error when ∼40 to ∼100 Pn arrivals are used to constrain the location. The effect of correlated error is mitigated by LLNL-G3Dv3, for which epicenter error steadily decreases to ∼4 km when 100 Pn arrivals are used. The median area of 0.95 epicenter probability bounds for ak135 and LLNL-G3Dv3 are 1811 and 758 km2, respectively. The ak135 ellipses are inflated to achieve the desired rate of true events occurring inside the probability region, whereas LLNLG3Dv3 error ellipses based on empirical residual distributions cover the true location at the expected rate because location bias is minimal.
... There is a general consensus that the use of 3D models improves seismic characterization (Antoun et al., 2008). Advances in computation and numerical methods have made it possible to capture increasingly broadband, full wave generation and propagation in 3D Earth models. ...
Advances in computation and numerical method have made it possible to simulate increasingly broadband, full wave propagation in 3D Earth models and use the resulting waveforms to image the Earth structure and characterize seismic sources. Here we report the development of multi-grid full-wave tomography and moment tensor inversion based on 3D strain Green's tensor databases. We collected and processed up to 21 years continuous seismic data from broadband seismic stations in the eastern hemisphere (latitude 55S-55N; longitude 30W-157E). We developed a new method to significantly improve the extraction of Empirical Green's Functions (EGF) from ambient seismic noise. Compared to the commonly used one-bit normalization, the new method improves the signal-to-noise ratio of EGFs by a factor of ~2 and thus increases the effective data recording duration by a factor of 4, assuming random local and instrument noise. It yields useful Rayleigh waves at periods up to 300 s for PASSCAL-type deployments of broadband stations and 600 s for permanent stations with very broadband sensors. We overcome the challenge of computing full-wave propagation in 3D models with a multi-grid approach that allows wave-speed solutions to be passed from coarse to fine wave-simulation (and inversion) grids, and vice versa. The result is a comprehensive P and S velocity model of the crust and upper mantle for the eastern hemisphere that contains regions with variable resolutions. In the upper mantle transition zone, the major findings to date include a deep-rooted low-velocity anomaly beneath the Eastern African Rift and Afar, the Tethyan slab, and a broad low-velocity anomaly beneath northern Eurasia. In the mid upper mantle, the model reveals various depth extents of the cratonic roots, and the remnant cratonic mantle in the wake of the Indian plate. In regions with fine grids and dense temporary arrays (e.g., southern Tibet), we observe details such as ductile extrusion of high-grade metamorphic rocks on the southern slope of the Himalaya. Ongoing work includes integration of earthquake arrivals and ambient noise data.
... Although the past years have seen considerable progress in developing global 3-D velocity models that give accurate traveltime predictions (e.g. Myers & Schultz 2000;Ritzwoller et al. 2003;Yang et al. 2004;Morozov et al. 2005;Murphy et al. 2005;Reiter et al. 2005;Flanagan et al. 2007;Zucca et al. 2009;Myers et al. 2010), 1242 I. Bondár and D. Storchak Figure 18. (Continued.) ...
The International Seismological Centre (ISC) is a non-governmental, non-profit organization with the primary mission of producing the definitive account of the Earth′s seismicity. The ISC Bulletin covers some 50 yr (1960–2011) of seismicity. The recent years have seen a dramatic increase both in the number of reported events and especially in the number of reported phases, owing to the ever-increasing number of stations worldwide. Similar ray paths will produce correlated traveltime prediction errors due to unmodelled heterogeneities in the Earth, resulting in underestimated location uncertainties, and for unfavourable network geometries, location bias. Hence, the denser and more unbalanced the global seismic station coverage becomes, the less defensible is the assumption (that is the observations are independent), which is made by most location algorithms.
To address this challenge we have developed a new location algorithm for the ISC that accounts for correlated error structure, and uses all IASPEI standard phases with a valid ak135 traveltime prediction to obtain more accurate event locations. In this paper we describe the new ISC locator, and present validation tests by relocating the ground truth events in the IASPEI Reference Event List, as well as by relocating the entire ISC Bulletin.
We show that the new ISC location algorithm provides small, but consistent location improvements, considerable improvements in depth determination and significantly more accurate formal uncertainty estimates. We demonstrate that the new algorithm, through the use of later phases and testing for depth resolution, considerably clusters event locations more tightly, thus providing an improved view of the seismicity of the Earth.
... There is a general consensus that the use of 3D models improves seismic characterization (Antoun et al., 2008). Advances in computation and numerical methods have made it possible to capture increasingly broadband, full wave generation and propagation in 3D Earth models. ...
There is a general consensus that 3D reference models can be used to isolate effects of wave propagation and thus help in improving characterization of seismic sources. Advances in computation and numerical method have made it possible to capture increasingly broadband, full wave generation and propagation in 3D earth models. The main objectives of this project are to construct hierarchical, multi-grid (resolution) joint P and S velocity models and the corresponding finite-difference strain Green's tensors (FDSGT) for rapid seismic moment determination. In the first year of a three-year project, we collected and processed up to 21 years continuous seismic data from broadband seismic stations in the eastern hemisphere (latitude 55S-55N; longitude 30W-157E). Empirical Green's functions derived from ambient seismic noise show clear Rayleigh waves over a broad frequency range. We measured over ~10,000 phase delays in 6 period bands ranging from 50 s to 600 s, which provide new constraints on the entire upper mantle, including the upper mantle transition zone. Several iterations of full-wave tomographic inversion have been carried out to obtain a joint P and S velocity model. Preliminary results clearly show high-velocity anomalies associated with plate subduction beneath Indonesia, southern Tibet, Iran, and the Hellenic arc. The African cratons and the west Australian craton appear as prominent high-velocity features. The eastern African Rift and Afar have deep-rooted low-velocity anomalies. Future work includes integration of earthquake arrivals and ambient noise data.
One branch of signal processing in geophysics has undergone significant long-term development due to the requirements of nuclear test ban monitoring. There was a burst of activity in the 1960s and 1970s due to the Limited Test Ban Treaty (1963), which banned tests in the atmosphere, underwater, and in outer space, and the Threshold Test Ban Treaty (1974), which placed a cap on explosive yield at 150 kt. These treaties drove testing underground and created requirements for detecting, locating, and identifying explosions, and estimating their yield through observations of seismic waves.
