Conference abstracts
L. Schardong, Y. Ben Horin, A. Ziv, S.C. Myers, H. Wust-Bloch, M.L. Begnaud, B. Young & Y. Radzyner, A crustal P-wave velocity model for Israel to improve IMS capabilities in the Middle East, CTBT Science and Technology conference, 2021, online, talk.
The Israeli National Data Center is responsible for monitoring and characterising the seismicity of the Eastern Mediterranean region. The accuracy of seismic locations is mostly affected by the velocity model used, and no clear picture of the variations in seismic velocities in Israel has emerged in the recent years. We gathered a large dataset of seismic travel times recorded in Israel and nearby countries. After quality control and joint relocation of over 30,000 natural and man-made seismic events, we produced a revised dataset of more than 500,000 arrivals. From this dataset, we inverted Pg and Pn travel times for a crustal velocity model of the area using the FMTOMO tomographic inversion package. In order to do this, we put together a 3-D starting model that consists of an ensemble of 1-D velocity profiles for the various tectonic settings observed in the region. We present images extracted from this model, as well as corresponding synthetic resolution tests to assess the quality of our results. This high-resolution model is to be integrated into the Regional Seismic Travel Time model and procedure in order to enhance the CTBT’s International Monitoring System capabilities in the Middle East.
L. Schardong, Y. Ben Horin, A. Ziv, H. Wust-Bloch & Y. Radzyner, P-wave tomographic model from local bulletin data for improved seismic location in and around Israel, EGU General Assembly, 2020, online, talk.
For the past 40 years, the Geophysical Institute of Israel has been in charge of the recording, monitoring and relocating of local earthquakes. Due to the variety of data analysts and data sources, as well as several network upgrades, the resulting bulletin data has to be completed and homogenised, and station metadata needs to be tracked down, and sometimes corrected. For those reasons, as well as because of the lack of consensus on an accurate model for seismic velocities in the area, published source locations are often poorly constrained. We present a homogenised Israeli bulletin, including natural and man-made explosion data. We extract sets of seismic sources with location accuracy greater than 5 km (GT5), as well as GT0 explosions.
We select a set of events with the highest network coverage, comprising (1) natural earthquakes, (2) man-made quarry or mine blasts, (3) GT5 earthquakes or explosions, and (4) GT0 explosions. We relocate them altogether using the BayesLoc package, a Bayesian, hierarchical, multi-event locator which produces, after source relocation, event-, station- and phase-specific correction terms. We put different a priori constraints on the different categories of seismic events, allowing poorly constrained origin parameters to improve thanks to the more accurate GT locations. BayesLoc also produces traveltime correction terms that can be used to correct systematic errors in the dataset, as well as error estimates.
Eventually, we invert this homogenised local traveltime dataset in order to invert for a P-wave crustal velocity model of Israel and its surroundings. To do so, we use the Fast Marching Tomography package, which allows the representation of a wide variety of input structures (starting model and geometry of layer boundaries) and can take many different types of input data. We show preliminary inversion tests and results that are in good agreement with past local studies.
This crustal model of Israel is ultimately to be used as a starting model in a larger tomographic study of the Eastern Mediterranean and Middle East region, where the Regional Seismic Travel Time approach is to be expanded, in order to improve the CTBT’s capabilities in monitoring the regional seismicity. Eventually, such a velocity model could also be used to relocate the whole earthquake catalogue more accurately, and improve the Earthquake Early Warning System currently in development in Israel.
We select a set of events with the highest network coverage, comprising (1) natural earthquakes, (2) man-made quarry or mine blasts, (3) GT5 earthquakes or explosions, and (4) GT0 explosions. We relocate them altogether using the BayesLoc package, a Bayesian, hierarchical, multi-event locator which produces, after source relocation, event-, station- and phase-specific correction terms. We put different a priori constraints on the different categories of seismic events, allowing poorly constrained origin parameters to improve thanks to the more accurate GT locations. BayesLoc also produces traveltime correction terms that can be used to correct systematic errors in the dataset, as well as error estimates.
Eventually, we invert this homogenised local traveltime dataset in order to invert for a P-wave crustal velocity model of Israel and its surroundings. To do so, we use the Fast Marching Tomography package, which allows the representation of a wide variety of input structures (starting model and geometry of layer boundaries) and can take many different types of input data. We show preliminary inversion tests and results that are in good agreement with past local studies.
This crustal model of Israel is ultimately to be used as a starting model in a larger tomographic study of the Eastern Mediterranean and Middle East region, where the Regional Seismic Travel Time approach is to be expanded, in order to improve the CTBT’s capabilities in monitoring the regional seismicity. Eventually, such a velocity model could also be used to relocate the whole earthquake catalogue more accurately, and improve the Earthquake Early Warning System currently in development in Israel.
L. Schardong, Y. Ben-Horin, A. Ziv, G.H. Wust-Bloch & Y. Radzyner, 3-D seismic velocity model of the Eastern Mediterranean region using body-wave tomography, CTBT Science and Technology conference, 2019, Vienna (AT), talk.
The Israeli National Data Center monitors and characterises the seismicity of the Eastern Mediterranean region as part of its mandate with the Comprehensive Nuclear-Test-Ban Treaty (CTBT). The velocity model is one of the factors that affects most the location accuracy. Three-dimensional tomographic studies of the Middle East region have been conducted in the past, but they are patchy with inhomogeneous resolution, due to limited data sets, irregular data quality, as well as to the absence of a reliable and efficient inversion technique. We developed a new, high-resolution body-wave velocity model of the Eastern Mediterranean region. Major efforts were applied into producing a high-quality body-wave traveltime dataset, essential in order to obtain a high-resolution velocity model. We gathered seismic data from local events, as well as from man-made explosions, mostly quarry blasts in Israel and Jordan. We used both Pg and Pn phases to constrain the crust and uppermost mantle. All events were relocated and the dataset was quality-controlled using the BayesLoc software. The tomographic inversion was performed using the Regional Seismic Travel Time software, which is used by CTBT-affiliated institutions to build the current global tomographic model. Our new velocity model will not only enhance seismic location capabilities of the CTBT, it will also be of significant importance for earthquake monitoring in the region and for the Earthquake Early Warning System being currently implemented in Israel.
L. Schardong, A.M.G. Ferreira, H.-J. van Heijst & J. Ritsema, Local velocity structure from the amplification of surface waves, AGU Fall Meeting, 2016, San Francisco (US), talk.
We apply a new method to build receiver-side amplification maps from the amplitude of surface waves recorded at nearby stations. From these maps, we extract single-station amplification dispersion curves that are subsequently inverted to retrieve the 1-D structure below each receiver using a neighbourhood algorithm approach. The technique is applied to Rayleigh-wave data measured in the 35-to-275-s period-range (good sensitivity down to ~400 km of depth), and recorded at 500+ stations in the Western continental U.S., and 80+ stations in the Kaapvaal craton area, and used to invert for shear-wave velocity. In both areas, we image commonly-observed large-scale features, such as the low shear-wave velocity regions corresponding to the active western U.S., or the fast signature of the Kaapvaal craton. By using amplitude data, we image smaller-scale features than those from classical travel-time tomography. Moreover, we present initial results of inversions for shear attenuation.
L. Schardong, A.M.G. Ferreira, H.-J. van Heijst & J. Ritsema, Local velocity structure from the amplification of surface waves, AGU Fall Meeting, 2015, San Francisco (US), poster.
We apply a new technique to retrieve local amplification from the amplitude of seismic surface waves, in various areas on Earth. By taking the ratio of amplitudes measured at two close-by locations, and by averaging over many recordings, we are able to isolate the receiver-side contribution from the effects of structures at the source and along the propagation path. The technique is applied to Rayleigh-wave data measured in the 35-275 s period-range, allowing us to construct amplification maps sensitive down to ~250 km depth. We assess the reliability of the method by performing various tests based on synthetic data. Since surface-wave amplification depth kernels demonstrate a strong sensitivity to shear velocity and attenuation, we invert for these parameters. To that purpose, we invert the observed amplification dispersion curves to generate best-fitting radial profiles using a neighbourhood algorithm approach. We employ this set of techniques to several regions of geological interest, and interpret the results in terms of local mantle dynamics.
L. Schardong, A.M.G. Ferreira, H.-J. van Heijst & J. Ritsema, An analysis of fundamental-mode surface-wave amplitude measurements, AGU Fall Meeting, 2014, San Francisco (US), poster.
Seismic tomography is a powerful tool to decipher the Earth's interior structure at various scales. Traveltimes of seismic waves are widely used to build velocity models, whereas amplitudes are still only seldomly accounted for. This mainly results from our limited ability to separate the various physical effects responsible for observed amplitude variations, such as focusing/defocussing, scattering and source effects. We present new measurements from 50 global earthquakes of fundamental-mode Rayleigh and Love wave amplitude anomalies measured in the period range 35-275 seconds using two different schemes: (i) a standard time-domain amplitude power ratio technique; and (ii) a mode-branch stripping scheme. For minor-arc data, we observe amplitude anomalies with respect to PREM in the range of 0-4, for which the two measurement techniques show a very good overall agreement. We present here a statistical analysis and comparison of these datasets, as well as comparisons with theoretical calculations for a variety of 3-D Earth models. We assess the geographical coherency of the measurements, and investigate the impact of source, path and receiver effects on surface wave amplitudes, as well as their variations with frequency in a wider range than previously studied.
L. Schardong, S. Chevrot & R.F Garcia, Global P- and S-wave tomography from finite frequency traveltime residual measurements, AGU Fall Meeting, 2013, San Francisco (US), talk.
There are two main ways to improve the resolution of tomographic images: building larger and higher quality datasets, and using better imaging theories, such as finite frequency theory or waveform inversion, instead of the more commonly used ray theory. A first step toward improving the resolution of global tomographic models is thus to construct a seismological database adapted to finite-frequency tomography or waveform inversion. We present a global dataset of finite frequency traveltime residuals. These measurements were obtained by cross-correlating observed and synthetic waveforms in several frequency bands. We exploit seismograms at significantly shorter periods than in other recent comparable works. We compute Green's functions with the GEMINI method and convolve them with high-frequency source-time functions. These source-time functions are inverted, along with focal depths, with a simulated annealing approach. We measure traveltime residuals on different P and S phases. We check the quality of our global database by performing preliminary tomographic inversions using ray theory. We use the subset of measurements performed in the 0.0725-Hz frequency range,
combined to ISC traveltimes, to build tomographic models for both P and S velocity anomalies. Resulting models are presented and compared to recently published global tomographic models, and some implications for finite frequency effects and physical properties of the deep Earth are discussed.
combined to ISC traveltimes, to build tomographic models for both P and S velocity anomalies. Resulting models are presented and compared to recently published global tomographic models, and some implications for finite frequency effects and physical properties of the deep Earth are discussed.
L. Schardong, N. Takeuchi, H. Kawakatsu & NECESSArray Project Team, Comparison of P-, SV- and SH-wave velocity models below Japan and northeast China, UK SEDI, 2014, London (UK), poster.
L. Schardong, N. Takeuchi, H. Kawakatsu & NECESSArray Project Team, Comparison of P-, SV- and SH-wave velocity models below Japan and Northeast China, SSJ Meeting, 2013, Yokohama (JP), talk.
H. Kawakatsu, L. Schardong, N. Takeuchi & NECESSArray Project Team, Comparison of P-, SV- and SH-wave velocity models below Japan and Northeast China, AGU Fall Meeting, 2013, San Francisco (US), poster.
The recent deployment of the NECESSArray seismic network in Northeast China has allowed new insights on the velocity structure of this formerly poorly resolved area. This experiment has brought new data to debate about the geodynamical context in this region (stagnant slab, origin of intraplate volcanoes, ...). We use a more efficient method for the measurement of traveltime residuals by cross-correlating observed and synthetic waveforms. Synthetic seismograms are convolved with high-frequency source-time functions inverted along with focal depths, following a simulated annealing approach. Thus, resulting modelled waveforms take more accurately account for the source effects. The method is used to perform measurements on direct P, SV and SH phases extracted from, respectively, vertical, radial and transverse band-pass-filtered records from NECESSArray and F-NET seismological networks. We finally invert the resulting datasets in order to obtain P-, SV- and SH-wave velocity models. To do so, we use a fast and efficient inversion method using ray theory. Observed structures in the three tomographic models show a very good geographical coherency. Nevertheless, some differences are observed below Songliao basin and Changbaishan volcanoes between, on the one side, P and SV models, and, on the other side, SH model. By measuring velocity ratios, we discuss anisotropy and thermal/compositional origin of these velocity anomalies.
L. Schardong, R.F. Garcia, S. Chevrot, N. Fuji & M. Calvet, Tomography of the lowermost mantle from core-diffracted body waves, SEDI Meeting, 2012, Leeds (UK), poster.
The D” layer plays a key role in the global dynamics of the Earth, as a thermal and chemical boundary layer. Thus, constraining in detail its structure is an important objective of seismic tomography. For this purpose, we have constructed a global data set of traveltime and amplitude perturbations of many seismic phases, measured in various frequency ranges. The method consists in cross- correlating observed with synthetic seismograms, computed with the GEMINI method. In order to compute synthetic seismograms as accurately as possible, we convolve GEMINI synthetic seismograms with high frequency source time functions, determined by waveform inversion of globally distributed body wave records. We present some results of the analysis of traveltime residuals of diffracted waves (Pdiff and Sdiff), which possess a strong sensitivity to D” layer heterogeneities, and are thus crucial to decipher the fine structures within this layer. A high resolution tomography of the lowermost mantle, including both P and Pdiff phases, and following the ray theory, will be presented, along with sensitivity kernels computed with new techniques, in prelude to the final high resolution finite frequency tomography.
L. Schardong, S. Chevrot, R.F Garcia & M. Calvet, A new approach to obtain improved P and S receiver functions globally: determination of crustal structure below seismic stations in the prospect of global tomography, AGU Fall Meeting, 2011, San Francisco (US), talk.
Classical receiver functions allow us to determine average crustal parameters beneath a seismological station, such as crustal thickness or Poisson ratio. We present a new way to compute receiver functions, by deconvolving seismic records by the source time function of each individual earthquake, determined by waveform inversion of seismological records distributed globally. This waveform inversion is performed with a simulated annealing approach. Our method allows us to detect P-to-S conversions and crustal multiples on radial components, like in classical receiver function approaches. However, it also allows us to detect P reflections from the top of the Moho on
vertical components, opening the way to apply seismic reflection techniques on teleseismic P wave records. We present results for several seismological stations, located in various tectonic environments. From the resulting simplified average crustal models, crustal corrections relative to ak135 model are computed, providing improved teleseismic traveltime residuals of P and S phases.
vertical components, opening the way to apply seismic reflection techniques on teleseismic P wave records. We present results for several seismological stations, located in various tectonic environments. From the resulting simplified average crustal models, crustal corrections relative to ak135 model are computed, providing improved teleseismic traveltime residuals of P and S phases.
L. Schardong, R.F Garcia, S. Chevrot & M. Calvet, Traveltime and amplitude measurements of core diffracted body waves: the first step to high-resolution tomography of the lowermost mantle, AGU Fall Meeting, 2010, San Francisco (US), poster.
The lowermost mantle, the D” layer, plays a key role in the global dynamics of the Earth, as a thermal and chemical boundary layer. It is thus essential to better constrain its detailed structure in order to better understand mantle convection, birth of hot spots, and stability of superplumes. Since diffracted waves (Pdiff and Sdiff) possess a strong sensitivity to D” layer heterogeneities, they should be crucial to decipher the fine structures in the D” layer. For this purpose, a global data base of broadband seismograms is being constructed with the objective of using it for high resolution tomography of the lower mantle. A method is presented to perform measurements of traveltime and amplitudes perturbations by cross-correlating observed with synthetic seismograms, calculated with the GEMINI method. The influence of the source time function is detailed. Source time functions including high frequency signals give a better fit between data and synthetics and improve the quality of the measurements. Regional variations of core diffracted traveltimes are presented in regions with good data coverage. These variations are discussed in terms of lower mantle dynamics and mineralogy.
Legend: myself, convenor (may be different)