Planned Products of the Mars Structure Service for the InSight Mission to Mars

Abstract The InSight lander will deliver geophysical instruments to Mars in 2018, including seismometers installed directly on the surface (Seismic Experiment for Interior Structure, SEIS). Routine operations will be split into two services, the Mars Structure Service(MSS) and Marsquake Service (MQS), which will be responsible, respectively, for defining the structure models and seismicity catalogs from the mission. The MSS will deliver a series of products before the landing, during the operations, and finally to the Planetary Data System (PDS) archive. Prior to the mission, we assembled a suite of a priori models of Mars, based on estimates of bulk composition and thermal profiles. Initial models during the mission will rely on modeling surface waves and impact-generated body waves independent of prior knowledge of structure. Later modeling will include simultaneous inversion of seismic observations for source and structural parameters. We use Bayesian inversion techniques to obtain robust probability distribution functions of interior structure parameters. Shallow structure will be characterized using the hammering of the heatflow probe mole, as well as measurements of surface wave ellipticity. Crustal scale structure will be constrained by measurements of receiver function and broadband Rayleigh wave ellipticity measurements. Core interacting body wave phases should be observable above modeled martian noise levels, allowing us to constrain deep structure. Normal modes of Mars should also be observable and can be used to estimate the globally averaged 1D structure, while combination with results from the InSight radio science mission and orbital observations will allow for constraint of deeper structure

Estimation Of Earthquake Site Effects By Array Processing Of Microtremors In Yeşilyurt And Avcilar

Tez (Yüksek Lisans) -- İstanbul Teknik Üniversitesi, Fen Bilimleri Enstitüsü, 2002Thesis (M.Sc.) -- İstanbul Technical University, Institute of Science and Technology, 2002Bu çalışmada, Yeşilyurt Hava Harp Okulu Kampüsü ve İstanbul Üniversitesi Avcılar Kampüsü’ nde, mikrotremor kayıtlarının dizilim işleme yöntemi ile analizinden elde edilen S-dalgası hız yapıları kullanılarak deprem yer tepkileri belirlenmiştir. Bu amaçla bir grup alıcıdan oluşan dairesel ve doğrusal dizilimler ile kaydedilen mikrotremorlerin, frekans-dalgasayısı spektral analizine dayanan, frekans ortamı ışın biçimlendirici tekniği kullanılarak, her iki çalışma alanına ait faz hızı dispersiyon eğrileri elde edilmiştir. Faz hızı dispersiyon verilerine ters çözüm uygulanarak her iki çalışma alanı için S-dalgası hız yapıları elde edilmiştir. Son olarak, çalışma alanları için deprem yer tepkisi, EERA programı kullanılarak modellenmiştir. S-dalgası hız yapıları, sondaj verileri, önceden yapılmış sismik kırılma çalışmalarından ve pasif kaynaklı yüzey dalgası yöntemlerinden elde edilen hız yapıları ile karşılaştırılmıştır. Modellenen yer tepkisi eğrileri, mikrotremor ölçümlerinden bulunan spektral oranların yorumlanmasıyla elde edilen yer büyütmeleri ile karşılaştırılmıştır ve elde edilen sonuçların daha önce yapılan çalışmalarla uyumlu olduğu gözlenmiştir. Bu çalışmanın sonuçları göstermiştir ki Yeşilyurt’ taki çalışma alanının S-dalgası hız yapısı, 40 m’ lik yumuşak sediman ve yaklaşık 10 m’ de yer alan düşük hız zonundan oluşmaktadır. Baskın frekans 1 Hz civarındadır ve 2.2-6 arası büyütme hesaplanmıştır. Avcılar’ daki çalışma alanındaki S-dalgası hız yapısı, en az 200 m derinliğe kadar düşük hıza sahip, kalın bir sediman katmanına işaret etmektedir. Yer tepkisi modellemesi, büyütmenin en çok 1 Hz civarında olduğunu göstermiş, bunun yanında 0.4 ve 1.7 Hz’ lerde de doruklar gözlemlenmiştir ve 2.5-4 kat arasında değişen büyütme değerleri hesaplanmıştır.In this study earthquake site response at the Yeşilyurt Campus of Air Force Academy and the Avcılar Campus of İstanbul University are analyzed using array recordings of microtremors. On this purpose the phase velocity dispersion curves can be obtained by a frequency domain beamforming approach, which is based on the frequency-wavenumber spectral analysis of microtremors measured across circular and linear arrays. Dispersion data are inverted for the S-wave velocity structures and earthquake site effects of the study areas are computed using EERA. Shear-wave velocity structures are compared with borehole data and velocity structures obtained from previously conducted seismic refraction and surface wave surveys. Site response curves computed for layered models are compared with observed site amplifications inferred from spectral ratios obtained using microtremor measurements. The results of this study show that, the S-wave velocity structure at the Yeşilyurt site consists of 40-m-thick soft soils with a low velocity zone present at about 10 m depth. The dominant frequency is found to be around 1 Hz and ground motion is found to be amplified about 2.2-6 times. The S-wave velocity structure at the Avcılar site indicates a rather thick sediment deposition with significantly low velocities at least to the depths of 200 m. The modeling of site amplification at Avcılar suggests that most of the amplification will occur at about 1 Hz with other peaks located at 0.4 and 1.7 Hz and ground motion is found to be amplified about 2.5-4 times.Yüksek LisansM.Sc

Sensitivity analysis of seismic waveforms to upper-mantle discontinuities using the adjoint method

Poster presented on Apri 24, 2017

Sensitivity analysis of seismic waveforms to upper-mantle discontinuities using the adjoint method

Using spectral-element simulations of wave propagation, we investigated the sensitivity of seismic waveforms, recorded on transverse components, to upper-mantle discontinuities in 1-D and 3-D background models. These sensitivity kernels, or Fréchet derivatives, illustrate the spatial sensitivity to model parameters, of which those for shear wave speed and the surface topography of internal boundaries are discussed in this paper. We focus on the boundaries at 400 and 670 km depth of the mantle transition zone. SS precursors have frequently been used to infer the topography of upper-mantle discontinuities. These seismic phases are underside reflections off these boundaries and are usually analysed in the distance range of 110°–160°. This distance range is chosen to minimize the interference from other waves. We show sensitivity kernels for consecutive time windows at three characteristic epicentral distances within the 110°–160° range. The sensitivity kernels are computed with the adjoint method using synthetic data. From our simulations we can draw three main conclusions: (i) The exact Fréchet derivatives show that in all time windows, and also in those centred on the SS precursors, there is interference from other waves. This explains the difficulty reported in the literature to correct for 3-D shear wave speed perturbations, even if the 3-D structure is perfectly known. (ii) All studies attempting to map the topography of the 400 and 670 km discontinuities to date assume that the traveltimes of SS precursors can be linearly decomposed into a 3-D elastic structure and a topography part. We recently showed that such a linear decomposition is not possible for SS precursors, and the sensitivity kernels presented in this paper explain why. (iii) In agreement with previous work, we show that other parts of the seismograms have greater sensitivity to upper-mantle discontinuities than SS precursors, especially multiply bouncing S waves exploiting the S-wave triplications due to the mantle transition zone. These phases can potentially improve the inference of global topographic variations of the upper-mantle discontinuities in the context of full waveform inversion in a joint inversion for (an)elastic parameters and topography

Sensitivity analysis of seismic waveforms to upper-mantle discontinuities using the adjoint method

Using spectral-element simulations of wave propagation, we investigated the sensitivity of seismic waveforms, recorded on transverse components, to upper-mantle discontinuities in 1-D and 3-D background models. These sensitivity kernels, or Fréchet derivatives, illustrate the spatial sensitivity to model parameters, of which those for shear wave speed and the surface topography of internal boundaries are discussed in this paper. We focus on the boundaries at 400 and 670 km depth of the mantle transition zone. SS precursors have frequently been used to infer the topography of upper-mantle discontinuities. These seismic phases are underside reflections off these boundaries and are usually analysed in the distance range of 110°–160°. This distance range is chosen to minimize the interference from other waves. We show sensitivity kernels for consecutive time windows at three characteristic epicentral distances within the 110°–160° range. The sensitivity kernels are computed with the adjoint method using synthetic data. From our simulations we can draw three main conclusions: (i) The exact Fréchet derivatives show that in all time windows, and also in those centred on the SS precursors, there is interference from other waves. This explains the difficulty reported in the literature to correct for 3-D shear wave speed perturbations, even if the 3-D structure is perfectly known. (ii) All studies attempting to map the topography of the 400 and 670 km discontinuities to date assume that the traveltimes of SS precursors can be linearly decomposed into a 3-D elastic structure and a topography part. We recently showed that such a linear decomposition is not possible for SS precursors, and the sensitivity kernels presented in this paper explain why. (iii) In agreement with previous work, we show that other parts of the seismograms have greater sensitivity to upper-mantle discontinuities than SS precursors, especially multiply bouncing S waves exploiting the S-wave triplications due to the mantle transition zone. These phases can potentially improve the inference of global topographic variations of the upper-mantle discontinuities in the context of full waveform inversion in a joint inversion for (an)elastic parameters and topography

Double-difference measurements in global full-waveform inversions

International audienceWe demonstrate double-difference (DD) tomography, a method involving differential measurements between stations, for 2-D and 3-D adjoint inversions based on realistic source–receiver distributions, with a focus on the global scale. We first present 2-D synthetic inversion results using cross-correlation traveltime and L2 waveform difference objective functions. Introducing a weighting term to DD objective functions based on the number of measurement pairs per station speeds up convergence and reduces bias in the final inverted model due to uneven data coverage. We next demonstrate frequency-dependent multitaper DD measurements in a 3-D experiment with real earthquake data by computing global-scale gradients. At the global scale, careful selection of station pairs is required for differential measurements in terms of geographical distance or geological context. In our technique, if no suitable pairs are found for a particular station, the DD measurement reduces to a classical misfit measurement. Furthermore, we compare 2-D and 3-D DD results with those from corresponding conventional misfits. By exploiting previously unused information in the recorded wavefield, DD tomography shows promise for balancing the gradient and speeding up convergence, especially around dense regional seismic networks

Balancing unevenly distributed data in seismic tomography: a global adjoint tomography example

International audienceSUMMARY The uneven distribution of earthquakes and stations in seismic tomography leads to slower convergence of nonlinear inversions and spatial bias in inversion results. Including dense regional arrays, such as USArray or Hi-Net, in global tomography causes severe convergence and spatial bias problems, against which conventional pre-conditioning schemes are ineffective. To save computational cost and reduce model bias, we propose a new strategy based on a geographical weighting of sources and receivers. Unlike approaches based on ray density or the Voronoi tessellation, this method scales to large full-waveform inversion problems and avoids instabilities at the edges of dense receiver or source clusters. We validate our strategy using a 2-D global waveform inversion test and show that the new weighting scheme leads to a nearly twofold reduction in model error and much faster convergence relative to a conventionally pre-conditioned inversion. We implement this geographical weighting strategy for global adjoint tomography

Global adjoint tomography: First-generation model

International audienceWe present the first-generation global tomographic model constructed based on adjoint tomography, an iterative full-waveform inversion technique. Synthetic seismograms were calculated using GPU-accelerated spectral-element simulations of global seismic wave propagation, accommodating effects due to 3D anelastic crust & mantle structure, topography & bathymetry, the ocean load, ellipticity, rotation, and self-gravitation. Fréchet derivatives were calculated in 3D anelastic models based on an adjoint-state method. The simulations were performed on the Cray XK7 named ‘Titan’, a computer with 18,688 GPU accelerators housed at Oak Ridge National Laboratory. The transversely isotropic global model is the result of 15 tomographic iterations, which systematically reduced differences between observed and simulated three-component seismograms. Our starting model combined 3D mantle model S362ANI (Kustowski et al. 2008) with 3D crustal model Crust2.0 (Bassin et al. 2000). We simultaneously inverted for structure in the crust and mantle, thereby eliminating the need for widely used ‘crustal corrections’. We used data from 253 earthquakes in the magnitude range 5.8~ ≤ ~Mw~ ≤ ~7.0. For the first 12 iterations, we combined ∼30 s body-wave data with ∼60 s surface-wave data. The shortest period of the surface waves was gradually decreased, and in the last three iterations we combined ∼17 s body waves with ∼45 s surface waves. We started using 180 min-long seismograms after the 12th iteration and assimilated minor- and major-arc body and surface waves. The 15th iteration model features enhancements of well-known slabs, an enhanced image of the Samoa/Tahiti plume, as well as various other plumes and hotspots, such as Caroline, Galapagos, Yellowstone, and Erebus. Furthermore, we see clear improvements in slab resolution along the Hellenic and Japan Arcs, as well as subduction along the East of Scotia Plate, which does not exist in the starting model. Point-spread function tests demonstrate that we are approaching the resolution of continental-scale studies in some areas, for example underneath Yellowstone. This is a consequence of our multi-scale smoothing strategy, in which we define our smoothing operator as a function of the approximate Hessian kernel, thereby smoothing gradients less wherever we have good ray coverage, such as underneath North America