Article Abstracts

JJ. Leveque, A. Maggi, A. Souriau, 2010. Seismic tomography of the southern California crust based on spectral-element and adjoint methods. Antarctic Science (in press).

The French-Italian Concordia (CCD) seismological station at Dome C is one of two observatories set up on the ice cap inside the Antarctic continent. We analyze the seismic signal due to ambient noise at this station, and at 3 temporary stations 5 km away from Concordia, in order to specify the ice properties beneath them. A method based on the horizontal to vertical (H/V) spectral ratio, commonly used to analyze soil response in seismic regions, has been applied to the Antarctic stations. The main peak in the spectral ratios is observed at frequencies 6.7 Hz to 8 Hz at the Dome C stations, but it is not observed at another station on the ice cap, QSPA, where the sensor is buried at 275 m depth. This peak can be explained by a 23 meter thick unconsolidated snow or firn layer with a low S-velocity of 0.7 km/s, overlying a consolidated layer with S-velocity 1.8 km/s. Despite the non-uniqueness of the solutions obtained by fitting the H/V spectra , this model is preferred because the depth of the velocity contrast coincides with the density at which ice particles arrange themselves in a continuous, dense lattice. A small variability of this structure is observed around Dome C.

C. Tape, Q. Liu, A. Maggi, J. Tromp, 2010. Seismic tomography of the southern California crust based on spectral-element and adjoint methods. Geophysical Journal International, 180, 433-462.

We iteratively improve a three-dimensional tomographic model of the southern California crust using numerical simulations of seismic wave propagation based on a spectral element method (SEM) in combination with an adjoint method. The initial 3D model is provided by the Southern California Earthquake Center. The dataset comprises three component seismic waveforms (i.e., both body and surface waves), filtered over the period range 2–30 s, from 143 local earthquakes recorded by a network of 203 stations. Time windows for measurements are automatically selected by the FLEXWIN algorithm. The misfit function in the tomographic inversion is based on frequency-dependent multi-taper travel-time differences. The gradient of the misfit function and related finite-frequency sensitivity kernels for each earthquake are computed using an adjoint technique. The kernels are combined using a source subspace projection method to compute a model update at each iteration of a gradient-based minimization algorithm. The inversion involved 16 iterations, which required 6800 wavefield simulations. The new crustal model, m16, is described in terms of independent shear (VS) and bulk-sound (VB) wave-speed variations. It exhibits strong heterogeneity, including local changes of ±30% with respect to the initial 3D model. The model reveals several features that relate to geologic observations, such as sedimentary basins, exhumed batholiths, and contrasting lithologies across faults. The quality of the new model is validated by quantifying waveform misfits of full-length seismograms from 91 earthquakes that were not used in the tomographic inversion. The new model provides more accurate synthetic seismograms that will benefit seismic hazard assessment.

E. Stutzmann, M. Schimmel, G. Patau, A. Maggi, 2009. Global climate imprint on seismic noise. Geochemistry, Geophysics, Geosystems, 10, Q11004, doi: 10.1029/2009GC002619.

In the absence of earthquakes, oceanic microseisms are the strongest signals recorded by seismic stations. Using the GEOSCOPE global seismic network, we show that the secondary microseism spectra have global characteristics that depend on the station latitude and on the season. In both hemispheres, noise amplitude is larger during local winter, and close to the equator noise amplitude is stable over the year. There is an excellent correlation between microseism amplitude variations over the year and changes in the highest wave areas. Considering the polarisation of the secondary microseisms, we show that stations in the northern hemisphere and close to the equator record significant changes of the secondary microseism source azimuth over the year. During northern hemisphere summer, part or all of the sources are systematically located further toward the south than during winter. Stations in French Guyana (MPG) and in Algeria (TAM) record microseisms generated several thousand km away in the South Pacific Ocean and in the Indian Ocean, respectively. Thus, secondary microseism sources generated by ocean waves which originate in the southern hemisphere can be recorded by northern hemisphere stations when local sources are weak. We also show, considering a station close to Antarctica, that primary and secondary microseism noise amplitudes are strongly affected by changes of the sea ice floe, and that sources of these microseisms are in different areas. Microseism recording can therefore be used to monitor climate changes.

C. Tape, Q. Liu, A. Maggi, J. Tromp, 2009. Adjoint tomography of the southern California crust. Science, 325, 988-992.

Using an inversion strategy based on adjoint methods, we developed a three-dimensional seismological model of the southern California crust. The resulting model involved 16 tomographic iterations, which required 6800 wavefield simulations and a total of 0.8 million central processing unit hours. The new crustal model reveals strong heterogeneity, including local changes of ±30% with respect to the initial three-dimensional model provided by the Southern California Earthquake Center. The model illuminates shallow features such as sedimentary basins and compositional contrasts across faults. It also reveals crustal features at depth that aid in the tectonic reconstruction of southern California, such as subduction-captured oceanic crustal fragments. The new model enables more realistic and accurate assessments of seismic hazard.

A. Maggi, C. Tape, M. Chen, D. Chao, J. Tromp, 2009. An automated data-window selection algorithm for adjoint tomography. Geophysical Journal International, 178, 257-281.

We present FLEXWIN, an open source algorithm for the automated selection of time windows on pairs of observed and synthetic seismograms. The algorithm was designed specifically to accommodate synthetic seismograms produced from 3-D wavefield simulations, which capture complex phases that do not necessarily exist in 1-D simulations or traditional traveltime curves. Relying on signal processing tools and several user-tuned parameters, the algorithm is able to include these new phases and to maximize the number of measurements made on each seismic record, while avoiding seismic noise. Our motivation is to use the algorithm for iterative tomographic inversions, in which the synthetic seismograms change from one iteration to the next. Hence, automation is needed to handle the volume of measurements and to allow for an increasing number of windows at each model iteration. The algorithm is sufficiently flexible to be adapted to many tomographic applications and seismological scenarios, including those based on synthetics generated from 1-D models. We illustrate the algorithm using data sets from three distinct regions: the entire globe, the Japan subduction zone, and southern California.

A. Maggi, E. Debayle, K. Priestley, G. Barruol, 2006. Azimuthal anisotropy of the Pacific Ocean. Earth and Planetary Science Letters, 250, 53-71.

Azimuthal anisotropy is the dependence of local seismic properties on the azimuth of propagation. We present the azimuthally anisotropic component of a 3D SV velocity model for the Pacific Ocean, derived from the waveform modelling of over 56,000 multi-mode Rayleigh waves followed by a simultaneous inversion for isotropic and azimuthally anisotropic Vsv structure. The isotropic Vsv model is discussed in a previous paper (Maggi et al., 2006). The azimuthal anisotropy we find is consistent with the lattice preferred orientation model (LPO): the hypothesis of anisotropy generation in the Earth's mantle by preferential alignment of anisotropic crystals in response to the shear strains induced by mantle flow. At lithospheric depths we find good agreement between fast azimuthal anisotropy orientations and ridge spreading directions recorded by sea-floor magnetic anomalies. At asthenospheric depths we find a strong correlation between fast azimuthal anisotropy orientations and the directions of current plate motions. We observe perturbations in the pattern of seismic anisotropy close to Pacific hot-spots that are consistent with with the predictions of numerical models of LPO generation in plume-disturbed plate motion-driven mantle flow. These observations suggest that perturbations in the patterns of azimuthal anisotropy may provide indirect evidence for plume-like upwelling in the mantle.

A. Maggi, E. Debayle, K. Priestley, G. Barruol, 2006. Multi-mode surface waveform tomography of the Pacific Ocean: A closer look at lithospheric cooling. Geophysical Journal International, 166, 1384-1397.

We present a regional surface waveform tomography of the Pacific upper mantle, obtained using an automated multi-mode surface waveform inversion technique on fundamental and higher mode Rayleigh waves, to constrain the Vsv structure down to ~40 km depth. We have improved on previous implementations of this technique by robustly accounting for the effects of uncertainties in earthquake source parameters in the tomographic inversion. We have furthermore improved path coverage in the south Pacific region by including Rayleigh wave observations from the French Polynesian PLUME deployment. This improvement has led to imaging of vertical low velocity structures associated with hotspots within the South Pacific Super-Swell region. We have produced an age-dependent average cross-section for the Pacific Ocean lithosphere and found that the increase in Vsv with age is broadly compatible with a half-space cooling model of oceanic lithosphere formation. We cannot confirm evidence for a Pacific-wide reheating event. Our synthetic tests show that detailed interpretation of average Vsv trends across the Pacific Ocean may be misleading unless lateral resolution and amplitude recovery are uniform across the region, a condition that is difficult to achieve in such a large oceanic basin with current seismic stations.

A. Maggi, K. Priestley, 2005. Surface waveform tomography of the Turkish-Iranian Plateau. Geophysical Journal International, 160, 1068-1080.

The Middle East is a tectonically complex region consisting of terrains as diverse as Precambrian shields and young orogens. The dominant tectonic feature is the Turkish--Iranian plateau, a recently elevated, seismically active region along the Zagros-Bitlis suture, which results from the collision of the Arabian plate with Eurasia. In this study, we use surface-waveform tomography to elucidate the upper mantle shear-wave velocity structure beneath the Turkish--Iranian plateau and adjacent regions. The main large-scale feature in the tomographic model is a low shear wave velocity (less than 4.5 km/sec) anomaly in the uppermost mantle (approx. 100 km depth) beneath the Plateau. This low velocity feature correlates with a long-wavelength free-air gravity anomaly and with recent volcanism whose geochemistry has a lower lithospheric mantle signature. Seismology, gravity and volcanism all suggest the presence of a thin lithosphere and warm upper mantle beneath the Turkish--Iranian plateau. The upper mantle low shear-wave velocity zone, the high free-air gravity and deep lithospheric source depth for the basaltic volcanism are all consistent with a partial delamination of the lithosphere due to earlierlithospheric thickening resulting from the continental collision.

S. Pilidou, K. Priestley, J. Jackson and A. Maggi, 2004. The 1996 Cyprus earthquake: a large, deep event in the Cyprean Arc. Geophysical Journal International, 158, 85-97.

We examine the source mechanisms and depths of the largest earthquake that has occurred in the vicinity of Cyprus in the last 50 years, the 1996 October 9 earthquake (M-w=6.8) and its principal aftershock on October 10 (M-w=5.8). Being the first large event in the area for which seismic data from the global digital network are available, it provides an excellent opportunity to study the complex tectonic processes of the eastern Mediterranean. We modelled the source mechanisms and depths of the earthquakes by a least-squares body waveform fitting procedure. The waveform fits resulting from our minimum-misfit solutions are considerably better than those predicted by previous studies. We show strong evidence that both events were at depths of 76-85 km rather than 20-30 km, as has been suggested by other studies. These earthquakes form a group indicating that the Mediterranean lithosphere south of the Hellenic and Cyprean arcs is in east-west compression as it is subducted to the north and northeast. We additionally examine the source mechanism and depth of the 1999 August 11 earthquake (M-w=5.6), which occurred at about 11 km with a reverse dip-slip mechanism. It probably reflects crustal shortening between the African and Turkish plates.

R. Saunders, R. Kneissl, K. Grainge, W. Grainger, M. Jones, A. Maggi, R. Das, A. Edge, A. Lasenby, G. Pooley, S. Miyoshi, T. Tsuruta, K. Yamashita, Y. Tawara, A. Furuzawa, A. Harada and I. Hatsukade, 2003. A measurement of H0 from Ryle Telescope, ASCA and ROSAT observations of Abell 773. Mon. Not. R. Astron. Soc., 341, 937-940.

We present new Ryle Telescope (RT) observations of the Sunyaev-Zel'dovich (SZ) decrement from the cluster Abell 773. The field contains a number of faint radio sources that required careful subtraction. We use ASCA observations to measure the gas temperature and a ROSAT HRI image to model the gas density distribution. Normalising the gas distribution to fit the RT visibilities returns a value of H0 of 77 (+19,-15) km/sec/Mcp for an Einstein-de Sitter universe, or 85 (+20,-17) km/sec/Mpa for a flat model with Omega = 0.7. The errors quoted include estimates of the effects of the principal errors: noise in the SZ measurement, gas temperature uncertainty and line-of-sight depth uncertainty.

A. Maggi, K. Priestley and J. Jackson, 2002. Focal depths of moderate to large earthquakes in Iran. Journal of Seismology and Earthquake Engineering, 4:2-3, 1-10.

Accurate focal depth estimates are essential for the correct interpretation of seismicity data in terms of regional tectonics and earthquake hazard assessment. Published global earthquake catalogues are a common source of focal depth information, but how accurate are they? We compare estimates of focal depths from the Harvard CMT catalogue and the Engdahl et. al (1998) relocations of the ISC catalogue with those determined by teleseismic waveform inversion methods, and find that the catalogues can be in error by up to 60 km.

A. Maggi, J.A. Jackson, K. Priestley and C. Baker, 2000. A re-assessment of focal depth distributions in southern Iran, the Tien Shan and northern India: do earthquakes really occur in the continental mantle?, Geophysical Journal International, 143, 629-661.

We investigate the depth distribution of earthquakes within the continental lithosphere of southern Iran, the Tien Shan and northern India by using synthetic seismograms to analyse P and SH body waveforms. In the Zagros mountains of southern Iran earthquakes are apparently restricted to the upper crust (depths of less than 20 km), whereas in the Tien Shan and northern India they occur throughout the thickness of the continental crust, to depths of 40-45 km. We find no convincing evidence for earthquakes in the continental mantle of these regions, in spite of previous suggestions to the contrary, and question whether seismicity in the continental mantle is important in any part of the world. In some regions, such as Iran, the Aegean, Tibet and California, seismicity is virtually restricted to the upper continental crust, whereas in others, including parts of East Africa, the Tien Shan and northern India, the lower crust is also seismically active, though usually less so than the upper crust. Such variations cannot reliably be demonstrated from published catalogue or bulletin locations, even from ones in which depth resolution is generally improved. In contrast to the oceanic mantle lithosphere, in which earthquakes certainly occur, the continental mantle lithosphere is, we suggest, virtually aseismic and may not be significantly stronger than the lower continental crust. These variations in continental seismogenic thickness are broadly correlated with variations in effective elastic thickness, suggesting that the strength of the continental lithosphere resides in the crust, and require some modification to prevalent views of lithosphere rheology.

A. Maggi, J.A. Jackson, D. McKenzie and K. Priestley, 2000. Earthquake focal depths, effective elastic thickness, and the strength of the continental lithosphere, Geology, 28:6, 495-598.

Almost all earthquakes on the continents are confined within a crustal layer that varies in thickness(Ts) from about 10 to 40 km, and are not in the mantle. Variations in Ts correlate with variations in the effective elastic thickness (Te), both of them having similar values, although Te is usually the smaller of the two. These observations suggest that the lower crust, at. least in some places, is stronger than the mantle beneath the Moho, contrary to most models of continental rheology. Thus the strength of the continental lithosphere is likely to be contained within the seismogenic layer, variations in the thickness of this strong layer determining the heights of the mountain ranges it can support. The aseismic nature of the continental mantle and the lower crustal seismicity beneath some shields are probably related to their water contents.

D.D. Dunlap, A. Maggi, M.R. Soria, L. Monaco, 1997. Nanoscopic structure of DNA condensed for gene delivery, Nucleic Acids Research, 25:15, 3095-3101.

Scanning force microscopy was used to examine DNA condensates prepared with varying stoichiometries of lipospermine or polyethylenimine in physiological solution. For the first time, individual DNA strands were clearly visualized in incomplete condensates without drying. Using lipospermine at sub- saturating concentrations, discrete nuclei of condensation were observed often surrounded by folded loops of DNA. Similar packing of DNA loops occurred for polyethylenimine-induced condensation. Increasing the amount of the condensing agent led to the progressive coalescence or aggregation of initial condensation nuclei through folding rather than winding the DNA. At over-saturating charge ratios of the cationic lipid or polymer to DNA, condensates had sizes smaller than or equal to those measured previously in electron micrographs. Polyethylenimine condensates were more compact than lipospermine condensates and both produced more homogeneously compacted plasmids when used in a 2-4-fold charge excess. The size and morphology of the condensates may affect their efficiency in transfection.

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