Helle Pedersen

Seismic monitoring of the Indian Ocean tsunami

The 26 December 2004 Sumatra-Andaman earthquake of Mw 9.3 triggered a massive tsunami in the Indian Ocean. We here report on observations of the Indian Ocean tsunami at broadband seismic stations located on islands in the area. The tsunami induces long-period (>1000 s) signals on the horizontal components of the sensor. Frequency-time analysis shows that the long-period signals cannot be due to seismic surface waves, but that it arrives at the expected time of the tsunami. The waveforms are well correlated to tide gauge observations at a location where both observations are available. To explain the signals we favour tilt due to coastal loading but we cannot at the present stage exclude gravitational effects. The density of broadband stations is expected to increase rapidly in the effort of building an earthquake monitoring system. They may unexpectedly become useful tsunami detectors as well.

Important findings expected from Europe's largest seismic array

An international, interdisciplinary project, which 2 years ago deployed the largest dense seismic antenna ever in Europe, expects in the next 2 years to present important findings on the lithosphere and asthenosphere of a portion of the Trans-European Suture Zone (TESZ). Final processing is currently under way of the data from the array of 120 seismographs along a 900-km-long by 100-km-wide strip from Gottingen, Germany, in the south, through Denmark, to Stockholm, Sweden in the north, across the northwestern part of the TESZ (Figure 1). Project Tor is a teleseismic tomography experiment with interdisciplinary data exploitation. It extends across the broad TESZ boundary between two markedly different lithospheric domains.These are (1) Proterozoic Europe, with Precambrian crust in Sweden and eastern Europe, and (2) Phanerozoic central Europe, with most of the crust influenced by the Caledonian and Variscan orogenies and only small areas of relic Precambrian crust. The project is designed to investigate the deep lithosphere traces of the broad-scale geology of the TESZ area, including the Tornquist Zone, from which Project Tor has its name. It is part of EUROPROBE, a major Earth science program of the European Science Foundation, which is run by a regional committee of the International Lithosphere Program.

Improving surface-wave group velocity measurements by energy reassignment

Surface waves are increasingly used for shallow seismic surveys—in particular, in acoustic logging, environmental, and engineering applications. These waves are dispersive, and their dispersion curves are used to obtain shear velocity profiles with depth. The main obstacle to their more widespread use is the complexity of the associated data processing and interpretation of the results. Our objective is to show that energy reassignment in the time-frequency domain helps improve the precision of group velocity measurements of surface waves. To show this, full-waveform seismograms with added white noise for a shallow flat-layered earth model are analyzed by classic and reassigned multiple filter analysis (MFA). Classic MFA gives the expected smeared image of the group velocity dispersion curve, while the reassigned curve gives a very well-constrained, narrow dispersion curve. Systematic errors from spectral fall-off are largely corrected by the reassignment procedure. The subsequent inversion of the dispersion curve to obtain the shear-wave velocity with depth is carried out through a procedure combining linearized inversion with a nonlinear Monte Carlo inversion. The diminished uncertainty obtained after reassignment introduces significantly better constraints on the earth model than by inverting the output of classic MFA. The reassignment is finally carried out on data from a shallow seismic survey in northern Belgium, with the aim of determining the shear-wave velocities for seismic risk assessment. The reassignment is very stable in this case as well. The use of reassignment can make dispersion measurements highly automated, thereby facilitating the use of surface waves for shallow surveys

Changes in the lithospheric structure across the Sorgenfrei-Tornquist Zone inferred from dispersion of Rayleigh waves

Abstract Surface wave analysis was used to study possible lateral changes in the lithospheric/upper mantle structure across the Sorgenfrei-Tornquist Zone (STZ), which is part of the transition zone between the ancient Baltic Shield and younger western Europe. Records of long period surface waves from earthquakes at teleseismic distances were analysed. The data originated from long period permanent stations in southeastern Norway and from broadband NARS stations installed in Denmark between 1983 and 1986. This configuration made it possible to compare the lithospheric structure north and south of the STZ by analysis of surface wave dispersion between pairs of closely spaced stations in the same tectonic unit. Phase velocities of fundamental mode Rayleigh waves were determined by calculating cross spectra of pairs of records. The phase velocity dispersion curves show significant differences between the two tectonic units north and south of STZ. Linearized inversion of the observed dispersion curves, to obtain models of shear wave velocity in the crust and upper mantle, shows a low velocity zone (LVZ) at a depth of about 110 km beneath the western part of Denmark, just south of the STZ. A similar LVZ was not identified beneath southeastern Norway, north of the STZ. If a low velocity zone is present beneath the Baltic Shield in this area it must be very weak. The lateral changes in lithospheric-asthenospheric structures take place over a distance of less than 500 km. Our results therefore suggest that the STZ coincides with a transition zone in the upper mantle.

Dispersion estimation from linear array data in the time-frequency plane

We consider the problem of estimating the dispersion of a wave field from data recorded by a linear array of geophones. The fact that the data we are looking at may contain several propagating waves make this even more challenging. In this paper, a new algorithm is proposed to solve this issue. Currently, there are two methods for estimating wave dispersion described in the literature. The first method estimates the group delay function from the time-frequency representation (TFR) of each sensor separately. It is efficient as long as the patterns of the different waves do not overlap in the time-frequency plane. The second method estimates the dispersion from the two-dimensional (2-D) Fourier transform of the profile (or more generally from a velocity-frequency representation). This assumes that the dispersion is constant along the entire sensor array. It is efficient as long as the patterns of the waves do not overlap in the frequency domain. Our method can be thought of as a hybrid of the above two methods as it is based on the construction of a TFR where the energy of waves that propagate at a selected velocity are amplified. The primary advantage of our algorithm is the use of the velocity variable to separate the patterns of the propagating waves in the time-frequency plane. When applied to both synthetic and real data, this new algorithm gives much improved results when compared with other standard methods.

Imaging the D″ reflector with noise correlations

The lowermost mantle of the Earth is characterized by seismic structures that range from a few tens to thousands of kilometers. At present, it is difficult to test hypotheses put forward to explain seismic observations due to poor seismic coverage, as particular earthquake-station geometries are needed. We demonstrate here that seismic noise correlations can be used to robustly image deep-mantle reflections with larger stacked amplitudes of reflected waves compared with earthquake data. In a comparison between noise and earthquake data, we find that the arrival times and the slowness of reflected waves, both sampling a region beneath Siberia, agree with those for a reflector at 2530 km depth, and the small amplitude reflections are sufficiently clear in the noise correlations to compare them reliably with synthetic data. Our data open exciting prospects for illuminating new target zones in the deep mantle to further constrain the dynamics and mineralogy of the deep Earth.

Simultaneous group and phase correction for the estimation of dispersive propagating waves in the time-frequency plane

The paper deals with seismic signal processing. We propose a new algorithm which performs the dispersion filter estimation using a linear array of geophones. This estimation enables a robust characterization and extraction of the dispersive propagating waves from a seismic profile. The novelty of our method is the use of a simultaneous time delay and phase shift correction in order to estimate the dispersion filter. The resulting algorithm is semi-automatic and requires very few input parameters. The main advantage of our algorithm is that the signals are analysed in the time-frequency domain using all sensors simultaneously while classical approaches are either multi-sensor in the frequency domain or single sensor in the time-frequency domain. Validations on synthetic and real data show the reliability of the algorithm.

3 – Upper mantle structure of the Aegean derived from two-station phase velocities of fundamental mode Rayleigh waves

Fundamental mode Rayleigh waves generated by 380 teleseismic events were analyzed over the period range 10-100 s, in order to study the structure of the lithosphere and upper mantle of the Aegean region. Using the two-station method, 255 reliable phase velocity dispersion curves were calculated over 35 profiles and further inverted to obtain a new model of S-wave velocity with depth. S-wave velocities are resolved to a depth of 180 km. Important features are defined, such as a not completely amphitheatric geometry for the western (≈25° dipping angle) and eastern segments (≈35° dipping angle) of the subducted slab. In north Aegean, high velocities associate with the North Aegean Trough, which westernmost tip correlates with a high velocity anomaly in eastern continental Greece. This zone of high velocity contrast is extended in depth, dips southwards with an angle ≈350 and intersects with the subducted slab at an area where the direction of major tectonic axes changes from ENE-WSW to NNW-SSE towards the continental massif. In Central and North Aegean, where back arc extension and crustal thinning occur, the predominant low velocities observed could be interpreted by upper mantle high thermal flow and partial melting.

Improving surface-wave group velocity measurements by energy reassignmentReassignment Applied to Surface Waves

Surface waves are increasingly used for shallow seismic surveys—in particular, in acoustic logging, environmental, and engineering applications. These waves are dispersive, and their dispersion curves are used to obtain shear velocity profiles with depth. The main obstacle to their more widespread use is the complexity of the associated data processing and interpretation of the results. Our objective is to show that energy reassignment in the time-frequency domain helps improve the precision of group velocity measurements of surface waves. To show this, full-waveform seismograms with added white noise for a shallow flat-layered earth model are analyzed by classic and reassigned multiple filter analysis (MFA). Classic MFA gives the expected smeared image of the group velocity dispersion curve, while the reassigned curve gives a very well-constrained, narrow dispersion curve. Systematic errors from spectral fall-off are largely corrected by the reassignment procedure. The subsequent inversion of the dispersion curve to obtain the shear-wave velocity with depth is carried out through a procedure combining linearized inversion with a nonlinear Monte Carlo inversion. The diminished uncertainty obtained after reassignment introduces significantly better constraints on the earth model than by inverting the output of classic MFA. The reassignment is finally carried out on data from a shallow seismic survey in northern Belgium, with the aim of determining the shear-wave velocities for seismic risk assessment. The reassignment is very stable in this case as well. The use of reassignment can make dispersion measurements highly automated, thereby facilitating the use of surface waves for shallow surveys

Improving group velocity measurements by energy reagissignment.

Surface waves are increasingly used for shallow seismic surveys—in particular, in acoustic logging, environmental, and engineering applications. These waves are dispersive, and their dispersion curves are used to obtain shear velocity profiles with depth. The main obstacle to their more widespread use is the complexity of the associated data processing and interpretation of the results. Our objective is to show that energy reassignment in the time-frequency domain helps improve the precision of group velocity measurements of surface waves. To show this, full-waveform seismograms with added white noise for a shallow flat-layered earth model are analyzed by classic and reassigned multiple filter analysis (MFA). Classic MFA gives the expected smeared image of the group velocity dispersion curve, while the reassigned curve gives a very well-constrained, narrow dispersion curve. Systematic errors from spectral fall-off are largely corrected by the reassignment procedure. The subsequent inversion of the dispersion curve to obtain the shear-wave velocity with depth is carried out through a procedure combining linearized inversion with a nonlinear Monte Carlo inversion. The diminished uncertainty obtained after reassignment introduces significantly better constraints on the earth model than by inverting the output of classic MFA. The reassignment is finally carried out on data from a shallow seismic survey in northern Belgium, with the aim of determining the shear-wave velocities for seismic risk assessment. The reassignment is very stable in this case as well. The use of reassignment can make dispersion measurements highly automated, thereby facilitating the use of surface waves for shallow surveys