Hanneke Paulssen

Complex lithospheric structure under the central Baltic Shield from surface wave tomography

Complex lithospheric structure under the central Baltic Shield from surface wave tomography

Variability of P660s phases as a consequence of topography of the 660 km discontinuity

Abstract Records from the broad-band NARS-NL array (deployed in the Netherlands, western Germany and Belgium) have been analyzed for P to S converted phases from the 660 km discontinuity (P660s). No individual seismogram from this array shows a clear P660s phase. Stacks of (up to 113) crosscorrelograms show P660s energy that is weak in comparison with similar stacks elsewhere. Very high amplitude observations of P660s in single seismograms from the same region have been made in a previous study ( Paulssen, 1985 , Geophys. Res. Lett., 12: 709–712; Paulssen, 1988 , J. Geophys. Res., 93: 10489–10500). Such variations may be due to relief on the 660 km discontinuity. In this paper we quantify the effects of different topographies on the time delays, amplitudes, and waveforms of the direct P wave as well as on the P660s converted wave. Synthetic P and P660s waveforms are calculated by applying a Kirchhoff-Helmholtz integration over the wave fields at the 660 km discontinuity interface. For topography on scales of a few hundreds of kilometers, the effects on the P wave are very small compared with those on the P660s phases. Time delays, focused and defocused amplitudes, and waveform distortion of P660s phases influenced by such topography diminish the coherence of P660s in (synthetic) stacks. Our results show that topography of the 660 km discontinuity with dominant scale lengths of a few hundreds of kilometers and variations in depth of ±15 km to ±25 km, is consistent with the pattern of P660s observations in single seismograms and stacks.

New array monitors seismic activity near the Gulf of California in Mexico

The Gulf of California rift forms a geologically young and active plate boundary that links the San Andreas strike-slip fault system in California to the oceanic spreading system of the East Pacific Rise. Although this is a classical example of a transform-rift plate boundary, the tectonic evolution of the Gulf of California and surrounding regions is complex and poorly understood due to a lack of geological and geophysical data. In 2002, the Network of Autonomously Recording Seismographs(NARS)-Baja network was installed.lt consists of 19 broadband seismic stations deployed in the Baja-California and Sonora provinces of Mexico (Figure 1). Since NARS-Baja surrounds the Gulf of California rift system, it is ideal for constraining earthquake faulting processes and the crust-mantle structure of the region. Moreover, NARS-Baja, in combination with permanent Mexican and U.S. arrays, forms a unique linear array in excess of 4000 km that should lend itself ideally to seismological studies of the North American-Pacific plate boundary on a larger scale. NARS-Baja is planned to operate for at least 5 years. To promote involvement from the entire research community the data collected from the stations will be made available immediately following routine data quality checks.

Finite-Frequency SKS Splitting: Measurement and Sensitivity Kernels

Splitting of SKS waves caused by anisotropy may be analyzed by measuring the splitting intensity, i.e., the amplitude of the transverse signal relative to the radial signal in the SKS time window. This quantity is simply related to structural parameters. Extending the widely used cross-correlation method for measuring travel-time anomalies to anisotropic problems, we propose to measure the SKS-splitting intensity by a robust cross-correlation method that can be automated to build large high-quality datasets. For weak anisotropy, the SKS-splitting intensity is retrieved by cross-correlating the radial signal with the sum of the radial and transverse signals. The cross-correlation method is validated based upon a set of Californian seismograms. We investigate the sensitivity of the SKS-splitting intensity to general anisotropy in the mantle based upon a numerical technique (the adjoint spectral-element method) considering the full physics of wave propagation. The computations reveal a sensitivity remarkably focused on a small number of elastic parameters and on a small region of the upper mantle. These fundamental properties and the practical advantages of the measurement make the cross-correlation SKS-splitting intensity particularly well adapted for finite-frequency imaging of upper-mantle anisotropy.

The crustal structure in Iberia inferred from P-wave coda

Abstract The crustal velocity-structure at four locations in the Iberian Peninsula is inferred from P-wave coda recorded by broad band stations. The crustal responses of the stations are derived from the P-wave coda of deep, teleseismic events, and the most pronounced features of the P and S velocity-structure are obtained by waveform modeling of the vertical and radial component of these responses. The method is applied to data of NARS stations in Toledo, Alicante, and Granada in Spain, and Manteigas in Portugal. The optimum models of the waveform inversions are carefully evaluated, and compared to results of deep seismic sounding experiments. In general, the depths of large velocity gradients are well-resolved, but the actual values of the seismic velocities are less constrained.

The NARS-DEEP Project

The NARS-DEEP project involves the deployment of broadband seismological stations in Russia, Belarus and the Ukraine. Six stations were installed in 1995, two in 1997, and more stations were to be installed from 1998 onwards. The NARS-DEEP project was initiated as a temporary deployment, but funding from the European Community enabled the stations to become permanent. In this paper we present first results of the NARS-DEEP project. A surface waveform inversion for the upper mantle structure along a profile from Egypt to Spitsbergen shows evidence for strong variations in the shear-velocity structure for the different tectonic units. The most striking features of the model comprise strong lithospheric anisotropy beneath the Eastern Mediterranean and a lithospheric thickness of approximately 200 km beneath the shield areas. The crustal structure beneath the seismic station in St. Petersburg was investigated using the receiver function method. The results of a Monte Carlo inversion for the receiver functions of this station show evidence for a sedimentary layer with a thickness of less than 1 km overlying an upper crustal layer extending to a depth of approximately 16 km. The Moho depth cannot be resolved from the receiver functions of this station.

On PP‐P differential travel time measurements

This study investigates the stability of PP-P travel time measurements using a waveform crosscorrelation method on both broadband and long-period data. This study finds correspondence between 50% of the PP-P travel times read from broadband and long-period data, but also finds 50% of the measurements differ more than 1.0 second. The inconsistent measurements are demonstrated to be due to two causes. (1) Diffraction near the core-mantle boundary causes dispersion of the P phase which yields an underestimate of the PP-P travel time determined from long-period data. This effect is identified at epicentral distances as small as 88°and may amount to several seconds. (2) Interference of the PP phase with secondary arrivals causes distortion of the PP waveform and produces a non-systematic measurement error. This effect are most clearly seen on broadband seismograms and may even hamper an unambiguous broadband PP-P travel time measurement. Although strong high-frequency effects associated with interference are filtered out on the long-period data, the long-period PP-waveform may still be contaminated. We conclude that the accuracy of PP-P travel time measurements is on the order of l s for both broadband and long-period seismograms.

Steeply reflected ScSH precursors from the D″ region

We present evidence for precursors to the ScS and SScS (SDS and sSDS) phases observed at epicentral distances smaller than 30°. These precursors are intermittently observed in broadband recordings from the six Incorporated Research Institutions for Seismology stations used (western Pacific region and South America). They appear approximately 35–50 s ahead of the ScS arrival on the transverse component and are independent of hypocentral depth. The observed precursors are characterized by relatively large amplitudes in the frequency band from 0.05 to 0.2 Hz. A more detailed analysis is restricted to a subsidiary data set from station SNZO (New Zealand). The coherency, frequency dependence, slowness, polarity, and polarization of the precursors are discussed. Many explanations for SDS can be rejected, and we conclude that a reflector 180 km above the core-mantle boundary causes these occasionally strong precursors. The large SDS/ScS amplitudes at low frequencies require an unrealistically large impedance contrast for a one-dimensional model. We test the possibility of focusing the SDS phase by a discontinuity with topography and show that structure with scale lengths of ∼l9°–25° (1200–1600 km, i.e., larger than the Fresnel zone) can account for large intermittently observed SDS amplitudes with their geometrical reflection points within the same Fresnel zone. This is surprising, since it is often assumed that scale lengths smaller than the Fresnel zone must be responsible for variations within a Fresnel zone. The limited data set permits no conclusions about the global properties of this phase or its implied reflector.

A Shallow Seismic Velocity Model for the Groningen Area in the Netherlands

The province of Groningen in the Netherlands holds one of the worlds largest natural gas fields, and it has been an important source of energy for Western Europe for many decades. The seismicity in recent years called for a better understanding of the local subsurface, and therefore a dense network of 70 boreholes was installed in early 2015. Each borehole is equipped with four geophones and a surface accelerometer. In this study, data from this network are used to determine the shallow velocity structure that is important for the quantification of the seismic hazard and accurate source localizations. Compressional and shear wave velocity profiles with uncertainties are derived for each of the 200 m deep boreholes using passive seismic interferometry applied to local event data. The resulting seismic velocity distributions are presented as contour maps for 50 m depth intervals. The maps show strong lateral variations, where areas of low VP/VS ratio correspond to regions of sedimentary infill. The shear wave velocities were derived using the transverse component seismograms. Because the sensor orientations of the borehole geophones were unknown, they had to be determined first. This was done using a novel method based on cross correlations between the geophones and their colocated surface accelerometer. In addition, by extensive cross-correlation analysis over the network, several installation inconsistencies were identified and resolved.

P and S Velocity Structure in the Groningen Gas Reservoir From Noise Interferometry

Noise interferometry has proven to be a powerful tool to image seismic structure. In this study we used data from 10 geophones located in a borehole at ∼3 km depth within the Groningen gas reservoir in the Netherlands. The continuous data cross-correlations show that noise predominantly comes in from above. The observed daily and weekly variations further indicate that the noise has an anthropogenic origin. The direct P wave emerges from the stacked vertical component cross-correlations with frequencies up to 80 Hz and the direct S wave is retrieved from the horizontal components with frequencies up to 50 Hz. The measured inter-geophone travel times were used to retrieve the P- and S-velocity structure along the borehole and a good agreement was found with well log data. In addition, from the S-wave polarizations, we determined azimuthal anisotropy with a fast direction of N65∘W±18∘ and an estimated magnitude of (4±2)%. The fast polarization direction corresponds to the present direction of maximum horizontal stress measured at nearby boreholes, but is also similar to the estimated paleostress direction.