Jan Šílený

Models of seismic anisotropy in the deep continental lithosphere

Seismological observations (SKS-wave polarizations, systematic P-residual variations, azimuthal dependence of Pn- and surface-wave velocities or a dispersion of surface waves) are not consistent with isotropic, if laterally heterogeneous, upper-mantle structure. Therefore, an anisotropy should be considered as an a priori aspect of future large-scale studies of mantle structure. Most studies of anisotropy, however, have assumed horizontal or vertical axes of symmetry, but such orientations cannot explain bipolar patterns of spatial variations of P residuals, which we have observed at many seismological stations. On the basis of the petrophysical properties of real upper-mantle rocks we consider anisotropy formed either by hexagonal or by orthorhombic aggregates composed of olivine, orthopyroxene, and clinopyroxene. Rotations of the aggregates about vertical and horizontal axes allow us to find the three-dimensional orientations of symmetry axes that fit combinations of both P and S seismological observations in Central Europe and in western North America. The orientations with plunging symmetry axes (velocity extremes) seem to be consistent across large, spatially uniform tectonic units and change abruptly at important suture zones.

Inversion of shear-wave splitting parameters to retrieve three-dimensional orientation of anisotropy in continental lithosphere

Abstract A method for inversion of splitting parameters of shear waves was designed to retrieve the three-dimensional orientation of an inclined anisotropic medium. Data consist of the polarization vector of the fast shear wave and the time delay between the fast and slow shear waves obtained from three-component records of teleseismic shear waves. The model to be determined by the inversion of these splitting parameters is a homogeneous anisotropic layer where axes of hexagonal or orthorhombic symmetry generally plunge. The method determines the orientation of the symmetry axes and the thickness of the anisotropic layer beneath a region on the receiver side. Tests with synthetic data contaminated by random noise of various levels demonstrated a good performance of the method even for an irregular distribution of events. Inversions assuming a hexagonal model appeared to be fairly stable with respect to increasing noise variance, whereas an orthorhombic model yielded rather scattered solutions. The method applied to a selected distribution of events appeared to be unstable when deciding on the symmetry of the medium when noise is superimposed on the data to be inverted. If the anisotropic symmetry is fixed by adding supplementary information, e.g. from the P-residual pattern, the inversion of the splitting parameters of shear waves can recover the orientation of the symmetry axes of the hexagonal or orthorhombic model with a high confidence. An inverted real data set containing the shear-wave polarization vectors and time delays collected in south Sweden yields, in accordance with the distribution of the P-residuals, an inclined symmetry axis of the hexagonal anisotropy dipping to the SE.

Resolution of non-double-couple mechanisms; simulation of hypocenter mislocation and velocity structure mismodeling

Mechanisms of earthquakes and induced seismic events are frequently found using the moment tensor description rather than the traditional double couple (DC), as it allows for nonshear source phenomena. However, non-DC source components are sensitive to event mislocations and inexact velocity models of the crust. Inaccuracies such as these can generate spurious non-DC components in the mechanism, which should be taken into account during the interpretation. We perform a synthetic case study simulating seismic observations at Soultz-sous-Forets, Alsace, hot dry rock (HDR) site. Synthetic P and S amplitudes for several shear-tensile source models are inverted, assuming hypocenter mislocation and velocity structure mismodeling in several types of station coverage. Satisfactory reconstruction of the source mechanism is achieved, except for rather extreme model simplification and extremely poor station coverage. Thus, our results suggest that non-DC mechanisms can be successfully resolved in local studies with reasonable station configuration, when errors in event location and velocity profile are realistic.

Seismic Anisotropy and Velocity Variations in the Mantle beneath the Saxothuringicum-Moldanubicum Contact in Central Europe

We report on results of a passive seismic experiment undertaken to study the 3-D velocity structure and anisotropy of the upper mantle around the contact zone of the Saxothuringicum and Moldanubicum in the western margin of the Bohemian Massif in central Europe. Spatial variations of P-wave velocities and lateral variations of the particle motion of split shear waves over the region monitor changes of structure and anisotropy within the deep lithosphere and the asthenosphere. A joint interpretation of P-residual spheres and shear-wave splitting results in an anisotropic model of the lithosphere with high velocities plunging divergently from the contact of both tectonic units. Lateral variations of the mean residuals are related to a southward thickening of the lithosphere beneath the Moldanubicum.

An array study of lithospheric structure across the Protogine zone, Värmland, south-central Sweden — signs of a paleocontinental collision

Abstract A small seismological array was installed on both sides of the Protogine Zone (PZ) in Varmland, south-central Sweden, to study the structure of the mantle lithosphere and lateral variations of its anisotropy. No distinct isotropic velocity anomalies were detected by tomography in the upper mantle around the PZ. The observed velocity variations depending on the direction of propagation can be explained by anisotropy within the subcrustal lithosphere on both sides of the suture. The best solution of a joint analysis of anisotropic inferences from teleseismic P-residual spheres and an inversion of shear-wave splitting parameters, resulted in 3D self-consistent anisotropic models of blocks of the subcrustal lithosphere. The anisotropic structures within the lithosphere are approximated by hexagonal models ( k P =5%) with low-velocity symmetry axes. The high-velocity planes dip to the E in a region westward of the PZ and to the NW eastward of the PZ. The PZ can be interpreted as a steep and narrow suture cutting the whole lithosphere and separating the two anisotropic blocks of different origin.

Joint interpretation of upper-mantle anisotropy based on teleseismic P-travel time delays and inversion of shear-wave splitting parameters

Abstract We inverted shear-wave splitting parameters and simultaneously analysed delays of teleseismic longitudinal waves to obtain a self-consistent three-dimensional (3-D) image of the anisotropic upper mantle beneath the continents. Efficiency of the simultaneous 3-D analyses of P-residual spheres and shear-wave polarizations is demonstrated on data from two regions, southern Sweden and the western USA. The anisotropic inferences of the subcrustal lithosphere are, to a first approximation, represented by homogeneous hexagonal or orthorhombic media with plunging symmetry axes.

Scenario of the January 1997 West Bohemia Earthquake Swarm

In order to learn more about the nature of the dynamic processes taking place in the West Bohemia/Vogtland earthquake swarm region, we investigated the temporal and spatial variations of the source mechanisms of the January 1997 swarm beneath Nový Kostel (NKC). Visual analyses of WEBNET seismograms of over 800 events revealed that a specific feature of this swarm was the occurrence of eight classes of multiplet events. The result of single-source, absolute moment tensor inversion of the P and SH peak amplitudes of a subset of 70 events representing all multiplet classes indicated that eight statistically significant types of mechanisms occurred during the swarm. Two of them, typesAandBin our denotation, comprised all ML≥ 1.3 events and predominated in the swarm. TypeAwere pure strike-slip mechanisms or strike-slip mechanisms containing a small normal component, with a nearly pure double-couple source. For classBevents, oblique-thrust faulting and non-double-couple components significant at a fairly high confidence level were typical. TypeAevents predominated in the southern subcluster of the swarm, whereas most of typeBevents occurred in the subcluster northwards from NKC. This indicates that two major seismogenic planes were active during the swarm. The swarm essentially developed in four phases: in the first, typeAevents prevailed and the southern plane was active; during the second, characterised by the occurrence of both typeAandBevents (the former in the southern, the latter predominantly in the northern subcluster), the activity of the swarm culminated; in the third and fourth, the occurrence of typeBevents in the northern plane predominated, and only weak single events occurred southwards from NKC. Mechanisms of typesAB,C,D,E,FandG, which were typical for ML≤1.2 events, occurred randomly throughout the swarm. TypeABevents were identified in both the southern and northern clusters, typeC,E,FandGmechanisms only southwards from NKC. TypeDevents exhibited a large scatter of hypocentres which fell in neither the southern nor the northern cluster. Focal mechanisms like those reported in this study and with analogous temporal and spatial variations were observed by other authors already fifteen years ago in the 1985/86 earthquake swarm and may, therefore, be typical for the region under study.

Moment tensors of the January 1997 earthquake swarm in NW Bohemia (Czech Republic): double-couple vs. non-double-couple events

Abstract We investigated moment tensors (MTs) of 70 events of the earthquake swarm which occurred in January 1997 in NW Bohemia. A refined location using the master-event procedure shows that all the foci clustered in a volume of less than 0.5 km3 comprising two compact clusters—the southern and northern ones. The results of single-source, absolute-moment tensor inversion of the P- and SH-peak amplitudes reveal two types of the source mechanisms, A and B in our denotation, which dominated in the swarm. Type A implies an oblique normal faulting with a nearly pure double-couple (DC) source. For the B type, an oblique-thrust faulting and a combined source [double-couple combined with the isotropic (ISO) and compensated linear-vector dipole (CLVD) components] are typical. Magnitudes of the non-double-couple components of MT appear unrelated to the ML magnitude of the event. The proximity of hypocentres of A and B events guarantees the non-double-couple source mechanisms of the B events not to be an artefact of a mismodelling of the medium. To exclude finiteness of the focus or station-site effects as possible causes of spurious non-double-couple components of MTs of the B events, the residuals of the peak amplitudes across the set of the B events were analysed and the jack-knife test was applied. The A and B events separate in time and space. Consequently, three major phases of swarm activity can be distinguished. In the first, only the southern cluster was active and A events prevailed, while B events dominated in the northern cluster in the third phase. Both A and B events occurred (the former in the southern cluster, the latter in the northern one) during the second phase. The initiation of the B events in the northern cluster are reflected in a pronounced increase in the non-double-couple components of the MTs, which points to tensile-source mechanisms as a consequence of a hypothesised fluid injection.

COMPARISON OF ABSOLUTE AND RELATIVE MOMENT TENSOR SOLUTIONS FOR THE JANUARY 1997 WEST BOHEMIA EARTHQUAKE SWARM

Moment tensor solutions for 70 clustered events of the 1997 West Bohemia microearthquake swarm, as calculated by two different methods, are given. The first method is a single-event, absolute moment tensor inversion which inverts body-wave peak amplitudes using synthetic Green functions. The second method is a multiple-event, relative method for which Green functions are “reduced” to 2 geometrical angles of rays at the sources. Both methods yield similar moment tensors, which can be divided into at least two or three different classes of focal solutions, indicating that, during the swarm activity, different planes of weakness were active. The major source component of most events is a double couple. However, the deviations from the double-couple mechanisms seem to be systematic for some classes of solutions. Error analysis was based on transforming the estimate of the standard deviation of amplitudes extracting from the seismograms into confidence regions of the absolute moment tensor. They show that the non-DC components are significant at a fairly high confidence level.

Seismic events with non-shear component. II: Rock bursts with implosive source component

The rock burst mechanism of an implosive character is introduced and discussed; it is based on the idea that the compressional component of the stress field around the loaded stress concentrator can participate in the rock burst event. For easier classification of the rock burst from this viewpoint, a set of P-wave radiation patterns of the rock burst with a varying portion of the implosive component and varying propagation velocity of the displacements is presented and discussed. To support the credibility of the rock burst mechanism of an implosive character the seismic signals of the rock bursts from the mid-Bohemian coal district with negative signs of the P-wave first onsets are presented.