S-wave Splitting Research Articles

Calculated and observed seismic behaviour of exposed upper mantle

In order to determine the seismic properties of rocks representing the continental crust/upper mantle transition, a structural and petrographical study of peridotites of the Ivrea-Verbano zone (N. Italy) has been undertaken. The seismic behaviour (Vp, Vs, birefringence, orientation of the fast shear-wave polarisation plane) has been calculated from the modal composition of rock samples, together with the crystallographic preferred orientation and the single crystal elastic constants, using the methods of Mainprice (1990). It is shown that the intrinsic anisotropy of peridotites is mainly due to the lattice preferred orientation (LPO) of olivine and to the competing effects due to the LPO of orthopyroxene. If present, well-oriented hydrous phases such as phlogopite, leads to both P- and S-waves velocities becoming drastically reduced, but the anisotropy and the S-wave splitting (mainly on the foliation plane) is increased. An attempt has been made to apply this petrologic and microstructural approach to understand the seismic properties of ultramafic rocks with the aid of a mobile seismic network set up in the Ivrea-Verbano zone. Five recording units with 3-component geophones were installed on the most representative lithologies of the Ivrea-Verbano zone: kinzigites, gabbros, stronalites, and on metapelites of the adjacent outcrop of the Serie dei Laghi. During five months the seismic network recorded 49 events that were mainly regional earthquakes. The preliminary analysis of the first arrival revealed a high-velocity anomaly near the Finero ultramafic massif. Rough calculations indicated that this anomaly could be due to a shallow-depth, peridotitic body about six kilometres thick. S-wave polarisation analysis was carried out on the horizontal components of the seismograms of four regional earthquakes that showed a first arrival propagating almost vertically at the recording sites. Four earthquakes were chosen in order to reconstruct the orientation of the polarisation plane of the shear phases. The preliminary results of this analysis have shown a qualitatively good agreement between the orientation of the polarisation plane of the fast shear-wave and the strike of the foliation at the station near Finero, located on kinzigites of the Ivrea-Verbano zone, although the relationships between the local structural setting and the polarisation orientations at the other recording sites is not yet so clear.

Crustal “bright spots” and anisotropy from multi-component P- and S-wave measurements in southern Germany

Controlled shear-wave experiments have focussed on deep crustal targets that were earlier recognized by DEKORP P-wave surveys in southern Germany (Black Forest, Upper Palatinate). A prominent P-wave reflection (“bright spot”) at 8 km depth at the KTB deep drilling site is not seen on a comparable S-wave reflection section, in contrast to deeper reflections. This behaviour can be explained either by purely compositional effects (layer with anomalously high quartz content) or by fluid accumulation (“reservoir”) in a fracture zone. A 20 km deep, 10 km long prominent P-wave reflection in the southwestern Black Forest is only partially accompanied by corresponding S-wave reflections. The influence of the physical state of the rock e.g. stress-related cracking, in addition to possible compositional variations is suggested to explain the contrasting P- and S-wave reflection strength. Seismic anisotropy of 10% magnitude, revealed by S-wave splitting in vertical seismic profiling data at the KTB to more than 4 km depth, is attributed to sub-vertically dipping and NW-striking foliation.

Using shear waves to predict lateral variability in vertical fracture intensity

During the last five years, the use of shear‐wave splitting in geophysical exploration has made the transition from research project to very promising field technique. The measurement of the azimuth of the fast shear‐wave and analysis of the S-wave splitting (or birefringence) are now commonplace. Furthermore, a very timely practical application is suggested by the recent proposal that reflectivity on post‐rotation S1 and S2 data sets should indicate lateral variation of anisotropy. This could be of great value because the determination of lateral variation in fracture intensity in a vertically fractured sedimentary layer is necessary to insure high efficiency in a horizontal drilling program designed to drain the fractures.

Anisotropy of Vp and Vs in an amphibolite of the deeper crust and its relationship to the mineralogical, microstructural and textural characteristics of the rock

Laboratory seismic measurements of compressional ( V p )- and shear-wave ( V s ) velocities have been carried out on a deformed amphibolite from the Ivrea Zone at pressures and temperatures up to 600 MPa and 600°C, respectively. Amphibolite is considered to be an important rock of the deeper crust. From crystallographic orientations of the component minerals (hornblende, plagioclase and diopside) and the respective elastic constants, we calculated the average elastic constants (Voigt average) for the rock specimen. Compressional- and shear-wave velocities in three orthogonal directions (normal and parallel to foliation) measured at high confining pressure (600 MPa) are in good agreement with the calculated velocity surface. This suggests that seismic-wave velocity anisotropy is controlled by preferred orientations of minerals in rocks (texture) at high confining pressure, where the effect of microcracks on seismic wave velocities is eliminated. In the amphibolite sample, the velocity anisotropy is dominated by the texture of hornblende because of the strong preferred orientation and large volume fraction of this mineral species. A large S-wave splitting is documented in the directions parallel to the foliation (particularly in the lineation) which strongly argues against the microcrack origin of S-wave splitting, at least at conditions of high effective pressure.

S-wave anisotropy in the upper mantle under a volcanic area in Japan

Most ultra-basic nodules that probably originated in the upper mantle have anisotropic petrofabric and elastic properties1,2. Within seismology, however, the Earth is generally assumed to be isotropic and this assumption is appropriate for most seismological studies. Thus, rocks can be considered isotropic on large scales although anisotropic on a scale of hand specimen3. This interpretation is questionable, however, in light of the increasing resolution of seismic observations. Many reliable reports on seismic anisotropies have been accumulating: for example, azimuthal anisotropy in Pn velocities in oceanic basins4, and in the continent5 and the back-arc basin6, and an S-wave splitting of the SH and SV waves revealing an anisotropic structure as deep as 190 km (ref. 7). These suggest that some unsuccessful detections of the Earth's anisotropies may be due to lack of appropriate observations. We describe here the recent discovery of shear wave anisotropy in a volcanic area in Japan, which was revealed by an improvement in the seismographic observation systems.

Seismic body waves in anisotropic media: synthetic seismograms

Summary. Synthetic seismograms and particle motion diagrams are computed for simple, layered Earth models containing an anisotropic layer. The presence of anisotropy couples the P, SV and SH wave motion so that P waves incident on the anisotropic layer from below produce P, SV and small-amplitude SH waves at the surface both the P velocity and the amplitudes of the converted phases vary with azimuth. Significant SH amplitudes may be generated even when the wavelength of the P wave is much greater than the thickness of the anisotropic layer. Incident SV or SH waves may each generate large amplitudes of both SV and SH motion. This strong coupling is largely independent of the degree of velocity anisotropy of the medium. The arrivals from short-period S waves exhibit S-wave splitting, but arrivals from longer period S waves superpose into a modified waveform. This strong coupling does not allow the arrival of separate phases with pure SV and SH polarization except along directions of symmetry where the motion decouples.

Seismic body waves in anisotropic media: propagation through a layer

Summary. The square-root energy ratios and pulse shapes are presented for P, SV and SH waves transmitted through a layer of orthorhombic olivine between two isotropic half-spaces. Off incident planes of symmetry, incident P waves generate two small amplitude SH waves (one from each interface), whose amplitudes decrease slowly with increasing period. Incident SV (orSH) waves can generate large amplitude SH (or SV) waves which decrease rapidly with increasing period. For incident S waves, many pulses not present in isotropic models are generated, often of large relative amplitude, with many of the transmitted S pulses showing evidence of double arrivals, either in the form of S-wave splitting, or a modification of the shape of the input waveform.

Premonitory seismic-wave phenomena before earthquakes near Fairview Peak, Nevada

abstract Anomalous variations in three different seismic processes have been observed before an earthquake of magnitude 4 on May 14, 1971 in the Fairview Peak region of central Nevada. The data used are the three-component seismograms from Tonopah (TNP) and Battle Mountain (BMN) as well as the vertical-component seismograms from several other seismographic stations. The observed precursory phenomena are (1) reorientation of the compressive stress axis: evidence for this is based on clear reversals of first motion of Pg at certain stations together with reversal of wave form of the SV component of Sg at BMN; (2) vertical migration of hypocenters: the PmP phase is often distinctly observed in central Nevada and the observed temporal variations in t(PmP)−t(Pg) at TNP indicate generally upward migration of foci before the main event; (3) changes in the extent of S-wave splitting: a large precursory increase is observed along the tensile stress direction and a small premonitory decrease along the compression direction. Similar but unidentical results have been obtained before another earthquake of magnitude 4 whereas all events of magnitude 3.5 or larger have been preceded by anomalous S-wave splitting along the tension direction. The various observed pre-earthquake processes appear to be interrelated and may be explained in terms of recent laboratory and theoretical results when applied to the tectonics of central Nevada. It seems highly desirable to attempt simultaneous observations of anomalous changes in more than one seismic process.

Comments on “S-wave splitting: A key to earthquake prediction?” By A. Ryall and W. U. Savage

Letter| December 01, 1974 Comments on “S-wave splitting: A key to earthquake prediction?” By A. Ryall and W. U. Savage Indra N. Gupta Indra N. Gupta Mackay School of Mines University of NevadaReno, Nevada 89507 Search for other works by this author on: GSW Google Scholar Bulletin of the Seismological Society of America (1974) 64 (6): 1997–2001. https://doi.org/10.1785/BSSA0640061997 Article history received: 31 May 1974 first online: 03 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Tools Icon Tools Get Permissions Search Site Citation Indra N. Gupta; Comments on “S-wave splitting: A key to earthquake prediction?” By A. Ryall and W. U. Savage. Bulletin of the Seismological Society of America 1974;; 64 (6): 1997–2001. doi: https://doi.org/10.1785/BSSA0640061997 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search nav search search input Search input auto suggest search filter All ContentBy SocietyBulletin of the Seismological Society of America Search Advanced Search This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not currently have access to this article.

S-wave splitting: Key to earthquake prediction?

abstract This paper presents a detailed analysis of S-wave data for two areas in western Nevada. Some of these data were used by Gupta (1973b, c) as the basis for his claim that large premonitory changes in the extent of stress-induced S-wave splitting are observed for moderate-sized earthquakes in Nevada. Analysis of particle motion for an earthquake sequence near Slate Mountain indicates that changes in S-wave splitting did not occur during that sequence. For the Mina area, comparison of S-wave signatures for 158 events occurring over a 3-year period resulted in the identification of numerous events that would be considered anomalous by Gupta's criteria, but these were not followed by larger earthquakes. The present state of knowledge on crustal structure and seismic source parameters in the western Basin and Range province is not sufficient to discriminate between stress-induced velocity anisotropy and the many other factors that contribute to the complexity of S-wave signatures.

Premonitory Variations in S -Wave Velocity Anisotropy before Earthquakes in Nevada

Application of nonhydrostatic stress to rock induces velocity anisotropy, causing the S wave to split into two components traveling with somewhat different velocities. Large premonitory changes in the extent of S-wave splitting have been observed for two earthquakes in Nevada. Observations of the difference between the two S-wave velocities may provide a simple method for pedicting earthquakes.