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1

On the origin of upper crustal shear wave anisotropy at Samos Island, Greece

Kaviris G., Spingos I., Kapetanidis V., Papadimitriou P., Voulgaris N.

Acta Geophysica

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2021

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Vol. 69, no. 3

1051--1064

EN

Shear-wave splitting is associated to diferent sources in the upper crust. Preferentially oriented minerals, stress-aligned microcracks and tectonic structures have all been identifed as causes of seismic anisotropy in the upper crust. However, distinguishing between them and discovering the actual origin of the splitting efect has important implications; changes in the anisotropic properties of the medium related to the behavior of fuid-flled microcracks could have potential connections to the occurrence of an impending signifcant earthquake. The recent 2020 Samos Mw =6.9 event and its associated sequence was a great opportunity to study shear-wave splitting in the area. The spatial constrains in such studies, i.e., the requirement of events located very close to the receivers, did not permit exploring local anisotropy in the past, due to a severe lack of suitable data. To establish a background of splitting, we searched for any appropriate earthquake in a fve-year period preceding the mainshock. We performed an automatic analysis on over 200 event-station pairs and obtained 164 high-quality splitting observations between January 2015 and November 2020. Results indicated a strong connection to local structures; Sfast polarization axes seem to align with faults in the area. However, we also observed a period of increasing and decreasing time-delays, associated with an Mw =6.3 earthquake that occurred on June 2017 near Lesvos Island. The latter behavior implies the possibility of stress-induced anisotropy in the area. Thus, the Samos Island could be represented by two diferent sources of splitting; structures to the NW and microcracks to the SE.

2

Lithospheric mantle anisotropy from local events beneath the Sunda–Banda arc transition and its geodynamic implications

Syuhada Syuhada, Hananto Nugroho D., Abdullah Chalid I., Puspito Nanang T., Anggono Titi, Febriani Febty, Soedjatmiko Bogie

Acta Geophysica

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2020

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Vol. 68, no. 6

1565--1593

EN

Shear wave splitting analysis to characterise lithospheric mantle anisotropy has been performed to provide better knowledge about lithospheric deformation and mantle fow beneath the Sunda–Banda arc transition, Indonesia. The tectonic setting of the study area is very complex characterised by the transition from subduction along Sunda arc to collision in Banda arc. The splitting measurements show lateral and vertical variation in the fast directions of the S-waves in this region. When the splitting results are analysed through 2D delay-time tomography and spatial averaging, systematic patterns in delay times and fast polarisation become more visible. In the subduction domain, the spatial averages of fast directions are dominated by two distinct fast polarisations: perpendicular and parallel to the plate motion for shallow and deep events, respectively. The results suggest that anisotropy in this area is not only controlled by anisotropic source related to the simple mantle fow model, but also by anisotropic fabric in the mantle deformed under infuence of high stresses, high water contents and low temperatures. In addition, there might also be contribution from the anisotropic body in the upper layer. In the collision domain, spatially averaged fast directions show mostly perpendicular to the plate motion for all deep levels. For shallow level in this region, this trend is mainly governed by the lithospheric deformation process due to the continent-arc collision as also shown by delay time tomographic inversion. For deeper part of the region, the result of tomographic inversion and spatial averaging reveals a high anisotropy followed by rotational pattern of fast directions in the north of Timor. We suggest that this pattern might be related to the induced mantle fow due to lateral tearing of the slab.

3

Separation of split shear waves based on a hodogram analysis of HTI media

Yang Y., Lu J., Wang Y.

Acta Geophysica

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2018

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Vol. 66, no. 4

643--658

EN

Although the shear-wave birefringence phenomenon affects the imaging of converted shear waves, it also provides a considerable amount of information on subsurface fracture development. Therefore, it is significant to separate split shear waves before seismic interpretation and reservoir prediction. In this paper, we propose a new method of split shear waves separation based on the polarization directions derived from hodogram analysis. Through the hodogram analysis, we find that the split shear-wave particle motions are within the range of a specific and fixed rectangle, which have relations with the fracture azimuth in strata. In addition, we found that a couple of split shear waves can only be fitted to the unique trajectory rectangle through the theoretical derivation. Based on this, we establish the trajectory rectangle through the wave vector calculation and calculate the fracture azimuth according to the fact that the one edge of the trajectory rectangle is along or perpendicular to the fracture azimuth. Synthetic data analysis shows that the calculation accuracy of fracture azimuth under the constraint of trajectory rectangle is less affected by the time delay between split shear waves than using the method of eigenvector–eigenvalue decomposition (EED). Therefore, we can obtain better results for separation of split shear waves using our method than using EED. Eventually, we propose an approach of layer stripping to deal with the problem that shear wave split several times due to the situation that different strata have different fracture azimuths. Synthetic data test indicates that our method can achieve higher calculation efficiency and faster convergence speed than the conventional eigenvector–eigenvalue decomposition method, even though the data are of a low signal-to-noise ratio. Moreover, field data applications show the effectiveness and potential of our method.

4

Shear Wave Splitting Analysis to Estimate Fracture Orientation and Frequency Dependent Anisotropy

Gholami R., Moradzadeh A., Rasouli V., Hanachi J.

Acta Geophysica

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2016

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Vol. 64, no. 1

76--100

EN

Shear wave splitting is a well-known method for indication of orientation, radius, and length of fractures in subsurface layers. In this paper, a three component near offset VSP data acquired from a fractured sandstone reservoir in southern part of Iran was used to analyse shear wave splitting and frequency-dependent anisotropy assessment. Polarization angle obtained by performing rotation on radial and transverse components of VSP data was used to determine the direction of polarization of fast shear wave which corresponds to direction of fractures. It was shown that correct implementation of shear wave splitting analysis can be used for determination of fracture direction. During frequencydependent anisotropy analysis, it was found that the time delays in shearwaves decrease as the frequency increases. It was clearly demonstrated throughout this study that anisotropy may have an inverse relationship with frequency. The analysis presented in this paper complements the studied conducted by other researchers in this field of research.

5

Parallel Implementation of Ray Tracing Procedure in Anisotropic Medium

Pieta A., Dwornik M.

TASK Quarterly : scientific bulletin of Academic Computer Centre in Gdansk

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2012

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Vol. 16, No 1-2

135-143

EN

This article describes a parallel implementation of a ray tracing algorithm in a heterogeneous anisotropic geological medium. The shortest path method, which was used for calculations, can give ray path and travel time of seismic wave propagation even for a random and discontinuous velocity field. The high precision required in such calculations was obtained by employing a dense computational grid. This led to a significant increase in the computational effort of the algorithm. The procedure was parallelized using domain decomposition. The results show that the parallel performance of the ray tracing procedure strongly depends on the assumed geological method and differs between media with and without anisotropy of seismic wave propagation.

6

Interpretacja danych sejsmicznych w oparciu o zdjęcie wieloskładnikowe

Śliż K.K.

Geologia / Akademia Górniczo-Hutnicza im. Stanisława Staszica w Krakowie

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2001

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T. 27, z. 2--4

589-601

PL

Wieloskładnikowe zdjęcie sejsmiczne (3C) wnosi nowe możliwości do geologicznej interpretacji danych sejsmicznych. Fale poprzeczne lepiej odwzorowują budowę geologiczną pod ciałami solnymi, bazaltowymi oraz obszarami zniekształcanymi przez strefy i kominy gazowe. Analiza danych wieloskładnikowych dostarcza wiedzy na temat anizotropii utworów geologicznych oraz typu cieczy złożowej. Interpretacja geologiczna na poziomie wieloskładnikowym pozwala na rozdzielenie facji sedymentacyjnych, określenie kontaktu woda - ropa, jak również na uzyskanie przybliżonych wartości współczynnika porowatości utworów skalnych. Otrzymujemy także nowe możliwości identyfikacji stref uskokowych, ich zasięgów oraz kierunków przepuszczalności. W artykule wprowadzone zostaną podstawy interpretacji wieloskładnikowej na przykładzie zdjęcia 3D/3C w oparciu o atrybut sejsmiczny Vp/Vs

EN

Multicomponent seismic data (3C) brings new dimensions into geological interpretation. Shear waves could image area below salt, basalt bodies, and disturbed by gas zones or chimneys. Analysis of multicomponet data provides information about anisotropy and type of pore fluids. Geological interpretation on multicomponent level allows lithology differentiation, definition of water - oil contact and estimation of rock porosity. We also obtain new possibilities of identification fractures and faults zones, theirs ranges and directions of permeability. The article introduces to multicomponent interpretation based on 3D/3C data. An example of Vp to Vs ratio interpretation is shown to prove its ability

7

Seismological station Kalwaria Pacławska (KWP): first year of activity

Wiejacz P., Suchcicki J., Jankowska W.

Acta Geophysica Polonica

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2001

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Vol. 49, nr 2

189-201

EN

The Institute of Geophysics, Polish Academy of Sciences, in cooperation with the GeoForschungsZentrum Potsdam, has set up in 1999 a new seismological broadband station at Kalwaria Pacławska (KWP), southeast Poland. The site was chosen as there are few seismological broadband stations in the area. Those that exist are either situated to the west or have outdated equipment and report data irregularly, as it is the case with the stations to the east. Therefore, Kalwaria Pacławska is very valuable to monitor local and regional seismicity in the area. The site is located on the edge of the Carpathians and has proven to be very interesting. During the first year of activity many seismic events from different parts of the world have been registered, the most noteful being perhaps the disastrous M = 7.4 earthquake that hit Izmit, Turkey. Surprisingly, some mining events from Upper Silesia and Lubin get registered fairly well on KWP. Attempt to describe the anisotropy beneath the station using SKS waves of events from 90-130 degree distance has yielded an interesting though ambiguous result. Hence, the anisotropy beneath KWP should be an issue of further study.