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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

Stress studies in the Central Alborz by inversion of earthquake focal mechanism data

Pourbeyranvand S.

Acta Geophysica

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2018

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Vol. 66, no. 6

1273--1290

EN

The Alborz is one of the most important seismotectonic provinces in Iran. Furthermore, emplacement of Tehran as a mega city in southern part of the Alborz intensifies the seismic vulnerability in this area. In this study, the focal mechanism data from teleseismic and local seismic networks are used for stress tensor inversion. The earthquake focal mechanisms in the Central Alborz are divided into several groups with respect to their location. Two different stress tensor inversions, linear and nonlinear, are used for obtaining the principal stress orientations. The results show spatial variations in tectonic stress field, consistent with fault orientations and faulting mechanisms. The maximum compressional stress directions obtained in this study are confirmed by fast S-wave polarization axes reported by a previous shear wave splitting study. The maximum horizontal stress directions are also compared with GPS strain rates. The results indicate a partitioning of deformation in the area due to regional stresses along preexisting faults.

4

Mantle dynamics beneath Greece from SKS and PKS seismic anisotropy study

Kaviris G., Fountoulakis I., Spingos I., Millas C., Papadimitriou P., Drakatos G.

Acta Geophysica

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2018

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Vol. 66, no. 6

1341--1357

EN

SKS and PKS splitting parameters were determined in the broader Greek region using data from 45 stations of the Hellenic Unified Seismological Network and the Kandilli Observatory and Earthquake Research Institute, utilizing teleseismic events that occurred between 2010 and 2017. Data were processed for shear-wave splitting with the Minimum Energy Method that was considered the optimal. The results generally confirm the existence of anisotropic zonation in the Hellenic subduction system, with alternating trench-normal and trench-parallel directions. The zonation is attributed to the upper and lower olivine fabric layers that can, potentially, be present in the subduction zone. At the edges of this zone, two possible toroidal flow cases have been identified, implying the existence of tears that allow the inflow of asthenospheric material in the mantle wedge. The high number of null measurements in the KZN and XOR stations indicates a possible anisotropic transition zone between the fore-arc and back-arc areas. SKS and PKS splitting results are jointly interpreted, given that they yield similar values in most cases.

5

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.

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