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Seismic site characterization considering directional near‑field seismogenic active faults in Aswan area, Egypt

Thabet Mostafa, Omar Khaled, Mohamed Abdel Nasser, Osman Mohamed

Acta Geophysica

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2023

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Vol. 71, no. 3

1119--1148

EN

Aswan area in South Egypt experiences continuous seismic activity due to seismogenic active faults, particularly Kalabsha and Seiyal active EW faults. The seismic site characterization is not properly identified, although the presence of high-density distribution of earthquake stations. The present study investigates fourteen earthquake stations of the Egyptian National Seismic Network, as well as six microtremor measurement sites. We analyzed ground motions due to seismogenic active faults recorded at surface from these fourteen earthquake stations. We measured microtremors for up to 120 min with portable seismometers at six sites in the vicinity of the High Dam area. The horizontal-to-vertical spectral ratios of earthquakes (EHVSR) and microtremors (MHVSR), their plots as a function of frequency and direction of motion, and diffuse field inversion are used in the study. Therefore, we could provide an obvious understanding of the site characterization including resonance frequencies, directional amplifications, and back-calculated subsurface velocity structures at these stations and sites. We found three predominant amplification directions of NS, EW, and NE–SW due to the horizontal components of the seismic waves. These amplification directions are near-transversal ~ transversal to the NS and EW strikes of the active fault system in the study area. In time–frequency analyses of the records, this directionality is observed clearly for S-wave and surface wave time windows. We validated the diffuse field inversion process not only using fitting between observed and inverted EHVSRs and MHVSRs, but also using available geological 2D cross sections and hydrological information in Aswan area.

2

New evidences of Holocene tectonic and volcanic activity of the western part of Lake Sevan (Armenia)

Avagyan Ara, Sahakyan Lilit, Meliksetian Khachatur, Karakhanyan Arkadi, Lavrushin Vasily, Atalyan Tatul, Hovakimyan Hayk, Avagyan Seda, Tozalakyan Petros, Shalaeva Evgenia, Chatainger Christine, Sokolov Sergey, Sahakov Arkadi, Alaverdyan Gagik

Geological Quarterly

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2020

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Vol. 64, No. 2

288--303

EN

The purpose of this paper is to present new data on active geological processes in the Lake Sevan Basin and to show its multidisciplinary aspects. The investigations of its structures, recent lake sediments, and lake floor gas emission allow a better understanding of the history of geological development and the recent tectonic and volcanic activity of the basin. This paper summarizes underwater investigations of active geological features of Lake Sevan, undertaken for the first time in Armenia. More than 30 aligned underwater-source related gas emission points were discovered. The gas contains carbon dioxide of volcanic or volcanogenic-metamorphogenic origin and can be related to unloading of deep fluid systems. This allows defining the Noratus-Kanagegh Fault segment trace below the recent Lake Sevan floor sediments. The discovery of the subaqueous segment of active fault shows the presence of another natural hazard of lake tsunami related to possible future co-seismic rupture. The recent sediments of the northwestern Lake Sevan coastline are sandwiched between two blocky lava flows. The radiocarbon dating of bones of bovine mammals (with entire skull), found ~15 cm from the cover of the lake sediments, suggests that the upper blocky basaltic-andesite layer can be a result of eruption younger than ~3400 years BP. About 80 m of the Noratus sequence sediments have been sampled for palaeomagnetic study and the age of 3.1-2.3 Ma for the lower part (42 m) is obtained. The upper and post-Gelasian activity of the Noratus-Kanagegh Fault is proven by a cross-cutscoria layer of 2.30 ±0.15 Ma K/Ar age.

3

Holocene activity and seismic hazard analysis of faults in Damxung, Tibet

Yang Gang, Tang Yong, Lei Dongning, Hu Qing, Wu Jianchao

Acta Geophysica

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2020

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Vol. 68, no. 3

597--604

EN

Based on interpretation of remote sensing and geological feld survey result, three secondary faults are distributed in parallel in Damxung of the southeastern piedmont fault of Nyainqentanglha mountain, which form landform profle across normal faults of graben due to cut platform margin of grade I terrace scarp formation. Through researching the displacements and the collected sediments dates, it is obtained that the horizontal slip rate is about 0.6 mm/a (millimeter per year), and the vertical slip rate is about 1.7 mm/a since 4 ka B.P. (kilo years before present). The northern margin of Damxung basin is a seismic gap, which continuously accumulated strain energy is not released in the M7.5 Damxung earthquake and the M8.0 Yangbajain earthquake. The fracture length of the seismic space can be regarded as the minimum length of potential surface rupture. The maximum moment magnitude of potential earthquakes is at least M6.8.in northern margin of Damxung basin the normal fault magnitude estimation formulas from Wells and Coppersmith (Bull Seismol Soc Am 84(4):974–1002, 1994).

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Evaluation of seismic hazard using tectonic fault data: case of Beni-Chougrane Mountains (Western Algeria)

Refas Soraya, Safa Aissa, Zaagane Mansour, Souidi Zahera, Hamimed Abderahmane

Mining Science

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2019

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Vol. 26

123--145

EN

In the world, people are increasingly exposed to natural hazards such as earthquakes. To this end, seismic risk mapping remains an essential topic of study in order to minimize their destructive effects. These maps are needed for both seismic risk management and for the design of infrastructure. The challenge is to take into account local information provided by seismic sources (historical seismicity) as well as information related to active tectonic faults. In this article, we calculated the seismic risk in the Mascara Mountains (western Algeria) using the geometric characteristic of known faults. This study is based on an important collection of a tectonic database of these faults (Nature, geometry and geological context). This information is relevant for their seismic potential. Indeed, by including these formations we tried to compute the seismic risk this region characterized by weak seismicity. Our results show more or less alarming facts. Indeed, the magnitude values calculated are between 4.85 and 7.25, whereas the magnitudes obtained by experimental seismicity do not exceed 6 on the Richter scale. The values of the maximum ground acceleration (PGA) are between 0.03 and 0.28 g. These results were compared with assessments made on the basis of historical seismicity; the maximum values obtained do not exceed 0.2 g. The higher values of magnitude calculated from the active faults is due to: (i) the nature of the faults (inverse, normal and strike slip), (ii) the geometry (length and depth) and (iii) that some of these faults may have an aseismic character.

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Tectonic geomorphology and neotectonic setting of the seismically active South Wagad Fault (SWF), Western India, using field and GPR data

Maurya D. C., Chowksey V., Tiwari P., Chamyal L. S.

Acta Geophysica

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2017

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Vol. 65, no. 6

1167--1184

EN

The South Wagad Fault (SWF) is an E–W trending fault that delimits the Wagad uplift comprising Mesozoic rocks in its northern upthrown block and Neogene–Quaternary sediments in the southern downthrown block. Detailed GPR investigations were carried out at seven sites selected after field studies. All profiles clearly showed the lithological contrast across the fault. The sharp amplitude contrast of the radar waves along a vertical to sub-vertical line is interpreted as the near surface trace of the SWF. As the Quaternary sediments are not displaced, we infer that no large magnitude earthquake has occurred along the SWF in late Quaternary. We attribute the low magnitude of neotectonic activity along the SWF to gentle warping of the Tertiary rocks in the southern downthrown block and greater accumulation of compressive stresses along the nearby KMF with an opposite structural setting. This is consistent with the observed variable levels of ongoing seismicity in the region around the SWF.

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Continuum theory of defects and gravity anomaly

Yamasaki K., Nagahama H.

Acta Geophysica Polonica

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1999

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Vol. 47, nr 3

239-257

EN

Based on the mathematical equivalence between the crack field and the continuous dislocation field, we briefly review continuum theory of defects from the view point of differential geometry. Then we derive a new differential geometric equation of static gravity change for anelastic effect due to the fault (dislocation) density. This equation shows that high gradient of dilatancy caused by the concentration of fault (dislocation) density accompanies high gradient of gravity change near the boundary between positive and negative gravity anomalies. This agrees with the characteristic distribution patterns; the distribution of short-wavelength gravity anomaly, active faults and shallow seismic activities overlap one another in the northeast Japan. Moreover, we discuss: (I) dynamic gravity anomaly related to earthquakes; (II) local gravity anomaly near the edges of an active fault; (III) differential geometric interpretation of gravity anomaly caused by the dislocation density; (IV) differential geometric relationship between gravity anomaly and magnetic anomaly (Poisson's relation).