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Vrancea intermediate-depth focal mechanism catalog: a useful instrument for local and regional stress field estimation

Andreea Craiu, Marius Craiu, Marius Mihai, Florinela Manea Elena, Alexandru Marmureanu

Acta Geophysica

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2023

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Vol. 71, no. 1

29--52

EN

The Vrancea seismic zone, located in the bend region of the South-Eastern Carpathians, is a unique area with both crustal and intermediate-depth seismic activity and is known as one of the most active seismic area in Europe. Moderate crustal seismicity is recorded all over the Carpathian region, but the far more intense activity occurs in a small subcrustal seismogenic volume beneath the SE‐bend of the Carpathian arc with about 20×50 km lateral and 110 km vertical extent (70–180 km depth). A unique slab geometry, likely preserved until the present, causes stress localization due to the slab bending and subsequent stress release resulting in large mantle earthquakes in the region. The main focus of this study is to determine the focal mechanisms for events with a magnitude larger than 2.7, between 2005 and 2020 and evaluate the current stress field along the Vrancea subcrustal region, from the derived fault plane solutions. The main style of faulting for Vrancea subcrustal events presents a predominant reverse one, with two main earthquakes categories: the first one with the nodal planes, oriented NE–SW parallel with the Carpathian Arc and the second one with the nodal planes, oriented NW–SE perpendicular on the Carpathian Arc. The results of stress inversion indicate a dominant thrust faulting style, with an average stress regime index of 2.87. The stress pattern shows similar partitioning with vertical extension in the slab and no preferred orientation in the overlying crust, showing a transition regime from the extensional regime in the Moesian Platform to the compressional regime in the Vrancea subcrustal zone.

2

Differential and source terms locations of the 2015 Teresva (East Carpathians) series and their tectonic implications

Gnyp Andriy, Malytskyy Dmytro

Acta Geophysica

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2021

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Vol. 69, no. 6

2099--2112

EN

Earthquakes of the 2015 Teresva series have been relocated using differential arrivals only of their P-waves at the same set of seismic stations and source-specific station terms. At least six distinct groups had been identified in the series as a result of single linkage clustering analysis of cross-correlations between their waveforms. Differential arrivals were estimated separately in each group, and not directly relative to the master event, but through the chains of events with the largest cross correlations. Time drift at some Ukrainian stations had been detected by comparing intervals between the first P-waves from the same earthquakes at pairs of stations and taken into account, assuming a linear drift rate. The relocated epicenter of the main MSH3.5 earthquake was only~2.3 km to the east of the macroseismic one and almost exactly at the intersection of the two major local faults, perpendicular and parallel to the Carpathians arc. The almost linear alignment of the other earthquakes in azimuth~320° almost coincided with the parallel fault and with the nodal plane of the almost purely strike-slip focal mechanism estimated for the strongest earthquake from its first polarities at 26 stations, and by moment tensor inversion of the ground displacement amplitudes and duration of the first P-wave pulses at 18 stations. A very interesting oscillatory (cyclic) pattern of the epicenter migration along the SE–NW axis, obtained as a result of relocation, was also confirmed by variations in S-relative to P-wave delays: During the cycle, the epicenters gradually shifted to NW and at the beginning of the new cycle returned.

3

Earthquake source dynamics and kinematics of the Eastern Indian Shield and adjoining regions

Singh Rashmi, Khan Prosanta Kumar, Singh A. P.

Acta Geophysica

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2020

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Vol. 68, no. 2

337--355

EN

The Eastern Indian Shield (EIS) consists of two cratonic nuclei, namely Singhbhum craton and Chhotanagpur Granitic Gneissic terrain. This area contains several crisscross faults, lineaments, shear zones, numerous hot springs and three major rivers (e.g., Ganga, Brahmaputra and Damodar). The area is regionally covered by 7 seismic stations and jointly recorded 16 events from the study area, and less noisy waveforms of 4 events were used for focal mechanism analysis using the Cut and Paste method. The focal parameters of these 4 events were compiled with results of 8 events computed by diferent workers for the study area. To understand the detailed tectonics, focal mechanisms of 21 events for the Himalayan segment were taken from CMT Harvard catalog of duration 1976–2017. Spatial variations of operative stress felds for major tectonic domains were analyzed in the present study based on stress inversion of focal mechanism parameters. We observed strike-slip-dominated movements in the EIS, which changes partially into extension in the northeast part between the Ganga and Brahmaputra Rivers. Thrust-dominated movements evidenced by focal mechanisms and the pure compression in the western segment of the Himalaya in the north might be promoting shear movements in the EIS and adjoining regions. The pure strike-slip in the eastern segment of the Himalaya and its deeper level account for lateral shearing and eastward movements of diferent tectonic blocks. The normal faulting earthquakes in the northeast part might be indicating stretching in the basement because of convergence of Indian lithosphere beneath the Myanmar plate.

4

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.

5

Preliminary Results of Anthropogenic Seismicity Monitoring in the Region of Song Tranh 2 Reservoir, Central Vietnam

Wiszniowski J., Giang N. V., Plesiewicz B., Lizurek G., Van D. Q., Khoi L. Q., Lasocki S.

Acta Geophysica

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2015

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Vol. 63, no. 3

843--862

EN

Song Tranh 2 hydropower plant and the reservoir containing backed up water are located in the Quang Nam province (Central Vietnam). The region experiences unusual seismic activity related to the reservoir impoundment, with earthquakes of magnitude up to 4.7. In result of cooperation between the Institute of Geophysics, Vietnam Academy of Sciences and Technology and the Institute of Geophysics, Polish Academy of Sciences a seismic network has been built to facilitate seismic monitoring of the Song Tranh 2 area. The network, operating since August 2013, consists of 10 seismic stations. Here we show that the network is sufficient for advanced data processing. The first results of monitoring of the earthquake activity in Song Tranh 2 area in the period between 2012 and 2014, especially the completeness of catalogs, study and comparisons between water level and the seismic activity suggest direct connection between reservoir exploitation and anthropogenic seismicity.

6

Mining Induced Seismic Event on an Inactive Fault

Lizurek G., Rudziński Ł., Plesiewicz B.

Acta Geophysica

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2015

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Vol. 63, no. 1

176--200

EN

On 19 March 2013, a tremor shook the surface of Polkowice town where the Rudna Mine is located. This event, of ML = 4.2, was the third most powerful seismic event recorded in the Legnica Głogów Copper District (LGCD). Inhabitants of the area reported that the felt tremor was bigger and lasted longer than any other ones felt in the last couple of years. Analysis of spectral parameters of the records from in-mine seismic system and surface LUMINEOS network along with broadband station KSP record were carried out. The location of the event was close to the Rudna Główna Fault zone; the nodal planes orientations determined with two different approaches were almost parallel to the strike of the fault. The mechanism solutions were also obtained as Full Moment Tensor from P-wave amplitude pulses of underground records and waveform inversion of surface network seismograms. The results from the seismic analysis along with macroseismic survey and observed effects from the destroyed part of the mining panel indicate that the mechanism of the event was complex rupture initiated as thrust faulting on an inactive tectonic normal fault zone. The results confirm that the fault zones are the areas of higher risk, even in case of carefully taken mining operations