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1

Icequakes in Hans glacier, Spitsbergen: Source parameters of icequake series

100%

Górski M.

Acta Geophysica Polonica

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2003

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tom Vol. 51, nr 4

399-407

EN

The data of icequakes series recorded in spring and summer seasons at Hans glacier, Spitsbergen, have been analysed. The determination of seismicity of the glacier, is based on temporary measurement networks placed on glacier surface. The following source parameters are analysed: seismic moments, apparent stresses, stress drops, sources diamensions and radiated seismic energies. A comparison of scalling relations for source parameters odf spring and summer series of icequakes has been made.

2

Temporal distribution, source parameters, and depth discrimination of earthquakes from the Abu Dabbab region, Egypt

100%

Hassoup A. , Niewiadomski J. , Gibowicz S.

Acta Geophysica Polonica

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2003

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tom Vol. 51, nr 2

135-146

EN

Records of earthquakes from the Abu Dabbab region in Egypt, situated about 25 km west of the Red Sea coast, were collected from the Aswan Seismograph Network (ASN). The temporal distribution of these events shows several sequences of the foreshock-main shock-aftershock type. Four such sequence occurred in 1984 and 1985. the slope of the frequency-amplitude relation (b-value) for the four sequences ranges form 1.8 to 2.4, reflecting co-seismic deformation with time in the vicinity of the source area. Source parameters were estimated for ten events from the same area, which occurred between 1998 and 2001 and had magnitude ranging from 3.0 to 4.2.the spectral plateau, corner frequency, seismic moment, source dimension, and stress drop were calculated. A good correlation is found between the logarithm of seismic moment and the local duration magnitude determined by the ASN. The stress drop is not uniform and ranges between 0.1 and 6.8 MPa. In addition, the relative decay of the amplitude of S waves from the Abu Dabbab earthquakes with the epicentral distance is examined from the records of various stations of the ASN. It was found the rater of decay can be divided into two distinct types, related to different paths between the hypocenters and the stations, which in turn indicate different depths of the studied events.

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Source characteristics and tectonic implications of moderate earthquake, northeastern Cairo Prefecture

100%

Abdel-Fattah A. K. , Badawy A.

Acta Geophysica Polonica

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2004

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tom Vol. 52, nr 1

29-43

EN

Using local seismograms of the Egyptian National Seismological Network (ENSN), source characteristics of a moderate earthquake Mw = 4.5 (28 December 1999) are analyzed. In this analysis, the Empirical Green's Function (EGF) deconvolution technique is applied. The records of an appropriate aftershock are taken as the EGF and are used to deconvolve the mainshock seismograms, thus obtaining a Relative Source Time Function (RSTF) at each station. The deconvolution is performed using P waves in frequency domain. From the time-domain analysis of the RSTF, the resulting source time functions indicate a simple rupture process. The azimuthal dependencies of the RSTF pulse amplitudes and widths are used to estimate rupture velocity and rupture direction for the mainshock. The azimuth of rup-ture direction is obtained using a global optimization method. We found that the rupture direction of the main event propagated toward S175 E with an averaged rupture velocity around 0.75 Vs. The result obtained for rupture direction is in agreement with one of the nodal planes of focal mechanism. From the rupture directivity analysis, focal mechanism and geological evidence it follows that the investi-gated event reflects a reactivation of a NW-SE Oligocene deep-seated normal faulting with sinistral movement. Source parameters were estimated using RSTFs of the mainshock, including seismic moment of 2.85×1015 Nm, fault radius of 344 m, fault length 1460 m, and static stress drop of 3.071 MPa.

4

Seismic doublets and multiplets at Polish coal and copper mines

75%

Gibowicz S.

Acta Geophysica

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2006

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tom Vol. 54, no. 2

142-157

EN

The following criteria for selection of doublets at Polish coal mines were accepted: the difference in magnitude (based on seismic moment) of two events not larger than 0.15, the distance between their hypocenters not greater than 150 m, and the time interval between their occurrence not longer than 10 days. Similarly, the criteria for seismic events at copper mines are: the difference in magnitude not exceeding 0.15, the distance not greater than 200 m, and the time interval not longer than 20 days. Seismic events from the Wujek and Ziemowit coal mines that oc-curred between 1993 and 1995, and seismic events from the Polkowice copper mine that occurred between 1994 and 1996 and from the Rudna copper mine that occurred between 1994 and 2004 were considered. Their source parameters and focal mechanisms were known in most cases from previous studies. Altogether 108 seismic pairs from coal mines and 118 pairs from copper mines were found, forming doublets, triplets and quadruplets, within the magnitude range from 0.7 to 3.5. The distance and time intervals between two events forming pairs are not dependent on magnitude of these events. The focal mechanism of seismic events forming pairs is similar in over 60 percent of pairs at coal mines and in about one third of pairs at copper mines. Spa-tial distributions of doublets in particular sections of coal and copper mines display dominant linear trends, characteristic for a given area, which are often in conformity with the direction of nodal planes determined by fault plane solution of one or both the events forming a doublet. In such cases, the rupture plane can be discriminated among the nodal planes.

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Comparison of source parameters estimated in the frequency and time domains for seismic events at the Rudna copper mine, Poland

75%

Domański B. , Gibowicz S. J.

Acta Geophysica

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2008

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tom Vol. 56, no. 2

324-343

EN

Source parameters estimated in the frequency domain for 100 selected seismic events from the Rudna copper mine, with moment magnitude ranging from 1.4 to 3.6, were collected to study their scaling relations and to compare them with the parameters estimated in the time domain. The apparent stress and static stress drop, corrected for the limited bandwidth recording, increase slightly in a similar manner with increasing seismic moment. The ratio of apparent stress to static stress drop, a measure of radiation efficiency, is practically constant and its mean value is close to 0.1. For 37 seismic events, with moment magnitude between 1.9 and 3.4, source parameters were estimated in the time domain from relative source time functions, that displayed unilateral rupture propagation, and their rupture velocity could be estimated. It ranges from 0.23 to 0.80 of shear wave velocity and is almost independent of seismic moment. The fault length, estimated from the average source pulse width and rupture velocity, is clearly dependent on seismic moment and is smaller than the source radius estimated from the corner frequency on the average by about 25 percent. There is no correlation between the values of static stress drop estimated in the frequency and time domains, but the time domain stress drop is in general similar to that estimated in the frequency domain. The apparent stress increases with increasing rupture velocity, and the ratio of apparent stress to static stress drop seems also to depend on rupture velocity.

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The 2021-2022 Mw 6.0 Bejaia Bay, NE Algeria, earthquake sequence: tectonic implications at the Algerian margin between lesser and greater Kabylian blocks

63%

Yelles-Chaouche A. , Abacha I. , Boulahia O. , Beldjoudi H. , Aidi C. , Bendjama H. , Tikhamarine E.-M. , Mohammedi Y. , Chami A. , Chimouni R. , Kherroubi A.

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tom Vol. 72, no. 2

529--551

EN

On March 18, 2021, at 00:04 UTC, a strong earthquake (Mw 6.0) hit Bejaia city, 200 km east of Algiers. Its epicenter was 15 km northeast of Cap Carbon in Bejaia Bay, making it the largest earthquake recorded offshore since the devastating earthquake (Mw 6.8) in Boumerdes on May 21, 2003. The earthquake had a maximum intensity of VII (EMS 98), triggering hundreds of aftershocks and damaging 2000 houses and social infrastructures, as well as causing several rock falls along the rocky coastline, but no human casualties were reported. The waveform inversion and spectral analysis of the mainshock and its largest aftershocks indicate an alignment along an E-W thrust fault plane offshore, dipping southward, the mainshock seismic moment of M0 = 9.7e + 17 N.m. corresponding to a magnitude Mw = 6.0. The aftershocks illuminated a surface 22 km long (N-S) and 12 km wide; their statistical parameters were assessed by Guttenberg-Richter relationship, Omori decay, and temporal clustering. The b-value is estimated at 0.83, the p value at 0.95, and the n-value at 0.75 (i.e., 75% triggered events), which follows classical patterns of aftershock sequences and suggests the sequence tectonic genesis. Furthermore, previous studies showed that the epicentral area was positively charged by coseismic static stresses transferred from recent events in the Bejaia-Jijel margin. Exactly one year later, on March 19, 2022, a moderate earthquake struck the same epicentral zone, 3 km NW of the first shock, generating a moment M0 = 8.5e + 16 N.m. corresponding to a magnitude Mw = 5.3; its focal mechanism also revealed an E-W striking reverse fault with a small strike-slip component. The present-day local stress field is characterized by a contractional tectonic regime (R ‘ = 2.99 ± 0.24) and d orientation (N345°E) consistent with the maximum regional compressive stress direction (NNW-SSE). The 2021-2022 Bejaia Bay seismic sequence underlined the active tectonics linking the major E-W offshore thrust fault system and the NW-SE strike-slip Babors Transverse Fault system. This sequence, along with a number of other earthquakes that occurred around the Lesser Kabylia Block (LKB) over the last decade, likely highlights the ongoing incipient subduction process between Africa and Eurasia along the northern Algerian margin.

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Determination of characteristics of the microearthquakes occurring in Lesser and Higher Himalaya India

63%

Goswami N. , Gupta S. C.

Acta Geophysica

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2022

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tom Vol. 70, no 1

1--13

EN

The present study aims to estimate the frequency-dependent attenuation relations and source parameters of local earthquakes that occurred in the Garhwal Lesser and the Higher Himalaya around Uttarkashi town, India. The local earthquake data of 234 events recorded from a 12-station digital telemetered local seismological network in the region during 2008 to 2017 have been used for this purpose. The first data set of 126 events occurred around Uttarkashi in the Lesser Himalaya, the second data set of 73 events located northwest of Uttarkashi in the Higher Himalaya and the third data set of 35 events located about 35 km east of Uttarkashi in the vicinity of 1991 Uttarkashi earthquake (Ms~6.8) in the Higher Himalaya. The source parameters such as: seismic moment (Mo), source radius (r) and stress drop (∆σ), have been estimated from the displacement spectrum of the SH component of S-wave by applying Brune’s circular source model (Brune in J Geophys Res 75: 4997 5009, 1970). The attenuation characteristics are examined by estimating the quality factor (Qc) from the decay of coda waves of vertical component record using the Single Backscattering model (Aki and Chouet in J Geophys Res 80: 23 3322 3342, 1975). The Qc estimated at different values of frequency (1.5 to 24 Hz) have shown that Qc a function of frequency. Here, we obtain the following frequency-dependent attenuation relationships: Qc=110 f 0.99 (using first and second data sets) and Qc=142 f 0.87 (using third data set). The source parameters of 234 events show that more than 96% of events (227 events) have a low stress drop (less than 10 bars), five events have a stress drop between 10 and 100 bars and two events have exceptionally high stress drop of 271 and 532 bars. This shows that the region produces, by and large, low stress drop events. The variation in stress drops with depth demonstrates no clear increase or decrease in stress drop with focal depth. The Mo-fc scaling relations have been obtained for events with magnitudes two and above, Mo=2.7× 1016 fc −3.98 (for data set 1), Mo=1.4× 1014 fc −2.74 (for data set 2), and Mo=1.1× 1015 fc −3.28 (for data set 3).

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Source parameters of seismic events potentially associated with damage in block 33/34 of the Kiirunavaara mine (Sweden)

63%

Nordström E. , Dineva S. , Nordlund E.

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tom Vol. 65, no. 6

1229--1242

EN

Forty-six mining-induced seismic events with moment magnitude between −1.2 and 2.1 that possibly caused damage were studied. The events occurred between 2008 and 2013 at mining level 850–1350 m in the Kiirunavaara Mine (Sweden). Hypocenter locations were refined using from 6 to 130 sensors at distances of up to 1400 m. The source parameters of the events were re-estimated using spectral analysis with a standard Brune model (slope −2). The radiated energy for the studied events varied from 4.7 × 10 −1 to 3.8 × 10 7 J, the source radii from 4 to 110 m, the apparent stress from 6.2 × 10² to 1.1 × 10 6 Pa, energy ratio (Es/Ep) from 1.2 to 126, and apparent volume from 1.8 × 10³ to 1.1 × 10 7 m³. 90% of the events were located in the footwall, close to the ore contact. The events were classified as shear/fault slip (FS) or nonshear (NS) based on the Es/Ep ratio (>10 or <10). Out of 46 events 15 events were classified as NS located almost in the whole range between 840 and 1360 m, including many events below the production. The rest 31 FS events were concentrated mostly around the production levels and slightly below them. The relationships between some source parameters and seismic moment/moment magnitude showed dependence on the type of the source mechanism. The energy and the apparent stress were found to be three times larger for FS events than for NS events.