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1

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

Abdel-Fattah A.K., Badawy A.

Acta Geophysica Polonica

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2004

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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.

2

The accuracy of source parameters estimated from the source time function of seismic events at Rudna copper mine in Poland

Domański B., Gibowicz S.

Acta Geophysica Polonica

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2003

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Vol. 51, nr 4

347-367

EN

The source time function of43 seismic events from Rudna copper mine was retrieved using empirical Greem's function deconvolution technique in the frequency and time domains. Thirty four events were studied preciously and nine new events were added, with moment magnitude raging from 2.1 to 3.6. the records of smaller events from the same area, with moment magnitude ranging from 1.5 to 2.7, were accepted as Grees's functions. Bith methods, the spectral division and the projected Landweber deconvolution, provided consistent results, but from an error analysis it follows that the classec spectral division solutions seem to be more reliable than the Lndweber solution. The relative source time functions retrieved from the records of a number of seismic stations (from 10 to 42) by the spectral division for 32 events and by the Landweber deconvolution for 36 events display directivity effects, implying unilaterally propagating ruptures. The rupture propagation direction and rupture velocity were estimated from the distribution of pulse widths and pulse maximum amplitudes as a function of station azimuths. The rupture velocity ranges from 0.25 to 0.9 of the shear wave velocity. Its values can be divided into two distinct sets: low velocity values in comparison with those from natural earthquakes, between 0.25 and 0.6 of shear wave velocity, and high velocity values greater than 0.6 of the shear wave velocity. The rupture velocity depends also to some extent on the rupture direction.

3

Source time function of seismic events at Polish coal mines derived by empirical Green's function approach

Gibowicz S., Domański B., Nita B., Wiejacz P.

Acta Geophysica Polonica

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2003

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Vol. 51, nr 1

1-22

EN

The empirical Gren's function deconvolution techniques in the frequency and time domains were applied to retrieve the source time functions from the records of P waves of 25 seismic events that occurred in 1994 and 1995 at Wujek coal mine and of 5 events that occurred in 1994 at Ziemowit coal mine. The selected events were located within the underground seismic networks composed at Wujek mine 14 vertical sensors, situated at a depth between 300 and 740 m and within the mining area of about 8 km2. the network at Ziemowit mine was composed of 16 vertical seismometers located at a depth between 430 and 620 m in the southern part of the mine. Moment magnitude of selected events ranged from 1.1 to 2.2. the records of several smaller events from the same area and with the similar source mechanism, with moment magnitude ranging from 0.3 to 1.7 , were accepted as empirical Green's functions. Both applied methods, the spectral division and the projected Landweber deconvolution, provided consistent and stable results. The relative source time functions of 6 events at Wujek mine and of 3 events at Ziemowit mine indicate that the rupture source propagated unilaterally, either along or perpendicularly to the longwall extension where the events were originated. The rupture velocity ranged from 0.4 to 0.8 of the S-wave velocity, which is distinctly lower than its typical value reported from natural earthquakes.

4

An application of the pseudo-spectral technique to retrieving source time function

Dębski W., Domański B.

Acta Geophysica Polonica

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2002

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Vol. 50, nr 2

207-221

EN

In this paper we discuss an application of pseudo-spectral approach to retrieving source time functions in a framework of the empirical Green's function technique. The method consists in a decomposition of the source time function into a set of suitably chosen base functions with decomposition coefficients estimated by a genetic algorithm based optimizer. The method, being essentially nonlinear, is compared with another nonlinear source time function deconvolution technique, namely the projected Landweber technique, by app1ying both algorithm to a Lubin copper mine seismic event. The pseudo-spectral method performs slightly better than the projected Landweber approach in the considered case, leading to smoother solutions.

5

Comparison of source time functions retrieved in the frequency and time domains for seismic events in a copper mine in Poland

Domański B., Gibowicz S.

Acta Geophysica Polonica

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2001

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Vol. 49, nr 2

169-188

EN

The extraction of source time functions based on empirical Green's functions is especially convenient for source studies of seismic events in mines, where underground seismic networks are situated in the source area and are often composed of a large number of sensors. The empirical Green's function deconvolution techniques were applied to retrieve the source time functions from the records of P waves of several seismic events that occurred in 1998 at Rudna copper mine in Poland. The selected events were located within the underground network composed of 32 vertical sensors situated at a depth of about 1 km; the size of the network is about 10 by 10 km. Their moment magnitude ranged from 2.7 to 3.2. The records of smaller events from the same area and with similar source mechanism, with moment magnitude from 2.2 to 2.4, were accepted as empirical Green's functions. The results of classic deconvolution approach in the frequency domain are reported elsewhere. The results of the application of projected Landweber deconvolution in the time domain to the same set of data are described here. The projected Landweber approach is an iterative deconvolution technique, allowing introduction of physical constrains on the final source time function. This technique successfully overcomes the instability effects of the deconvolution process inherent in the frequency domain and provides stable and reliable relative source time functions retrieved at several stations. Although the results obtained in the frequency and time domains were found to be similar, the time domain approach provides more objective determination of the source time function duration, essential for a proper determination of the source dimension. The relative source time functions retrieved by both methods display directivity effects in several cases, implying unilateral rupture propagation. The rupture propagation direction and rupture velocity were estimated from the distribution of pulse widths and pulse maximum amplitudes as a function of the station azimuths.

6

Focal mechanism and source time function of selected aftershocks of the Egion 1995 earthquake, Gulf of Corinth, Greece

Gibowicz S., Domański B., Wiejacz P.

Acta Geophysica Polonica

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1999

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Vol. 47, nr 1

1-25

EN

Numerous aftershocks, which followed the 1995 Egion earthquake of magnitude Ms = 6.2, were recorded by the seismic network PATNET of the University of Patras and by a temporary local network EGINET. The well recorded aftershocks were located by the Patras Seismological Centre, University of Patras. The complete data set, received from the Centre, contains the records of over a hundred events. The spatial distribution of their epicentres forms an E-W elongated cloud with a central cluster and a few stray-away epicentres west and north-west from the cloud. The events from the central cluster, which is the source area of the main shock, that occurred during the first decade of August 1995, were selected for the analysis. Additionally, a few events from the small cluster located west of the main one were also analysed. Altogether, it was possible to perform the moment tensor inversion in the time domain for 29 aftershocks from the main cluster and for 4 events from the small cluster. The selected aftershocks have moment magnitude ranging from 1.7 to 3.2. The source time function of the larger events was determined by the empirical Green's function technique. The records of several pairs of large-small aftershocks, close to each other and with similar focal mechanism, were selected for deconvolution. The source time function was recovered for 5 larger aftershocks from the central cluster and for 3 events from the small cluster. The small number of available stations used for the moment tensor inversion limit stable results to the constrained double couple solutions. The solutions show a mixture of various types of the focal mechanism, though dip-slip events prevail over strike-slip events, and normal faulting prevails over reverse faulting. The pulse widths of relative source time functions depend on the station azimuth, indicating that the ruptures propagated unilaterally. The azimuth of rupture direction ranges from about 90 to 250 degrees with the dominant direction of about 200 degrees. The fracture velocity ranges from 0.4 to 0.8 of the shear wave velocity. The obtained results indicate complexity of the stress distribution in the source area of the Egion earthquake.