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

Domański B., Gibowicz S. J.

Acta Geophysica

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2008

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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 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 approach to investigate earthquake rupture processes

Abdel-Fattah A.

Acta Geophysica Polonica

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2003

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Vol. 51, nr 3

257-269

EN

A linear least-squares inversion technique was developed to estimate earthquake rupture processes by considering 1D line source model propagating unilaterally. For the propesed model, the azimuthal variations of Source Time Functions (STF's) depend on the rupture direction, the average rupture velocity and rupture dimension. These parameters represent the unknown parameters of the proposed model that are estimated by performing a simulationeous linear least-squares inversion approach. The azimuth of rupture propagation direction and the rupture velocity are estimation from the distribution of the STF pulse widths or from their maximum amplitudes as a function of the station azimuths. In order to confirm the applicability of this approach, the model is tested with a series of synthetic data. Beside this test, the approach is also applied to several earthquakes ruptured unilaterally. The results of inversion strongly suggest that small earthquakes occur with varying rupture velocity similar to large earthquakes.

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.