SOURCE TIME FUNCTIONS OF SEISMIC EVENTS INDUCED AT A COPPER MINE IN POLAND: EMPIRICAL GREEN'S FUNCTION APPROACH IN THE FREQUENCY AND TIME DOMAINS

摘要

Empirical Green's function deconvolution technique was applied to retrieve the source time functions from the records of P waves from 17 seismic events that occurred in 1996, 1998 and 1999 at Rudna copper mine in Poland. The events were located within the underground seismic network composed of tens of 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 2.1 to 3.3. The records of smaller events from the same area and with similar source mechanism (based on moment tensor inversion), with moment magnitude ranging from 1.5 to 2.4, were accepted as empirical Green's functions. The relative source time functions were retrieved by the classical deconvo-lution in the frequency domain and by the projected Landweber deconvolution in the time domain. The Landweber approach is an iterative technique allowing introduction of physical constraints (positivity, causality, finite duration) on the final source time function. The relative source time functions retrieved at several stations by both methods are quite similar, although the Landweber deconvolution provides more objective determination of their duration. The relative source time functions display directivity effects in 15 cases, implying unilateral rupture propagation in the source. The direction of the rupture propagation and the rupture velocity were estimated from the distribution of pulse widths and pulse maximum amplitudes as a function of station azimuths. The rupture velocity is variable, ranging from 0.3 to 0.9 of the shear wave velocity. The source pulse widths is considerably smaller than the reciprocals of the corresponding corner frequencies. Consequently, the source dimensions, represented by the fault length, are smaller than those estimated from seismic spectra, ranging from 75 to 550 m.