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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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Earthquake doublets and multiplets in the Fiji-Tonga-Kermadec region

Gibowicz S. J., Lasocki S.

Acta Geophysica Polonica

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2005

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Vol. 53, nr 3

239-274

EN

We investigated all the pairs of shallow, intermediate and deep earthquakes in the Harvard CMT catalogue that occurred between 1977 and July 2003 at a centroid distance of less than 40, 60 or 90 km and within a time interval of 200, 300 or 450 days for small, medium and large events, respectively. For the Fiji-Tonga-Kermadec area the Harvard catalogue lists 1022 shallow, 410 intermediate and 633 deep earthquakes of moment magnitude from 4.9 to 8.0. The magnitude threshold, above which the catalogue is complete, is 5.3-5.4, and the number of earthquakes of magnitude above this value is 691 for shallow, 329 for intermediate and 476 for deep events, respectively. The proportion of earthquakes, however, associated with doublets and multiplets against the total number of earthquakes is approximately the same in both data sets and therefore all earthquake pairs were considered regardless of their magnitude. We found 208 pairs of shallow, 31 of intermediate and 92 of deep events with moment magnitude from 5.0 to 7.2. To check whether these earthquakes in pairs are really coupled and not connected by chance, the possibility of their occurrence in an uncorrelated Poissonian catalogue was considered. It was assumed that in such a catalogue the inter-event time is exponentially distributed, the earthquake magnitude follows the Gutenberg -Richter relation, and the distribution of centroid distances between the events in pairs is controlled by its non-parametric kernel estimate. The probability of appearance of the observed proportion of doublets of shallow earthquakes in the Poisson-ian catalogue was found to be very low, 5.64×10-6, confirming the non-random character of observed pairs. The low probability of occurrence in a semi-randomcatalogue, created by randomising centroid locations in the actual dataset, indicates also high importance of the distance criterion used for a doublet specification. The time interval criterion is significantly less important, although the double events that occur shortly one after another are the least probable events to be uncorrelated. In general, shallow earthquakes tend to form the pairs at smaller distances and within shorter time intervals than deep earthquakes. Both the distance and the time intervals do not depend on magnitude of involved events. The largest number of pairs of deep earthquakes is observed at a depth of about 600 km, and only a few pairs are found at a depth of 350-400 km. The proportion of deep events associated with doublets and multiplets against the number of all events increases also with depth. From comparison of focal mechanism of earthquakes in pairs, measured by the 3D rotation angle, it follows that deep earthquakes forming pairs have more di-verse focal mechanism than shallow events. The rotation angle for three quarters of shallow earthquake pairs and only for about one third of deep earthquake pairs is smaller than 30 degrees.

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Professor Adam M. Dziewoński the 1998 Crafoord Prize winner

Gibowicz S. J.

Acta Geophysica Polonica

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1998

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Vol. 46, nr 2

229-231