Modelling of multiple events using empirical Green's functions: method, application to swarm earthquakes and implications for their rupture propagation

Details Modelling of multiple events using empirical Green's functions: method, application to swarm earthquakes and implications for their rupture propagation Modelling of multiple events using empirical Green's functions: method, application to swarm earthquakes and implications for their rupture propagation

Dec 2005

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005geoji.163..991f&link_type=abstract

Geophysical Journal International, Volume 163, Issue 3, pp. 991-1005.

Astronomy and Astrophysics

Astronomy

4

Earthquake Location, Inverse Problem, Microearthquakes, Rupture Propagation, Synthetic Seismograms, Waveform Analysis

Scientific paper

Recent studies of source-time functions (STFs) of small earthquakes have shown that some of the ML < 3 events may display complicated waveforms indicating multiple rupturing episodes. The STFs of such earthquakes consist of several pulses whose relative positions provide information on the mutual position of the subevents. I have used the waveform modelling method to analyse multiple events in order to disclose the geometry of the rupture. The P and S waveforms of multiple events (MEs) are modelled as the sum of waveforms of single subevents with different hypocentre coordinates and scalar moments. To construct the waveform of each single event composing the ME, the waveform of a co-located small event is used as an empirical Green's function (EGF). Assuming similar focal mechanisms of the subevents and of the EGF, the method seeks the coordinates and origin times of the subevents and their relative seismic moments. The non-linear problem is solved using the genetic algorithms method. Synthetic tests have shown that the method is capable of locating reliably up to three subevents with an accuracy better than 40 m. The method was applied to the records of the 2000 earthquake swarm in NW-Bohemia/Vogtland in Central Europe. By the EGF deconvolution, 54 MEs were identified in the magnitude range from 1.2 to 3.3, and 18 of them were successfully modelled as double or triple events with separate rupture positions. The separation of subsources reached 100 ms in time and 320 m in space. The relative positions of the subevents with respect to the orientation of the fault indicate that most of them occurred on a common fault plane. The space-time separation of the subevents corresponds to a speed of 3.0 +/- 0.9 km s-1, a value typical for rupture propagation of large earthquakes. The later subevents occur farther than the nominal rupture radius of the first subevent, and their mutual distance scales with magnitude. These observations suggest that the analysed MEs share a common fault surface and that their subevents represent individual rupture episodes. The angular distribution of the position vectors of later subevents indicates that many of them result from slip-parallel rupture growth, while some of the ruptures propagate upwards.

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