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FOCAL MECHANISM AND SOURCE DIRECTIVITY FOR VRANCEA INTERMEDIATE DEPTH EARTHQUAKES
Abstract
The focal mechanism and rupture directivity for intermediate-depth earthquakes occurred in the Vrancea source (Romania) are determined using FMNEAR algorithm [1]. The method is based on the waveform inversion of near-source seismic waveforms and on a linear finite source model. When the FMNEAR algorithm is started the first parameters that are determined are the moment magnitude (Mw), the double couple focal mechanism (strike, dip, rake) and the distribution of seismic moment along strike. This provides a first order estimate of rupture length and directivity. The depth of the source is also explored. A band-pass filter is automatically applied for each individual component of the recorded waveform (N, E, Z). The inversion is made in successive steps combining grid searches for (strike, dip, rake) parameters and simulation to determine rupture onset times and the shape of the local source time functions. Also a confidence index is defined. The method was tested on seismic events with magnitude higher than Mw = 3.5 recorded by the Romanian seismic network (broadband stations and accelerometers). Tests were made on events recorded by a large number of welldistributed stations and also for less favorable cases to evaluate the capability of the algorithm to constrain focal mechanism and source time function. In its present configuration, FMNEAR method is implemented in near real-time.
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References5
Delouis, Bertrand. "FMNEAR: Determination of focal mechanism and first estimate of rupture directivity using near-source records and a linear distribution of point sources." Bulletin of the Seismological Society of America (2014).
Oncescu, Mihnea-Corneliu, and Klaus-Peter Bonjer. "A note on the depth recurrence and strain release of large Vrancea earthquakes." Tectonophysics 272.2-4 (1997): 291- 302.
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Cristian Neagoe, Liviu Marius Manea, Constantin Ionescu, Romanian Complex Data Center for Dense Seismic network, Annals of Geophysics, 54, 1, 2011; DOI: 10.4401/ag-4809
Delouis, B., J. Charlety, and M. Vallée (2009a). A method for rapid determination of moment magnitude Mw for moderate to large earthquakes from the near-field spectra of strongmotion records (MWSYNTH), Bull. Seismol., DOI: 10.1785/0120080234.
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