FMNEAR: Determination of Focal Mechanism and First Estimate of Rupture Directivity Using Near-Source Records and a Linear Distribution of Point Sources,

Description

The FMNEAR method is based on both the waveform inversion of near-source seismic records and on a linear finite-source model. The primary source param-eters that are determined are the moment magnitude (Mw), the double-couple focalmechanism (strike, dip, and rake), and the distribution of seismic moment along strike,which provides a first-order estimate of rupture length and directivity. Source depth isalso explored. An advantage of the approach is the use of the finite-source modeldirectly within the search of the focal mechanism. This strategy allows the inclusionof strong-motion records at short distances for moderate to very large earthquakes(Mw>5:5). However, for moderate to small earthquakes (Mw<5:5), the source issimplified to a single point source. A specific band-pass filter is automatically adaptedfor each individual component of the seismograms (north, east, vertical).The linear finite source is represented by point sources aligned along strike, each pointsource being characterized by a local source time function (STF). The inversion iscarried out in successive steps combining fast grid searches on the (strike, dip, andrake) parameters and simulated annealing to determine rupture onset times and theshape of the localSTFs. The nonlinear grid search offers the possibility of assessingthe degree of uniqueness of thesolution, and a confidence indexisdefined. The methodwas tested on 11 earthquakes worldwide, ranging fromMw4.5 to 9.0. Test cases in-corporatecontinentaleventsrecordedbyalargenumberofwell-distributedstationsandless favorable cases. In its present form, the FMNEAR approach is very well adapted tocontinental earthquakes surrounded by seismic stations, but it can also give informativeresults in more difficult configurations. The FMNEAR method is now ready for fullyautomated determinations and is already implemented in near real time.

Abstract

International audience

Additional details