FMNEAR: Determination of Focal Mechanism and First Estimate of Rupture Directivity Using Near-Source Records and a Linear Distribution of Point Sources,
Résumé
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 (
M
w
), the double-couple focal
mechanism (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 is
also explored. An advantage of the approach is the use of the finite-source model
directly within the search of the focal mechanism. This strategy allows the inclusion
of strong-motion records at short distances for moderate to very large earthquakes
(
M
w
>
5
:
5
). However, for moderate to small earthquakes (
M
w
<
5
:
5
), the source is
simplified to a single point source. A specific band-pass filter is automatically adapted
for each individual component of the seismograms (north, east, vertical).
The linear finite source is represented by point sources aligned along strike, each point
source being characterized by a local source time function (
STF
). The inversion is
carried out in successive steps combining fast grid searches on the (strike, dip, and
rake) parameters and simulated annealing to determine rupture onset times and the
shape of the local
STF
s. The nonlinear grid search offers the possibility of assessing
the degree of uniqueness of thesolution, and a confidence indexisdefined. The method
was tested on 11 earthquakes worldwide, ranging from
M
w
4.5 to 9.0. Test cases in-
corporatecontinentaleventsrecordedbyalargenumberofwell-distributedstationsand
less favorable cases. In its present form, the FMNEAR approach is very well adapted to
continental earthquakes surrounded by seismic stations, but it can also give informative
results in more difficult configurations. The FMNEAR method is now ready for fully
automated determinations and is already implemented in near real time.