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MOMENT TENSOR INVERSION OF POSSIBLY MULTIPLE
EVENTS AT REGIONAL DISTANCES Petra Adamová1, Jirí
Zahradník1, George Stavrakakis2 1Charles
University in Prague 2National Observatory of
Athens, Institute of Geodynamics email
adamova_at_karel.troja.mff.cuni.cz
Abstract Moment tensor inversion of possibly
multiple events is studied. All experiments are
performed with new code ISOLA, based on
time-domain iterative deconvolution of multiple
point-source subevents. It is extension of
Kikuchi and Kanamori (1991) for complete waveform
inversion of regional data (Zahradník et al.,
2005). The inversion is tested and applied to two
selected M5 events from western Greece near the
town Amfilochia (December 31, 2002) and near the
town Vartholomio (December 2, 2002). High
non-shear CLVD component was reported by major
agencies for them. Both these earthquakes have
been interpreted as double events. Methodical
lessons emerging from the applications are
formulated.

• Steps of inversion
• Point source first approximation of the
  centroid depth (fig. (a))
• Point source optimization of the horizontal
  position of the centroid C in the depth found in
  step 1 (fig. (b))
  
  
  
• Multiple point source plane passing through the
  centroid C of the step 2, using strike and dip
  found in step 2 (fig. (c))

• Method ISOLA
• Iterative deconvolution of multiple point source
  based on Kikuchi and Kanamori method (1991) our
  modification for regional and local data, ISOLA
  Fortran code (Zahradnik et al., 2005)
• Least-square inversion of waveforms to retrieve
  moment tensor
• Optimization of waveform fit by grid search over
  trial source positions in space and time

• Data
• stations from the network of NOA (National
  Observatory of Athens, IG) Lennartz Le-3D/20 sec
• 1 station (SER) jointly operated by the Charles
  University Prague and University of Patras
  Guralp CMG-3T/100 sec

• Application Vartholomio earthquake, December 2,
  2002 Mw5.5
• step 1 point solution (f 0.04 0.08 Hz)
  Optimum depth 17 km
• step 2 centroid in horizontal plane 3
  km to the East and 3 km to the North from the NOA
  epicenter
• step 3 split into subevents not successful

red stars epicenters of Amfilochia and
Vartholomio earthquakes black triangles stations
used for Amfilochia and/or Vartholomio earthquake
Correlation and focal mechanism as a function of
the depth.

• Application Amfilochia earthquake, December 31,
  2002 Mw 4.5
• step 1 point solution (f 0.04-0.08 Hz)
  Optimum depth 17km
• step 2 centroid in horizontal plane (f
  0.04-0.08 Hz) 3 km to the West and 3
  km to the South from the SED epicenter
• step 3 multiple point source (f 0.04 0.3
  Hz) 2 subevents

17 km
Correlation and focal mechanism as a function of
the depth.
displacement blue observed red synthetic
without centroid shift black synthetic with
centroid shift
17 km
displacement blue observed red synthetic with
centroid shift black synthetic without centroid
shift

• Discussion
• Delicate instrument. Needs a lot of care in
  physical justification of subevents.
• Source multiplicity may explain apparently high
  non-DC component.
• Joint interpretation of the centroid moment
  tensor and hypocenter position may help to
  identify the fault plane.

References Kikuchi, M. and H. Kanamori (1991).
Inversion of complex body waves-III, Bull. Seis.
Soc. Am. 81, 6, 2335-2350. Zahradník et al.
(2005). Iterative Deconvolution of Regional
Waveforms and a Double-Event Interpretation of
the 2003 Lefkada Earthquake, Greece, Bull. Seis.
Soc. Am. 95, 1, 159-172. Adamová, P. (2006).
Earthquake source process and its complexity,
Master thesis. Charles University in Prague.
Free mechanism
Fixed mechanism
Multiple point solution, red points surface
projection of the trial source positions.
(Second nodal plane appears less likely candidate
for the fault plane.)
Acknowledgements This work was supported by
projects 004043(GOCE)-3HAZ-CORINTH and GAUK
279/2006/B-GEO/MFF. We thank E. Sokos for ISOLA
Matlab GUI.
blue observed displacement black synthetic
displacement