PROBLEMATIC NONSHEAR MECHANISM OF MODERATE EARTHQUAKES IN WESTERN GREECE

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PROBLEMATIC NON-SHEAR MECHANISM OF MODERATE
EARTHQUAKES IN WESTERN GREECE

• J. Zahradník, E. Sokos
• Charles University Prague
• National Observatory of Athens

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Data source broad-band Le-3D/20sec waveforms
from the National Observatory of Athens
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Motivation W. Greece Mgt5 events DC lt
60sometimes reported by major agencies

• collision
• transform f.
• subduction

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Vartholomio (near Zakynthos)Dec. 2,
2002ETH-SED Mw5.7DC55 !(HRV
DC58,Mednet DC44)
Zakynthos
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Questions to be answered

• Can the data be explained with DC100 ?
• If we accept an explanation with a non-DC
  component, is it so large as reported by major
  agencies ?
• Can we estimate the uncertainty of the DC?
• Is there any relation between the DC error bars
  and the quality of the match between data and
  synthetics?
• Can the multiple source explain the non-DC ?

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Method

• moment-tensor inversion (minimization of the L2
  waveform misfit by the weighted least-square
  method)
• optimization of the source position and time
  (maximization of the correlation by the
  space-time grid search)
• a single point source for f lt 0.1 Hz, and
  multiple point sources for f lt 0.3 Hz

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6 NOA stations, f0.05 to 0.1 Hz
blue data black synthetics for crustal model of
Haslinger et al. (1999)
weights proportional to 1/A were applied
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Are you surprised by the quality of the match
although we work only with T lt 20 ?

• A trick After obtaining a preliminary solution,
  we refine it using
• artificially aligned waveforms
• (i.e. a station and component-dependent shift by
  a few seconds) !
• Justification a synthetic test.

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Forward modeling uncertain locationand crustal
model justifies the waveformalignment
(artificial shift by a few seconds)
epicenter shift 5 km to N or E

epicenter shift 3 crustal models
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The waveforms remain basically unchanged. The
shifts are needed mainly because of the uncertain
crustal structure.
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Resolving depth and fault-plane solution
very stable strike, dip, rake for all this depth
range, much less stable DC
opt. depth 17 km Mo0.16e18 Nm Mw5.4
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100 DC matches data also well(only 0.05 worse)
we cannot see the difference
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Going into large details Optimum correlation
is not compatible with 100 DC
trial source position
trial time shift
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Forcing DC to be gt 90 decreases correlation.
What is an acceptable range ?
top curves DC bottom curves
correlation
thick curves refer to the optimum depth
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The optimum correlation indicates DC 80 to 90
black all tested depths and all time
shifts blue optimal depth, all time shifts
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Uncertainty (6 data subsets by repeatedly
removing one station)
here we have only the optimum solutions for each
data subset
we need more details ...
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Uncertainty (6 data subsets by repeatedly
removing one station)
red error bars (a relative measure) /- one
sigma taken (formally) as acceptable
solutions however, we need them in 2D,
i.e. for a range of depths and shifts
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2D Acceptable solutions are retrieved for trial
source positions and time shifts in a range
around the optimum correlation
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2D acceptable solutions and their distribution
reveal the DC uncertainty
green DC-percentage (with red error
bars) blue correlation
For optimum depth of 17 km we get DC 77 to 95
However, 17 km is not strongly preferred
... so DC 72 to 97
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Compare the uncertainDC percentage with very
stablestrike-dip-rake
DC 72 to 97
black nodal lines all 2D acceptable
solutions (a range of depths and time shifts)
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Without artificial waveform alignment lower
DC, more uncertainty
DC 60 to 95 (65 to 90 at optimal depth)
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How the result may change in a different crustal
model ?
the artificial time shifts kept fixed as for
Haslinger et al. model, i.e. not optimal for
Novotny et al. model!
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How the result may change in a different crustal
model ?
the artificial time shifts kept fixed as in the
previous model !
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The uncertainty of DC is still 72 to 97, but
the depth is problematic
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In any case, for this event we systematically
find DC gt 70 (i.e. higher than reported for
this earthquake by major agencies).

• Can we explain the DC by means of a multiple
  source ?

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Fixing the opt. source position and increasing
frequency (f lt 0.3 Hz) 3 subevents
2-sec time delay between sub 1 and 2 sub 3 is
unstable
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Subevents 1 and 2 similar strike and dip, but
different rake
Consider sub 1 and 2 as 100 DC (but unequal !),
and sum up their moment tensors Result sub 1
2 provides DC 77 to 93, analogous to the
previous single-source study. Multiplicity
seems to explain the non-DC mechanism.
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We have to understand the space-time complexity
of the source

• We searched multiple point sources in both
  possible fault-planes 1 and 2, passing through
  the epicenter,
• but the results were not good.

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Innovation 3D (volume) point source optimization

• We allow the single-source position to vary not
  only in depth, but also in a horizontal plane.
• We do not find an optimized hypocenter,
• but a major slip patch!

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The new trial fault plane is given by the patch
(point 3) and the known strike(303o)
old epicenter
patch
5 trial source positions at each depth (16,17,
and 18 km)
the old epicenter now appears slightly off the
plane (location error)
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the subevent time separation is stable (2 2
sec), and the focal mechanism as well, incl.
sub 3
2 2 sec
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The optimized fault plane stabilized the
solution a lot.
RLS
VLS
Sub 1,2,3 are not separated more than 2 km
from each other, but the delay is 22 seconds.
small distance, large delay. Rupture propag.
with arrest ? A multiple event !
ITM
removed station
EVR
JAN
KEK
trial vertical plane 8 x 2 km
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Are the 3 concentrated subevents in agreement
with empirical relations (Somerville et al.,
1999) ?

• Mo 0.16e18 Nm implies the rupture area A 30
  km2, and the (total) area of asperities Aa 7km2
  (e.g. 2.5 x 2.5 km), similar to our result.

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Conclusions

• The data can be explained with DC100.
• Statistically, the DC is 70 to 95, much larger
  than reported by major agencies.
• It cannot be excluded that (for this event) a
  better crustal model can still increase DC.
• Vartholomio earthquake consisted of 3 nearby, but
  enough delayed subevents (we have 3
  earthquakes, not 1 with 3 patches).
• Multiplicity provides a partial explanation of
  the non-DC mechanism.

http//seis30.karlov.mff.cuni.cz
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http//seis30.karlov.mff.cuni.cz
Thank you !
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Lefkada Mw6.2August 14,2003faultsand
stations
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Final solution 2-4 major subeventsexplain
the two aftershock clusters
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DC or non-DC ?
The tensorial sum sub 1 sub 2, (both 100
DC) imitates the HRV non-DC solution ! (J.
Sileny)
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AmfilochiaDecember 31, 2002ETH-SEDMw5.0DC3
0
!
This is one of the three Mgt4.5 events that
occurred within 1 hour
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NOA network (N. Melis)