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Cythera M6.7 earthquake (January 8, 2006) in southern Aegean: uneasy retrieval of the upward rupture propagationJ. Zahradnik, J. Jansky, V. Plicka, E. Sokos Charles University in Prague University of Patras
EMSC • Diverse centroid • position • Unclear aftershock • pattern • Unclear fault • Low DC%: • ETH 60%, Mednet 56%
Inconsistent hypocenter and centroid moment solution nodal planes, and centroid in the middle
Teleseismic recordsKikuchi-Kanamori method • pP: depth • P: complexity P pP
Bottom trace = synthetics (K & K) simple event and complex event
Regional records and EGF method apparent source time functions prove complexity ... Neighborhood Algorithm provides two slip patches (similar to M. Vallée) plane 2 strike ~70° plane 1 strike ~200°
Lower misfit identifies the fault plane: strike ~70°
Relocation • 30 teleseismic stations, pP-P: depth 90 km • 21 regional stations (Greece + Italy), P and S • Wadati diagram: Vp/Vs=1.75 • Optimization of RMS: Vp/Vs=1.75 • Relocation of regional data with first approximation of depth = 90 km and Vp/Vs=1.75 with various azimuthal and epic.distance weighting schemes
Free depth: This is uncertainty of mainshock location, not the aftershocks !
hypo1 EMSC this study We relocated hypocenter 15 km South, 10 km East and 25 km below EMSC.
ISOLA code (Fortran & Matlab) multiple point-source moment tensors Full waveform modeling of regional records Free on web:
Iterative deconvolution (Kikuchi and Kanamori) modified for regional recordsMoment tensor (deviatoric, or DC-constrained)at each trial space-time position by minimization of the L2 waveform misfit(least squares)Optimum space-time positionof subevents by maximization of the waveform correlation (grid search)
Free BB waveformdata(Internet) Our LTK station soon on Orfeus, too.
Hierarchic grid search of centroid f < 0.1 Hz EMSC EMSC epic. is just the coordinate origin search 1 search 2, etc.
Accurate centroid location needed for usable DC% search 1 search 2
Getting more accurate centroid makes DC% to converge search 2 search 3… DC% 10-30 only !
We found centroid25 km East of EMSC epicenterand the DC% has convergedto 10-30%.Does it imply that the source is actually strongly non-DC ?Not !
DC-constrained solution is an equivalent model deviatoric DC-constrained M EMSC and Mednet M Note different optimal source position.
Can we better justify our centroid position and MT ?Remember the inconsistency for Mednet centroid and EMSC hypocenter:
Our CMT is fully consistent with our relocation. Far from being trivial!5 … and it identifies the fault plane as the “red” nodal plane, strike ~ 80°
The BB first-motion polarities are consistent with the CMT solutiono5 Red: this study Black: others
Where’s complexity found in EGF analysis and teleseismic modeling ? For f < 0.1 Hz, in addition to stable subevent 1 (1.1e19 Nm) the waveforms clearly indicate subevent 2, 6-sec later, comparable size ! (1.1e19 Nm) Sub 1 ? Sub 2 ? M Solution for sub2 is not unique.
Seeking sub 2 in the fault plane of sub 1: DC-constrainedpo5 X 1 depth 60 km 2 X depth 85 km depth 72 km X…EMSC X…this study
A double-event interpretation: • Subevent 1: 1.10e19 Nm strike, dip, rake: (84, 64, 121)=(209 40 43) • Subevent 2(6 sec later): 0.87e19 Nm strike, dip, rake: (61, 86, 52)=(326, 38, 174) Depths Sub 1: 60 km Sub 2: 76 km Hyp.: 85 km EMSC 1 2 this study
Possible explanation of the apparently large non-DC: Summing up MT of these two 100% DC events provides a non-DC solution strike, dip, rake: (82, 70, 94) 1.6e19 Nm, DC%=57 near to long-period Mednet CMT strike, dip, rake: (81, 67, 139) 1.4e19 Nm, DC%=56 But Mednet centroid is too far… 1 2 M
Can we identify fault plane of subevent 2 ? 5 1 1 2 2 strike 61° strike 326° Nodal plane with strike 326° passes through the hypoc. !
Nodal plane with strike 326° passes through the hypoc. ! hypo5
hypo5 1 2 2 1 common hypoc. Depths Sub 1: 60 km Sub 2: 76 km Hyp.: 85 km Hypothesis: both patches (on different fault planes) nucleated close to the same point, and ruptured upward, sub 2 being delayed with respect to sub 1.
Another possibility:fixed DC focal mechanism (that of sub 1).It moves sub 2 close to sub 1. X 1 2 Depth 60 km depth 69 km X depth 72 km X…EMSC X…this study
Another possibility:fixed DC focal mechanism (that of sub 1)moves sub 2 close to sub 1 1 2 now we do not need the left segment depth 60 km depth 69 km … but how to explain low DC % and why the 6-sec delay ?
strike, dip, rake: 84° 64°, 121° (for both) Interpretation I: Fixed mechanism Varred= 52% 69 and 60 km x hypocenter depth 85 km (this study) 84°, 64°, 121° Interpretation II: DC-constrained Varred=64% !! 329°, 36°, 179° 60 km x 72 km 85 km
Methodical lesson and Cythera model • Relocation and CMT inversion in same model enabled identification of the fault plane (strike ~80°) of the main patch. • Hierarchic space-time grid search lead to convergence of the DC% to a low value. • 100% DC-constrained solution provided a double-event model and explained the low DC% as only apparent non-DC. • Rupture started at depth 85 km. Most stable slip patch was centered 35 km apart, at depth 60 km. • Second large patch was delayed by 6 sec. Position and mechanism not unique. Possibly on a different fault plane. http://geo.mff.cuni.cz
The most important essence is visions and dreams http://geo.mff.cuni.cz
„Blind“ experiment on slip inversion from synthetic data (EC projekt SPICE) Data = synthetics for a „secret“ slip distribution
New BB satellite network Patras Univ. green= Trillium and CMG = status May 21
ITSAK-GR 2006-2009 starts right these days = a new EC Marie Curie RTN accelerographs near Cythera available ! Ch. Papaioannou
The fault plane is needed for correct identification of the slip patches apparent source time functions from EGF method ... and slip patches by NA algorithm