PROBLEMATIC NON-SHEAR MECHANISM OF MODERATE
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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 Data source: broad-band Le-3D/20sec waveforms from the National Observatory of Athens Motivation: W. Greece M>5 events DC% < 60% sometimes reported by major agencies • collision • transform f. • subduction Vartholomio (near Zakynthos) Dec. 2, 2002 ETH-SED: Mw=5.7 DC%=55 ! (HRV: DC%=58, Zakynthos Mednet: DC%=44) Questions to be answered • Can the data be explained with DC%=100 ? • 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 ? 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 < 0.1 Hz, and multiple point sources for f < 0.3 Hz 6 NOA stations, f=0.05 to 0.1 Hz blue: data black: synthetics for crustal model of Haslinger et al. (1999) weights proportional to 1/A were applied Are you surprised by the quality of the match although we work only with T < 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. Forward modeling: uncertain location and crustal model justifies the waveform alignment (artificial shift by a few seconds) epicenter shift 5 km epicenter shift + 3 to N or E crustal models Resolving depth and fault-plane solution opt. depth 17 km: very stable strike, dip, rake Mo=0.16e18 Nm for all this depth range, Mw=5.4 much less stable DC% 100% DC matches data also well (only 0.05 worse) we cannot see the difference Going into large details: Optimum correlation is not compatible with 100% DC trial time shift Forcing DC% to be > 90% decreases correlation. What is an “acceptable” range ? top curves: DC% bottom curves: correlation thick curves refer to the optimum depth 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 “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% Compare the uncertain DC percentage with very stable strike-dip-rake DC%: 72 to 97 % black nodal lines: all “2D” acceptable solutions (a range of depths and time shifts) Without artificial waveform alignment: lower DC%, more uncertainty DC%= 60 to 95% (65 to 90% at optimal depth) 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! The uncertainty of DC% is still 72 to 97%, but the depth is problematic In any case, for this event we systematically find DC% > 70% (i.e. higher than reported for this earthquake by major agencies). Can we explain the DC% by means of a multiple source ? Fixing the opt. source position and increasing frequency (f < 0.3 Hz): 3 subevents 2-sec time delay between sub 1 and 2; sub 3 is unstable 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. 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. 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! The new trial fault plane is given by the patch (point 3) and the known old epicenter strike o patch (303 ) 5 trial source positions the old epicenter now at each depth appears slightly off (16,17, and 18 km) the plane (location error) the subevent time separation is stable (2 + 2 sec), and the focal mechanism as well, incl. sub 3 2 + 2 sec The optimized “fault plane” RLS stabilized the solution a lot. VLS Sub 1,2,3 are not separated ITM more than ~2 km from each other, but the delay EVR is 2+2 seconds. = small distance, large delay. JAN Rupture propag. with arrest ? KEK A multiple event ! trial vertical plane 8 x 2 km Conclusions • The data can be explained with DC%=100. • 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 http://seis30.karlov.mff.cuni.cz Thank you !