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The January 2010 Efpalio earthquake sequence in Western Corinth Gulf:

The January 2010 Efpalio earthquake sequence in Western Corinth Gulf: epicenter relocations, focal mechanisms, slip models. Sokos E. 1 , Kiratzi A. 2 , Serpetsidaki A. 1 , Tselentis G. 1 , Novotny O. 3 , Jansky J. 3 , Zahradnik J. 3.

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The January 2010 Efpalio earthquake sequence in Western Corinth Gulf:

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  1. The January 2010 Efpalio earthquake sequence in Western Corinth Gulf: epicenter relocations, focal mechanisms, slip models Sokos E. 1, Kiratzi A.2, Serpetsidaki A.1, Tselentis G.1, Novotny O.3, Jansky J.3, Zahradnik J.3 1 Department of Geology, Seismological Laboratory, University of Patras, Greece 2 Department of Geophysics, Aristotle University of Thessaloniki, Greece 3 Charles University in Prague, Czech Republic

  2. CORINTH GULF Dec 2009 to Aug 2010 THESS catalog, using HUSN stations (Hellenic Unified Seismic Network)

  3. REGIONAL FRAMEWORK Very active extensional region Both large and small events Cooperative efforts Bernard et al. Tectonophysics 426 (2006), 7–30.

  4. REGIONAL FRAMEWORK A lot is known about the faults on the southern coast, much less for the northern coast. Hence importance of the Efpalio study. Bernard et al., Tectonophysics 426 (2006) 7–30.

  5. SEQUENCE Jan 18 - Jan 23 LOCATION HYPOINVERSE code, stations up to 120 km temporary UPSL network included (starting Jan 19)

  6. SEQUENCE Jan 18 - Jan 23 LOCATION HYPOINVERSE code, stations up to 120 km temporary UPSL network included (starting Jan 19)

  7. TWO M5 MAINSHOCKS Two mainshocks Jan 18 (Mw~5.3), Jan 22 (Mw~5.2), both normal mechanisms

  8. MAINSHOCK HYPOCENTERS RELOCATED BY 3D GRID SEARCH single global minimum of the L2 misfit, depth 8-10 km similar results in two 1D crustal models

  9. WAVEFORM INVERSION - CENTROID MOMENT TENSOR 9 near-regional stations f < 0.1 Hz 2 crustal models varied station sets

  10. CENTROID POSITION C-H distance of about 4 km, consistent with the assumed M5 size Centroid depth C H Stable preference of C depth shallower (~5 km) than H (~ 9km)

  11. TWO MAINSHOCKS - SMALL BUT STABLE DIFFERENCE IN THE NODAL PLANES North-dipping plane South-dipping plane Two events = two faults ! Which are the fault planes? Jan 18 Jan 22

  12. FINITE-EXTENT SOURCE (forward modeling the nearest-station record, f<1 Hz) No preference of the south- or north-dipping plane. Radial rupturing possible. Higher freq. needed, but synthetic Green f. then not reliable.

  13. SLIP DISTRIBUTION BY EMPIRICAL GREEN FUNCTIONS North-dipping plane (no preference) South-dipping plane However, horizontal position of main slip patch is stable, well resolved. Jan 18 Jan 22

  14. HYPOCENTER AND CENTROID INDICATE THE FAUL PLANE Centroid Hypocenter of Jan 18 by different methods Green: strike 102°, dip 55° (south-dipping plane; preferred) Red: strike 270°, dip 36° north-dipping plane

  15. RELOCATION (HYPODD) before relocation after relocation

  16. RELOCATION (HYPODD) – MAIN FINDINGS

  17. RELOCATION (HYPODD) – MAIN FINDINGS Gap around the Jan18 mainshock

  18. RELOCATION (HYPODD) – MAIN FINDINGS Gap between the ‘northern’ and ‘southern’ clusters

  19. RELOCATION (HYPODD) – MAIN FINDINGS Part of Jan18 aftershocks clustered in place of Jan22 (= ‘preshocks’ of Jan22)

  20. RELOCATION (HYPODD) – MAIN FINDINGS Part of Jan18 aftershocks clustered in place of Jan22 (= ‘preshocks’ of Jan22)

  21. RELOCATION (HYPODD) – MAIN FINDINGS Gap around the Jan18 mainshock Gap between the ‘northern’ and ‘southern’ clusters Part of Jan18 aftershocks clustered in place of Jan22 (= ‘preshocks’ of Jan22) Almost simultaneous activation of several faults.

  22. FOCAL MECHANISMS OF AFTERSHOCKS ISOLA software Normal mechanisms are prevailing. Other mechanisms are also present, confirming activation of several faults.

  23. SYNTHESIS – Jan 18 Jan 18 event ruptured the south-dipping plane. Aftershocks outside main slip patch, activation of another fault to the NE (place of Jan 22). Methodical problem to be improved: Solve the shift between main slip patch (color isolines) and centroid (red).

  24. SYNTHESIS – Jan 22 • Jan22 also on south-dipping • plane (slightly different • geometry than Jan18). • Similarly to Jan18: • no aftershocks in the • main slip patch. • no aftershocks • towards west.

  25. SYNTHESIS – Jan 18, Jan 22

  26. Hypocenter Jan 18: 21.91277 38.41260 8.9 km Jan 22: 21.94719 38.43420 9.5 km Centroid Jan 18: 21.9416000000 38.4220173190 5.0 km strike, dip, rake: 270. 36. -100. strike, dip, rake: 102. 55. -83. Jan22: 21.9645110296 38.4490409549 5.0 km strike, dip, rake: 282. 42. -67. strike, dip, rake: 72. 52. -109.

  27. CONCLUSION Accurate relocation, combined with the near-regional, low-frequency centroid moment-tensor preferred the south-dipping planes for the Jan18 and 22 mainshocks of M5. The mainshocks ruptured from a depth ~9km upwards (centroid depth ~ 5km) and eastward.

  28. CONCLUSION Accurate relocation, combined with the near-regional, low-frequency centroid moment-tensor preferred the south-dipping planes for the Jan18 and 22 mainshocks of M5. The mainshocks ruptured from a depth ~9km upwards (centroid depth ~ 5km) and eastward. Slip inversion by means of empirical Green functions provided stable positions of the main slip patches (a single patch for each event). The two mainshocks had almost no aftershocks in the main slip region. Jan18 event activated aftershock cluster (preshocks) at place of Jan22 event. Sequence occurred on a complex system of faults of different strike and dip.

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