The January 2010 Efpalio earthquake sequence in Western Corinth Gulf: epicenter relocations, focal mechanisms, slip models 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

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

THESS catalog, using HUSN stations (Hellenic Unified Seismic Network) and HYPOINVERSE code CORINTH GULF Dec 2009 to Aug 2010

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

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

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

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

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

MAINSHOCK HYPOCENTER - MORE METHODS Lat uncertainty smaller than 1 km Long uncertainty about 2 km Star: Grid Search Circle: Athens Univ. Small symbols: varying methods and data

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

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

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

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.

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

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

RELOCATION (HYPODD) before relocation after relocation

RELOCATION (HYPODD) – MAIN FINDINGS

Gap around the Jan18 mainshock

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

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

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

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.

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

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).

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.

SYNTHESIS – Jan 18, Jan 22

Hypocenter Jan 18: km Jan 22: km Centroid Jan 18: km strike, dip, rake: strike, dip, rake: Jan22: km strike, dip, rake: strike, dip, rake:

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.

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.