Quick fault-plane identification by a geometrical method: The M w 6.2 Leonidio earthquake, 6 January 2008, Greece and some other recent applications J. Zahradnik, F. Gallovic Charles University in Prague E. Sokos, A. Serpetsidaki, G-A. Tselentis University of Patras

Why we need to know the fault plane ? Shake maps Aftershock prediction Stress field

Why we need to know the fault plane quickly? Shake maps Aftershock prediction

Which nodal plane is the fault plane ? Aftershock distribution Finite-extent source models; waveform modeling Geometrical configuration of hypocenter (H) and centroid (C)

Which nodal plane is the fault plane ? Aftershock distribution … too slow Finite-extent source models; waveform modeling Geometrical configuration of hypocenter (H) and centroid (C)

Which nodal plane is the fault plane ? Aftershock distribution … too slow Finite-extent source models; waveform modeling … too slow Geometrical configuration of hypocenter (H) and centroid (C)

Which nodal plane is the fault plane ? Aftershock distribution … too slow Finite-extent source models; waveform modeling … too slow Geometrical configuration of hypocenter (H) and centroid (C) … quick enough !

H-C method H and C are in the same plane (I or II) of the conjugated fault-plane solutions. H-C distance must be larger enough; M>6. Multiple H and C solutions (uncertainty) help to prefer one of the two planes.

H-C method – continuation Success depends on the particular focal mechanism: Easy case of strike slip (Epicenter is sufficient) Good case of one horiz. plane (H depth not very critical) Bad case; inclined planes (H depth is critical)

H-C method – continuation Problematic applications: A segmented fault A symmetric case H on intersection of I and II

H-C method applied to five M>6 events in 2008

H-C method applied to five M>6 events in 2008 This presentation: 2 examples

Example 1

M 6.2 Leonidio, Jan 6, 2008 depth km

Waveform modeling for CMT 10 near-regional BB stations f < 0.07 Hz

HYPOCENTER CENTROID

„Collective“ solutions = including uncertainties of H and CMT H H red, green: nodal planes of three CMT solutions

The weakly dipping nodal plane identified as the fault plane Strike 213° Dip 34° Rake 5° The ‘green’ nodal plane is the fault plane because it encompasses the (uncertain) hypocenter.

animation

Practical output: Report to EMSC within 1 week after the earthquake report_jan06.pdf (in Earthquake News & Highlights)report_jan06.pdf

Consistence with the regional stress field (Kiratzi & Papazachos, 1995) T of this earthquake T of regional field

sub-horizontal slip vector Slip vector and regional stress field allow us to resolve the traction and evaluate the Coulomb Failure Function. Validation without aftershocks ?

The Coulomb Failure Function supports the sub-horizontal slip. TVS: tangential traction parallel to slip TVN: normal traction CFF=TVS+  TVN TVN negative TVN positive

Nodal plane TVSTVNCFF I II CFF larger for plane II because TVN is positive

Example 2

Mw 6.3 Andravida June 8, 2008 depth ~ 20 km Strike 210° Dip 85° Rake 179°

Mw 6.3 Andravida June 8, 2008 depth ~ 20 km H: UPSL and THE C: Harvard H: UPSL C: Mednet Strike 210, a right-lateral strike slip fault

Report to EMSC 7 hours after the earthquake report_june08.pdf report_june08.pdf

Abundant aftershocks (24-hours, NOA) validate the quick fault- plane guess (7 hours)

Strong-motion accelerograms (NOA) reveal a different duration: Amaliada dist. ~25 km Patras dist, ~35 km

Patras Amaliada Does a simple finite-extent source model based on the H-C result explain the data?

Typical finite-source synthetics reproduce the duration and support H-C results Amaliada: backward Patras: forward

Further support: azimuthal variation of the differential travel time t’-t t … hypocentre t‘ … asperity first brake (After Takenaka et al., 2005)

Conclusion H-C method is a simple tool for quick identification of the fault plane Applicable with ‘manual’ locations and CMT agency solutions (within a few hours) Collective solutions account for uncertainty through scatter in the H and C solutions So far the best validated: June 8, 2008 Andravida (=> rupture propagation to NE)

Full paper and e-supplement: Seism. Res. Letters, 79, , 2008 Try also a 3D ‘animation’ tool ( hcplot.m ).

H-C geometrical method applied to five M>6 events in 2008 =========================================== Event Fault plane less likely Report strike dip rake to EMSC =========================================== Leonidio Jan week Methoni Feb day Methoni Feb day Andravida Jun hours Rhodos Jul days ===========================================