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

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 aboutthe faults on thesouthern coast,

much less for the northern coast.

Hence importance ofthe Efpalio study.

REGIONAL FRAMEWORK

Bernard et al., Tectonophysics 426 (2006) 7–30.

HYPOINVERSE code, stations up to 120 kmtemporary UPSL network included (starting Jan 19)

SEQUENCE Jan 18 - Jan 23LOCATION

HYPOINVERSE code, stations up to 120 kmtemporary UPSL network included (starting Jan 19)

SEQUENCE Jan 18 - Jan 23LOCATION

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

TWO M5 MAINSHOCKS

single global minimumof the L2 misfit, depth 8-10 km

similar results in two 1D crustal models

MAINSHOCK HYPOCENTERS RELOCATED BY 3D GRID SEARCH

6 8 10 12 14 16

depth (km )

mis

fit

Even date-time: 20100118 1:32:39Displacement (m). Inversion band (Hz) 0.05 0.06 0.08 0.1

ObservedSynthetic

Gray waveforms weren't used in inversion.

Blue numbers are variance reduction

-101

x 10-3 NS

SE

R -0.05

-101

x 10-3 EW

0.96

-101

x 10-3 Z

0.78

-101

x 10-4

TR

Z 0.89

-101

x 10-4

0.95

-101

x 10-4

0.55

-101

x 10-4

MA

M 0.68

-101

x 10-4

0.97

-101

x 10-4

0.89

-4-2024

x 10-5

DS

F 0.74

-4-2024

x 10-5

0.62

-4-2024

x 10-5

0.39

-101

x 10-4

DR

O 0.82

-101

x 10-4

0.78

-101

x 10-4

0.88

-101

x 10-4

EV

R 0.06

-101

x 10-4

0.93

-101

x 10-4

0.80

-101

x 10-4

GU

R 0.65

-101

x 10-4

0.93

-101

x 10-4

0.83

-4-2024

x 10-5

PD

O 0.49

-4-2024

x 10-5

0.74

-4-2024

x 10-5

-2.68

0 100 200 300

-505

x 10-5

Time (sec)

LT

K 0.93

0 100 200 300

-505

x 10-5

Time (sec)

0.89

0 100 200 300

-505

x 10-5

Time (sec)

0.40

WAVEFORM INVERSION - CENTROID MOMENT TENSOR

9 near-regional stationsf < 0.1 Hz2 crustal modelsvaried station sets

CENTROID POSITION

Stable preference of C depthshallower (~5 km) than H (~ 9km)

C-H distance of about4 km, consistent withthe assumed M5 size

HC

Centroid depth

TWO MAINSHOCKS - SMALL BUT STABLEDIFFERENCE IN THE NODAL PLANES

North-dipping plane

South-dippingplane

Jan 18 Jan 22

Two events= two faults !

Which arethe fault planes?

FINITE-EXTENT SOURCE (forward modeling the nearest-station

record, f<1 Hz)

54

50 60 70 80 90 100tim e (s)

-0 .004

-0 .002

0

0.002

0.004

dis

pla

cem

en

t (m

)

SER _Zblack: datacolor: synt

blue: fin ite_25pink: fin ite_43green: fin ite_hypred: po in t source

50 60 70 80 90 100tim e (s)

-0 .004

-0 .002

0

0.002

0.004

dis

pla

cem

en

t (m

)

SER _Zblack: datacolor: synt

blue: fin ite_25pink: fin ite_49green: fin ite_hypred: po in t source

-1 .5 -1 -0 .5 0 0.5 1 1.5

-1

-0 .5

0

0.5

1

-1.5 -1 -0.5 0 0.5 1 1.5

-1

-0.5

0

0.5

1

No preference of thesouth- or north-dippingplane.

Radial rupturingpossible.

Higher freq. needed, but synthetic Green f.then not reliable.

SLIP DISTRIBUTION BY EMPIRICAL GREEN FUNCTIONS

North-dipping plane

South-dippingplane

Jan 18 Jan 22

However, horizontal position of main slip patchis stable, well resolved.

(no preference)

HYPOCENTER AND CENTROID INDICATE THE FAUL PLANE

Hypocenter of Jan 18by different methods

-20

0

20-15 -10 -5 0 5 10 15

-20

-15

-10

-5

0

5

10

North-South (km)

jan18

East-West (km)

Dep

th

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

RELOCATION (HYPODD) – MAIN FINDINGS

Gap around the Jan18mainshock

RELOCATION (HYPODD) – MAIN FINDINGS

Gap between the ‘northern’and ‘southern’ clusters

RELOCATION (HYPODD) – MAIN FINDINGS

Part of Jan18 aftershocksclustered in place of Jan22(= ‘preshocks’ of Jan22)

RELOCATION (HYPODD) – MAIN FINDINGS

Part of Jan18 aftershocksclustered in place of Jan22(= ‘preshocks’ of Jan22)

RELOCATION (HYPODD) – MAIN FINDINGS

Gap around the Jan18mainshock

Gap between the ‘northern’and ‘southern’ clusters

Part of Jan18 aftershocksclustered in place of Jan22(= ‘preshocks’ of Jan22)

Almost simultaneous activation

of several faults.

FOCAL MECHANISMS OF AFTERSHOCKS

ISOLA software

Normal mechanismsare prevailing.

Other mechanismsare also present,confirming activationof several faults.

SYNTHESIS – Jan 18

Jan 18 event rupturedthe south-dipping plane.

Aftershocks outsidemain slip patch,activation ofanother fault to the NE(place of Jan 22).

Methodical problemto be improved:

Solve the shift betweenmain slip patch (color isolines)and centroid (red).

SYNTHESIS – Jan 22

Jan22 also on south-dippingplane (slightly differentgeometry than Jan18).

Similarly to Jan18:• no aftershocks in the main slip patch.• no aftershocks towards west.

SYNTHESIS – Jan 18, Jan 22

HypocenterJan 18: 21.91277 38.41260 8.9 kmJan 22: 21.94719 38.43420 9.5 km

CentroidJan 18: 21.9416000000 38.4220173190 5.0 kmstrike, dip, rake: 270. 36. -100. strike, dip, rake: 102. 55. -83.

Jan22: 21.9645110296 38.4490409549 5.0 kmstrike, dip, rake: 282. 42. -67. strike, dip, rake: 72. 52. -109.

CONCLUSION

Accurate relocation, combined with the near-regional, low-frequencycentroid moment-tensor preferred the south-dipping planes for theJan18 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-frequencycentroid moment-tensor preferred the south-dipping planes for theJan18 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 positionsof 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.