1. Two M5 earthquakes in Corinth Gulf, January 2010
J. Janský, O. Novotný, J. Zahradník - Prague
E. Sokos - Patras

2. Outline Records Hypocenter Centroid and moment tensor Finite source
Faults ?

5. Stations and records

9. peak accel. 1.7 m/s2 Intensity 6 to 7 acceleration velocity
displacement
the near-field ramp

10. Pre-shock

11. Possible signal from hypocenter (P)
before signal from asperity (P’).
This is not the anti-alias filter effect !

12. This
is the
filter
effect.

13. Zooming the first arrival.

14. S? S?
The near-field ramp. Where is S ?

16. Stations and records – a summary:
Clipping of CMG-3T, double P arrival (P, P’?), unclear S onset,
a clear near-field ramp at SER (thanks to CMG-5T)…

17. Location

19. One event located by many methods.

23. Grid-search location.

25. Grid search combined
with station-differences;
trade off between depth
and origin time is eliminated.

26. One event located by many methods,
agencies included.
Jan 18

27. One event located by many methods
Jan 22

29. The two events slightly shifted one to another.
Independently supported
by a relative location of Jan 22
with respect to Jan 18.

30. Location - summary:
Conjugate gradients, grid search, relative location, all providing a stable epicenter location. Two events close to each other. Depth ~10 km.

31. Centroid and moment tensor

32. Distances < 100 km

33. Jan 18 Frequency < 0.2 Hz Specific role of the nearest station.
Excluded from inversion.
Jan 18
Frequency < 0.2 Hz

34. Jan 22
strike, dip, rake
Jan 18: Plane 1: Plane 2:
Jan 22: Plane 1: Plane 2:

35. independent of crustal models.
Centroidal depth ~ 3-5 km,
independent of crustal models.
(Lat , Lon , model ON)

36. Uncertainty of
the centroid position

37. Centroid and moment tensor - summary
Two normal events, slightly different positions and mechanisms, small centroid depths (~ 4 km).

38. Fault plane

39. Jan 18: H-C consistency; fault dipping to South

40. Jan 18
Yes ! No

41. Jan 18, Jan 22
Jan 18 ??
Yes ! No

42. Jan 22: H-C consistency achieved when
C shifted by 0.02° to North.

43. If the H depths are 8 km, both H’s are in the same plane
determined by the strike and dip of Jan 22.

44. Caution: These are the two likely fault planes,
not the conjugate nodal planes !

46. Fault plane - summary Jan 18 fault plane dipping to South,
Jan 22 a bit more problematic
(a Northward 0.02° shift of C would be needed, but allowed within the uncertainty limits).
Relation to surface faults is unclear.

47. Fault plane – additional data ?

48. Preliminary location of the sequence (Patras Univ.)

49. Finite-fault modeling
(forward simulation for SER) 25 43
Finite-source effects
are evident, but
no simple preference
can be made.

52. Finite fault (additional data) – summary
Relocation of the sequence by relative methods would be useful. Finite-source effects were found but their interpretation is non-unique.
The near stations are few, only forward simulation is possible.

53. Interesting observation
Fast long-period waves were observed at many stations for these events, perhaps leaking modes PL. How to use them in the source and crustal structure studies?

55. Source depth 5.5 km, source duration 1 sec: able
to simulate the 5-sec waves. It is a structural effect.

56. Concluding remarks Routine epicenter location is surprisingly good.
Details needed for local tectonic interpretation are ‘on the edge’ of the available data.
It seems possible to formulate an H-C consistent model of both events.
How to produce the whole class of possible (non-unique) models automatically?
Combining the geometry constrains and finite-source effects is a good strategy.
The near-filed effects and the long-period waves (PL?) provide interesting additional data.

57. Thanks for your attention !