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

Outline Records Hypocenter Centroid and moment tensor Finite source Faults ?

Stations and records

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

Pre-shock

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

This is the filter effect.

Zooming the first arrival.

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

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)…

Location

One event located by many methods.

Grid-search location.

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

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

One event located by many methods Jan 22

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

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

Centroid and moment tensor

Distances < 100 km

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

Jan 22 strike, dip, rake Jan 18: Plane 1: 108 57 -74 Plane 2: 260 37 -113 Jan 22: Plane 1: 76 38 -108 Plane 2: 279 54 -76

independent of crustal models. Centroidal depth ~ 3-5 km, independent of crustal models. (Lat 38.4250, Lon 21.9187, model ON)

Uncertainty of the centroid position

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

Fault plane

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

Jan 18 Yes ! No

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

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

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

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

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.

Fault plane – additional data ?

Preliminary location of the sequence (Patras Univ.)

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

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.

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?

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

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.

Thanks for your attention !