An analysis of the Kefalonia seismic sequence of Jan Feb. 3, 2014

Transcription

1 An analysis of the Kefalonia seismic sequence of Jan Feb. 3, 2014 Alessandro Caporali 1),Carine Bruyninx 2),Rui Fernandes 3),Athanassios Ganas 4),Ambrus Kenyeres 5),Martin Lidberg 6), Guenter Stangl 7), Holger Steffen 6), Joaquin Zurutuza 1) 1) Department of Geosciences, University of Padova, Italy 2)Royal Astronomical Observatory, Brussels, Belgium 3)University of Corvilha, Portugal 4) National Observatory of Athens, Greece 5)Satellite Geodetic Observatory FOMI, Budapest, Hungary 6) Lantmateriaet, Gaevle, Sweden 7) BEV-OeAW, Graz, Austria

2 Outlook Tectonic setting and seismicity of the KTZ The long term velocities from GPS data How geodetic and seismological data set an upper limit to a static stress drop The January/February 2014 seismic sequence Conclusion: the regional stress from Coulomb Failure and distribution of the aftershocks is close to the upper limit of the static stress drop

3 Kokinou et al., 2006 KTZ accommodates continental thrust on the NE and oceanic subduction to the SW S. Yolsal-Çevikbilen, T. Taymaz, 2012

4 KTZ consists of the main branch of ca 90 km, and a northern Lefkada branch of some 40 km Dextral strike slip fault Thrust events at both edges Recurrent seismic activity (M=6.5 event every 10 yrs on average) Max reported magnitude 7.4 agrees with fault length Shaw and Jackson, 2010 Louvari et al. 1999

5 projected velocity (mm/year) Topography (m) Regional strain rate from GNSS geodesy Graz + Padova multiyear solution Velocity inversion along profile ca. 4 cm/yr The velocity gradient is interpreted as distributed shearing across this part of western Greece, which is released by slip on a number of shallow faults (Floyd et al., 2010) Apulia Ionian Sea ,00-400,00-300,00-200,00-100,00 0,00 100,00 200,00 distance from point of maximum shear rate (km) Vel_orthog_profile Topo (m) 225±20 nstrain/yr Ionian islands/peloponnesos mm/yr

6 Constraints on stress drop and mmax: Combine geodetic shear strain rate with the statistical seismicity of the area to infer a relation between stress drop and Mmax Kostrov formula reformulated in terms of integrals across the magnitude spectrum From seismic catalog Wells and Coppersmith empirical formulas for Rupture Area and Displacement Conservation of energy requires the thickness of the seismic volume to be inversely proportional to the stress drop Brune s formula, up to a form factor Shear strain rate associated with seismicity g is the average stress drop that we expect in the case of the exact balance of the geodetic and seismic strain rates across the catalogue time span and the magnitude range, which, in our case, is [ ] for the KTZ seismic zone. (Caporali et al., 2011) The last equation shows that for a seismic province of given strain rate and Mmax, the larger the stress drop the smaller is a, as it can be expected.

7 Statistical seismicity 87 events of [5.1<m<7.4] from 1862 to 2014 (from Votsi et al., 2011) 1,0 0,5 0,0-0,5 5 5,5 6 6,5 7 7,5 magnitudo m a -10%, b a + 10%, b -1,0-1,5-2,0-2,5-3,0 log(n>m) =( /-0.07)m + (4.91+/- 0.43) Time interval: /- 1 a The estimated a and b are such that we can expect a M=6.5 event in this area approximately every 10 years (Papadimitriou and Papazachos 1985). The maximum magnitude on KTZ is estimated 7.4, corresponding to rupture along the entire faults (Louvari et al., 1999) The regional stress drop should consequently be < 0.4 MPa

8 Latitude (degree) KTZ seismic sequence of 2014 Fault plane solutions and coseismic displacements PONT SPAN VLSM RLSO m Computed Measured Longitude (degree) Coulomb Apr :47:24 balkans.mat Map view grid Depth: 0.00 km

9 Time series of Selected GPS sites PONT, VLMN (Graz analysis) consistent with SW motion on a 20 deg striking vertical dextral fault

10 Stress transfer from source fault ( ) to receiver fault ( ) contour plots stacked at steps of 3 km Coulomb stress excess is more positive (ca 0.5 Mpa) on the top part of the fault No tapering applied to slip on source fault Kefalonia island Source fault ( at 13:55) 10 km MPa Mw = at 18:45 and 17 km depth: unlikely to have been triggered by the 13:55 event

11 Cumulative Coulomb stress change and migration of the aftershocks along strike N30E at 10 km mean depth r=4 bar (0.4MPa) Use aftershocks as indicators of preferred planes whose normal makes an angle to the largest principal stress 1 tan 2 being the coefficient of friction Fault plane of the event Fault plane of the event r=100 bar (10 Mpa) If aftershocks indicate areas closer to failure, then a small regional stress is more in keeping with the data

12 Conclusions Study area with very high steady state shear strain rate, measured by GNSS (EUREF + Italian+Greek stations, Graz+Padova processing): 225 +/- 20% nstrain/yr Historical seismicity gives estimates for yearly release of strain rate in terms of Gutenberg Richter s a and b Size of KTZ sets an upper limit to M max Depending of the value of GR s a (+/-10%) the regional stress drop should have an upper bound of 0.4 Mpa, if M max =7.4. Coseismic displacements measured accurately and modeled with the Okada approach: agreement between Fault Plane Solution and Geodesy Matching the hypocenters of the aftershocks to areas of highest likelyhood of Coulomb failure along optimal right lateral shear stress orientations suggests that the regional deviatoric stress is comparable to the stress drop, and that the static friction coefficient is 0.36