The Cephalonia, Ionian Sea (Greece), sequence of strong earthquakes of January-February 2014: a first report

On 26.1.2014 and 3.2.2014 two strong earthquakes of M w 6.0 and M w 5.9 ruptured the western Cephalonia Isl., Ionian Sea (Greece), at the SSW-wards continuation of the Lefkada segment of the Cephalonia Transform Fault Zone (CTFZ), causing considerable damage and a variety of ground failures. High-precision relocation of the aftershocks implies that the seismogenic layer was of 35 km in length (L) striking NNE-SSW, of 10 km maximum in width and 15 km in thickness. Two aftershock spatial clusters were revealed at north ( L 1 ~10 km) and at south ( L 2 ~25 km). However, no time correlation was found between the two clusters and the two strong earthquakes. Fitting the temporal evolution of aftershocks to the Omori-law showed slow aftershock decay. Fault plane solutions produced by moment tensor inversions indicated that the strong earthquakes as well as a plenty of aftershocks (M w ≥4.0) were associated with dextral strikeslip faulting with some thrust component and preferred fault planes striking about NNE-SSW. Average fault plane parameters obtained for the three largest events are: strike 21(±2) 0 , dip 65.5(±3) 0 , slip 173(±3) 0 . Broadband P-wave teleseismic records were inverted for understanding the rupture histories. It was found that the earthquake of 26.1.2014 had a complex source time function with c. 62 cm maximum slip, source duration of ~12 s and downwards rupture. Most of the slip was concentrated on a 13x9 km fault rupture. The earthquake of 3.2.2014 had a relatively simple source time function related with one big patch of slip with maximum slip c. 45 cm, with 10 s source duration. The rupture was directed upwards which along with the shallow focus (~5 km) and the simple source time function may explain the significantly larger (0.77 g) PGA recorded with the second earthquake with respect to the one recorded (0.56 g) with the first earthquake. Most of the slip was concentrated on a 12x6 km fault rupture. Maximum seismic intensity ( I m ) of level VII and VIII to VIII+ was felt in Lixouri town and the nearby villages from the first and the second earthquake, respectively. The rupture histories and the increased building vulnerability after the damage caused by the first shock may account for the larger I m caused by the second shock. However, the ground failures area of the second earthquake was nearly half of that of the first earthquake, which is consistent with the faster attenuation of ground acceleration away from the meizoseismal area caused by the second earthquake with respect to the first one. From that the 2014 earthquakes ruptured on land western Cephalonia we suggested to revise the CTFZ geometry in the sense that the Lefkada CTFZ segment does not terminates offshore NW Cephalonia but extends towards SSW in western Cephalonia.

[1]  Chen Ji,et al.  Source Description of the 1999 Hector Mine, California, Earthquake, Part I: Wavelet Domain Inversion Theory and Resolution Analysis , 2002 .

[2]  George E. Randall,et al.  Efficient calculation of complete differential seismograms for laterally homogeneous earth models , 1994 .

[3]  D. Helmberger,et al.  A Procedure for Modelling Shallow Dislocation Sources , 2007 .

[4]  Thomas H. Heaton,et al.  Inversion of strong ground motion and teleseismic waveform data for the fault rupture history of the 1979 Imperial Valley, California, earthquake , 1983 .

[5]  K. Pitilakis,et al.  Liquefaction case histories and empirical relations of earthquake magnitude versus distance from the broader Aegean region , 2005 .

[6]  Thomas H. Heaton,et al.  The 1971 San Fernando earthquake: A double event? , 1982 .

[7]  Gerassimos A. Papadopoulos,et al.  Magnitude–distance relations for earthquake-induced landslides in Greece , 2000 .

[8]  C. DAVISON,et al.  The Earthquakes in Greece , 1894, Nature.

[9]  G. Danuser,et al.  The strain field in northwestern Greece and the Ionian Islands: results inferred from GPS measurements , 1995 .

[10]  Yann Hello,et al.  Western Hellenic subduction and Cephalonia Transform: local earthquakes and plate transport and strain , 2000 .

[11]  D. Panagiotopoulos,et al.  Evidence for transform faulting in the Ionian sea: The Cephalonia island earthquake sequence of 1983 , 1985 .

[12]  Anastasia Kiratzi,et al.  The Cephalonia Transform Fault and its extension to western Lefkada Island (Greece) , 1999 .

[13]  Crustal stress and seismic activity in the Ionian archipelago as inferred by satellite- and ground-based observations, Kefallinìa, Greece , 2006, Geological Society, London, Special Publications.

[14]  F. Omori,et al.  On the after-shocks of earthquakes , 1894 .

[15]  K. Konstantinou,et al.  Routine Regional Moment Tensor Inversion for Earthquakes in the Greek Region: The National Observatory of Athens (NOA) Database (2001–2006) , 2010 .

[16]  S. Hartzell,et al.  Importance of model parameterization in finite fault inversions: Application to the 1974 Mw 8.0 Peru Earthquake , 1993 .

[17]  N. Frazer.  Seismic Wave Propagation in Stratified Media , 1987 .

[18]  E. Papadimitriou,et al.  3D crustal structure from local earthquake tomography around the Gulf of Arta (Ionian region, NW Greece) , 1999 .

[19]  Gerassimos A. Papadopoulos,et al.  Magnitude-distance relations for liquefaction in soil from earthquakes , 1993, Bulletin of the Seismological Society of America.

[20]  Yuji Yagi,et al.  Introduction of uncertainty of Green's function into waveform inversion for seismic source processes , 2011 .

[21]  D. Wald,et al.  Spatial and temporal distribution of slip for the 1992 Landers, California, earthquake , 1994, Bulletin of the Seismological Society of America.

[22]  Anastasia Kiratzi,et al.  The 14 August 2003 Lefkada Island (Greece) earthquake: Focal mechanisms of the mainshock and of the aftershock sequence , 2005 .

[23]  B. Gutenberg,et al.  Frequency of Earthquakes in California , 1944, Nature.

[24]  Gerassimos A. Papadopoulos,et al.  Nonlinearity of the magnitude‐frequency relation in the Hellenic Arc‐Trench System and the characteristic earthquake model , 1993 .

[25]  T. Utsu.  A statistical study on the occurrence of aftershocks. , 1961 .

[26]  N. N. Ambraseys,et al.  Earthquakes in the Mediterranean and Middle East: A Multidisciplinary Study of Seismicity up to 1900 , 2009 .

[27]  Maria Sachpazi,et al.  Variation of seismic coupling with slab detachment and upper plate structure along the western Hellenic subduction zone , 2004 .

[28]  Pengcheng Liu,et al.  The 1994 Northridge, California, earthquake: Investigation of rupture velocity, risetime, and high‐frequency radiation , 1996 .

[29]  S. Hartzell,et al.  Fault-slip distribution of the 1995 Colima-Jalisco, Mexico, earthquake , 1999 .

[30]  T. Iwata,et al.  Rupture Process of the 1999 Kocaeli, Turkey, Earthquake Estimated from Strong-Motion Waveforms , 2002 .

[31]  A. Ganas,et al.  The Lefkada, Sea (Greece), shock (Mw 6.2) of 14 August 2003: Evidence for the characteristic earthquake from seismicity and ground failures , 2003 .