High-precision relocation of seismic sequences above a dipping Moho: the case of the January–February 2014 seismic sequence on Cephalonia island (Greece)

Abstract

Abstract. Detailed velocity structure and Moho mapping is of crucial importance for a high precision relocation of seismicity occurring out of, or marginal to, the geometry of seismological networks. Usually the seismographic networks do not cover the boundaries of converging plates such as the Hellenic arc. The crustal thinning from the plate boundary towards the back-arc area creates significant errors in accurately locating the earthquake, especially when distant seismic phases are included in the analysis. The case of the Cephalonia (Ionian Sea, Greece) sequence of JanuaryFebruary 2014 provided an excellent example where the hypocentral precision was greatly affected by the crustal thinning from the plate boundary at the Ionian sea towards the Aegean sea. This effect was examined in detail by testing various velocity models of the region in order to determine an optimal model. Our tests resulted in the adoption of a velocity model that resembles the crustal thinning of the region. Then, a relocation procedure was performed in the Cephalonia sequence for the time period of 26 January to 15 May 2014 by applying probabilistic non-linear location algorithms. The high-precision relocation resulted in an improved spatial distribution of the seismicity with respect to the preliminary locations and provided a reliable basis to examine seismotectonic implications of the Cephalonia sequence.

Rupture process of the 2014 Cephalonia, Greece, earthquake doublet (Mw6) as inferred from regional and local seismic data

“…This choice is based on the fact that this epicenter: a) is supported by the first-motion polarities at near station DMLN (Z+, N+, E+), which imply that the epicenter lies south-west of the station and b) is in agreement with the provisions imposed by the collocation of hypocenter and fault plane position . Our preferred solution is in disagreement with the relocations proposed by other authors (Karakostas et al, 2014;Karastathis et al, 2015), which are closer to our epicenter obtained from the local data.…”

“…This test also showed that the epicenter defined by the use of the local stations (Fig. 2a), close to that of Karastathis et al (2015), is much less favored. This stems from the fact that even a large increase in the depth still leaves the hypocenter distance from the fault too large (~5 km) and, moreover, such a hypocenter would project closer to the conjugate WNW-ESE trending nodal plane.…”

“…b) Relocated epicenters of the two major events: the black star denotes the epicenter of the 1st event on Jan 26, 2014 and the black diamond denotes the epicenter of the 2nd event on Feb 3, 2014. For comparison we have included the relocated epicenters for the 1st (gray star) and 2nd (gray diamond) events reported in Karastathis et al (2015). The CMT solutions for the two events, as calculated here and as reported in GCMT catalog are also shown (beach-balls marked accordingly).…”

“…No loss of life was reported and the constructions performed remarkably well (GEER/ EERI/ATC report, 2014). The 2014 sequence attracted the attention of the scientific community and a number of publications are already available (Benekos et al, 2015;Boncori et al, 2015;Karakostas et al, 2014;Karastathis et al, 2015;Papadopoulos et al, 2014;Sakkas and Lagios, 2015;Valkaniotis et al, 2014). Despite the fact that the sequence was well recorded by the regional networks in Greece, there are still unresolved issues.…”

“…Karakostas et al (2014) relocated the sequence and concluded that the two major shocks were related to two adjacent fault segments, striking almost N-S and dipping to the east. Karastathis et al (2015) relocated the sequence using the equal differential time and probabilistic non-linear approaches, accounting for the effects of the laterally varying crustal structure. They showed that the January 26 and February 3 events could be related with fault planes dipping to east and west, respectively.…”