The 2013 earthquake swarm in Helike, Greece: seismic activity at the root of old normal faults

Abstract

The Corinth Rift in Central Greece has been studied extensively during the past decades, as it is one of the most seismically active regions in Europe. It is characterized by normal faulting and extension rates between 6 and 15 mm yr −1 in an approximately N10E • direction. On 2013 May 21, an earthquake swarm was initiated with a series of small events 4 km southeast of Aigion city. In the next days, the seismic activity became more intense, with outbursts of several stronger events of magnitude between 3.3 and 3.7. The seismicity migrated towards the east during June, followed by a sudden activation of the western part of the swarm on July 15th. More than 1500 events have been detected and manually analysed during the period between 2013 May 21 and August 31, using over 15 local stations in epicentral distances up to 30 km and a local velocity model determined by an error minimization method. Waveform similarity-based analysis was performed, revealing several distinct multiplets within the earthquake swarm. High-resolution relocation was applied using the double-difference algorithm HypoDD, incorporating both catalogue and cross-correlation differential traveltime data, which managed to separate the initial seismic cloud into several smaller, densely concentrated spatial clusters of strongly correlated events. Focal mechanism solutions for over 170 events were determined using P-wave first motion polarities, while regional waveform modelling was applied for the calculation of moment tensors for the 18 largest events of the sequence. Selected events belonging to common spatial groups were considered for the calculation of composite mechanisms to characterize different parts of the swarm. The solutions are mainly in agreement with the regional NNE-SSW extension, representing typical normal faulting on 30-50 • north-dipping planes, while a few exhibit slip in an NNE-SSW direction, on a roughly subhorizontal plane. Moment magnitudes were calculated by spectral analysis of S waves, yielding b-values between 1.1 and 1.2 in their frequency-magnitude distribution. The seismic moment release history indicates swarm-like activity during the first phase, which could have acted as a preparatory stage for the second phase (after 12 July) that presented a more typical main-shock-aftershock behaviour. The spatiotemporal analysis reveals that the swarm has occurred in a volume that is likely related with the extension at depth of the NNEdipping Pirgaki normal fault, outcropping ∼8 km to the south. The slow velocity of eastward migration of the epicentres during June implies triggering by fluids. The situation appears different in the second phase of the sequence, which was probably triggered by a build-up of stress during the first one. The relatively deep hypocentres of the 2013 swarm, compared 2044

Preliminary Shear-Wave Splitting Results in the Broader Aigion Area (Greece) During 2013

“…During 2013, a period of extended significant unrest was observed in the broader area of the WGoC, featuring an intense seismic swarm located off the coast of Aigion between May and August (Chouliaras et al, 2015;Kapetanidis et al, 2015). The total annual seismicity presents a curved distribution from Rio -Antirion in the west to Antikyra in the east, roughly delineated by the epicentres in Fig.…”

“…Location errors were very satisfactory, having mean values equal to 0.6 km for the horizontal (ERH) and 0.9 km for the vertical (ERZ) one, while mean RMS was equal to 0.13 sec. Hypocenters of events belonging to the May -August seismic swarm were adopted by Kapetanidis et al (2015). The earthquakes selected for the study meet certain strict criteria.…”

“…One of the most well-known seismic events of the recent decades is the 15 June 1995 (Ms=6.2) Aigion earthquake (Bernard et al, 1997a). Since the occurrence of the Aigion event, the seismic energy in the Western GoC (WGoC) is mainly expressed by the generation of seismic swarms and not specific strong events (Lyon-Caen et al, 2004;Duverger et al, 2015;Kapetanidis et al, 2015). The two strongest events of this period were located at the northern part of the GoC, close to Efpalio city, in 2010, both with a magnitude of Mw = 5.1 (Kapetanidis and Papadimitriou, 2011;Ganas et al, 2013).…”

“…In the framework of the current study, an anisotropy analysis was conducted in the WGoC during 2013, including the period of the Helike seismic swarm (Chouliaras et al, 2015;Kapetanidis et al, 2015). The data were recorded by stations belonging to both the HUSN and the CRLN.…”

Relocation of recent seismicity and seismotectonic properties in the Gulf of Corinth (Greece)

“…On 2010 January 18, an earthquake doublet occurred near Efpalio beneath the north coasts of the westernmost part of Corinth Gulf with two M w = 5.5 events (Karakostas et al 2012;Sokos et al 2012;Ganas et al 2013). On 2013 May 22, an earthquake swarm that initiated near Aigio, with more than 1500 earthquakes detected in three months (Chouliaras et al 2015;Kapetanidis et al 2015;Mesimeri et al 2016;Kaviris et al 2017).…”