The Psachna (Evia Island) Earthquake Swarm of June 2003 (original) (raw)
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The 2013 earthquake swarm in Helike, Greece: seismic activity at the root of old normal faults
Geophysical Journal International, 2015
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 C The Authors 2015. Published by Oxford University Press on behalf of The Royal Astronomical Society. by guest on July 26, 2015 http://gji.oxfordjournals.org/ Downloaded from The 2013 earthquake swarm in Helike, Greece 2045
Spatio-temporal properties and evolution of the 2013 Aigion earthquake swarm (Corinth Gulf, Greece)
Journal of Seismology, 2015
The 2013 Aigion earthquake swarm that took place in the west part of Corinth Gulf is investigated for revealing faulting and seismicity properties of the activated area. The activity started on May 21 and was appreciably intense in the next 3 months. The recordings of the Hellenic Unified Seismological Network (HUSN), which is adequately dense around the affected area, were used to accurately locate 1501 events. The double difference (hypoDD) technique was employed for the manually picked P and S phases along with differential times derived from waveform crosscorrelation for improving location accuracy. The activated area with dimensions 6 × 2 km is located approximately 5 km SE of Aigion. Focal mechanisms of 77 events with M ≥ 2.0 were determined from P wave first motions and used for the geometry identification of the ruptured segments. Spatio-temporal distribution of earthquakes revealed an eastward and westward hypocentral migration from the starting point suggesting the division of the seismic swarm into four major clusters. The hypocentral migration was corroborated by the Coulomb stress change calculation, indicating that four fault segments involved in the rupture process successively failed by stress change encouragement. Examination of fluid flow brought out that it cannot be unambiguously considered as the driving mechanism for the successive failures.
Relocation of the 2001 Earthquake Sequence in Aegion, Greece
Studia Geophysica et Geodaetica, 2004
The western part of the Corinth Gulf attracts attention because of its seismically active complex fault system and considerable seismic hazard. Close to the city of Aegion, damaged by the M L 6.2 earthquake of 1995, a sequence of small earthquakes occurred from February to May 2001. The sequence, comprising 171 events of M L 1.8 to 4.7, was recorded by a short-period network of the University of Patras, PATNET. As most stations have single component-recording, the S-wave arrival time readings were scarce. A subset of 139 events was recorded by at least 5 stations, and in this study we limit ourselves just to that subset. A preliminary location is performed by a standard linearized kinematic approach, with several starting depths and crustal models. Then the mainshock is relocated , and finally it is used as a master event to locate the remaining events. The mainshock relocation is performed by a systematic 3D grid search, and the trade-off between depth and origin time is eliminated by a special procedure, the so-called station difference (SD) method. In the SD method, instead of inverting arrival times directly, their intra-station differences are employed. The station corrections, determined from the master event, are also used. As a result, the subset is imaged as a relatively tight cluster, occupying space of about 5 by 5 km horizontally and 10 km vertically, with the mainshock inside (at a depth of 7 km). The results should be interpreted with caution, mainly as regards the "absolute" depth position of the cluster. A more accurate location would require a local network with both P and S readings.
2013
On 10 April 2007, three moderate earthquakes with Mw = 4.9-5.1 occurred in the vicinity of Trichonis Lake (W. Greece). A local network composed of 12 three-component digital seismographs was installed in the epicentral area and recorded more than 1600 events. The double-difference algorithm HYPODD, incorporating both catalogue and waveform cross-correlation differential travel-time data, was applied for the successful relocation of 1490 earthquakes. The latter led to the distinction of a main NW-SE trending and NE-dipping zone, as well as of three neighboring faults; a conjugate NW-SE striking and SW-dipping marginal fault mapped along the northeastern flanks of the lake; a E-W trending and south-dipping low-angle normal fault, possibly related to the major Agrinio Fault Zone (AFZ), parallel to the northern bank of the lake; a NE-SW striking and NW-dipping normal fault, likely related to a segment of the active Evinos fault, located south of the lake. Calculation of the Coulomb stre...
Bulletin of The Seismological Society of America, 2008
In early April 2007, a series of moderate earthquakes (M L 4.1-4.8) occurred in the area of Trichonis Lake in western Greece. The earthquake activity was well recorded by the Hellenic Broadband Seismic Network (HL) operated by the National Observatory of Athens. Initial locations for 156 events of the swarm showed a diffuse image of seismicity. Subsequently, 101 events are precisely relocated, calculating source-specific station terms and differential travel times from waveform cross correlation. Uncertainties in relocations are estimated with a bootstrap approach by randomly weighting the original picks and the differential times. Additionally, waveforms of seven out of the eight largest earthquakes of the swarm were inverted in order to derive regional moment tensor solutions. The results showed a tight northnorthwest-south-southeast cluster located on an offshore extension of a similarly oriented fault trace mapped onshore. Moment tensor solutions indicate normal faulting with a substantial component of left-lateral strike-slip motion. It is possible that this identified fault forms part of a link that connects the Gulf of Corinth rift system to the west-northwest-east-southeast fault zone south of Trichonis basin.
2016
In early April 2007, a series of moderate earthquakes (ML 4.1–4.8) oc-curred in the area of Trichonis Lake in western Greece. The earthquake activity was well recorded by the Hellenic Broadband Seismic Network (HL) operated by the Na-tional Observatory of Athens. Initial locations for 156 events of the swarm showed a diffuse image of seismicity. Subsequently, 101 events are precisely relocated, calcu-lating source-specific station terms and differential travel times from waveform cross correlation. Uncertainties in relocations are estimated with a bootstrap approach by randomly weighting the original picks and the differential times. Additionally, wave-forms of seven out of the eight largest earthquakes of the swarm were inverted in order to derive regional moment tensor solutions. The results showed a tight north-northwest–south-southeast cluster located on an offshore extension of a similarly oriented fault trace mapped onshore. Moment tensor solutions indicate normal fault-ing wi...