A. Serpetsidaki

Iterative Deconvolution of Regional Waveforms and a Double-Event Interpretation of the 2003 Lefkada Earthquake, Greece

The moment tensor inversion for multiple point sources, based on Kikuchi and Kanamori (1991), was extended to full waveform data at regional (or local) distances. The new code proved to be efficient for retrieving major source contributions of the 2003 Lefkada, Greece, earthquake. The source model was derived from five three-component regional stations (epicentral distances <140 km), at periods 10–20 s. Two main events dominated the rupture process, one at the Lefkada Island (comprising three subevents of total moment 0.9 × 1018 N m) and the other at the Cephalonia Island (comprising one subevent of 0.5 × 1018 N m). Their spatial and temporal separation is 40 km and 14 s, respectively. They can be understood as two earthquakes. The uncertainty estimate based on reduced data sets (repeatedly excluding a station) shows that the Cephalonia subevent and the major Lefkada subevent are very well resolved regarding their position, time, and focal mechanism. The source model explains well the aftershock distribution, characterized by two clusters at the Lefkada and Cephalonia Islands, respectively. The focal mechanisms of the two main subevents are predominantly right-lateral strike slip of south-southwest–north-northeast orientation. The Cephalonia subevent occurred on a less steeply dipping fault with a small thrust component. Large deviations from pure double couple were found but interpreted as artifacts. The new software developed in this article (Fortran code and Matlab graphic user interface) is freely available. Online material: Color graphics and 3D visualization of the 2003 Lefkada earthquake sequence.

Asperity break after 12 years: The Mw6.4 2015 Lefkada (Greece) earthquake

The Mw6.4 earthquake sequence of 2015 in western Greece is analyzed using seismic data. Multiple point source modeling, nonlinear slip patch, and linear slip inversions reveal a coherent rupture image with directivity toward the southwest and several moment release episodes, reflected in the complex aftershock distribution. The key feature is that the 2015 earthquake ruptured a strong asperity, which was left unbroken in between two large subevents of the Mw6.2 Lefkada doublet in 2003. This finding and the well-analyzed Cephalonia earthquake sequence of 2014 provide strong evidence of segmentation of the major dextral Cephalonia-Lefkada Transform Fault (CTF), being related to extensional duplex transform zones. We propose that the duplexes extend farther to the north and that the CTF runs parallel to the western coast of Lefkada and Cephalonia Islands, considerably closer to the inhabited islands than previously thought. Generally, this study demonstrates faulting complexity across short time scales (earthquake doublets) and long time scales (seismic gaps).

Local high-resolution passive seismic tomography and Kohonen neural networks — Application at the Rio-Antirio Strait, central Greece

A high-resolution passive seismic investigation was performed in a 150 km 2 area around the Rio-Antirio Strait in centralGreeceusingnaturalmicroearthquakesrecordedduringthreemonthsbyadense,temporaryseismicnetworkconsisting of 70 three-component surface stations. This work was part of the investigation for a planned underwater rail tunnel, and it gives us the opportunity to investigate the potential of this methodology. First, 150 well-located earthquake events were selected to compute a minimum 1D velocity model for the region. Next, the 1D model served as the initialmodelfornonlinearinversionfora3DP-andS-velocity crustal structure by iteratively solving the coupled hypocenter-velocity problem using a least-squares method. The retrieved Vp and Vp/Vs images were used as an input to Kohonen self-organizing maps SOMs to identify, systematicallyandobjectively,theprominentlithologiesintheregion. SOMs are unsupervised artificial neural networks that map the input space into clusters in a topological form whose organization is related to trends in the input data. This analysis revealed the existence of five major clusters, one of which may be related to the existence of an evaporite body not shown in the conventional seismic tomography velocity volumes. The survey results provide, for the first time, a 3D model of the subsurface in and around the Rio-Antirio Strait. It is thefirst time that passive seismic tomography is used together with SOM methodologies at this scale, thus revealing themethod’spotential.

Seismicity and Seismotectonics in Epirus, Western Greece: Results from a Microearthquake Survey

During a twelve-month passive tomography experiment in Epirus, in northwestern Greece, a total of 1368 microearthquakes were located. The most accu- rately located events and focal mechanisms are used here to understand the seismo- tectonics of the area. The seismicity shows a clear association with the main, previously defined deformation zones. A total of 434 well-defined focal mechanisms were also used for the determination of the stress pattern in the area. The computed stress-field pattern is quite complex close to the surface and almost homogeneous at depths below 15 km. For these depths, the stress field is purely compressional in a west-southwest direction, whereas for shallow depths it is transpressional or even extensional for some smaller areas. The abrupt change in the stress pattern, which occurs as depth increases, suggests the existence of a detachment surface, which is provided by the evaporites that have intruded into the upper layers through the thrust zones. The presence of the evaporites and their lateral extent is mapped by the seismicity distribution and con- firmed by seismic tomography. Based on the findings, we estimate a possible total evaporite thickness of almost 10 km at least for the central part of the study area. Such a result is important for the oil exploration efforts that have just started in Epirus.

Relocation of the 2001 Earthquake Sequence in Aegion, Greece

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 ML 6.2 earthquake of 1995, a sequence of small earthquakes occurred from February to May 2001. The sequence, comprising 171 events of ML 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 sub-set of 139 events was recorded by at least 5 stations, and in this study we limit ourselves just to that sub-set. A preliminary location is performed by a standard linearized kinematic approach, with several starting depths and crustal models. Then the mainshock is re-located, 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 sub-set 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.

Seismicity and Seismotectonics Study in Southwestern Albania Using a Local Dense Seismic Network

Abstract The Albanides represent a complex orogen made up of a heterogeneous tectonic nappe pile of Paleozoic, Mesozoic, and Cenozoic domains. Albania is tectonically active, and moderate to strong earthquakes have occurred in the past. However, abundant microseismicity has not been monitored and studied by a dense seismic network. During this study, a seismic network of 40 stations was deployed in southern Albania for one year. A total of 2113 microearthquakes were well located. The most accurately located events and 810 focal mechanisms were used in order to define the seismotectonics and the stress pattern in the area. Results indicate that thrust and strike‐slip faulting both exist in southwestern Albania, suggesting a continuation of the complex tectonic setting of the neighboring northwestern Greece to the north.

Seismotectonics of Lower Assam, Northeast India, Using the Data of a Dense Microseismic Network

Northeast India has been subjected to extensive compressional forces, mainly in north–south and east–west directions resulting from the convergence of the Indian plate with the Eurasian and Burmese plates, respectively. The area is characterized as one of the most seismically active regions of the world; however, the lower Assam valley’s microseismicity has not been monitored and studied intensively by a dense seismic network during the past. During this study, a seismic network of 76 stations was deployed in northeastern India for one year. Hundreds of microearthquakes were recorded. The most accurately located events, moment tensor solutions, and focal mechanisms were used in order to define the seismotectonic and stress regime in the area.