Efthimios Sokos

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.

Methane and hydrogen sulfide seepage in the northwest Peloponnesus petroliferous basin (Greece): Origin and geohazard

Gas seepages along the Ionian coast of the northwestern Peloponnesus (Greece), at Killini, Katakolo, and Kaiafas reflect deep hydrocarbon-generation processes and represent a real hazard for humans and buildings. Methane microseepage, gas concentration in offshore and onshore vents, and gas dissolved in water springs, including the isotopic analysis of methane, have shown that the seeps are caused by thermogenic methane that had accumulated in Mesozoic limestone and had migrated upward through faults, or zones of weakness, induced by salt diapirism. A link between local seismicity and salt tectonics is suggested by the analyses of hypocenter distribution. Methane acts as a carrier gas for hydrogen sulfide produced by thermal sulfate reduction and/or thermal decomposition of sulfur compounds in kerogen or oil. Methane seeps in potentially explosive amounts, and hydrogen sulfide is over the levels necessary to induce toxicological diseases and lethal effects.

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.

Upper crustal structure of the western Corinth Gulf, greece, inferred from arrival times of the January 2010 earthquake sequence

The western part of the Corinth Gulf attracts attention due to its seismically active fault system and considerable seismic hazard. A moderate size earthquake occurred close to the town of Efpalio on January 18, 2010, followed by a sequence of smaller earthquakes. In the present paper we use this sequence to derive a local structural model for the region in the vicinity of Efpalio. The model is based on the minimization of traveltime residuals. In particular, we used arrival times from 51 selected events recorded on January 19 and 20 by at least 5 stations at epicentral distances less than about 25 km. A variant of the method of conjugate gradients has been used for this purpose. In comparison with several previous models, the new model is characterized by higher velocities to a depth of about 8 km. The velocity ratio in the model is vP / vS = 1.83. The hypocentres of the selected earthquakes lay at depths between about 5 and 9 km, but their distribution is rather irregular.

Tsunami Hazards Associated with the Perachora Fault at Eastern Corinth Gulf, Greece

We investigate the tsunami hazard associated with the Perachora fault at eastern Corinth Gulf, Greece. Realistic faulting parameters are used to model expected coseismic displacements of the seafloor. This study also investigates the effect that rupture complexity has on the local tsunami wave field. Several earthquake scenarios are used as initial conditions for tsunami stimulation either considering the numerous published constant slip models or taking into account the fault rupture complexity with the help of a realistic finite-extent k 2 stochastic kinematic source model with k-dependent rise time. The obtained results indicate that there is sub- stantially more variation in the local tsunami wave field derived from the inherent complexity of the shallow fault zone than predicted by a simple elastic dislocation model. Rupture complexity, as represented by heterogeneous slip-distribution pat- terns, has an important effect on near-field tsunami records. Tsunami hazards as- sessments based on only a few scenario earthquakes may not accurately account for natural variation in tsunami amplitude caused by earthquake rupture complexity.

Earthquake location from P-arrival times only: problems and some solutions

Selected problems related to accurate hypocenter locations are discussed in the difficult case that only reliable P-wave readings are available. Near stations are usually only few, and often have a poor azimuthal coverage. As such, they are insufficient because the inversion is highly ill-posed, and the epicenter position strongly trades-off with depth. Thus more distant stations are also needed to obtain the correct epicenter. However, joint use of near and distant stations present another difficulty; it may yield a significantly incorrect depth estimate in case that the crustal model is not fully appropriate. In practice, the erroneous depth often remains unrecognized. An indication of the depth problem can be obtained by analyzing the travel-time residuals at individual stations. It is also useful to check fully independent depth estimates, for example those from the centroid-moment-tensor analysis. If the problematic crustal model is detected, and it is not easy to find a better one, the near- and distant station effects should be decoupled (a two-step location): the epicenter is calculated from all stations, kept fixed, and the source depth is grid-search beneath the epicenter by means of the near stations. The ideas are applied to the Mw 5.2 Efpalio (Western Greece) earthquake of January 18, 2010, and the following aftershock sequence.

The Virtual Seismologist in SeisComP3: A New Implementation Strategy for Earthquake Early Warning Algorithms

The feasibility of earthquake early warning (EEW) is now widely recognized. However, EEW systems that are in operation or under evaluation worldwide have significant variations and are usually operated independently of routine earthquake monitoring. We introduce a software that allows testing and evaluation of a well‐known EEW algorithm directly within a widely used earthquake monitoring software platform. In the long term, we envision this approach can lead to (1) an easier transition from prototype to production type EEW implementations, (2) a natural and seamless evolution from very fast EEW source parameter estimates with typically large uncertainties to more delayed but more precise estimates using more traditional analysis methods, and (3) the capability of seismic networks to evaluate the readiness of their network for EEW, and to implement EEW, without having to invest in and maintain separate, independent software systems. Using the Virtual Seismologist (VS), a popular EEW algorithm that has been tested in real time in California since 2008, we demonstrate how our approach can be realized within the widely used monitoring platform SeisComP3. Because this software suite is already in production at many seismic networks worldwide, we have been able to test the new VS implementation across a wide variety of tectonic settings and network infrastructures. Using mainly real‐time performance, we analyze over 3200 events with magnitudes between 2.0 and 6.8 and show that, for shallow crustal seismicity, 68% of the first VS magnitude estimates are within ±0.5 magnitude units of the final reported magnitude. We further demonstrate the very significant effect of data communication strategies on final alert times. Using a Monte Carlo simulation approach, we then model the best possible alert times for optimally configured EEW systems and show that, for events within the dense parts of each of the seven test networks, effective warnings could be issued for magnitudes as small as M 5.0.

Scisola: Automatic Moment Tensor Solution for SeisComP3

An automatic moment tensor is a significant product of regional seismic networks and an essential piece of information for real‐time seismological applications, like shake maps or tsunami warning. In this article, we present scisola, a new software for automatic moment tensor (MT) retrieval, based on the ISOLated Asperities (ISOLA) MT inversion code and SeisComP3 real‐time processing system. Scisola connects SeisComP3 with the ISOLA FORTRAN codes by retrieving event, station, waveform, and instrumental response data from SeisComP3 and passing the information to ISOLA. The Green’s function calculation and centroid spatiotemporal grid search are done in parallel mode in scisola, thus the computational time is significantly reduced. The user has full control of all calculation aspects, for example, frequency range of inversion and station selection through the creation of magnitude‐based rules. Scisola is programmed in Python and provides a complete graphical user interface (GUI) and a database for storing the results. The automatic solution is stored in the database, and the user is able to revise it through a GUI. The software provides a complete logging of processing steps, extended graphical output, a text file useful for e‐mail dissemination, and a handful of quality indexes of the solutions. The code’s performance was tested against manual MT solutions and proved to be efficient.

Fitting Waveform Envelopes to Derive Focal Mechanisms of Moderate Earthquakes

Focal mechanisms of shallow crustal earthquakes with magnitude 4 or less remain unresolved in many regions of the world due to the sparseness of seismic networks. Relatively distant stations must be used, but modeling those waveforms is difficult due to imprecision of seismic-velocity models. The waveform inversions also suffer from instrumental problems, such as flipped channel sign and errors in gain or timing. In such situations, waveforms should therefore be substituted by a robust characteristic which still carries information about focal mechanism and moment magnitude. We propose inversion of waveform envelopes. Three application examples demonstrate the performance of the method for earthquakes in various ranges of epicentral distance and frequency. In the examples, the focal mechanisms of the sample earthquakes are known from the waveform inversion approach based on near-source stations. The near-source stations are then ignored completely. Use of the remaining stations fails to provide the true mechanism through waveform inversion, but a correct estimate of the solution is found using envelopes. This method may be of interest to seismologists who need to compute focal mechanisms for stress-field studies in sparse networks, where traditional waveform inversion and amplitude inversion methods often fail. Electronic Supplement: Figures and text illustrating robustness of envelopes relative to waveforms, problems when increasing frequency, importance of the posterior polarity constraint, and synthetic tests addressing the effects of noise and imprecise