Vassilis Karakostas

Properties of the 2003 Lefkada, Ionian Islands, Greece, Earthquake Seismic Sequence and Seismicity Triggering

On 14 August 2003, Lefkada Island (Central Ionian) was strongly af- fected by an Mw 6.2 earthquake. A dense temporary seismic network was installed one day after and accurately located hundreds of aftershocks that defined in detail the main rupture, as well as the activity distribution in the neighboring fault segments. The main rupture occupied the northwestern part of the coastline and trends north- northeast-south-southwest in agreement with regional tectonics. Regional network locations were appropriately calibrated using the local network data, allowing the relocation of the mainshock and strong (M 4.5 or larger) aftershocks during the first day. Intense aftershock activity took place up to 40 km beyond the southern end of the main rupture. Theoretical static stress changes from the mainshock give a preliminary explanation for the aftershock distribution aside from the main rupture, as well as triggering of seismicity in the nearby Kefalonia fault, providing evidence for future seismic hazard ensuing from this fault.

Synthetic earthquake catalogs simulating seismic activity in the Corinth Gulf, Greece, fault system

The characteristic earthquake hypothesis is the basis of time-dependent modeling of earthquake recurrence on major faults. However, the characteristic earthquake hypothesis is not strongly supported by observational data. Few fault segments have long historical or paleoseismic records of individually dated ruptures, and when data and parameter uncertainties are allowed for, the form of the recurrence distribution is difficult to establish. This is the case, for instance, of the Corinth Gulf Fault System (CGFS), for which documents about strong earthquakes exist for at least 2000 years, although they can be considered complete for M ≥ 6.0 only for the latest 300 years, during which only few characteristic earthquakes are reported for individual fault segments. The use of a physics-based earthquake simulator has allowed the production of catalogs lasting 100,000 years and containing more than 500,000 events of magnitudes ≥ 4.0. The main features of our simulation algorithm are (1) an average slip rate released by earthquakes for every single segment in the investigated fault system, (2) heuristic procedures for rupture growth and stop, leading to a self-organized earthquake magnitude distribution, (3) the interaction between earthquake sources, and (4) the effect of minor earthquakes in redistributing stress. The application of our simulation algorithm to the CGFS has shown realistic features in time, space, and magnitude behavior of the seismicity. These features include long-term periodicity of strong earthquakes, short-term clustering of both strong and smaller events, and a realistic earthquake magnitude distribution departing from the Gutenberg-Richter distribution in the higher-magnitude range.

Rupture zones in the area of the 17.08.99 Izmit (NW Turkey) large earthquake (Mw 7.4) and stress changes caused by its generation

The Mw 7.4 Izmit earthquake of 17 August 1999 struck a part ofthe North Anatolian fault in the area of Izmit Bay (NW Turkey). Historicalinformation shows that the fault which moved during the generation of thisearthquake consists of two fault segments moved during the generation oflarge (M ∼ 7) earthquakes in 1719 and 1754, respectively. Since then onlythe central part (between Izmit and Lake Sapanca) of this fault ruptured bythe generation of a smaller shock (M = 6.6) in 1878.The spatial stress variations based on the calculation of changes in theCoulomb Failure Function (ΔCFF) associated with this earthquake aresupported by the distribution of strong aftershock foci. Large positive valuesof ΔCFF to the east and west of the mainshock epicenter are inagreement with the notion that secondary faults were triggered there by thegeneration of the main event. Large positive values of ΔCFF are alsoobserved in the adjacent western fault segment where the 1766 event wasgenerated, evidencing the occurrence of the next strong earthquake in thissegment.

Major active faults of SW Bulgaria: implications of their geometry, kinematics and the regional active stress regime

Abstract Southwest Bulgaria is an intracontinental region between the Dinaro-Hellenic and Balkan mountain ranges that has experienced infrequent, but strong and destructive earthquakes. The general geometric and kinematic characteristics of the major faults, mainly the active ones, are investigated, as the seismic activity is insufficient to describe thoroughly the active crustal deformation associated with the faulting. The results suggest a major rupture zone with a length of more than 50 km. The east-west-striking Kochani-Kroupnik-Bansko ‘rupture zone’ was potentially associated with the large 1904 Kroupnik earthquakes, and has been found to transect the region joining the Kochani, Kroupnik and Bansko faults. In addition, a long-term slip rate ranging from 0.14 to 0.7 mm a−1 has been estimated for some large faults in the region using morphotectonic features. The most active faults are normal ones striking WNW-ESE to ENE-WSW, whereas the NNW-SSE- to NW-SE-striking faults tended to act as barriers to the growth of the former faults, as they do not exhibit much indication of recent reactivation. The stress regime determined is extensional with the least principal stress axis (σ3) subhorizontal and oriented north-south. The fact that the active faults show geometric and kinematic characteristics, as well as estimated long-term slip rates, similar to those of the active faults of central and eastern Macedonia and Thrace (Northern Greece) suggests that both of these regions share a single contemporary stress field.

A Non-Extensive Statistical Physics View in the Spatiotemporal Properties of the 2003 (Mw6.2) Lefkada, Ionian Island Greece, Aftershock Sequence

Investigation of the spatiotemporal properties of the 2003 Lefkada seismic sequence is performed through non-extensive statistical physics. Information on highly accurate aftershock source parameters became feasible from the recordings of a portable digital seismological network that was installed and operated in the study area, during the evolution of the seismic sequence. Thus, the spatiotemporal distribution of aftershocks onto the main and neighboring fault segments was investigated in detail, enabling the recognition of four distinctive seismicity clusters separated by less active patches. The aftershock spatiotemporal properties are studied here, using the ideas of non-extensive statistical physics (NESP). The cumulative distribution functions of the inter-event times and the inter-event distances are presented using the data set in each seismicity cluster, and the analysis results in values for the statistical thermodynamic qT and qD parameters for each cluster, where qT varies from 1.16 to 1.47 and qD from 0.42 to 0.77 for the inter-event times and distances distributions, respectively. These values confirm the complexity and non-additivity of the spatiotemporal evolution of seismicity, and the applicability of the NESP approach in investigating aftershocks sequence. The temporal pattern is discussed using the closely connected to NESP approach of superstatistics, which is based on a superposition of ordinary local equilibrium statistical mechanics. The result indicates that the temporal evolution of the Lefkada aftershock sequence in clusters A, B and C is governed by very low number of degrees of freedom, while D is a less organized seismicity structure with a much higher number of degrees of freedom.

A detailed analysis of microseismicity in Samos and Kusadasi (Eastern Aegean Sea) areas

A detailed investigation of microseismicity and fault plane solutions are used to determine the current tectonic activity of the prominent zone of seismicity near Samos Island and Kusadasi Bay. The activation of fault populations in this complex strike-slip and normal faulting system was investigated by using several thousand accurate earthquake locations obtained by applying a double-difference location method and waveform cross-correlation, appropriate for areas with relatively small seismogenic structures. The fault plane solutions, determined by both moment tensor waveform inversions and P-wave first motion polarities, reveal a clear NS trending extension direction, for strike slip, oblique normal and normal faults. The geometry of each segment is quite simple and indicates planar dislocations gently dipping with an average dip of 40–45°, maintaining a constant dip through the entire seismogenic layer, down to 15 km depth.