Alexandros Chatzipetros

Seismic fault geometry and kinematics of the 13 May 1995 Western Macedonia (Greece) earthquake

Abstract During the devastating earthquake of 13 May 1995, in the Kozani-Grevena area (Western Macedonia, Greece), many surface ruptures formed in the epicentral area. Most of these fractures were due to faulting, but some were secondary ground ruptures and landslides. Geological field work in the area has shown that the Aliakmon river neotectonic fault consists of several (three or more) fault strands: the Servia, the Rymnio and the Paleochori-Sarakina strands. Using geological criteria, all of these fault strands were judged to be active faults affecting recent (Holocene) deposits and scree. The main new surface fractures caused by the earthquake, and particularly those clearly of tectonic origin, follow systematically the traces of the last two neotectonic fault strands, forming a new fracture line. This tectonic line, trending ENE-WSW (N 70 °), coincides with the focal mechanism solution and the satelite image major lineament. Both the geological and instrumental seismological data suggest that the seismogenic fault is a segment of the Aliakmon river neotectonic fault zone situated among the villages of Rymnio, Paleochori, Sarakina, Kentro and Nisi. The total length of the reactivated fault segment is about 30km long overall and is separated from the non-activated Servia fault segment by a geometrical seismic segment barrier near the village of Goules. The seismic fault is a normal fault trending ENE-WSW and dipping to NNW, with high angle at the surface and low angle at depth. The majority of the epicentres of the seismic sequence were distributed on the hangingwall of this reactivated fault segment. Additionaly a series of subparallel antithetic surface fractures, mainly striking E-W or ENE-WSW and dipping to the South, following previous neotectonic strike-slip faults, were reactivated during the earthquake with the geometry of normal faults antithetic to the main seismic fault. The most important of these are the Chromio-Varis-Myrsina fracture line (length 15km), along the Vourinos corridor dextral strike-slip structure and the Felli fracture line (length 6 km) along the Felli sinistral strike-slip fault. An interpretation of the geometry and kinematics of the reactivated faults is shown in the proposed geological model with simplified cross sections.

Understanding the 13 May 1995 western Macedonia earthquake: A paleoseismological approach

Abstract Paleoseismological research by means of trenching in the area that was affected by the Kozani-Grevena strong (Ms = 6.6) earthquake sequence, revealed evidence for past reactivations of the same seismogenic fault. Five trenches were excavated along the Palaeochori-Sarakina part of the fault, in which three surface faulting paleoevents were identified at ca. 8.97, 36.7 and 72.5 ka BP (TL dates). Recurrence interval based on these datings is about 30 ka, which is very long, verifying the ‘low seismicity’ status of the area. On this basis, the 1995 earthquake was an out of sequence event, because the elapse time since the last major event is 8.97 ka instead of 30. Assuming a constant rate of strain accumulation, this would also explain the small amount of surface displacement that was observed during the 1995 earthquake (maximum 18 cm, usually up to 10 cm) in respect to the displacements observed in the trenches (> 25 cm) for previous paleoevents.

GEOMETRY, KINEMATICS AND MORPHOTECTONICS OF THE YANQING-HUAILAI ACTIVE FAULTS (NORTHERN CHINA)

Field studies along the Huailai and Yanqing mutually parallel neotectonic fault zones lead to a description of morphotectonics and palaeoseismicity, as well as the geometry and segmentation of these structures. The study emphasizes the kinematics of the region. The faults belong to the tectonic interaction between the Shanxi rift system and the North China fault system. Although the general strike of the zones is NE–SW, the Yanqing fault is divided into five segments of NNE–SSW, ENE–WSW and E–W trends, and the Huailai fault into three segments of ENE–WSW and E–W trends. They are mainly pure dip-slip normal faults, while less often they show oblique-slip components. The mesostructural quantitative palaeostress analyses (local neotectonic–active stress pattern) indicate a regional NNW–SSE-oriented extension (σ3), locally deviated to N–S, NW–SE, WNW–ESE, depending on the fault segment strike. The great deviation of fault strikes from the mean and their relation with the basement joint and thrust systems lead to the hypothesis that the neotectonic faults, which have a capacity for strong earthquakes, follow pre-existing inherited structures.

Evidence for late Holocene activity along the seismogenic fault of the 1999 Izmit earthquake, NW Turkey

Abstract During the strong 1999 Izmit (Kocaeli) earthquake, 100 km long east-west-striking (N80°–100°), right-lateral, fault traces were formed. In the epicentral area the seismic ruptures did not follow any known or mapped fault traces, but morphology and tectonostratigraphic evidence from trenches reveal pre-existing earthquake-related features, e.g. elongated valleys, shutter ridges, high-angle slopes, scarplets and stream offsets. In the Gölcuk Peninsula a characteristic NW-SE-trending extensional fault segment emerged at the surface with a 1.5–2 m maximum vertical displacement and a 0.30 m right-lateral component. The resulting coseismic fault scarp was mapped in detail, and two trenches were excavated at the Dëniz Evler site. The 1999 displacement at this site was 1.50 m, whereas the penultimate event displaced the same sediments by 0.70 m, and a previous event by 0.20 m. Displacement is not characteristic, as fault-associated soft (recent) deltaic deposits, and the fault itself, are typically not coseismic, but rather a secondary accommodation structure in geometric consistency with the right-lateral main displacement zone. The data were compared with similar results from the Aşãgı Yuvacik, Kular Yaylacik and Acisu sites between Izmit and Sapanca Lake. The same fault segment seems to have been activated and produced surface ruptures including during the earthquakes of AD 1509, AD 989 and AD 554, plus two prehistoric events. The palaeoseismological results provide clear evidence for repeated reactivation of the same fault or fault segments during historical seismic events.

Throw partitioning across normal fault zones in the Ptolemais Basin, Greece

Abstract The total throw across a fault zone may not occur entirely on a single fault strand but may be distributed onto several strands or may be accommodated by distributed deformation within or adjacent to the fault zone. Here we conduct a quantitative analysis of the partitioning of throw into three components, the throw accommodated by: (a) the largest fault strand; (b) subsidiary faults; and (c) continuous deformation in the form of bed rotation in sympathy with the fault downthrow direction. This analysis is applied to seven seismic-scale fault zones at outcrop resolution (maximum throw 50 m) that were mapped over a four-year period during open-cast lignite mining within the late Miocene–Pliocene Ptolemais Basin, West Macedonia, Greece. The analysis shows that the fault zones offsetting the lignite–marl sequence are more localized at higher throws with progressively more of the total throw accommodated by the largest fault strand. Normal drag, which can account for up to 12 m of the total throw, accommodates a lower proportion of the total throw on larger faults. It appears that initial fault segmentation is the main control on the degree of, and spatial variation in, fault throw partitioning.

Ground fissures in the area of Mavropigi Village (N. Greece): Seismotectonics or mining activity?

In the beginning of July 2010, a ground fissure was observed in the field near the village of Mavropigi (Northern Greece) and specifically in its NW side. Later on (early September), a second ground fissure was perceived, close and almost parallel to the first one and very close to the limits of the lignite exploitation mine (by the Public Power Corporation, PPC). It was observed that the village of Mavropigi slides away slowly towards the PPC lignite mine. Geological, seismological, as well as geotechnical survey in the field indicated that the phenomenon is related to the coal mining exploitation in the near vicinity of the village rather than to any seismotectonic activity in the surrounding area.

Simulation of Off-Fault Surface Effects from Historical Earthquakes: The Case of the City of Thessaloniki (Northern Greece)

Thessaloniki, the second largest city in Greece, is located at the northeastern shore of Thermaikos Gulf, a shallow water body and remnant of the Axios valley sedimentary system that has received massive sedimentation. The broader region is surrounded by neotectonic fault zones, several of which can be considered as active, based on their kinematic data compatibility with the active stress field and the frequent microseismicity. Part of the city is built on basement rocks of primarily Mesozoic age, but most of it is founded on the sedimentary filling of the Axios basin that has been active since Eocene. Historical records indicate that Thessaloniki, has suffered several earthquake disasters, both from sources close to the city, as well as from more distant sources (e.g. 1829 M7 Drama and 1904 M7 Kresna events). The event of 1759 (MMI IX; M6.5) is reported as the most destructive for Thessaloniki with a postulated epicenter at the Axios River estuary in Thermaikos Gulf. We combine the most updated tectonic and seismological data for the broader Thessaloniki area to postulate a scenario of a moderate-magnitude earthquake and model the expected strong shaking and off-fault coseismic surface effects, with emphasis on liquefaction phenomena that match the damage patterns described in historical archives. In particular, we delineate zones prone to liquefaction occurrence and evaluate the potential of the geological units. The latter is achieved based on data provided by in situ tests and the former based on information provided by geological maps regarding the age and depositional process of the sediments.

An Assessment of the Earthquakes of Ancient Troy, NW Anatolia, Turkey

According to the intermittent archeological excavations, carried out from 1871 up to the present, there exist nine different layers of settlements in Troy. Although there is some archeological evidence which indicates that some of these layers, especially Troy III (B.C. 2200‐2050) and Troy VI (B.C. 1800‐1275) have been damaged by one or more earthquakes, no multidisciplinary geoscientific research has been carried out so far on the active faults which could have caused these earthquakes.