Riccardo Caputo

The use of electrical resistivity tomographies in active tectonics: examples from the Tyrnavos Basin, Greece

Abstract A 2D Electrical Resistivity Tomography (ERT) survey was carried out in the tectonically active Tyrnavos Basin, Eastern Thessaly, Greece. The principal aims of this research are to test the efficiency of this relatively new geoelectrical technique when applied to the recognition and the geometrical characterisation of active faults and to improve our tectonic knowledge of the investigated area. We therefore carried out several tests performing the geophysical prospecting across morphotectonic scarps or fault traces along which the Late Quaternary tectonic activity is well documented by previous structural, stratigraphic, morphotectonic and palaeoseismological researches. The tests concerned the electrode spacing, the maximum depth of investigation, the quality-to-costs ratio, etc. In a second phase of the survey, we applied this geophysical methodology to specific sites along the major tectonic structures bordering the Tyrnavos Basin, in order to obtain, firstly, a better tectonic knowledge of the area, secondly, to solve particular and local geological problems and, thirdly, to help deciding between ambiguities left unsolved by superficial surveys. Accordingly, numerous ERT with different electrode spacing (from 2 up to 50 m) and depth of investigation (from 0.5 to 120 m) were performed with a dipole-dipole array using a multielectrode system, with 32 electrodes equally spaced along a straight line, for data acquisition. Combining advanced technologies for data acquisition and new tomographic techniques for resistivity data inversion, we obtain a large data-set of high-resolution electrical images of the subsurface across active faults. General and particular results concerning the application of ERT techniques for mapping subsurface active faults are discussed, while advantages and limits in applying this geophysical methodology are emphasised.

Late Cainozoic geodynamic evolution of Thessaly and surroundings (central-northern Greece)

Abstract In the framework of the late Alpide deformation of Greece and of the recent and active extensional tectonism of the Aegean region, the geotectonic evolution of the Thessaly region (central-northern Greece) has been examined, using a quantitative and qualitative structural analysis; stratigraphie, sedimentological, morphotectonic and seismological data. The geometry of the faults, their architecture and the knowledge of the stress pattern are used to explain some aspects of the tectonics and crustal dynamics of Thessaly and the surrounding area. The oldest compressional phases taken into account show a mean ENE-WSW trending direction of shortening and have been defined as late Alpide (early Aquitanian and Langhian). A later (Late Miocene-Pliocene) NE-SW oriented extensional phase has been related to the Hellenic post-orogenic collapse which developed behind the collisional front between the Aegean (Eurasia) and African plates. This phenomenon diachronically migrated from the east (Central Macedonia, Thermaikos Basin) towards the west (Epirus, Albania) where it is still active. As a consequence of this second phase, the area forms a basin and range like structure. The third, and last, phase (Middle Pleistocene-present) is characterized by a N–S direction of extension and affects the entire Aegean region. It generated new E–W trending basins, superimposed on the inherited ones. This gave as a final result, the complex block pattern we can see today. The recent and active right-lateral strike-slip movements along the North Aegean Trough seem to stop in the Sporades Basin and do not affect the uppermost crust of mainland Greece. A further WNW-ESE directed extension observed occasionally in central and northern Greece could be explained by local events or as block-related deformation.

The large tsunami of 26 December 2004 : Field observations and eyewitnesses accounts from Sri Lanka, Maldives Is. and Thailand

Post-event field surveys were conducted and measurements were taken in Sri Lanka and Maldives about two weeks after the catastrophic Indian Ocean tsunami of 26 December 2004. The measurements taken were cross-checked after interviewing with local people. In the southwest, south and east coastal zones of Sri Lanka maximum water levels ranging from h = 3 m to h = 11 m a.m.s.l. were estimated. The highest values observed were in the south of the island: Galle h ∼ 10 m, Hambantota h ∼ 11m. Maximum inundation of d ∼ 2 km was observed in Hambantota. The heavy destruction and thousands of victims caused in coastal communities, buildings and infrastructure, like railways and bridges, is attributed not only to physical parameters, like the strength of the tsunami hydrodynamic flow, coastal geomorphology and the wave erosional action in soil, but also to anthropogenic factors including the increased vulnerability of the non-RC buildings and the high population density. Local people usually described the tsunami as a series of three main waves. The leading wave phase was only a silent sea level rise of h ≤ 1.5 m and d ≤ 150 m, while the second wave was the strongest one. The first two waves occurred between 09:00 and 09:30 local time, depending on the locality. It is well documented that near Galle, southern part, the strong wave arrived at 09:25:30. In the west coast the third wave was a late arrival which possibly represents reflection phases. In Maldives, three waves were also reported to arrive between 09:00 and 09:30 local time. Maximum water level was only h ∼ 3 m in Laamu Atoll, which is interpreted by the wave amplitude damping by the coral reef to the east of the island complex as well as to that the tsunami did not arrived at high tide time. Damage was observed in several islands of Maldives but this was minimal as compared to the heavy destruction observed in Sri Lanka. About 25 Greek eyewitnesses, who happened to experience the tsunami attack in Padong and Blue Lagoon Port of Phuket island as well as in Maya Bay, Phi-Phi islands, Thailand, were interviewed on the basis of a standard questionnaire. The first sea motion was a retreat of at least 100 m. Then, two main waves arrived, the first being the strong one occurring at about 09:55–10:05 local time, with h ∼ 6m in Padong causing significant destruction and human victims. The collected information clearly indicates that the tsunami propagated as the leading crest wave to the west side, e.g. in Sri Lanka and Maldives, and as the leading trough wave to the east, e.g. in Thailand.

The Pliocene-Quaternary tecto-sedimentary evolution of the Larissa Plain (Eastern Thessaly, Greece)

In order to understand the present-day morphological, geographical and environmental patterns of the Larissa Plain, a large amount of historical, archaeological, sedimentologi-cal, stratigraphic, t...

QUICK SURVEY OF THE POSSIBLE CAUSES OF DAMAGE ENHANCEMENT OBSERVED IN SAN GIULIANO AFTER THE 2002 MOLISE, ITALY SEISMIC SEQUENCE

On October 31 and November 1, 2002, two earthquakes of magnitude 5.4 and 5.3 hit the area at the border between the Molise and Puglia regions in Southern Italy. The damage pattern in the epicentral area qualified the quake as an intensity VII MCS event, although providing a notable exception relevant to the small village of San Giuliano di Puglia. Since the first macroseismic survey, it appeared clear that in S. Giuliano the intensity was two degrees higher with respect to three neighbouring villages located within a radius of 3 km. Soon after the quake, our team started a campaign of microtremor HVSR measurements (Horizontal to Vertical Spectral Ratio), then we installed accelerometers and carried out damage and geological surveys. Finally, we performed a geoelectrical tomography and two profiles of Vs velocity with depth using the NASW technique (Noise Analysis of Surface Waves). The preliminary observations indicate that ground motion amplification is present in S. Giuliano within the frequency band that may affect building. A strong velocity contrast 20 m deep causes the predominant peak. More amplification could be due to more complicated, 2D effects. As regards the damage pattern, it divides S. Giuliano in three zones showing different characteristics and seismic behaviour. A building-by-building survey is still under way to better evaluate vulnerability variations in different zones of the village. However, the acquired data so far is sufficient to propose site amplification as a possible cause of the damage enhancement observed in S. Giuliano.

ARCHAEOLOGICAL EVIDENCES OF PAST EARTHQUAKES: A CONTRIBUTION TO THE SHA OF THESSALY, CENTRAL GREECE

Thessaly, Central Greece, like most of the Aegean realm is characterised by a diffuse seismic activity. Both instrumental and historic data are revised and, with few exceptions, this information substantiates that the local seismicity is characterised by a shallow seismogenic depth that can generate important destructions though distributed to limited areas. However, historical sources like diaries and church books or, for more recent events, newspapers are almost completely lacking before the 17th century. On the other hand, recent palaeoseismological surveys carried out along some of the active faults of Eastern Thessaly clearly document the occurrence of past earthquakes during latest Pleistocene and Holocene times. The present research is an attempt of partially filling the gap of our knowledge on the seismicity of the area. At this regard, we followed the methodological approaches of the Historical Seismology and the Archaeoseismology. In particular, we focused our investigations on the interpretation of both direct and indirect evidences issued from Greek inscriptions, monuments and settlements of Ancient and Byzantine times. Special emphasis is given to the recently discovered Great Theatre of Larissa that shows important evidences of seismically induced damages. We analyse all this information with the intention of reconstructing the late Holocene seismic evolution of the northeastern sector of Thessaly, while the final aim of this article is the seismic hazard assessment of this region, whose major urban centre is Larissa that represents the third town of Greece.

Paleoseismicity of the Souli Fault, Epirus, Western Greece

Abstract The E-W trending left-lateral Souli Fault, an old Alpine structure in Epirus, northwestern Greece, affecting Lias-Eocene carbonate and siliceous rocks and Oligocene-Burdigalian flysch exhibits geological evidence of past individual coseismic events. Along some sectors of the fault, or in associated structures, young morphotectonic features have been recognised thereby emphasising the recency of tectonic activity. In a natural paleoseismological section, at least three distinct seismic events pre- and post-dating wedge layers of 20,000 to 26,000 yr (C14) lignite type sediments, have been recognised.

The use of HVSR measurements for investigating buried tectonic structures: the Mirandola anticline, Northern Italy, as a case study

The Mirandola anticline represents a buried fault-propagation fold which has been growing during Quaternary due to the seismogenic activity of a blind segment belonging to the broader Ferrara Arc. The last reactivation occurred during the May 2012 Emilia sequence. In correspondence with this structure, the thickness of the marine and continental deposits of the Po Plain foredeep is particularly reduced. In order to better define the shallow geometry of this tectonic structure, and hence its recent activity, we investigated in a depth range which is intermediate between the surficial morphological observations and seismic profiles information. In particular, we carried out numerous passive seismic measurements (single-station microtremor) for obtaining the horizontal-to-vertical spectral ratio. The results of a combined analysis of the peak frequency and its amplitude nicely fit the available geological information, suggesting that this low-cost geophysical technique could be successfully applied in other sectors of wide morphologically flat alluvial plains to investigate blind and completely buried potential seismogenic structures.

An integrated study to recent tectonics in Central Japan: seismological, geodetic, morphotectonic and structural data compared

Abstract The abundance of seismological, geodetic, morphotectonic and structural data for Central Japan allows an integrated approach to the study of recent tectonics. This study has been conducted in a region where an apparent uniformity of geodynamic conditions existed during Quaternary times. Four independent data sets have been collected: minor and major earthquakes, mesoscale faults and large scale morphological dislocations. All of them have been reduced to a common format including fault plane orientation, direction and sense of movement. The analysis has been carried out by applying different methodologies commonly employed in structural geology in order to obtain both qualitative and quantitative estimates of the stress field, where the latter kind of analyses also provides the principal stress directions and the ratio R = (σ 1 − σ 2 ) (σ 1 − σ 3 ) . The results of each data set are internally consistent. But above all, the similarity between the three principal stress directions of the four data sets is straightforward, showing a clear strike-slip regime with an ESE-WNW-trending σ1 and a NNE-SSW-trending σ3 whose averages are 288°/05° and 198°/10°, respectively. Numerous geodetic data from the region show exactly the same direction of maximum contraction and broadly confirm previous large-scale stress trajectories maps. By considering characteristic depth, age and fault dimension for each fault set, it is possible to understand better the recent (Late Quaternary-Present) tectonic evolution of the area where a progressive NNE-SSW restraining appears to occur. Due to the possible westward migration of the compressional zone associated with the Itoigawa-Shizuoka Tectonic Line, or the southern increasing constrain produced by the Philippine Sea Plate, this geodynamic process will eventually produce a permanent swap of the σ2 and σ3 axes.

Mapping the Palaeo-Piniada Valley, Central Greece, Based on Systematic Microtremor Analyses

The application of seismic noise-based techniques has become particularly popular in the last decades, as they are not invasive and do not require large teams or expensive equipments. The Horizontal to Vertical Spectral Ratio (HVSR) is commonly used not only in seismic microzoning studies as far as from noise recording constraining the fundamental resonant frequency, but it is also possible to infer the depth of the bedrock knowing the average shear wave velocity of the overlying sedimentary cover, or viceversa (i.e. resonance equation). For the purposes of the present research, more than 300 single-station noise measurements were carried out across the Piniada Valley (Central Greece), along and between several transects planned roughly perpendicular to the mean valley trend. To understand the palaeogeographic and tectonic evolution of this area, we needed an estimation of the geometry at depth of the bedrock underlying the fluvial deposits of the present-day Pinios River. As a result, for each measured site, we calculated the depth of the bedrock and, afterwards, such values were opportunely interpolated for obtaining a 3D model of the palaeo-Piniada Valley documenting for the first time the recent (Late Quaternary) inversion of the topographic gradient.