D. Pantosti, A. Frepoli and G. Frepoli

The 20 April earthquake produced only very few phenomena of this kind. Some fissures, or small slightly visible ground cracks, appeared in Kato Pelli along the coast for a length of 50-60 m. Moreover, small surface ground cracks appeared at other localities along the way to Martino; in particular, on road sections next to bridges, where the road itself is built on artificial fill the strike of these cracks was SE-NW and their length never exceeded 20 m. Some more important cracks formed close to the villages of Masi and Malessina but also in these cases they never exceeded 200m. These cracks were open 4 to 6 cm with no evidence for vertical movement.

Similar cracks occurred along the road for Livanates in Pharmakorhevma, also in this case they were minor features and occurred on artificial fill. In short, the cracks and the changes of the surface were so meaningless that if the earthquake of the 27th would not have occurred, nobody would have ever mentioned them.

Things change with the earthquake of April 27th because it produced interesting and very long ruptures accompained by subsidence, sliding, collapses, and displacements of the ground.

The main rupture follows the trench of the Euboean gulf, parallel to the coastline for a total length of 55-60 km. This rupture starts to the SE at Gatza Cape, in the Skroponery Bay and ends to the NW in the alluvial plain between Agios Kostantinos and Molos. Along its length, this rupture cuts the following formations from bottom to top: Cretaceous dolomites, limestones and scists; Neogene marls, gravels and sandstones; recent alluvium and also serpentinites and volcanic rocks. This rupture cuts the Cretaceous limestones with very small displacements close to the Skroponeri and Pasari Mts., and also close to Halmyra (table 15) where 30 cm displacement was observed for a distance of 200 m. Further to the NE the limestones are cut with very small displacements and for very limited distance. On the contrary, the Neogene strata were cut for long distance and with important displacements; this is even more important in the alluvial deposits.

The strike of the rupture is SE-NW: it starts at Gatza Cape, cut the southwest slope of Mt. Skroponeri, the corresponding slope of Mt. Pasari; from here, continuing to the SE [NW?] , it gets to the only spring exisiting in Martino, which is located to the NE of the village in the old mesa that in the past collapsed and become a basin; then, it climbs on the road that joins Martino with Proskyna and cut it at a distance of 5 minutes NW of the village of Martino, it runs close to the locality of Chiliadu, that was the site of the ancient town of Korseia, it cuts the northeastern slopes of Chlomos Mts. which are located about half an hour from Proskyna, it continues along the Atalanti plain, goes through the localities of Halmyra and bending to the west runs next to the slopes of Chlomos Mts. and on top of the town of Atalanti, and then it cut the northeastern slopes of Mt. Rhoda. From this point the rupture bends to the NW running between the localities of Kalamaki and Arkudari, cuts the northeastern slopes of Mt. Epiknemis, runs to the SW of Hagios Kostantinos, close to this locality and to Thronion (Table 16).

Some other ruptures occurred besides this rupture that separated a long and wide region of central Greece. First of all that one starting next to Mulkia, a site in front of the soutern edge of the Atalanti island, that cuts transversally the Atalanti plain from NE to SW and meets the main rupture at the southeast edge of Mt. Rhoda. This rupture (Mulkia) extended for 7km and produced a 30-50cm subsidence of the NW of the Atalanti plain. Moreover, it distupted and interrupted the road that leads from Atalanti to Proskyna because of some radial cracks that cross each other several times in a way that it is not possible anymore to ride it by carriage, and even on a horse it is possible to ride only very carefully. The opening of the cracks is generally 15-20 cm, in some cases it is 5 cm with increases up to 25 cm.

One other fissure is in the town of Atalanti. This rupture, slightly out of the town in the Atalanti plain, branches from the main rupture with a southeast strike, cuts the southeast area of the town and joins again the main rupture next to the Pasari spring, in this way enclosing an elliptic region 800 m-long and 300 m-wide. This region shows a change in elevation of 1-1.5 m in depth [a subsidence of 1-1.5 m; i.e., the Atalanti graben] (table 17).

One other rupture that encircle a similar elliptic region formed to the SW of Kiparissi. The hill of Tertiary age, encircled by this rupture trends SE-NW, extends for more than 2 km and is 800 m wide. This rupture can be considered also as a bifurcation of the main rupture because the main axis of the ellipse has the same strike of the main rupture. The change in elevation produced by the subsidence of this region is 35-40 cm. The rupture has an opening of 25-45 cm.

Moreover, we have to mention a rupture close to the village of Skenderaga (now Megaplatanos) that, similarly to the previous ones, produced the subsidence of an elongated area that was 500m long and 35-45 m wide. The subsidence was 15-20 m [huge landslide] and caused the damming of a creek that now forms a small lake.

According to a report to the Ministry of Interiors from prof. Dr. Mitsopulos some ruptures formed also around the village of Charma which is formed by 20 houses and is located on the left side of the 30m-deep gorge where the Boaggeios (vulg. Platania) river gushes. The Tertiary clay outcropping in the area is cut from the gorge. The gorge is surrounded by cracks parallel to its axis. There is the real danger that the slope where the houses are built may collapse and bury the people along with their houses.

Moreover, in a report to the Ministry of Interiors, Prof. Dambergis informs that "after the visit to Therma I went to Hagios Georgios, which is the harbor of the town of Gialtra, half of an hour away, where the sandy beach subsided in the sea for 6-10 m for an extent of 300 m. The subsided beach is about 11 m wide and is grooved by several small fissures. Similar fissures can be observed also on the ground 400 m on land. Where the ground subsided in the sea, several emissions of solphydric acid occurred".

According to another report of the engineer of the Euboean province at the Ministry of Interiors, it seems that landsliding occurred due to ruptures filled with water in the town of Messonta of Demos Telethrion and at 500 m from the town of Anata.

Also the towns located on the Aetolyma peninsula have some ruptures. For example, the town of Masi is crossed by NE-SW fracture zones that, with the contribution of the spring water, produced an almost horizontal landsliding such as the fruit trees that were on that block are moved to a side of the gorge 1 meter lower and at a horizontal distance of 12 m. Similarly, because of ruptures with similar strike located on the highest edges of the Masi town, the ground subsided of 1 m for an extent of more than 150 m. This rupture did produce also the violent rock falls. Malessina too was not avoided by the formation of these ruptures, in fact, besides the old faults which produced a stepping configuration of the ground, it was crossed by a large amount of new ruptures in the market area with NE-SW direction. In particular, the low plateau threatens detachments and collapses of ground portion in the narrow gorge of Spelia. The ruptures which formed in Martino are meaningless and it is not worth to mention them. On the contrary, the ruptures in Larymna are very significant, they strike NW-SE, extend for more than 200 m, are still open, and some reddish water gushes from depth; this flooded a large portion of the fields. The cemented conglomerates outcropping there, are covered by clay and iron oxide that was mechanically dissolved and gave to the gushing marine water the red color.

After the description of all the ruptures, fractures, subsidence etc., I come back to the large rupture that I would call the "Locris rupture" to distinguish it from the others and from the others which are related to it.

The Locris rupture that produced the separation of a large block, did not produce the same amount of subsidence along its extent, but this is lower where the strata are formed by rocks are compacted and hard; however, the modification that it produced are very important because they produced cave collapses as for example in Skroponeria, not far from Larymna bay, rock falls, as for example next to Halmyra, Chiliadu, Pikraki, etc. In the looser strata of the Neogene formation and in the alluvium, the subsidence is larger but occurred silently and did not produced the disruption occurred in the Atalanti plain from ancient Chiliadu to the road to Levadia and further on.

The maximum throw of the rupture is 2 m, in general it is slightly more than 1 m with exception of the limestone where in general is 30 cm and sometimes only 5-6 cm. The distance from the edges of the ruptures is up to 4 m but in general is less than 1.5 m, sometimes is only 25 cm or even 5-6 as it is in the Cretaceous rocks.

Regarding the visible depth of the rupture we cannot be sure, but by considering the soil collapsed on it, at some locations it seems to be 15-20 m deep; however, in most cases, the visible depth does not exceed 2.5 m, sometimes it becomes significantly smaller as in the Cretaceous limestone. Although the limited visible depth, it is impossible that a rupture having such an extraordinary extent would be only superficial but it should penetrate more in the earth crust. It is very difficult to estimate how deep it would penetrate; however we can affirm with certitude that its depth in the central part is not less than 70.71 m; this depth should be found next to Atalanti. I calculated this depth by assuming that a fracture extending for an terrestrial arc of 60 km, as it is the Locris fracture from Skroponeria to Molos, for sure starting from the surface it should penetrate at least up to its chord. I calculated the distance of the chord from its center to the arc to be 70.71.m.

Prof. Mitzopulos thinks that the Locris zolla, which was separated from the fracture, does not represent a subsidence due to a tectonic displacement but a simple sliding toward NE. This would mean that the whole process is superficial. I would accept this opinion if the rupture was restricted to only a portion of the Atalanti plain; but I have the following to say against it:

1. The enormous extent of the Locris rupture, about 60 km, indicates that it is almost impossible that a portion of the ground could slide for a such a distance and not even perpendicularly to its length, in fact on the basis of our previous discussion we have demonstrated that slip to NW occurred that is the earthquake direction and not perpendicular to it.

2. The dip of the strata composing the Locris block along the rupture. In fact, if a landsliding took place, the tectonic conditions should allow it; so that, the sliding portion should have the same dip of the beds and for its whole length, in the direction of the sliding. In the case the bedding would be different at some locations, this difference should be very minor to allow the whole sliding to drag them with the rest. None of these cases corresponds to ours, on the contrary the beds dip in a opposite direction respect to the sliding and moreover the volcanic rocks of Roda Mt. are not even stratified; they show only some laminar slip oriented to the SW only by chance. Moreover, the beds of the Tertiary formation, sandstone, marls, marly limestones, and conglomerate, which outcrop to the NW of Rhoda Mt. up to Livanates and beyond, have a dip to the S or SW, whereas, at the same time, and this is the most important thing, all the limestone beds, along with the Tertiary formations overlying them all along the length of the rupture strike NW-SE and dip 10-35° SW, thus in the opposite direction respect to the sliding.

3. The non uniform petrographic and geologic composition of the Locris block. As already pointed out, and as it is possible to infer from the geologic map of the region hit by the earthquake, it is very difficult to envision that beds of a such a different composition may have slided jointly and for a such extent.

4. The width of this stretch of ground (10-12 km) is such that the friction, caused by the sliding on a large surface is so important that it would not allow sliding from the beginning.

5. If such a sliding was to the SW, that is toward the Kopais lake and Thiva plain, Levadia, etc., I could even envision it, because several preliminary conditions would favor it, however I could not admit it easily. In our case, however, we are dealing with a case of a presumed sliding toward NE, against the possibilities derived by the preliminary conditions.

Thus, this is a seismogenic fracture, produced by a tectonic earthquake, with a length of 55-60 km, a visible depth of 15-20 m, a minimum depth of 70.71 m, a vertical displacement of 2 m, and a horizontal offset of a few cm toward the NW [left-lateral]. The rupture to maintain a NE-SW strike, thus parallel to the Eubean fracture zones, the subsidence of Kopais lake, the Levadia plain, the Melas river etc. and also to the fracture of the Gulf of Corinth, did not avoid the limestones nor the eruptive rocks of Rhoda Mt. etc. - a such a rupture cannot be seen as a simple fracture in the superficial structure but it should be considered as a displacement which produced vertical changes along with some horizontal movement.

In the following, I am considering all these fractures as seismic, those in the Atalanti plain, from Skenderaga, on the coast near Agios Kostantinos and Loggos whereas those of Achladion on the other coast, as well as all the fractures which produced modifications in the tectonics and in the vertical and horizontal setting of that region as simple dislocation fractures. If the Locris fracture would keep developing producing a further extension of the Eubean Gulf or a further detachment and subsidence of the Locris block, those locations at higher elevation would remain visible as islands - in that case anyone would claim the occurrence of a tectonic phenomenon and in particular one of relevance. What are the differences between what is caused by small fractures and main ones: subsidence of hollow shaped areas with consequent formation of lake, subsidence with formation of an island with immersion of portions of land in the sea waves etc. etc. In the case of major phenomena nobody would deny their tectonic origin, whereas the other fractures are explained as they collapsed under the effect of their weight and fall into the sea. If this is the case, how to explain that the sea bottom is deeper everywhere now?

For example, in the Corinth isthmus, that is commonly considered as a consequence of relative uplift, there is a large number of displacements and especially of small scale; nevertheless, nobody would consider them as something different because they contributed to the tectonic changes and to the vertical and horizontal setting, but this is only because these phenomena occurred in older geologic times.

Moreover, Prof. Mitsopulos and Dr. Papavasiliu said that only in a very far future, after myriads of centuries, which cannot scientifically determined [the myriads], the separation of the block could produce a subsidence, and thus a flooding, and a widening of the Euboean gulf or could form an island or, as they expressed in a different way, only in a million of year, the subsidence of the detached block may occur little by little.