The 20 – 29 May 2012 earthquake events occurred along Emilia Romagna segment of Po Plain valley (Pianura Padana), a NW – SE oriented basin along the Southern tectonic boundary of Adria plate, with Northern Apennines. Major earthquake sequence events’ focal mechanisms, evince the activation of low angle thrust structures, separating the earthquake activity on at least 2 distinct fault planes, with similar slip characteristics. Both planes seem to be on the same seismogenic source, on the Western part of 1570 events’ source, interpreting the delimitation of the seismic activity of the May – June 2012 earthquake sequence. Widely observed secondary effects were categorized with respect to spatial distribution, density and trend, to: i) individual liquefaction cases, ii)Strictly oriented liquefactions and lateral spreading due to exposure of unconformities to surfacial liquefaction-prone geological formations and iii)extended liquefactions of confined liquefiable formations (at a depth of≈10m) and lateral spreading along river strike. This depth resulted in less surfacial deformation and differential subsidence, as well as very limited structural damage. Only fences, pavements and other light weight structures have been heavily damaged, deformed and dislodged. The incurred structural damage is extremely heavy for the churches and the cultural heritage, as well as for the industrial infrastucture of the region, besides the death toll and injuries. The available strong motion records are not adequate in order to explain in a rational and reliable way the above mentioned heavy damage. Based on years’ experience in post earthquake investigations and on results of numerous full scale tests of structures on shaking tables, the authors concluded that the main reasons of the damage is the extremely high vertical ground shaking (of the order of 1.0 g) in combination with the moderate horizontal motions. Due to this very high value of the vertical ground shaking, impact phenomena are observed as well as total or partial loss of friction (due to the loss of gravitational forces).The incurred damage are grouped in general categories possessing similar characteristics. The majority of the observed damage of churches is mainly due to collapse of their roofing systems. This occurred due to resonance-like vertical vibration of the timber trusses of the roofs. This was proved by analysing representative dimensions of timber trusses with the overburden mass and, it was found that their fundamental periods are between 0.06 and 0.10 sec. On the other hand, a predominant period of the vertical component is, according to the recorded first event of 20th May 2012, 0.06 to 0.07 sec. Thus, intensive up and down motions are induced in the whole trusses and in their horizontal lower beams resulting in dislocations from their supports on the walls. On the other hand, it is well known that the arrival time between P (vertical) and S (horizontal) waves in epicentral regions is quite small. This resulted in a convolution between vertical and horizontal ground motions. The top of the walls, where are the supports of the trusses, were significantly displaced as free standing vertical cantilevers due to the loss of connection with the truss. In this way the trusses totally lost their supports and collapsed. In examining the debris one may guess what structure between the two (truss or wall) collapsed first. A proof of the dominance of the vertical component might be based, among others, on the response of bell towers that in the majority of the cases were not significantly damaged and on the fact that the mode of collapse is inside the ground plan of the building. Quite similar observations are also valid for the damaged industrial facilities. The natural vertical periods of the roofs are also, between 0.06 and 0.10 sec. The constructed supports are from functional point of view quite similar to those used for the support of the timber trusses in the churches (just simply supported cantilevers). Nevertheless, by using the conventional bolts in the supports, it is not certain whether or not the situation would be ameliorated. In order to prevent the effects of the high frequency impact type of the vertical ground motion a kind of absorbing elastomeric devices could be used in the supports of beams and trusses besides the conventional anchoring systems. Also, a kind of base isolation devices, that should not depend on friction at the base of the structures might be used. Finally, the partial collapse of the ceramic factory is attributed to the rhythmic phenomena of destruction. After the present communication was completed, it has just been released the important and technically very interesting vertical strong motion record of the second event, presenting a peak ground acceleration of the order of 0.9 g, a fact that comes in full agreement with the findings and arguments exposed in the present paper.
Earthquake reconnaissance
THE OCTOBER 23 (MW = 7.2) AND NOVEMBER 9 (MW = 5.7), 2011 VAN, TURKEY EARTHQUAKES. A GEOSCIENTIFIC AND ENGINEERING REPORT
On 23 October 2011 (13.41 LT) a disastrous earthquake with a magnitude of MW = 7.2 occurred in Van Province of eastern Turkey. That is the region where the Arabian Plate drifting towards north-northeast collides with the Eurasian Plate, while the other smaller plates in the region are moving apart as if pushed by the stronger Arabian. As a result, the seismic history of the region is quite rich usually resulting in severe losses. The death toll due to the main shock reached 604 people and injured 4,152, with at least 188 pulled out of the rubble of collapsed buildings. The most heavily damaged Ercis city was at a distance of 35-40 km to the north of the causative fault. The city of Van suffered much less damage, being to the south of the fault at a distance of 25-30 km. The damage was concentrated mainly in the old city centers of both cities. On the 9th of November 2011 (21.23 LT) a strike slip earthquake of MW = 5.7 occurred very close to the city center of Van, associated with a different fault. Due to this shock, an additional number of 40 people were killed and 260 injured, while the Bayram Hotel, in Van city, where the authors stayed from 25 to 29 October 2011, collapsed. The hotel building was visually inspected by the first author following a widely accepted methodology and it was considered to be earthquake safe. The available cross-checked information concerning the mechanism of the hotel’s collapse is, at first glance, contradicted by the strong motion records presented in the paper. Unfortunately, during the main shock in both Van and Ercis cities, there were no strong motion recordings. In order to infer some basic characteristics of the ground motion due to the main shock, strong motion records from an aftershock, occurring in the same focal volume with that of the main shock, were used. Observations of the response of structures immediately after the main shock were carried out by the authors during their reconnaissance trip and led towards the same goal. The heaviest damage was observed to engineered reinforced concrete buildings. There are some cases of modern and even recently built, multistoried buildings with rather good reinforcement detailing that suffered extended damage or even collapse. On the contrary, nearby non-engineered, low-rise simple or traditional masonry houses, weathered the earthquake in the epicentral region almost without any damage. Most of the reinforced concrete structures are quite flexible, without shear walls, and used a flat-slab constructional system of rather small thickness compared to their spans. In spite of those characteristics justifying high flexibility, no noticeable horizontal motion or pounding was observed between adjacent buildings. For this reason it was thought helpful to present a critical evaluation of the published Turkish seismic building codes since 1940. A crucial subject directly related to the incurred damage and discussed by the authors is the widespread practice of building construction in the region that it is not according to the Turkish earthquake code requirements. It is shown that due to the main shock the numerous and heavy losses in Ercis city were due to the dominance of a severe vertical seismic component, while those in Van city due to resonance phenomena caused by the relatively weak horizontal ground motion. The losses due to the event of 9th November in Van city are attributed to the catalytic function of the vertical seismic component. The resulting collapse is usually quite abrupt and does not allow occupants time to egress safely.
The complete Issue 1, 2012 could be downloaded from the home page.
The Haiti Earthquake Mw = 7.0 of January 12th 2010: structural and geotechnical engineering field observations, near-field ground motion estimation and interpretation of the damage to buildings and infrastructure in the Port-au-Prince area
The Mw 7.0 destructive earthquake that struck the Republic of Haiti on January 12, 2010 was one of the deadliest of the last century in the region. What has been concluded after a field study during the period from 16th to 22nd of January 2010 is that this event is mostly a man-made natural disaster. This is due to the very adverse social, political and economic conditions that existed before the earthquake that greatly contributed to the lack of any building-construction code and control of land use and the lack of any governmental – public services during the post-earthquake emergency. The primary effects of the earthquake were worsened due to the inability or absence of any preparedness for search and rescue and help the victims. The frozen response of the structures to the seismic event, observed during the field trip, gave valuable information about the ground motion, since instrumental records were unavailable. The response of a fence functioning as a primitive seismoscope was analyzed, resulting to an effective ground horizontal acceleration of about 0.6 to 0.7 g. Similarities between Haitian structures and their response with instrumented cases in other regions, resulted in the acceptance of a 1.0 g vertical acceleration for the site. Finally, the detailed investigation of the whole liquefaction phenomenon at the site, documented that this is starting with ejection of sand and water, before the large ground displacements take place and continues after the strong phase of the ground motion.
First remarks about the structural response of the constructions in the Abruzzo Earthquake
During the week after the main shocks two survey groups visited the area and prepared for ‘Ingegneria Si- smica’ the following preliminary reports on damage suffered by some masonry and r.c. buildings. The groups included researchers of various Universities and engineers belonging to Public Administations. Seismological data of the events may be found at the site http://itaca.mi.ingv.it/itacanet/.
For this paper is available an extended abstract after the text in Italian