On the other hand, the occurrence of the 1755 violent earthquake provoked fear, but also interest regarding this type of phenomena. The usual replicas allied to the increasing interest, raised the documentation of smaller tremors, which would probably otherwise, not be registered.
With regard to the intensity of the earthquakes, it was perceived that there were some violent earthquakes BC, but they were not properly documented. In the fourth century AD, violent earthquakes occurred, with magnitudes probably around 9.0 MS. These were followed by the eighteenth century earthquakes, in which the 1755 earthquake was recorded with a high of 8.0 MS.
Regarding the minimal intensities, the fourth century stands out due to the registration of the highest value recorded among the lowest magnitude impacts. The nineteen-century is the period with the lower minimum intensity registered, quite possibly because of the advances in seismography monitoring systems, which allowed the record of small shocks that would otherwise be imperceptible. In the twentieth century, with the advances of seismography, this evolution could be traced annually.
From an overall interpretation of the national historical record, the1755’s Lisbon earthquake, the 1909’s Benavente earthquake, the 1969’s Algarve earthquake and the 1980’s Azores earthquake, despite their different circumstances, are considered the most relevant occurrences, which is corroborated by the quantity and quality of the available documentation. The year 1909 stands out with great evidence, but equally significant values are registered in the years 1983 and 1984, with 115 and 143 earthquakes respectively, as well as in 1975 (116 occurrences). In 1969, there was 75 recorded earthquakes; and in 1978, there was 71 tremors. In 1931, 1951, 1954, 1964 and 1969 there are records of earthquakes with a maximum intensity level above 7. The Earthquake of Benavente, despite its dramatic consequences, recorded a much lower intensity (6.2). The information available regarding each of these earthquakes is scarce, and even scarcer in topics related to the main focus of this research: the consequences that these phenomena may have had, on the local building culture.
The records that mention these phenomena, when addressing its consequences, focus mainly on the human casualties registered. The exception is the 1755 earthquake. Following the disaster, all the Portuguese parishes were required to send to the prime minister, a written report of all the occurring damages. Only for the most recent tremors, there is available and diversified information on the issue. On the eighteen-century, the first journals started to publish in a more systematic way, and with them the first records of earthquakes in Portugal were reported nationally. As an example, the news from October 26th 1724, in the “Gazeta de Lisboa Occidental” reporting an earthquake felt in Lisbon, Porto, Elvas, Cantanhede, Santarém and Portimão, on the night from the 12th to 13th of October. These were the most ancient news on this subject found during this study. The availability of information multiplies exponentially in case of major disasters, with a great national and international dissemination impact, as it is the case of the Lisbon’s earthquake of 1755.
With regard to the perception of the earthquake by the people, it seems to be recurrent a reference to the noise that precedes the earthquake, and also a frequently mentioned strong smell associated with it. The panic of the population during the earthquake seems also to stand by similar standards. It is possible to identify, in the different descriptions, a first moment of shock and confusion, followed by moments of panic and running away; and often the recollection of disorientation, when the main shock is followed by replicas, or other associated phenomena such as a tsunami. The most serious consequences are undoubtedly the loss of human lives. In the case of Benavente’s earthquake in 1909 and during the earthquake of 1969, the number of indirect victims was higher than those of direct victims. The emotional shock of the quake was deadlier than its physical consequences.
The injuries produced by shock and trauma, caused by the fall of structures or landslides are also referred to, in all the news of earthquakes. As for the material damage, in all the cases there is a record of falling chimneys, the cracking of walls and ceilings, and the fall of attics and other structures. However, an inversion of priorities is also detected. In the eighteen-century, most reports focus on the description of damage in religious or public buildings; while in the twentieth century a greater concern is directed to the loss of housing. During the 1969 earthquake, it should also be highlighted the rising of registration of new concerns, as is the cut on the power supply and on communications, as well as on the traffic. However, it should be highlighted that historically, the first addressed measurements, in all the cases, still relate with the rescue of victims, the relocation of displaced inhabitants and on the repair of damage to the buildings. On a second phase, the reconstruction is then, considered.
Finally, with regard to the consequences of earthquakes in the local building culture, at an official level, the authorities tried to regulate reconstruction, when ensuring that effective seismic reinforcement measures were added into the rebuilding process. This happened during the Pombal reconstruction in 1755, the Benavente reconstruction plans in 1909, and on the regulation of the reconstruction in 1969. However, from these three cases, only the first plan from 1755 appears to have been effectively and fully implemented. In the two subsequent cases, the delay of official aid led to the rising of private initiative on most reconstructions. As illustrated in the case of Benavente, new building tend to reproduce the models, techniques and construction systems, practiced before the quake, with the exception of very few plans.
Regarding the reconstruction of Lisbon following the earthquake of 1755, Portuguese authorities were pioneer in the introduction of regulated seismic reinforced systems. With time and with the fading of memories concerning the disaster, builders have a tendency to overlook the initial care, altering the plans, which weaken the buildings resistance in case of future earthquakes.
The Lisbon region and the Algarve Region are located in the highest seismic hazard area from the Portuguese continent. To refer are also the regions of Tagus Lezíria and Setúbal Peninsula, located in the ‘Fault of Tagus Lower Valley’, which also present a high seismic vulnerability. The volcanic activity of the Azores archipelago and its particular geographical location turn these islands into one of the most vulnerable contexts, regarding seismic hazard impact. Also, the Coastal Alentejo region, presents a medium-high impact index, along with a significant frequency of occurrences. Finally, the Central Alentejo is depicted as a region of medium and low impact index, even if it presents an intense regularity of occurrences.
Major mainland faults
Macro seismic intensity in Portugal