Earthquake in the Azores Islands, Portugal
The 1998 Azores Islands earthquake (also known as the Faial earthquake) struck with an epicenter in the Atlantic Ocean, off the Azores Islands of Portugal at 05:19 local time. The deep mainshock, which measured 6.1 on the moment magnitude scale caused significant damage on Faial Island. At least 10 people died, 100 were injured, and 2,500 people were left without homes.
Background.
The Portuguese founded the first settlements on the islands in the 15th century. Since its founding, there has been at least 30 damaging and deadly earthquakes. The first recorded damaging earthquake occurred in 1522—it destroyed Vila Franca do Campo and caused over 5,000 deaths. Deadly earthquakes were also reported in 1787 and 1980. The largest recorded earthquake in the vicinity of the Azores was the 1941 magnitude 8.3 event which was associated with strike-slip faulting on the Eurasian–Nubian plate boundary.
Earthquake.
Tectonic setting.
The Azores Islands are situated atop a triple junction zone where the North American, Eurasian and African plates meet. The northern and southern arm of this junction is represented by the Mid-Atlantic Ridge while the Terceira Ridge forms its eastern arm. The Terceira Ridge joins the Gloria Fault and Azores–Gibraltar Transform Fault. With the exception of Flores and Corvo islands situated on the North American Plate, all of the Azores Islands lie along the western part of the Azores–Gibraltar Transform Fault. There is no agreed plate boundary type for the Azores–Gibraltar Transform Fault, which bounds the Eurasian and African plates. This feature has been considered a transform fault, leaky transform fault, oblique spreading center and rift by geologists.
Two zones of faulting occur around the 1998 earthquake epicenter; WNW–ESE and NNW–SSE trending systems with steep dip angles to the NNE or SSW and WSW or ENE, respectively. The WNW–ESE striking faults exhibit oblique-normal faulting with left-lateral slip. Faial Island, which rests above an elongated ridge connecting Pico Island, features a rift zone with approximately ESE striking faults with steep dip angles. Called the Pedro Miguel Graben, it hosts seven oblique-normal faults (right-lateral). South of the volcanic caldera and on the western part of Faial Island, similar-striking faults also occur.
Geology.
A strong foreshock was recorded at 05:01 which alerted the residents of Horta and other nearby settlements. The mainshock occurred 18 minutes later near the epicenter of the foreshock, and was the a result of pure strike-slip faulting. Faulting in the left-lateral direction ruptured along a north-northwest–south-southeast trending fault. The fault trend was also consistent with the interpretation of seismic data and aftershock distribution. The shock represented a rare and large earthquake of tectonic origin while most other earthquakes on the islands are associated with volcanism. The strike-slip faulting mechanism was also unusual as most earthquakes display normal faulting. A major aftershock sequence persisted for four months. An estimated 10,600 aftershocks were recorded, many of them perceivable by residents in the affected area. Aftershocks also occurred along some faults in the rift zone on Faial Island caused by their reactivation.
Damage.
The earthquake was assigned a maximum Mercalli intensity of VIII in the northeastern corner of Faial Island, where the greatest destruction was observed. A total of 3,909 buildings were damaged; a large number of them were on Faial Island while some occurred on Pico Island. Serious structural damage was recorded up to away from the epicenter. Most of these affected buildings were two or three storey masonry constructions. These damage also included the total collapse of masonry walls and large cracks appearing in exterior walls.
A large number of schools on both islands were moderately damaged; the most affected was a kindergarten in Salão, where its external masonry walls collapsed, leading to the demolition of the entire structure. At a school in Espalhafatos, the earthquake caused the separation of concrete and reinforced masonry components in the structure. Many of the schools inspected for damage were constructed of reinforced-concrete thus received slight or no damage.
Many churches which were severely damaged had been constructed of predominantly masonry materials. Only recently were three churches constructed with reinforced concrete. Common damage patterns associated with churches point to shear failure of walls. The Ribeirinha Church was one of the most seriously affected churches; the arch between the nave and apse completely collapsed.
Aftermath.
Rescuers spent the night retrieving the bodies of those killed in the rubble and looking for survivors. Concerned about aftershocks, residents of Faial and Pico islands slept inside tents provided by the government. The Portuguese Air Force deployed a Lockheed C-130 Hercules plane carrying tents and thousand blankets from Lisbon; medical teams and supplies also arrived. Prime minister António Guterres, visited Faial Island to survey the devastation. The United States ambassador to Portugal, Gerald S. McGowan, declared the earthquake a disaster on 17 July. In response, the Office of Foreign Disaster Assistance aided US$20,000 to the U.S. embassy for the Portuguese government to purchase lavatory items and distribute them to the affected communities. The Portuguese government sent medical doctors and nurses to treat the injured victims on the islands. Sniffer dogs and medical aid were also transported to help in rescue missions. The reconstruction, resettlement and recovery projects were undertaken by the regional government and estimated at €255 million. One of the largest recovery effort in the islands history, it involved constructing 670 prefabricated homes.
References.
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