The 1998 Azores Islands earthquake (also known as the Faial earthquake) struck with an epicenter in the

Atlantic Ocean The Atlantic Ocean is the second-largest of the world's five oceans, with an area of about . It covers approximately 20% of Earth's surface and about 29% of its water surface area. It is known to separate the " Old World" of Africa, Europe ...

, off the

Azores Islands ) , motto =( en, "Rather die free than subjected in peace") , anthem= ( en, "Anthem of the Azores") , image_map=Locator_map_of_Azores_in_EU.svg , map_alt=Location of the Azores within the European Union , map_caption=Location of the Azores wi ...

Portugal Portugal, officially the Portuguese Republic ( pt, República Portuguesa, links=yes ), is a country whose mainland is located on the Iberian Peninsula of Southwestern Europe, and whose territory also includes the Atlantic archipelagos of ...

at 05:19 local time. The shallow ( deep)

mainshock In seismology, the mainshock is the largest earthquake in a sequence, sometimes preceded by one or more foreshocks, and almost always followed by many aftershocks. Foreshock A foreshock is an earthquake that occurs before a larger seismic eve ...

, which measured 6.1 on the

moment magnitude scale The moment magnitude scale (MMS; denoted explicitly with or Mw, and generally implied with use of a single M for magnitude) is a measure of an earthquake's magnitude ("size" or strength) based on its seismic moment. It was defined in a 1979 pape ...

caused significant damage on the island of Faial and Corvo. At least 10 people died, 100 were injured, and 2,500 people were left without homes.

Tectonic setting

The Azores Islands mark the location where the

African African or Africans may refer to: * Anything from or pertaining to the continent of Africa: ** People who are native to Africa, descendants of natives of Africa, or individuals who trace their ancestry to indigenous inhabitants of Africa *** Ethn ...

(

Nubian Nubian may refer to: *Something of, from, or related to Nubia, a region along the Nile river in Southern Egypt and northern Sudan. *Nubian people *Nubian languages *Anglo-Nubian goat, a breed of goat * Nubian ibex * , several ships of the Britis ...

Eurasian Eurasia (, ) is the largest continental area on Earth, comprising all of Europe and Asia. Primarily in the Northern and Eastern Hemispheres, it spans from the British Isles and the Iberian Peninsula in the west to the Japanese archipelago ...

and

North American North America is a continent in the Northern Hemisphere and almost entirely within the Western Hemisphere. It is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South America and the Ca ...

plates meet at the ride-ridge-fault

Azores Triple Junction The Azores Triple Junction (ATJ) is a geologic triple junction where the boundaries of three tectonic plates intersect: the North American Plate, the Eurasian Plate and the African Plate. This triple junction is located along the Mid-Atlantic R ...

. To the west is the

Mid-Atlantic ridge The Mid-Atlantic Ridge is a mid-ocean ridge (a divergent or constructive plate boundary) located along the floor of the Atlantic Ocean, and part of the longest mountain range in the world. In the North Atlantic, the ridge separates the North Ame ...

; a 16,000-km-long

divergent boundary In plate tectonics, a divergent boundary or divergent plate boundary (also known as a constructive boundary or an extensional boundary) is a linear feature that exists between two tectonic plates that are moving away from each other. Divergent b ...

on the seafloor of the

which separates the

Eurasian Plate The Eurasian Plate is a tectonic plate that includes most of the continent of Eurasia (a landmass consisting of the traditional continents of Europe and Asia), with the notable exceptions of the Indian subcontinent, the Arabian subcontinent and ...

from the

North American Plate The North American Plate is a tectonic plate covering most of North America, Cuba, the Bahamas, extreme northeastern Asia, and parts of Iceland and the Azores. With an area of , it is the Earth's second largest tectonic plate, behind the Pacific ...

. East of the Azores is the

Azores–Gibraltar Transform Fault The Azores–Gibraltar Transform Fault (AGFZ), also called a fault zone and a fracture zone, is a major seismic zone in the Eastern Atlantic Ocean between the Azores and the Strait of Gibraltar. It is the product of the complex interaction between ...

, specifically the strike-slip Gloria Fault which serves as the boundary between the Nubian and Eurasian plates. The plate boundaries are seismically active and capable of producing large earthquakes. 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 Vila Franca do Campo () is a town and municipality in the southern part of the island of São Miguel in the Portuguese Autonomous Region of the Azores. The population of the municipality was 11,229 in 2011, in an area of 77.97 km². The town ...

and caused over 5,000 deaths. Deadly earthquakes were also reported in 1787 and

1980 Events January * January 4 – U.S. President Jimmy Carter proclaims a grain embargo against the USSR with the support of the European Commission. * January 6 – Global Positioning System time epoch begins at 00:00 UTC. * January 9 – ...

. 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

The earthquake off the coast of Faial Island occurred as a result of pure strike-slip faulting. The two fault plane solutions suggest the earthquake was the outcome of strike-slip faulting on either a northwest–southeast striking left-lateral fault, or a northeast–southwest striking right-lateral fault. Seismological data from the earthquake support the occurrence of a rupture on the latter solution, which is also consistent with the plate tectonics in the Azores Islands. The shock represented a rare and large earthquake of tectonic origin; most earthquakes on the islands are of volcanic origin. The strike-slip faulting mechanism was also unusual as most tectonic earthquakes display normal faulting.

Seismicity

At 05:01, a strong earthquake struck near the epicenter of the mainshock. It woke residents near the epicenter and at Horta. Following the mainshock was a major aftershock sequence which persisted for four months. An estimated 10,600 aftershocks were recorded in the four-month period, many of them perceivable by residents in the affected area. Analysing the locations of aftershocks associated with the mainshock, the event likely caused some

normal fault In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectonic ...

ing on the Faial Graben onshore on Faial Island.

Impact

The earthquake was assigned a maximum

Modified Mercalli intensity scale The Modified Mercalli intensity scale (MM, MMI, or MCS), developed from Giuseppe Mercalli's Mercalli intensity scale of 1902, is a seismic intensity scale used for measuring the intensity of shaking produced by an earthquake. It measures the eff ...

and

European macroseismic scale The European macroseismic scale (EMS) is the basis for evaluation of seismic intensity in European countries and is also used in a number of countries outside Europe. Issued in 1998 as an update of the test version from 1992, the scale is referred ...

intensity of VIII in the northeastern corner of Faial Island, where the greatest destruction was observed. Approximately 35 percent of all buildings on Faial Island were damaged or destroyed. On the nearby island of Pico, only 10 percent of its structures were affected, bringing the total number of affected structures to 3,909. Over 2,100 buildings up to 30 km away from the epicenter sustained serious structural damage. Serious damage included the complete 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 Salão is a ''freguesia'' ("civil parish") in the municipality of Horta on the island of Faial, the Portuguese Azores. The population in 2011 was 401, in an area of 10.40 km². History The settlement of Salão can be traced back to 1620, t ...

, 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 Reinforced concrete (RC), also called reinforced cement concrete (RCC) and ferroconcrete, is a composite material in which concrete's relatively low tensile strength and ductility are compensated for by the inclusion of reinforcement having hig ...

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 The nave () is the central part of a church, stretching from the (normally western) main entrance or rear wall, to the transepts, or in a church without transepts, to the chancel. When a church contains side aisles, as in a basilica-type ...

and

apse In architecture, an apse (plural apses; from Latin 'arch, vault' from Ancient Greek 'arch'; sometimes written apsis, plural apsides) is a semicircular recess covered with a hemispherical vault or semi-dome, also known as an ''exedra''. In ...

completely collapsed.

Aftermath

António Guterres António Manuel de Oliveira Guterres ( , ; born 30 April 1949) is a Portuguese politician and diplomat. Since 2017, he has served as secretary-general of the United Nations, the ninth person to hold this title. A member of the Portuguese Socia ...

, the then

Prime Minister of Portugal The prime minister of Portugal ( pt, primeiro-ministro; ) is the head of government of Portugal. As head of government, the prime minister coordinates the actions of ministers, represents the Government of Portugal to the other bodies of state, ...

, was flown to

Faial Island Faial Island (), also known in English as Fayal, is a Portugal, Portuguese island of the Central Group (Portuguese: ''Grupo Central'') of the Azores. The Capelinhos Volcano, the westernmost point of the island, may be considered the westernmost ...

to survey the devastation. The

United States ambassador to Portugal Bilateral diplomatic relations between the United States and Portugal date from the earliest years of the United States. Following the Revolutionary War, Portugal was the first neutral country to recognize the United States. On February 21, 1791 ...

Gerald S. McGowan Gerald Stephen "Gerry" McGowan (born September 3, 1946 Birmingham, Michigan) is an American lawyer, wireless industry entrepreneur, and diplomat. He was appointed United States Ambassador to Portugal on November 10, 1997, and was sworn in by Vice ...

, declared the earthquake a disaster on 17 July. In response, the

Office of Foreign Disaster Assistance The Office of U.S. Foreign Disaster Assistance (OFDA) was an organizational unit within the United States Agency for International Development (USAID) charged by the President of the United States with directing and coordinating international United ...

aided US$20,000 to the U.S embassy in for the

Government of Portugal The Government of Portugal is one of the four sovereignty bodies of the Portuguese Republic, together with the President of Portugal, President of the Republic, the Assembly of the Republic (Portugal), Assembly of the Republic and Judiciary of Port ...

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

Portuguese Air Force , colours = , colours_label = , march = , mascot = , anniversaries = 1 July , equipment = , equipment_label ...

deployed a

Lockheed C-130 Hercules The Lockheed C-130 Hercules is an American four-engine turboprop military transport aircraft designed and built by Lockheed (now Lockheed Martin). Capable of using unprepared runways for takeoffs and landings, the C-130 was originally design ...

transport plane from Lisbon with tents for 1,000 people and several thousand blankets.

References