1998 Azores Islands earthquake

1998 Azores Islands earthquake

Terceira Graciosa São Jorge
UTC time	1998-07-09 05:19:07
ISC event	1165443
USGS-ANSS	ComCat
Local date	July 9, 1998 (1998-07-09)
Local time	04:19:07
Magnitude	6.2 Mw
Depth	13.6 kilometres (8 mi)
Epicenter	12 km north of Horta 38°39′00″N 28°37′34″W / 38.650°N 28.626°W / 38.650; -28.626
Type	Strike-slip
Areas affected	Azores, Portugal
Max. intensity	VIII (Severe)
Aftershocks	10,600 recorded
Casualties	10 dead, 100 injured

The 1998 Azores Islands earthquake (also known as the Faial earthquake) struck the Azores Islands of Portugal at 05:19 local time. The shallow mainshock, which measured 6.2 on the moment magnitude scale caused significant damage on the island of Faial and Corvo, despite being centered offshore in the Atlantic Ocean.^[1] At least 10 people died and some 100 were injured in the earthquake, while 2,500 people were left without homes.^[2][3]

Tectonic setting[]

The Azores Islands mark the location where the African (Nubian), Eurasian and North American plates meet at the ride-ridge-fault Azores Triple Junction. To the west is the Mid-Atlantic ridge; a 16,000-km-long divergent boundary on the seafloor of the Atlantic Ocean which separates the Eurasian Plate from the North American Plate. East of the Azores is the Azores–Gibraltar Transform Fault, specifically the strike-slip Gloria Fault which serves as the boundary between the Nubian and Eurasian plates.^[4] The plate boundaries are seismically active as they accommodate elastic strain caused by movements of the plates, and releases them during large earthquakes. Two of the largest recorded earthquakes in the vicinity of the Azores were the M_w  8.1 1941 Gloria Fault and M_w  7.9 1975 North Atlantic earthquakes, which were associated with strike-slip faulting on the Azores–Gibraltar Transform Fault.^[5]

Earthquake[]

The M_w  6.2 earthquake off the coast of Faial Island occurred as a result of pure strike-slip faulting at a shallow depth of 13.4 km beneath sea level. 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. Earlier modeling of the earthquake, however, suggested the quake occurred on the former solution, with a fault dimension of 25 km by 12.5 km. The model finds two subevents with the former releasing most of the seismic at 8 km depth. A second subevent occurred at 7 km depth but less energy was released.^[6]

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.^[7] Analysing the locations of aftershocks associated with the mainshock, the event likely caused some normal faulting on the Faial Graben onshore on Faial Island.^[8]

The quake was assigned a maximum Modified Mercalli intensity scale and European macroseismic scale intensity of VIII in the northeastern corner of Faial Island, where the greatest destruction was observed.^[7]

Impact[]

Approximately 35% of all buildings on Faial Island were damaged or destroyed by the earthquake, while on the nearby island of Pico, only 10% 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 received serious structural damage.^[9] Some of the most serious damage included the complete collapse of masonry walls and large cracks appearing in exterior walls.^[9]

A large number of schools on both islands suffered moderate damage, while some were hit particularly hard; the most affected school was a kindergarten in Salão, where external masonry walls collapsed, leading to the demolition of the entire structure. At a school in Espalhafatos, the quake 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 during the quake. Older buildings, however, had more noticeable damage, while masonry buildings had more damage, with some collapsing.^[10]

The quake severely damaged many churches that were predominantly masonry constructed. Only three churches that were recently constructed were of the reinforced concrete type. Common damage patterns associated with the churches point to shear failure of walls. The Ribeirinha Church was one of the most seriously impacted churches, where the arch between the nave and apse had completely collapsed.^[11]

Aftermath[]

António Guterres, the then Prime Minister of Portugal, was flown to Faial Island to survey the devastation inflicted by the quake.^[12] 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 in for the Government of Portugal to purchase lavatory items and distribute them to the affected communities.^[13] 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.^[14]

See also[]

• List of earthquakes in 1998
• List of earthquakes in the Azores
• List of earthquakes in Portugal

References[]