1998 Azores Islands earthquake

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 shallow (13.6 km (8.5 mi) deep) mainshock, which measured 6.1 on the moment magnitude scale caused significant damage on the island of Faial and Corvo.[1] At least 10 people died, 100 were injured, and 2,500 people were left without homes.[2]

1998 Azores Islands earthquake
1998 Azores Islands earthquake is located in Azores
1998 Azores Islands earthquake
Terceira
Terceira
Graciosa
Graciosa
São Jorge
São Jorge
UTC time1998-07-09 05:19:07
ISC event1165443
USGS-ANSSComCat
Local dateJuly 9, 1998 (1998-07-09)
Local time04:19:07
Magnitude6.1 Mw
Depth13.6 kilometres (8 mi)
Epicenter12 km north of Horta
38.650°N 28.626°W / 38.650; -28.626
TypeStrike-slip
Areas affectedAzores, Portugal
Max. intensityVIII (Severe)
Aftershocks10,600 recorded
Casualties10 dead, 100 injured

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.[3][4] 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 and caused over 5,000 deaths. Deadly earthquakes were also reported in 1787 and 1980.[4] 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.[5]

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.[6]

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.[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]

Impact
The iconic Ribeirinha lighthouse after the earthquake

The earthquake 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] 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.[9] 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 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.[10]

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.[11]

Aftermath

António Guterres, the then Prime Minister of Portugal, was flown to Faial Island to survey the devastation.[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] The Portuguese Air Force deployed a Lockheed C-130 Hercules transport plane from Lisbon with tents for 1,000 people and several thousand blankets.

See also

References
  1. International Seismological Centre, On-line Bulletin, Thatcham, United Kingdom [Event 1165443 ].
  2. "UPI Focus: Strong earthquake kills 10 in Azores". United Press International. Lisbon, Portugal. 9 July 1998. Retrieved 26 August 2021.
  3. M. Fournier; C. Petit; N. Chamot-Rooke; O. Fabbri; P. Huchon; B. Maillot; C. Lepvrier (27 November 2008). "Do ridge–ridge–fault triple junctions exist on Earth? Evidence from the Aden–Owen–Carlsberg junction in the NW Indian Ocean" (PDF). Basin Research. 20 (4 (Special Issue ‐ Seafloor expression of tectonic and geomorphic processes)): 575–590. Bibcode:2008BasR...20..575F. doi:10.1111/j.1365-2117.2008.00356.x. S2CID 54036008.
  4. Gaspar, J. L.; Queiroz, G.; Ferreira, T.; Amaral, P.; Viveiros, F.; Marques, R.; Silva, C.; Wallenstein, N. (12 April 2011). "Geological Hazards and Monitoring at the Azores (Portugal)". Earthzine. Retrieved 26 August 2021.
  5. Maria Ana Baptista; Jorge Miguel Miranda; Josep Batlló; Filipe Lisboa; Joaquim Luis; Ramon Maciá (22 August 2016). "New study on the 1941 Gloria Fault earthquake and tsunami". Natural Hazards and Earth System Sciences. 16 (8): 1967–1977. Bibcode:2016NHESS..16.1967B. doi:10.5194/nhess-16-1967-2016. hdl:10400.21/7140.
  6. J.F. Borges; M. Bezzeghoud; E. Buforn; C. Pro; A. Fitas (2007). "The 1980, 1997 and 1998 Azores earthquakes and some seismo-tectonic implications" (PDF). Tectonophysics. Elsevier. 435 (1–4): 37–54. Bibcode:2007Tectp.435...37B. doi:10.1016/j.tecto.2007.01.008. hdl:10174/6383.
  7. Carlos Sousa Oliveira; Mónica Amaral Ferreira; Francisco Mota de Sá (2012). "The concept of a disruption index: application of the overall impact of the July 9, 1998 Faial earthquake (Azores Islands)" (PDF). Bulletin of Earthquake Engineering. 10 (1): 7–25. doi:10.1007/s10518-011-9333-8. S2CID 109897250.
  8. F.O. Marques; J. Catalão; A. Hildenbrand; A.C.G. Costa; N.A. Dias (21 October 2014). "The 1998 Faial earthquake, Azores: Evidence for a transform fault associated with the Nubia–Eurasia plate boundary?". Tectonophysics. Elsevier. 633: 115–125. doi:10.1016/j.tecto.2014.06.024.
  9. G. Zonno; C. S. Oliveira; M. A. Ferreira; G. Musacchio; F. Meroni; F. Mota-de-Sá; F. Neves (5 December 2009). "Assessing Seismic Damage Through Stochastic Simulation of Ground Shaking: The Case of the 1998 Faial Earthquake (Azores Islands)". Surveys in Geophysics. 31 (3): 361–381. doi:10.1007/s10712-009-9091-1. S2CID 54605828.
  10. Jorge Miguel Proença. "Damage in schools in the 1998 Faial earthquake in the Azores Islands, Portugal" (PDF). Institute of Structures, Territory and Construction Engineering, Higher Technical Institute, Portugal. {{cite journal}}: Cite journal requires |journal= (help)
  11. Guerreiro, L.; Azevedo, J.; Proença, J.; Bento, R.; Lopes, M. (2000). "Damage in Ancient Churches During the 9th of July 1998 Azores earthquake" (PDF). Proceedings of the 12th World Conference on Earthquake Engineering. Auckland, New Zealand.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  12. Brough, David (10 July 1991). "10 die, 90 injured in Azores earthquake". The Irish Times. Retrieved 26 August 2021.
  13. USAID (1 Jan 1999). "OFDA Annual Report for Fiscal Year 1998". ReliefWeb. Retrieved 26 August 2021.
  14. "10 killed, 100 injured by earthquake in Azores". Tampa Bay Times. 13 September 2005. Retrieved 26 August 2021.
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