1998 Azores Islands earthquake

1998 Azores Islands earthquake
1998 Azores Islands earthquake is located in Azores
1998 Azores Islands earthquake
Location within Azores
UTC time
  • 1998-07-09 05:19:07
ISC event
USGS-ANSS
Local date
  • 9 July 1998 (1998-07-09)
Local time
  • 05:19:07
Magnitude
Depth
  • 10 km (6 mi)
Epicenter12 km north of Horta
38°39′00″N 28°37′34″W / 38.650°N 28.626°W / 38.650; -28.626
TypeStrike-slip
Areas affectedAzores, Portugal
Max. intensityMMI VIII (Severe)
Aftershocks10,600 recorded
Casualties10 dead, 100 injured, 1,000 homeless

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 10 km (6.2 mi) deep mainshock, which measured 6.1 on the moment magnitude scale was preceded by a strong foreshock 18 minutes prior. The mainshock was caused by movement on a strike-slip fault. A highly productive aftershock sequence followed with more than 10,600 recorded within four months. These aftershocks were mainly distributed across two west-southwest and south-southeast striking faults that intersect orthogonally off the coast of Faial while some occurred on faults associated with the Pedro Miguel graben on the island. The regional tectonics and kinematics indicated the shock occurred on the west-southwest trending plane. Elastic dislocation modelling further suggested that interaction between the primary rupture and faults of the Pedro Miguel graben could explain the GPS and geodetic ground deformation. The shock also caused some surface ruptures on the graben faults.

The earthquake caused significant damage on Faial Island with the villages of Ribeirinha and Espalhafatos almost totally destroyed. At least 10 people died, 100 were injured, and 1,000 people were left without homes. More than 3,900 homes, most on Faial and some on Pico Island, were damaged. The severity of damage was due to the dominance of non-seismic resistant construction on the island. After the earthquake, the Azores Islands regional government began reconstruction works at a cost of €255 million.

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.[1] 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.[2]

Earthquake

Tectonic setting

The Azores Islands are situated atop a triple junction zone where the North American, Eurasian and African plates meet.[3] 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.[4] 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.[5]

Two zones of faulting occur around the 1998 earthquake epicenter. There are west-northwest–east-southeast and north-northwest–south-southeast trending fault systems with steep dip angles. The west-northwest–east-southeast striking faults exhibit oblique-slip faulting with sinistral and normal components.[5] On Faial Island, which rests above an elongated ridge connecting Pico Island, there is a rift zone (Pedro Miguel Graben) containing east-southeast trending faults that dip steeply.[3] The graben hosts seven active faults with normal and dextral slip components. South of the Caldeira Volcano caldera and on the western part of Faial Island, there are also faults with a similar strike.[5]

Geology

A strong foreshock was recorded at 05:01 which alerted residents near the epicenter area and those living in Horta. The mainshock occurred at 05:18 with an epicenter near that of the foreshock.[6] The earthquake was caused by movement on a strike-slip fault,[4] had a moment magnitude of 6.1, and depth of 10 km (6.2 mi).[7] It involved sinistral faulting along a north-northwest–south-southeast trending fault. The fault trend was also consistent with the interpretation of seismic data and aftershock distribution. While most earthquakes on the islands are associated with volcanism, the 1998 shock represented a rare and large one of tectonic origin.[4] The strike-slip faulting mechanism was also unusual as most earthquakes in the area occur because of normal faulting.[3]

In a 2002 journal article published by Geophysical Research Letters, GPS and geodetic data obtained two almost vertically-dipping rupture planes. The first was a west-southwest trending dextral fault with a dimension of 9.5 by 4.6 km (5.9 by 2.9 mi). The second plane was a south-southeast trending sinistral fault with a dimension of 9.6 by 4.2 km (6.0 by 2.6 mi). The dimensions of these faults are consistent with the expected size of the earthquake which they calculated to be Mw 6.1 which based on geodetic data. Both slip planes indicated coseismic displacement occurred at 2 to 6 km (1.2 to 3.7 mi) depth within the crust and peaking at ~1 m (3 ft 3 in).[8]

A major aftershock sequence persisted for four months with an estimated 10,600 events recorded. Many of them perceivable by residents in the affected area.[6] These aftershocks illuminated two fault planes trending west-southwest and south-southeast intersecting orthogonally. Some aftershocks occurred on the Ribeirinha and Lomba Grande faults of the Pedro Miguel graben. These aftershocks were located at depths of 3–13 km (1.9–8.1 mi) with the shallow ones occurring on the graben. The regional kinematics and damage distribution of the mainshock suggest the west-southwest plane was the source of the mainshock. Elastic dislocation modelling showed that a dextral fault with a dimension of 12 by 5.5 km (7.5 by 3.4 mi) slipping up to 1.17 m (3 ft 10 in), and slip on two graben faults (Ribeirinha and Lomba Grande faults) could reproduce the observed geodetic deformation, aftershock distribution, and damage. The rupture parameters of the graben faults were 2 by 1 km (1.24 by 0.62 mi) with 15 cm (5.9 in) of normal slip and 2.2 by 1 km (1.37 by 0.62 mi) with 34 cm (13 in) and 14 cm (5.5 in) of normal and sinistral displacement, respectively. The aftershocks on the south-southeast trending fault was likely a response to coulomb stress changes caused by the mainshock.[3]

Surface ruptures

Surface ruptures occurred on mapped faults in the northeastern part of Faial. They comprised discontinuous ground fractures up to 10 m (33 ft) long. These cracks are aligned with two primary fault trends of the area and showed various features. These included open fractures, some displaced to the right or left or vertically (normal), or a combination (oblique). The observed displacements were consistent with the slip sense of pre-existing faults. Small ridges occurred in areas where the fault's geometry limited the strike-slip component in favor of compression while pull-apart basins formed in areas where the fault bent caused localised extension. There was also ground deformation in the uplifted fault blocks of the Ribeirinha, Chã da Cruz and Lomba Grande faults. Dextral fractures and south-facing vertical steps on the order of several centimeters occurred near the Chã da Cruz Fault. There was also a 10–20 cm (3.9–7.9 in) scarp where the Lomba Grande Fault intersects a road.[9]

Ground motion

Strong ground motion map

The Modified Mercalli intensity on Faial ranged from V to VIII. The towns of Ribeirinha and Espalhafatos in northeastern Faial had damage consistent with intensity VIII.[5][6] There was also evidence of localised amplification in the area. The earthquake's intensity does not follow a simple seismic attenuation relationship with its distance from the epicenter. This was observed in Lombega, a village on southwest Faial which had intensity VII effects whereas the surrounding area was assigned intensity V. A similar macroseisimic characteristic was reported on Pico with the areas closest to the epicenter assigned intensity VI effects while Piedade at the eastern part experienced intensity III. Some areas of Pico experienced local VII effects including at Almagreira, whereas intensity IV was reported in the surrounding area. On São Jorge, areas closest to the epicenter was assigned intensity V while Topo at the eastern end experienced intensity III. The earthquake was also felt in Graciosa (IV), Terceira (III–IV), and São Miguel (III).[5]

Impact

The Ribeirinha lighthouse after the earthquake

A total of 10 people died, 100 were injured and 1,000 were homeless on Faial.[10][11] A total of 3,909 buildings were damaged; many of them were on Faial while some were damaged on Pico.[12] About 70 percent of homes on Faial was destroyed while 20 percent of them were damaged on Pico.[13]

The villages of Espalhafatos and Ribeirinha experienced near-total destruction (about 90 percent[5]) while some damage occurred in Pedro Miguel, Salão and Cedros. In these settlements, some 1,500 homes were heavily damaged and half had to be torn down. Recently-constructed buildings were undamaged even in areas of high damage. The building closest to the epicenter was the Ribeirinha lighthouse; its tower was heavily fractured to mid-height and rotated clockwise by 15–20 cm (5.9–7.9 in). The heavy damage to many homes was attributed to the traditional construction methods and materials used which were not designed for earthquakes. Many were made from stone and clay or mortar with wooden partition used as walls.[9] About 2,100 buildings damaged were located up to 30 km (19 mi) away from the epicenter. Most of these affected buildings had two or three floors. In some cases, their masonry walls completely collapsed or large cracks appeared in the exterior walls.[12]

Some schools on both islands had moderate damage. The worst-affected school was a kindergarten in Salão where its external masonry walls collapsed. The damage was so severe that it was later demolished. At a school in Espalhafatos, its concrete and reinforced masonry components separated. Many of the schools inspected for damage were constructed of reinforced-concrete thus received slight or no damage.[14] There were three churches constructed with reinforced concrete before the earthquake and did not sustain heavy damage. Many other churches that were severely damaged were predominantly constructed with masonry and a common damage pattern was shear failure of the walls. The Ribeirinha Church was one of the most worst-affected; the arch between the nave and apse completely collapsed.[15]

Aftermath

Rescuers spent the night retrieving the bodies of those killed in the rubble and looked for survivors. Concerned about aftershocks, residents of Faial and Pico islands slept inside tents provided by the government.[16] The Portuguese Air Force deployed a Lockheed C-130 Hercules plane carrying tents and thousand blankets from Lisbon; medical teams and supplies also arrived.[11] Prime minister António Guterres, visited Faial Island to survey the devastation.[17] 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 allocated US$20 thousand to the U.S. embassy for the Portuguese government to purchase lavatory items for the affected communities.[18] 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.[19]

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. About 72 percent of the reconstruction project was funded by the regional government while the remaining was from the nationwide public funds. The recostruction projects drastically changed the architecture of buildings on Faial as traditional stone and mud homes were replaced with ones built with blocks, iron and cement.[13]

See also

References

  1. ↑ 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.
  2. ↑ 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.
  3. 1 2 3 4 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. 633. Elsevier: 115–125. doi:10.1016/j.tecto.2014.06.024.
  4. 1 2 3 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. 435 (1–4). Elsevier: 37–54. Bibcode:2007Tectp.435...37B. doi:10.1016/j.tecto.2007.01.008. hdl:10174/6383.
  5. 1 2 3 4 5 6 Matias, L.; Dias, N. A.; Morais, I.; Vales, D.; Carrilho, F.; Madeira, J.; Gaspar, J. L.; Senos, L.; Silveira, A. B. (2007). "The 9th of July 1998 Faial Island (Azores, North Atlantic) seismic sequence". Journal of Seismology. 11: 275–298. doi:10.1007/s10950-007-9052-4.
  6. 1 2 3 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. Bibcode:2012BuEE...10....7O. doi:10.1007/s10518-011-9333-8. S2CID 109897250. Archived from the original (PDF) on 8 June 2024.
  7. ↑ International Seismological Centre, Bulletin of the ISC, Thatcham, United Kingdom [Event 1165443 ].
  8. ↑ Fernandes, R. M. S.; Miranda, J. M.; Catalão, J.; Luis, J. F.; Bastos, L.; Ambrosius, B. A. C. (2002). "Coseismic displacements of the MW = 6.1, July 9, 1998, Faial earthquake (Azores, North Atlantic)". Geophysical Research Letters. 29 (16): 21-1 – 21-4. doi:10.1029/2001GL014415.
  9. 1 2 Madeira, José; Brum da Silveira, António (2007). "Tectónica e sismicidade na ilha do Faial e o sismo de 9 de Julho de 1998". Boletim do Núcleo Cultural da Horta (in Portuguese). 16: 61–79.
  10. ↑ "Quake in Azores kills 10". The New York Times. Reuters. 10 July 1998. Retrieved 22 August 2026.
  11. 1 2 "UPI Focus: Strong earthquake kills 10 in Azores". United Press International. Lisbon, Portugal. 9 July 1998. Retrieved 26 August 2021.
  12. 1 2 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.
  13. 1 2 "Sismo de 1998 mudou a ilha do Faial para sempre" [The 1998 earthquake changed Faial Island forever] (in Portuguese). Observador. Lusa News Agency. 8 August 2016. Retrieved 22 August 2026.
  14. ↑ 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.
  15. ↑ 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.
  16. ↑ "A thousand homeless after quake". Reuters. The Straits Times. 10 July 1998. Retrieved 17 October 2024.
  17. ↑ Brough, David (10 July 1991). "10 die, 90 injured in Azores earthquake". The Irish Times. Retrieved 26 August 2021.
  18. ↑ USAID (1 January 1999). "OFDA Annual Report for Fiscal Year 1998". ReliefWeb. Retrieved 26 August 2021.
  19. ↑ "10 killed, 100 injured by earthquake in Azores". Tampa Bay Times. 13 September 2005. Retrieved 26 August 2021.
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