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2009 Dusky Sound earthquake

Coordinates: 45°45′43″S 166°33′43″E / 45.762°S 166.562°E / -45.762; 166.562
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2009 Dusky Sound earthquake
USGS ShakeMap
2009 Dusky Sound earthquake is located in New Zealand
2009 Dusky Sound earthquake
UTC time2009-07-15 09:22:29
ISC event15157724
USGS-ANSSComCat
Local date15 July 2009
Local time21:22 NZST (UTC+13)
Magnitude7.8 Mw
Depth12 km (7 mi)
Epicentre45°45′43″S 166°33′43″E / 45.762°S 166.562°E / -45.762; 166.562
TypeReverse fault
Areas affectedNew Zealand
Max. intensityMMI VI (Strong)[1]
Tsunami230 cm (91 in)
Aftershocks>1,000
CasualtiesNone

The 2009 Dusky Sound earthquake was a Mw 7.8 earthquake that struck a remote region of Fiordland, New Zealand, on 15 July at 21:22 local time (09:22 UTC). It had an initially–reported depth of 12 km (7.5 mi), and an epicentre near Dusky Sound in Fiordland National Park, which is 160 km (99 mi) north-west of Invercargill.[1][2] It was the country's largest earthquake magnitude since the 1931 Hawke's Bay earthquake, but caused only minor damage and there were no casualties. A tsunami accompanied the event and had a maximum measured run-up of 230 cm (91 in).

Earthquake

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New Zealand's Institute of Geological and Nuclear Sciences (GNS Science) initially measured the earthquake at ML 6.6. The United States Geological Survey's preliminary measurement was Mw  8.0 before it was revised to 7.8.[3]: 24  It was New Zealand's biggest earthquake magnitude since the 1931 Hawke's Bay earthquake,[4][5] and equal in magnitude to the 2016 Kaikōura earthquake.[6][7] The 2009 earthquake occurred after a sequence of seismic events in northern Fiordland since 1988—six earthquakes above magnitude 6.0 struck the region prior to 2009,[8] including a magnitude 7.2 earthquake in 2003.[9]

The hypocentre was near Resolution Island in Dusky Sound, Fiordland, at a depth that was initially reported as 12 km (7.5 mi).[10][11] The earthquake was the result of shallow reverse faulting at the subduction zone of the Indo-Australian plate and Pacific plate.[4] At this location, the Indo-Australian plate subducts obliquely beneath the Pacific plate along the Puysegur Trench at a rate of 34 mm (1.3 in) per year. The oblique interaction is also accommodated by strike-slip deformation along the Alpine Fault.[3]: 24 

An early model of the rupture calculated that it initiated at a depth of 30 km (19 mi) and propagated southwards and upwards before it ceased just 15 km (9.3 mi) beneath the surface.[3] Subsequent analysis showed that the rupture started at 38 km (24 mi) depth and ended at 8 km (5.0 mi).[3]: 24  The rupture occurred within a 36 km × 44 km (22 mi × 27 mi) region on the subduction zone. Maximum displacement was estimated at 8.9 m (29 ft) about 23 km (14 mi) beneath the surface, occurring about 16 seconds after the earthquake began. As the rupture mainly propagated away from the mainland, the measured shaking intensity was less than what would normally be expected given the earthquake's magnitude and shallow depth.[3]: 26  The earthquake's epicentre was in a remote region where few seismic stations were installed, and as a result, limited recordings of strong ground motion were made.[3]: 29 [12]

Shaking reportedly lasted for at least a minute,[13] and was felt throughout the South Island and in the lower North Island as far away as New Plymouth.[14] GNS Science described the ground motion as relatively slow with a "lower frequency shaking" as opposed to abrupt, "rolling" motion from higher frequency shaking. The ground motion characteristic resulted in relatively few landslides.[10][15]

Thousands of aftershocks occurred, with most being too small to be felt.[16] One of the larger aftershocks measured magnitude 6.1, and occurred 19 minutes after the mainshock.[17][18][19] Most of them were distributed at the two edges of the rupture. These aftershocks had focal mechanisms indicating reverse and strike-slip faulting.[8]

Effects

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NASA map showing epicentre of earthquake and its aftershocks

There was only minor damage and no human casualties.[14][5] A few buildings in Invercargill experienced minor damage,[20] and power outages occurred in several parts of the South Island.[21][22] At least 241 landslides were recorded over an area of about 5,600 km2 (2,200 sq mi).[3]: 37  By comparison, the number of landslides was half that recorded after the 2003 Fiordland earthquake.[3]: 39 

From 21:30, KiwiRail suspended rail services south of Oamaru and in Buller Gorge to check for damage. Services were operational again by midnight because no damage was found.[17] Prime Minister John Key responded that people could "thank their lucky stars" due to the lack of damage.[11] In 2018, The New Zealand Herald described the earthquake as a "forgotten monster", due to it being followed by much more destructive quakes, such as the 2011 Christchurch earthquake.[5]

By 2010, the Earthquake Commission paid out $6.1 million in earthquake compensation after receiving 5,219 claims.[23] The low level of damage, despite the magnitude of the earthquake, was attributed to its remote location, slow moment release rate, low-frequency shaking,[10][15] and New Zealand's building standards.[23]

Puysegur Point, on the southwestern tip of the island, moved westwards by 30 cm (12 in). Te Anau moved 10 cm (3.9 in), Bluff, 3 cm (1.2 in), and Dunedin, 1 cm (0.39 in).[24][25] Around the epicentre, a large area was uplifted by approximately 1 m (3 ft 3 in).[19] The earthquake is also believed to have raised the Coulomb stress in the southern, offshore part of the Alpine Fault.[25][8]

Tsunami

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Tsunami warnings were issued soon after the earthquake by authorities in New Zealand and Australia, as well as by the Pacific Tsunami Warning Center in Hawaii.[26] Civil defence officials in Southland also issued a "potential tsunami" warning, stating their concerns about widely varying measurements of the earthquake.[21] Reacting to the Pacific warnings, about 50 residents and tourists on Lord Howe Island were evacuated, and in Sydney a theatre in Bondi Beach was evacuated and residents were told to keep away from the shore.[27] The tsunami warnings were subsequently cancelled or reduced.[14] The tsunami reached a height of 1 m (3 ft 3 in) in Jackson Bay, 25 cm (9.8 in) at Charleston, 12 cm (4.7 in) at Dog Island, 14 cm (5.5 in) at Port Kembla, and 6 cm (2.4 in) at Spring Bay.[1] A maximum run-up of 230 cm (91 in) was recorded; however, the tsunami may have been higher in some areas that were not surveyed due to the remoteness of the affected region.[28]

See also

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References

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  1. ^ a b c ANSS. "M 7.8 – 97 km WSW of Te Anau, New Zealand 2009". Comprehensive Catalog. U.S. Geological Survey.
  2. ^ "New Zealand Earthquake Report – Jul 15 2009 at 9:22 pm (NZST)". GeoNet. 15 July 2009. Archived from the original on 18 July 2009. Retrieved 16 July 2009.
  3. ^ a b c d e f g h Fry, B.; Bannister, S.; Beavan, J.; Bland, L.; Bradley, Brendon; Cox, S.; Cousins, J.; Gale, N.; Hancox, G.; Holden, C.; Jongens, R.; Power, W.; Prasetya, G.; Reyners, M.; Ristau, J. (2010). "The Mw 7.6 Dusky Sound Earthquake of 2009: Preliminary Report". Bulletin of the New Zealand Society for Earthquake Engineering. doi:10.5459/bnzsee.43.1.24-40. hdl:10092/4559.
  4. ^ a b "Fiordland quake biggest for 80 years". GeoNet. 16 July 2009. Archived from the original on 18 July 2009. Retrieved 21 July 2009.
  5. ^ a b c "Nine years ago: NZ's forgotten magnitude 7.8 monster quake struck Fiordland". The New Zealand Herald. 18 July 2018. Retrieved 20 May 2024.
  6. ^ "M7.8 Kaikoura quake the biggest since the Dusky Sound jolt in 2009", GNS Science (media release), 15 November 2016, retrieved 1 June 2024
  7. ^ ANSS. "M 7.8 – 53 km NNE of Amberley, New Zealand 2016". Comprehensive Catalog. U.S. Geological Survey.
  8. ^ a b c Mahesh, P.; Kundu, Bhaskar; Catherine, J. K.; Gahalaut, V. K. (1 January 2011). "Anatomy of the 2009 Fiordland earthquake (Mw 7.8), South Island, New Zealand". Geoscience Frontiers. 2 (1): 17–22. doi:10.1016/j.gsf.2010.12.002. ISSN 1674-9871.
  9. ^ ANSS. "M 7.2 – 56 km NW of Te Anau, New Zealand 2003". Comprehensive Catalog. U.S. Geological Survey.
  10. ^ a b c "GeoNet M7.8 Dusky Sound Wed, Jul 15 2009". GeoNet. Retrieved 20 May 2024.
  11. ^ a b Gorman, P. (17 August 2009). "'Soft rocks' prevented earthquake damage". New Zealand: Stuff. Retrieved 21 May 2024.
  12. ^ Hamling, I. J.; Hreinsdóttir, S., Postseismic deformation following the 2009 Mw7.8 Dusky Sound earthquake (journal), EQC Toka Tū Ake, retrieved 5 June 2024
  13. ^ "Tsunami alert after NZ earthquake". BBC News. 15 July 2009. Retrieved 15 July 2009.
  14. ^ a b c NZPA, Fairfax (15 July 2009). "Fiordland quake measures 7.8". New Zealand: Stuff. Retrieved 15 July 2009.
  15. ^ a b Cowlishaw, S. (31 July 2009). "Quake claims rise to almost $3 million". New Zealand: Stuff. Southland Times. Retrieved 20 May 2024.
  16. ^ "Aftershocks from large quake continue in Fiordland". Radio New Zealand. 28 July 2009. Retrieved 21 May 2024.
  17. ^ a b "Aftershocks hit Fiordland after quake". Otago Daily Times. 17 July 2009. Retrieved 21 May 2024.
  18. ^ "Magnitude 6.1, Wed Jul 15 2009 9:41 PM". GeoNet. Retrieved 5 June 2024.
  19. ^ a b "Ground raised one metre by quake". Radio New Zealand. 18 July 2009. Archived from the original on 27 July 2009. Retrieved 21 July 2009.
  20. ^ "Canterbury earthquake facts and figures – New Zealand Parliament". parliament.nz. 25 June 2024. Retrieved 2 June 2024.
  21. ^ a b Dye, Stuart; NZPA (15 July 2009). "South Island quake prompts 'potential tsunami' warning". The New Zealand Herald. Retrieved 15 July 2009.
  22. ^ "Quake strikes off New Zealand's Southland". Xinhua News Agency. 15 July 2009. Archived from the original on 17 July 2009. Retrieved 15 July 2009.
  23. ^ a b Earthquake Commission 2009–2010 Annual Report (PDF) (Report). EQC Toka Tū Ake. 2010. pp. 8–9. Retrieved 24 May 2024.
  24. ^ Ramnarayan, A. (22 July 2009). "Earthquake brings New Zealand closer to Australia". The Guardian. ISSN 0261-3077. Retrieved 20 May 2024.
  25. ^ a b Gormon, P. (22 July 2009). "Our Australian cousins just got closer". The Press. New Zealand: Stuff. Archived from the original on 26 July 2009. Retrieved 21 July 2009.
  26. ^ Beckford, G. (15 July 2009). "New Zealand quake sparks tsunami warnings". Reuters. Retrieved 15 July 2009.
  27. ^ "Fiordland quake causes evacuation in Sydney". Television New Zealand News. 16 July 2009. Archived from the original on 18 July 2009. Retrieved 21 July 2009.
  28. ^ Uslu, B.; Power, W.; Greenslade, D.; Eblé, M.; Titov, V. (2011). "The July 15, 2009 Fiordland, New Zealand Tsunami: Real-Time Assessment". Pure and Applied Geophysics. 168 (11): 1963–1972. Bibcode:2011PApGe.168.1963U. doi:10.1007/s00024-011-0281-7. S2CID 140714530.

Further reading

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  • Rogers, A. (2013) [1996]. The Shaky Isles: New Zealand Earthquakes (2 ed.). Wellington: Grantham House. pp. 185, 186. ISBN 978-1-86934-119-0.
  • Wright, Matthew (2014). Living on shaky ground: The science and story behind New Zealand's earthquakes. Auckland: Random House (New Zealand). pp. 174, 175. ISBN 978-1-77553-688-8.
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