Tsunami Activity in New Zealand

A tsunami warning continues in place for New Zealand coastal areas following last night’s magnitude 8.8 earthquake in Chile.

Sunday 28th February 2010

A tsunami warning with waves of between 1 and 3 metres expected at coastal locations remains in force in New Zealand this morning. The tsunami was generated off the coast of South America when a magnitude 8.8 earthquake and several strong aftershocks struck Chile.

The warning has been in force since just before midnight when initial sea gauge measurements indicated that a tsunami had been generated by the earthquake. The Pacific Tsunami Warning Centre (PTWC) abruptly changed its outlook to severe and extended tsunami warnings to most Pacific Ocean nations at 11:45 p.m.

The Ministry of Civil Defence and Emergency Management (MCDEM) followed suit, and issued a warning that a threat to New Zealand existed at 12 minutes after midnight.

Regular bulletins have been issued by both the PTWC and MCDEM through the early hours of the morning, but authorities have been frustrated by the lack of sea level monitoring equipment available to give reports on the progress of the tsunami across the Pacific Ocean from South America. Sea level gauges off the coast of Chile continued reporting sea level disturbances with the highest measurements being 2.3 metres at Talcahuano, Chile at 7:53 p.m. and 1.29 metres at Valparaiso, Chile at 8:08 p.m.

The situation facing New Zealand remained unclear until four surges were detected at the Marquesas Islands from 6:41 a.m. Maximum sea level rises of 1.8 metres were measured by instruments..

In New Zealand, some commercial shipping put to sea to avoid being caught in port if a destructive disturbance occurred, and many private vessels and yachts did likewise.

The first sea level changes were detected at the Chatham Islands at about 7:30 a.m. with the sea level changes occurring at Raoul Island, East Cape, Gisborne, Napier and Castlepoint simultaneously at 8:50 a.m. As expected, the first changes were small, and subsequent surges have been increasing at an irregular rate.

Disturbances have continued at the Chatham Islands where maximum surges of 1 metre have been recorded. At nearby Pitt Island, surges had reached 1.5 metres by 9:50 this morning.

Some low-lying areas have been evacuated, with decisions being made locally. Most local authorities have left decisions on whether to evacuate to local residents at this stage, with Police blocking access to some areas to prevent sightseeing in high-risk areas.

In Gisborne, the sea level disturbances have followed a similar pattern to those in the Chatham Islands with sharp movements of water in Poverty Bay being observed over the last hour. GeoNet’s tsunami gauge at Gisborne indicates sea-level changes of about half a metre.

The massive earthquake struck at 7:34 p.m. New Zealand time last night. The magnitude 8.8 quake was located 325 km south-west of Chile’s capital Santiago, 115 km north-north-east of Concepcion. Shaking was was felt as far away as Sao Paulo in Brazil. Shaking was severe in Santiago where electrical supplies and water services have been disrupted. Damage is serious in Concepcion where highways and bridges have collapsed, as have buildings.

The earthquake occurred at the boundary between the Nazca and South American tectonic plates according to the U.S. Geological Survey, when the Nazca plate abruptly slid down and landward beneath the South American plate. Since 1973 there have been 13 earthquakes of magnitude 7.0 or greater on the Chilean coast.

The largest earthquake worldwide in at least 200 years, the magnitude 9.5 event of May 1960, occurred 230 km south of last night’s quake. This event claimed an estimated 1600 lives in Chile and generated a Pacific-wide tsunami which took another 200 lives in Japan, Hawaii and the Philippines. In New Zealand the largest tsunami of the century caused damage in Poverty Bay and Lyttelton with a maximum run-up of 3.5 metres.

In November 1922 a magnitude 8.4 earthquake struck about 870 km north of last night’s earthquake. This quake generated a 9 metre tsunami locally that inundated the coastal town of Coquimbo. The tsunami also crossed the Pacific Ocean but was measured at 20 cm in New Zealand.

Last night’s magnitude 8.8 earthquake ruptured the South American subduction zone between those two massive earthquakes.

Since the main earthquake, the Chilean subduction zone has experienced another 63 earthquakes of magnitude 4 or greater. This includes 52 events of magnitude 5 and seven magnitude 6 earthquakes. The largest aftershock was a magnitude 6.9 quake at 9:01 p.m. last night New Zealand time.

[Compiled from data supplied by the U.S. Geological Survey and its contributing agencies; the U.S. National Oceanic & Atmospheric Administration’s Pacific Tsunami Warning Centre; New Zealand’s Ministry of Civil Defence and Emergency Management; and GeoNet. Facts and figures from “Earthquakes,” G.A. Eiby, Heinemann Reed, 1989, and “Awesome Forces,” Hicks and Campbell, Te Papa Press, 1999. ]

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