Geological Summary for New Zealand Area, April 2009

Earthquake activity declined slightly. White Island and Mt. Ruapehu remained at Alert Level 1.

GeoNet, the U.S. Geological Survey and GNS Science reported 32 earthquakes in the New Zealand area between the Kermadec Islands in the north, and the Auckland Islands to the south during April 2009.

The magnitude distributions were as follows:
M6 to 6.9 (1), M5 to 5.9 (5), M4 to 4.9 (9) M3 to 3.9 (10).
An additional 7 events in the magnitude 2 range were deemed worthy of mention.

Only four earthquakes were reported near Raoul Island during April 2009. With magnitudes between 4.8 and 5.9, one of the quakes was 500 km deep, with the other three at 58 km or shallower.

The three earthquakes located near L’Esperance Rock in the southern Kermadecs included two quakes of magnitude 5.4 and 5.2 and one event of magnitude 6.1. The largest event was located 125 km north-north-east of the rock at a depth of 140 km at 7 minutes after midday on April 26th. With the epicentre 960 km north-east of Auckland, the event was not felt on the New Zealand mainland.

Five events were felt as part of the Matata earthquake swarm. The quakes, with magnitudes between 2.6 and 3.6 were located 10 km north or north-west of the coastal town at depths of 2 to 5 km.

A pair of earthquakes with magnitudes of 4.2 and 4.4 struck 40 km east of Gisborne on the 6th. They were part of a swarm of eleven shallow quakes of magnitude 3 or 4 that commenced on Friday the 3rd of April. They were a precursor to a larger quake of magnitude 5.0 which struck just before midnight on Wednesday the 8th. The foreshocks were 25 to 30 km deep with the main event slightly shallower at 20 km.

The main shock was felt strongly in Gisborne, triggering memories of the magnitude 6.6 earthquake of the 20th of December 2007 which was located nearby. During the 2007 quake, three buildings collapsed in Gisborne and many others were damaged. At the time of the magnitude 5.0 quake in April, many of the buildings damaged in 2007 were still awaiting permanent repair. Barriers were hastily re-erected around one of the seriously damaged buildings on the corner of Bright Street and Gladstone Road, known to many locals as Rosie’s corner. The barriers remained throughout the Easter break keeping pedestrians away from the weakened structure.

  Rosies corner April 2009  Cracked pillars, Gisborne, April 2009

Barriers were hastily re-erected around this damaged building on Gisborne’s main street following the magnitude 5 quake of the 8th of April.  The brick exterior shows damage sustained during the December 2007 earthquake.  [Wild Land photographs]

A pair of earthquakes struck within a few kilometres of Greytown on the 28th of April. The quakes of magnitude 3.2 and 4.3 were 12 and 30 km deep, respectively.

Only seven of April’s earthquakes were reported for locations in the South Island. The activity that commenced near Twizel at the end of March eased with only one event of magnitude 3.1 reported on the 17th of April.

Regular reporting of the status of New Zealand’s volcanoes ceased at the end of June 2007, with the closure of the Hazard Watch service. GNS Science now only issues bulletins which record significant changes in volcanic behaviour.

No Alert Bulletins were issued by GNS Science during April.

At the end of April 2009, New Zealand’s active volcano status can be summarised as follows:
Raoul Island (Alert Level 0).
White Island (Alert Level 1).
Mt Ngauruhoe (Alert Level 0).
Mt Ruapehu (Alert Level 1).

[Compiled from data supplied by GNS Science, US Geological Survey, GeoNet, and their contributing agencies.]

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